Encyclopedia of water science 9780849396274

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Volume I –II

Encyclopedia of

Water Science Second Edition

© 2008 by Taylor & Francis Group, LLC

Encyclopedias from Taylor & Francis Group Engineering Titles

Agriculture Titles Dekker Agropedia Collection (Eight Volume Set) ISBN: 978-0-8247-2194-7 Cat. No.: DK803X Encyclopedia of Agricultural, Food, and Biological Engineering Edited by Dennis R. Heldman ISBN: 978-0-8247-0938-9 Cat. No.: DK9381 Encyclopedia of Animal Science Edited by Wilson G. Pond and Alan Bell ISBN: 978-0-8247-5496-9 Cat. No.: DK2206 Encyclopedia of Pest Management Edited by David Pimentel ISBN: 978-0-8247-0632-6 Cat. No.: DK6323 Encyclopedia of Pest Management, Volume II Edited by David Pimentel ISBN: 978-1-4200-5361-6 Cat. No.: 53612 Encyclopedia of Plant and Crop Science Edited by Robert M. Goodman ISBN: 978-0-8247-0944-0 Cat. No.: DK1190 Encyclopedia of Soil Science, Second Edition (Two Volume Set) Edited by Rattan Lal ISBN: 978-0-8493-3830-4 Cat. No.: DK830X Encyclopedia of Water Science Edited by Stanley W. Trimble ISBN: 978-0-8493-9627-4 Cat. No.: DK9627

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© 2008 by Taylor & Francis Group, LLC

Volume I –II

Encyclopedia of

Water Science Second Edition

Stanley W. Trimble

Boca Raton London New York

CRC Press is an imprint of the Taylor & Francis Group, an informa business © 2008 by Taylor & Francis Group, LLC

Cover photograph courtesy of Edyta Zygmunt, University of Silesia.

CRC Press Taylor & Francis Group 6000 Broken Sound Parkway NW, Suite 300 Boca Raton, FL 33487‑2742 © 2008 by Taylor & Francis Group, LLC CRC Press is an imprint of Taylor & Francis Group, an Informa business No claim to original U.S. Government works Printed in the United States of America on acid‑free paper 10 9 8 7 6 5 4 3 2 1 International Standard Book Number‑13: 978‑0‑8493‑9627‑4 (Hardcover) This book contains information obtained from authentic and highly regarded sources. Reprinted material is quoted with permission, and sources are indicated. A wide variety of references are listed. Reasonable efforts have been made to publish reliable data and information, but the author and the publisher cannot assume responsibility for the validity of all materials or for the consequences of their use. Except as permitted under U.S. Copyright Law, no part of this book may be reprinted, reproduced, transmitted, or utilized in any form by any electronic, mechanical, or other means, now known or hereafter invented, including photocopying, microfilming, and recording, or in any information storage or retrieval system, without written permission from the publishers. For permission to photocopy or use material electronically from this work, please access www.copyright.com (http://www.copyright.com/) or contact the Copyright Clearance Center, Inc. (CCC) 222 Rosewood Drive, Danvers, MA 01923, 978‑750‑8400. CCC is a not‑for‑profit organization that provides licenses and registration for a variety of users. For organizations that have been granted a photocopy license by the CCC, a separate system of payment has been arranged. Trademark Notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation with‑ out intent to infringe. Library of Congress Cataloging‑in‑Publication Data Trimble, Stanley Wayne. Encyclopedia of water science / author, Stanley W. Trimble. ‑‑ 2nd ed. p. cm. Includes bibliographical references and index. ISBN 978‑0‑8493‑9627‑4 (alk. paper) ‑‑ ISBN 978‑0‑8493‑9617‑5 (alk. paper) ‑‑ ISBN 978‑0‑8493‑9616‑8 (alk. paper) 1. Water‑supply, Agricultural. 2. Water in agriculture. 3. Irrigation efficiency. I. Title. S494.5.W3E54 2007 627.03‑‑dc22 Visit the Taylor & Francis Web site at http://www.taylorandfrancis.com and the CRC Press Web site at http://www.crcpress.com

© 2008 by Taylor & Francis Group, LLC

2007035897

Encyclopedia of Water Science, Second Edition Stanley W. Trimble Editor-in-Chief Department of Geography, University of California, Los Angeles, Los Angeles, California, U.S.A.

Editorial Advisory Board Wil Burns Co-Chair, International Environmental Law Group, American Society of International Law, El Cerrito, California, U.S.A.

Biological & Agricultural Engineering, University of California, Davis, California, U.S.A.

Andrew Goudie St. Cross College, Oxford, U.K.

Clarence W. Richardson Laboratory Director, Grassland Soil and Water Research Laboratory, Agricultural Research Service, U.S. Department of Agriculture, Temple, Texas, U.S.A.

Mary Beth Kirkham Professor, Department of Agronomy, Kansas State University, Manhattan, Kansas, U.S.A. Miguel A. Marino Distinguished Professor of Hydrology, Civil and Environmental Engineering, and

Dennis Wichelns Rivers Institute at Hanover College, Hanover, Indiana, U.S.A.

v © 2008 by Taylor & Francis Group, LLC

Contributors

S.G.K. Adiku = Department of Soil Science, University of Ghana, Accra, Ghana Laj R. Ahuja = U.S. Department of Agriculture (USDA), Fort Collins, Colorado, U.S.A. J. David Aiken = Department of Agricultural Economics, Institute of Agriculture and Natural Resources, University of Nebraska, Lincoln, Nebraska, U.S.A. Tony Allan = King’s College London and School of Oriental and African Studies (SOAS), London, U.K. Richard G. Allen = Research and Extension Center, University of Idaho, Kimberly, Idaho, U.S.A. Leon Hartwell Allen Jr. = Crop Genetics and Environmental Research Unit, Agricultural Research Service (USDA-ARS), U.S. Department of Agriculture, University of Florida, Gainesville, Florida, U.S.A. Lal Khan Almas = Agricultural Business and Economics, West Texas A&M University, Canyon, Texas, U.S.A. Faye Anderson = School of Public Affairs, University of Maryland, College Park, Maryland, U.S.A. Michael A. Anderson = Department of Environmental Sciences, University of California–Riverside, Riverside, California, U.S.A. Randy L. Anderson = Northern Grain Insects Research Laboratory (NGIRL), Agricultural Research Service (USDA-ARS), U.S. Department of Agriculture, Brookings, South Dakota, U.S.A. A. N. Angelakis = Regional Foundation for Agricultural Research, National Foundation of Agricultural Research, Heracleion, Greece Ramo´n Aragu¨e´s = Agronomic Research Service, Diputacion General de Aragon, Zaragoza, Spain Jeffrey G. Arnold = Grassland Soil and Water Research Laboratory, U.S. Department of Agriculture (USDA), Temple, Texas, U.S.A. Francisco Arriaga = Department of Soil Science, University of Wisconsin–Madison, Madison, Wisconsin, U.S.A. Brent W. Auvermann = Texas A&M System Agriculture Research and Extension Center, Amarillo, Texas, U.S.A. James E. Ayars = U.S. Department of Agriculture (USDA), Parlier, California, U.S.A. James L. Baker = Department of Agricultural and Biosystems Engineering, Iowa State University, Ames, Iowa, U.S.A. John M. Baker = Agricultural Research Service (USDA-ARS), U.S. Department of Agriculture, University of Minnesota, St. Paul, Minnesota, U.S.A. Allah Bakhsh = Department of Irrigation and Drainage, University of Agriculture, Faisalabad, Pakistan Khaled M. Bali = Cooperative Extension Imperial County, University of California–Davis, Holtville, California, U.S.A. Javier Barragan = Department d’Enginyeria Agroforestal, University of Lleida, Lleida, Spain Dorothea Bartels = Botanisches Institut der Universitat, University of Bonn, Bonn, Germany Philip J. Bauer = U.S. Department of Agriculture (USDA), Florence, South Carolina, U.S.A. R. Louis Baumhardt = U.S. Department of Agriculture (USDA), Bushland, Texas, U.S.A. Steven Bednarz = Grassland Soil and Water Research Laboratory, U.S. Department of Agriculture (USDA), Temple, Texas, U.S.A.

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M.H. Behboudian = College of Sciences, Institute of Natural Resources, Massey University, Palmerston North, New Zealand Fethi BenJemaa = Office of Water Use Efficiency, California Department of Water Resources, Sacramento, California, U.S.A. Joseph K. Berry = W.M. Keck Visiting Scholar in Geosciences, University of Denver, Denver, Colorado, U.S.A. Stephen F. Blanchard = Office of Surface Water, U.S. Geological Survey (USGS), Reston, Virginia, U.S.A. George W. Bomar = Texas Department of Licensing and Regulation, Austin, Texas, U.S.A. Derek B. Booth = Center for Water and Watershed Studies, University of Washington, Seattle, Washington, U.S.A. John Borrelli = Department of Civil Engineering, Texas Tech University, Lubbock, Texas, U.S.A. David D. Bosch = Southeast Watershed Research Laboratory, Agricultural Research Service (USDA-ARS), U.S. Department of Agriculture, Tifton, Georgia, U.S.A. Thierry Boulard = Unit Plante et Systemes Horticoles, Institut National de la Recherche Agronomique (INRA), Avignon, France Vince Bralts = Agricutural and Biological Engineering, Purdue University, West Lafayette, Indiana, U.S.A. James R. Brandle = School of Natural Resources, University of Nebraska, Lincoln, Nebraska, U.S.A. Erik Braudeau = Se´questration du C dans les sols tropicaux, L’institut de Recherche pour le developpement-Montpellier, Fort de France, Martinique, France, and Centre de cooperation international en recherche agronomique pour le de´veloppement, Montpellier, France D. R. Bray = Department of Animal Sciences, University of Florida, Gainesville, Florida, U.S.A. David D. Breshears = Environmental Dynamics and Spatial Analysis Group, Los Alamos National Laboratory, Los Alamos, New Mexico, U.S.A. Sherry L. Britton = Plant Science and Water Conservation Research Laboratory (PSWCRL), Agricultural Research Service (USDA-ARS), U.S. Department of Agriculture, Stillwater, Oklahoma, U.S.A. Michael S. Brown = Division of Agriculture, West Texas A&M University, Canyon, Texas, U.S.A. Gary D. Bubenzer = Department of Biological Systems Engineering, College of Agriculture and Life Sciences, University of Wisconsin–Madison, Madison, Wisconsin, U.S.A. Graeme D. Buchan = Centre for Soil and Environmental Quality, Lincoln University, Canterbury, New Zealand Gerald W. Buchleiter = Water Management Research, Agricultural Research Service (USDA-ARS), U.S. Department of Agriculture, Fort Collins, Colorado, U.S.A. Robert D. Burman = Department of Agricultural Engineering, University of Wyoming, Laramie, Wyoming, U.S.A. Charles M. Burt = BioResource and Agricultural Engineering Department, California Polytechnic State University, San Luis Obispo, California, U.S.A. Tim P. Burt = Department of Geography, Science Laboratories, University of Durham, Durham, U.K. Amnon Bustan = Institutes for Applied Research, Ben-Gurion University of the Negev, Beer-Sheva, Israel Keith C. Cameron = Centre for Soil and Environmental Quality, Lincoln University, Canterbury, New Zealand Kenneth L. Campbell = Agricultural and Biological Engineering Department, University of Florida, Gainesville, Florida, U.S.A. Carl R. Camp Jr. = Coastal Plains Research Center, Agricultural Research Service (USDA-ARS), U.S. Department of Agriculture, Florence, South Carolina, U.S.A. Lucila Candela = Departmento Ingenieria del Terreno, Technical Univiersity of Catalonia – Universitat Politecnica de Catalunya (UPC), Barcelona, Spain Peter S. Carberry = Sustainable Ecosystems, Agricultural Production Systems Research Unit (APSRU), Commonwealth Scientific and Industrial Research Organisation (CSIRO), Toowoomba, Queensland, Australia

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Jesu´s Carrera = Technical University of Catalonia (UPC), Barcelona, Spain Dipankar Chakraborti = School of Environmental Studies, Jadavpur University, Calcutta, West Bengal, India J. W. Chandler = Department of Developmental Biology, University of Cologne, Cologne, Germany Andrew C. Chang = Department of Environmental Sciences, University of California–Riverside, Riverside, California, U.S.A. Andre Chanzy = Unite´ de Recherche Climat, Sol et Environnement, Institut National de la Recherche Agronomique (INRA), Avignon, France Philip Charlesworth = Davies Laboratory, Commonwealth Scientific and Industrial Research Organisation (CSIRO), Townsville, Queensland, Australia Alexander H.-D. Cheng = Department of Civil Engineering, University of Mississippi, University, Mississippi, U.S.A. George M. Chescheir = North Carolina State University, Raleigh, North Carolina, U.S.A. Uttam Kumar Chowdhury = School of Environmental Studies, Jadavpur University, Calcutta, West Bengal, India Gary A. Clark = Department of Biological and Agricultural Engineering, College of Engineering, Kansas State University, Manhattan, Kansas, U.S.A. Sharon A. Clay = Plant Science Department, South Dakota State University, Brookings, South Dakota, U.S.A. Albert J. Clemmens = U.S. Water Conservation Laboratory, Agricultural Research Service (USDA-ARS), U.S. Department of Agriculture, Phoenix, Arizona, U.S.A. Wayne Clyma = Water Management Consultant, Fort Collins, Colorado, U.S.A. Gretchen C. Coffman = Department of Environmental Health Sciences, University of California–Los Angeles, Los Angeles, California, U.S.A. Bonnie G. Colby = Department of Agricultural and Resource Economics, University of Arizona, Tucson, Arizona, U.S.A. W. Arden Colette = Agricultural Business and Economics, West Texas A&M University, Canyon, Texas, U.S.A. Ray Correll = Commonwealth Scientific and Industrial Research Organisation (CSIRO), Adelaide, South Australia, Australia Dennis L. Corwin = U.S. Department of Agriculture (USDA), Riverside, California, U.S.A. John E. Costa = U.S. Geological Survey (USGS), Vancouver, Washington, U.S.A. Robin K. Craig = College of Law, Florida State University, Tallahassee, Florida, U.S.A. Richard Cruse = Iowa State University, Ames, Iowa, U.S.A. Seth M. Dabney = National Sedimentation Laboratory, Upland Erosion Processes Research, Mid South Area, Agricultural Research Service (USDA-ARS), U.S. Department of Agriculture, Oxford, Mississippi, U.S.A. Jacob H. Dane = Department of Agronomy and Soils, Auburn University, Auburn, Alabama, U.S.A. Phillipe Debaeke = Unite d’Agronomie, Institut National de la Recherche Agronomique (INRA), Castanet-Tolosan, France Sherri L. DeFauw = University of Arkansas, Fayetteville, Arkansas, U.S.A. Jorge A. Delgado = Soil Plant Nutrient Research Unit, Agricultural Research Service (USDA-ARS), U.S. Department of Agriculture, Fort Collins, Colorado, U.S.A. G. R. Diak = Cooperative Institute for Meteorological Satellite Studies, University of Wisconsin–Madison, Madison, Wisconsin, U.S.A. Peter Dillon = Commonwealth Scientific and Industrial Research Organisation (CSIRO), Adelaide, South Australia, Australia Anisa Joy Divine = Resources Planning and Management Department, Imperial Irrigation District, Imperial, California, U.S.A. Pay Drechsel = West Africa Office, Cantonments, International Water Management Institute (IWMI), Accra, Ghana Tom L. Dudley = Marine Science Institute, Santa Barbara, California, U.S.A.

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William A. Dugas = Blackland Research and Extension Center, Texas Agricultural Experiment Station, Temple, Texas, U.S.A. Michael D. Dukes = Agricultural and Biological Engineering Department, University of Florida, Gainesville, Florida, U.S.A. Tim Dybala = Grassland Soil and Water Research Laboratory, U.S. Department of Agriculture (USDA), Temple, Texas, U.S.A. Simon O. Eching = California Department of Water Resources, Sacramento, California, U.S.A. Gabriel Eckstein = International Water Law Project, Silver Spring, Maryland, U.S.A. Bahman Eghball = Soil and Water Cons Research Unit, Agricultural Research Service (USDA-ARS), U.S. Department of Agriculture, Lincoln, Nebraska, U.S.A. Laura J Ehlers = Water Science and Technology Board, National Research Council, Washington, District of Columbia, U.S.A. F. A. El-Awar = Office of the Humanitarian Coordinator for Iraq (UNOHCI)—Baghdad, United Nations, New York, New York, U.S.A. Robert O. Evans = Department of Biological and Agricultural Engineering, North Carolina State University, Raleigh, North Carolina, U.S.A. Steven R. Evett = Agricultural Research Service (USDA-ARS), U.S. Department of Agriculture, Bushland, Texas, U.S.A. Norman R. Fausey = Soil Drainage Research Unit, Agricultural Research Service (USDA-ARS), U.S. Department of Agriculture, Ohio State University, Columbus, Ohio, U.S.A. Bruce K. Ferguson = School of Environmental Design, University of Georgia, Athens, Georgia, U.S.A. Charles W. Fetter Jr. = C. W. Fetter, Jr. Associates, Oshkosh, Wisconsin, U.S.A. Floron C. Faries Jr. = College of Veterinary Medicine, Texas A&M University, College Station, Texas, U.S.A. Martin V. Fey = Department of Soil Science, University of Stellenbosch, Matieland, Stellenbosch, South Africa Guy Fipps = Agricultural Engineering Department, Texas A&M University, College Station, Texas, U.S.A. Markus Flury = Center of Multiphase Environmental Research, Department of Crop and Soil Sciences, Washington State University, Pullman, Washington, U.S.A. Graham E. Fogg = Hydrology, Department of Land, Air and Water Resources (LAWR), University of California–Davis, Davis, California, U.S.A. James L. Fouss = Soil and Water Research Unit, Agricultural Research Service (USDA-ARS), U.S. Department of Agriculture, Baton Rouge, Louisiana, U.S.A. Kent Frame = California Department of Water Resources, Sacramento, California, U.S.A. Thomas G. Franti = Department of Biological Systems Engineering, University of Nebraska, Lincoln, Nebraska, U.S.A. Gary W. Frasier = U.S. Department of Agriculture (USDA), Fort Collins, Colorado, U.S.A. James R. Frederick = Department of Crop and Soil Environmental Science, Pee Dee Research and Education Center, Clemson University, Florence, South Carolina, U.S.A. Marcel Fuchs = Environmental Physics and Irrigation, Volcani Center, Institute of Soil, Water and Environmental Sciences, Agricultural Research Organization (ARO), Bet-Dagan, Israel Stuart R. Gagnon = University of Maryland, College Park, Maryland, U.S.A. Devin Galloway = U.S. Geological Survey (USGS), Sacramento, California, U.S.A. Anja Gassner = Institute of Science and Technology, Universiti Malaysia Sabah, Kota Kinabalu, Sabah, Malaysia David K. Gattie = Biological and Agricultural Engineering Department, College of Agricultural and Environmental Sciences, University of Georgia, Athens, Georgia, U.S.A. Glendon W. Gee = Battelle Pacific Northwest National Laboratory, Richland, Washington, U.S.A. Timothy R. Ginn = Department of Civil and Environmental Engineering, University of California– Davis, Davis, California, U.S.A.

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George Di Giovanni = Department of Plant Pathology and Mircobiology, Texas A&M University System Agriculture Research and Extension Center, El Paso, Texas, U.S.A. David A. Goldhamer = Kearney Agricultural Center, University of California, Parlier, California, U.S.A. Noel Gollehon = Resource Economics Division, U.S. Department of Agriculture (USDA), Washington, District of Columbia, U.S.A. Andrew S. Goudie = St. Cross College, Oxford, U.K. William L. Graf = University of South Carolina, Columbia, South Carolina, U.S.A. Stephen R. Grattan = Hydrologic Sciences, Department of Land, Air and Water Resources (LAWR), University of California–Davis, Davis, California, U.S.A. John R. Gray = Office of Surface Water, U.S. Geological Survey (USGS), Reston, Virginia, U.S.A. Patricia K. Haan = Biological and Agricultural Engineering Department, Texas A&M University, College Station, Texas, U.S.A. Ardell D. Halvorson = Agricultural Research Service (USDA-ARS-SPNRU), U.S. Department of Agriculture, Fort Collins, Colorado, U.S.A. Dorota Z. Haman = Agricultural and Biological Engineering, Institute of Food and Agricultural Sciences, University of Florida, Gainesville, Florida, U.S.A. Joel R. Hamilton = Department of Agricultural Economics, University of Idaho, Moscow, Idaho, U.S.A. Clayton L. Hanson = U.S. Department of Agriculture (USDA), Boise, Idaho, U.S.A. Gregory J. Hanson = Plant Science and Water Conservation Research Laboratory (PSWCRL), Agricultural Research Service (USDA-ARS), U.S. Department of Agriculture, Stillwater, Oklahoma, U.S.A. Mohamed M. Hantush = National Risk Management Research Laboratory, U.S. Environmental Protection Agency (US-EPA), Cincinnati, Ohio, U.S.A. Michael J. Hayes = National Drought Mitigation Center, University of Nebraska, Lincoln, Nebraska, U.S.A. Phillip D. Hays = Department of Geosciences, University of Arkansas, Fayetteville, Arkansas, U.S.A. John V. Headley = Aquatic Ecosystem Research Protection Branch, National Water Research Institute, Saskatoon, Saskatchewan, Canada Richard W. Healy = U.S. Geological Survey (USGS), Lakewood, Colorado, U.S.A. David L. Higgit = Department of Geography, National University of Singapore, Singapore, Singapore Robert W. Hill = Biological and Irrigation Engineering Department, Utah State University, Logan, Utah, U.S.A. Curtis H. Hinman = School of Agricultural, Human and Natural Resource Sciences, Washington State University, Tacoma, Washington, U.S.A. Michael C. Hirschi = University of Illinois, Urbana, Illinois, U.S.A. Kyle D. Hoagland = School of Natural Resources, University of Nebraska, Lincoln, Nebraska, U.S.A. Laurie Hodges = Department of Agronomy and Horticulture, University of Nebraska, Lincoln, Nebraska, U.S.A. Arjen Y. Hoekstra = Department of Water Engineering and Management, University of Twente, Enschede, The Netherlands Glenn J. Hoffman = Biological Systems Engineering, University of Nebraska, Lincoln, Nebraska, U.S.A. James E. Hook = NESPAL, University of Georgia, Tifton, Georgia, U.S.A. Jan W. Hopmans = Department of Land, Air and Water Resources (LAWR), University of California–Davis, Davis, California, U.S.A. Terry A. Howell = Conservation and Production Research Laboratory, Agricultural Research Service (USDA-ARS), U.S. Department of Agriculture, Bushland, Texas, U.S.A. Joel M. Hubbell = Idaho National Engineering and Environmental Laboratory (INEL), Idaho Falls, Idaho, U.S.A. Paul F. Hudak = Department of Geography, University of North Texas, Denton, Texas, U.S.A. Paul F. Hudson = Department of Geography and the Environment, University of Texas at Austin, Austin, Texas, U.S.A.

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Ray G. Huffaker = Department of Agricultural Economics, Washington State University, Pullman, Washington, U.S.A. Rodney L. Huffman = Department Biological and Agricultural Engineering, North Carolina State University, Raleigh, North Carolina, U.S.A. Frank J. Humenik = National Center for Manure and Animal Waste Management, North Carolina State University, Raleigh, North Carolina, U.S.A. John Hutson = School of Chemistry, Physics and Earth Sciences, Flinders University, Adelaide, South Australia, Australia Keith E. Idso = Center for the Study of Carbon Dioxide and Global Change, Tempe, Arizona, U.S.A. Sherwood B. Idso = U.S. Department of Agriculture (USDA), Tempe, Arizona, U.S.A. W. Jabre = Department of Soils, Irrigation and Mechinization, American University of Beirut, Beirut, Lebanon C. Rhett Jackson = Warnell School of Forest Resources, University of Georgia, Athens, Georgia, U.S.A. Sharad K. Jain = National Institute of Hydrology, Roorkee, Uttarakhand, India Bruce R. James = Natural Resource Sciences, University of Maryland, College Park, Maryland, U.S.A. A. D. Jayakaran = Belle Baruch Institute of Coastal Ecology and Forest Science, Georgetown, South Carolina, U.S.A. Dan Jaynes = National Soil Tilth Laboratory, Agricultural Research Service (USDA-ARS), U.S. Department of Agriculture, Ames, Iowa, U.S.A. Kevin Jeanes = ASK Laboratories, Inc., Amarillo, Texas, U.S.A. Gregory Jennings = Biological and Agricultural Engineering, North Carolina State University, Raleigh, North Carolina, U.S.A. Blanca Jimenez = Environmental Engineering, Universidad Nacional Auto´noma de Me´xico, Mexico City, Mexico Gregory L. Johnson = National Water and Climate Center, U.S. Department of Agriculture (USDA), Portland, Oregon, U.S.A. Ordie R. Jones = Amarillo, Texas, U.S.A. Rameshwar Kanwar = Agricultural and Biosystems Engineering Department, Iowa State University, Ames, Iowa, U.S.A. Fawzi Karajeh = Office of Water Use Efficiency, California Department of Water Resources, Sacramento, California, U.S.A. Andreas J. Karamanos = Laboratory of Crop Production, Agricultural University of Athens, Athens, Greece Timothy O. Keefer = Southwest Watershed Research Center, U.S. Department of Agriculture (USDA), Tucson, Arizona, U.S.A. Keith A. Kelling = Department of Soil Science, University of Wisconsin–Madison, Madison, Wisconsin, U.S.A. Rami Keren = Volcani Center, Institute of Soil, Water and Environmental Sciences, Agricultural Research Organization (ARO), Bet-Dagan, Israel Dennis C. Kincaid = Northwest Irrigation and Soils Research Laboratory, Pacific West Area, Agricultural Research Service (USDA-ARS), U.S. Department of Agriculture, Kimberly, Idaho, U.S.A. M. B. Kirkham = Department of Agronomy, Kansas State University, Manhattan, Kansas, U.S.A. Peter Kleinman = Pasture Systems and Watershed Management Research Laboratory, Agricultural Research Service (USDA-ARS), U.S. Department of Agriculture, University Park, Pennsylvania, U.S.A. Klaus Ku¨mmerer = Institute of Environmental Medicine and Hospital Epidemiology, University Hospital Freiburg, Freiburg, Germany Rai Kookana = Commonwealth Scientific and Industrial Research Organisation (CSIRO), Adelaide, South Australia, Australia John Kost = Department of Agronomy, Iowa State University, Ames, Iowa, U.S.A. Andrey G. Kostianoy = Ocean Experimental Physics Laboratory, P.P. Shirshov Institute of Oceanology, Russian Academy of Sciences, Moscow, Russia

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Demetris Koutsoyiannis = Department of Water Resources, School of Civil Engineering, National Technical University of Athens, Zographou, Greece Jacek A. Koziel = Texas A&M System Agriculture Research and Extension Center, Amarillo, Texas, U.S.A. Timothy P. Krantz = Salton Sea Database Program, University of Redlands, Redlands, California, U.S.A. William L. Kranz = Northeast Research and Extension Center, University of Nebraska, Norfolk, Nebraska, U.S.A. Tore Krogstad = Department of Soil and Water Sciences, Agricultural University of Norway, Aas, Norway William P. Kustas = Agricultural Research Service (USDA-ARS), U.S. Department of Agriculture, Beltsville, Maryland, U.S.A. John M. Laflen = Purdue University, Buffalo Center, Iowa, U.S.A. Rattan Lal = School of Natural Resources, Ohio State University, Columbus, Ohio, U.S.A. Freddie L. Lamm = Research and Extension Center, Kansas State University, Colby, Kansas, U.S.A. Matthias Langensiepen = Department of Modeling Plant Systems, Institute of Crop Science, Humboldt University of Berlin, Berlin, Germany Robert J. Lascano = Texas A&M University, Lubbock, Texas, U.S.A. Brian Leib = Biological Systems Engineering, Washington State University, Pullman, Washington, U.S.A. Jay A. Leitch = College of Business Administration, North Dakota State University, Fargo, North Dakota, U.S.A. Rick P. Leopold = National Resources Conservation Service, U.S. Department of Agriculture (USDA), Bryon, Texas, U.S.A. Delphis F. Levia = Department of Geography and Center for Climatic Research, University of Delaware, Newark, Delaware, U.S.A. Daniel G. Levitt = Science and Engineering Associates, Inc., Sante Fe, New Mexico, U.S.A. Vincenzo Levizzani = National Council of Research, Institute of Atmospheric Sciences and Climate, Bologna, Italy Hong Li = Citrus Research and Education Center, University of Florida, Lake Alfred, Florida, U.S.A. K.F. Andrew Lo = Department of Natural Resources, Chinese Culture University, Taipei, Taiwan Sally D. Logsdon = National Soil Tilth Laboratory, U.S. Department of Agriculture (USDA), Ames, Iowa, U.S.A. Hugo A. Loa´iciga = Department of Geography, University of California–Santa Barbara, Santa Barbara, California, U.S.A. Guy H. Loneragan = Feedlot Research Group, Division of Agriculture, West Texas A&M University, Canyon, Texas, U.S.A. Birl Lowery = Department of Soil Science, University of Wisconsin–Madison, Madison, Wisconsin, U.S.A. Richard Lowrance = Southeast Watershed Research Laboratory, Agricultural Research Service (USDA-ARS), U.S. Department of Agriculture, Tifton, Georgia, U.S.A. Xi Xi Lu = Department of Geography, National University of Singapore, Singapore, Singapore Stephan J. Maas = Texas Tech University, Lubbock, Texas, U.S.A. Babs Makinde-Odusola = Riverside Public Utilities, Riverside, California, U.S.A. Joseph R. Makuch = Water Quality Information Center, National Agricultural Library, Beltsville, Maryland, U.S.A. Kyle R. Mankin = Department of Biological and Agricultural Engineering, College of Engineering, Kansas State University, Manhattan, Kansas, U.S.A. Thomas Marek = Texas A&M System Agriculture Research and Extension Center, Amarillo, Texas, U.S.A. Miguel A. Marino = Hydrology, Civil and Environmental Engineering, and Biological & Agricultural Engineering, University of California–Davis, Davis, California, U.S.A.

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Dean A. Martens = Southwest Watershed Research Center, Agricultural Research Service (USDA-ARS), U.S. Department of Agriculture, Tucson, Arizona, U.S.A. Luciano Mateos = Instituto de Agricultura Sostenible, Consejo Superior de Investigaciones Cientıficas, Co´rdoba, Spain Donald K. McCool = Pacific West Area, Agricultural Research Service (USDA-ARS), U.S. Department of Agriculture, Pullman, Washington, U.S.A. Kevin B. McCormack = U.S. Environmental Protection Agency (US-EPA), Washington, District of Columbia, U.S.A. Richard McDowell = AgResearch Ltd., Invermay Agricultural Research Centre, Mosgiel, New Zealand Marshall J. McFarland = Texas A&M University, Stephenville, Texas, U.S.A. W. Allan McGinty = Texas A&M University System Agriculture Research and Extension Center, San Angelo, Texas, U.S.A. Gregory McIsaac = Natural Resources and Environmental Sciences, University of Illinois, Urbana, Illinois, U.S.A. Dan E. Mecklenburg = Ohio Department of Natural Resources, Division of Soil and Water Conservation, Columbus, Ohio, U.S.A. Mallavarapu Megharaj = Commonwealth Scientific and Industrial Research Organisation (CSIRO), Adelaide, South Australia, Australia Tessa Marie Mills = Environment and Risk Management Group, Horticulture and Food Research Institute of New Zealand, Palmerston North, New Zealand Paramjit Singh Minhas = Integrated Water Management, Indian Council of Agricultural Research (ICAR), Pusa, New Delhi, India M. Monirul Qadar Mirza = Adaptation and Impacts Research Group (AIRG), University of Toronto, Toronto, Ontario, Canada J. Kent Mitchell = Agricultural Engineering, University of Illinois, Urbana, Illinois, U.S.A. Steven B. Mitchell = School of the Environment, University of Brighton, Brighton, U.K. Rabi H. Mohtar = Department of Agricultural and Biological Engineering, Purdue University, West Lafayette, Indiana, U.S.A., and Centre de coope´ration international en recherche agronomique pour le de´veloppement, Montpellier, France Philip A. Moore Jr. = Poultry Production and Product Safety Research Unit, U.S. Department of Agriculture (USDA), Fayetteville, Arkansas, U.S.A. M. Susan Moran = Southwest Watershed Research Center, Agricultural Research Service (USDA-ARS), U.S. Department of Agriculture, Tucson, Arizona, U.S.A. James M. Morgan = Tamworth Centre for Crop Improvement, Tamworth, New South Wales, Australia James I.L. Morison = Department of Biological Sciences, University of Essex, Colchester, U.K. John R. Morse = Land Management and Water Conservation Research Unit, Agricultural Research Service (USDA-ARS), U.S. Department of Agriculture, Washington State University, Pullman, Washington, U.S.A. Miranda Y. Mortlock = Office of Economic and Statistical Research (OESR), Brisbane, Queensland, Australia Saqib Mukhtar = Biological and Agricultural Engineering Department, Texas A&M University, College Station, Texas, U.S.A. ˜oz-Carpena = Agricultural and Biological Engineering Department, University of Florida, Rafael Mun Gainesville, Florida, U.S.A. Ranjan S. Muttiah = Blackland Research and Extension Center, Texas Agricultural Experiment Station, Temple, Texas, U.S.A. Mahmood H. Nachabe = Department of Civil and Environmental Engineering, University of South Florida, Tampa, Florida, U.S.A. Ravendra Naidu = Commonwealth Scientific and Industrial Research Organisation (CSIRO), Adelaide, South Australia, Australia Mark A. Nearing = Southwest Watershed Research Center, Agricultural Research Service (USDA-ARS), U.S. Department of Agriculture, Tucson, Arizona, U.S.A.

© 2008 by Taylor & Francis Group, LLC

xv

David E. Nelson = Irrigation Engineering, P.E., Billings, Montana, U.S.A. Jerry Neppel = Department of Agronomy, Iowa State University, Ames, Iowa, U.S.A. Mary Nichols = Southwest Watershed Research Center, Agricultural Research Service (USDA-ARS), U.S. Department of Agriculture, Tucson, Arizona, U.S.A. John Nimmo = U.S. Geological Survey (USGS), Menlo Park, California, U.S.A. Janet G. Norris = Sterling C. Evans Library, Texas A&M University, College Station, Texas, U.S.A. Takashi Oguchi = Center for Spatial Information Science, Division of Spatial Information Analysis, University of Tokyo, Tokyo, Japan Iwao Ohtsu = Department of Civil Engineering, Nihon University College of Science and Technology, Tokyo, Japan Derrick M. Oosterhuis = Altheimer Laboratory, University of Arkansas, Fayetteville, Arkansas, U.S.A. Bill Orts = Cereal Processing Research Unit, Western Regional Research Center, U.S. Department of Agriculture (USDA), Albany, California, U.S.A. James D. Oster = Department of Environmental Sciences, University of California–Riverside, Riverside, California, U.S.A. Waite R. Osterkamp = Desert Laboratory, National Research Program, Water Resources Discipline (WRD), U.S. Geological Survey (USGS), Tucson, Arizona, U.S.A. Lloyd B. Owens = Agricultural Research Service (USDA-ARS), U.S. Department of Agriculture, Coshocton, Ohio, U.S.A. Ioan Paltineanu = Paltin International, Inc., Laurel, Maryland, U.S.A. Charalambos Papelis = Division of Hydrologic Sciences, Desert Research Institute, Las Vegas, Nevada, U.S.A. David B. Parker = Division of Agriculture, West Texas A&M University, Canyon, Texas, U.S.A. Henry (Hank) S. Parker = North Atlantic Area, Agricultural Research Service (USDA-ARS), U.S. Department of Agriculture, Wyndmoor, Pennsylvania, U.S.A. Dov Pasternak = International Crops Research Institute for the Semi-Arid Tropics, Niamey, Niger Kunal Paul = School of Environmental Studies, Jadavpur University, Calcutta, West Bengal, India William A. Payne = Crop Stress Physiology, Texas A&M System Agriculture Research and Extension Center, Amarillo, Texas, U.S.A. Kurt D. Pennell = School of Civil and Environmental Engineering, Georgia Institute of Technology, Atlanta, Georgia, U.S.A. Alain Perrier = Physique de l’Environnement et Re´gulations Biologiques des Echanges, Institut National Agronomique de Paris-Grignon (INAPG-INRA), Paris, France Kerry M. Peru = Environment Canada, National Hydrology Research Center (NWRI-NHRC), National Water Research Institute, Saskatoon, Saskatchewan, Canada Steven P. Phillips = U.S. Geological Survey (USGS), Sacramento, California, U.S.A. Joseph L. Pikul Jr. = Northern Grain Insect Research Laboratory, Agricultural Research Service (USDA-ARS), U.S. Department of Agriculture, Brookings, South Dakota, U.S.A. Suresh D. Pillai = Department of Poultry Science, Texas A&M University, College Station, Texas, U.S.A. Zvi Plaut = Volcani Center, Institute of Soil, Water and Environmental Sciences, Agricultural Research Organization (ARO), Bet-Dagan, Israel Daniel H. Pote = Dale Bumpers Small Farms Research Center, Agricultural Research Service (USDA-ARS), U.S. Department of Agriculture, Booneville, Arkansas, U.S.A. G. Erick Powell = Brockway Engineering, PLLC, Twin Falls, Idaho, U.S.A. Manzoor Qadir = Integrated Water and Land Management Program, International Center for Agricultural Research in the Dry Areas (ICARDA), Aleppo, Syria, and International Water Management Institute (IWMI), Colombo, Sri Lanka Quazi Quamruzzaman = Dhaka Community Hospital, Dhaka, Bangladesh Nancy N. Rabalais = W. J. DeFelice Marine Center, Louisiana Universities Marine Consortium, Chauvin, Louisiana, U.S.A. Nageswararao C. Rachaputi = Queensland Department of Primary Industries, Kingaroy, Queensland, Australia

© 2008 by Taylor & Francis Group, LLC

xvi

David E. Radcliffe = Department of Crop and Soil Sciences, University of Georgia, Athens, Georgia, U.S.A. Mohammad Mahmudur Rahman = School of Environmental Studies, Jadavpur University, Calcutta, West Bengal, India Liqa Raschid-Sally = West Africa Office, Cantonments, International Water Management Institute (IWMI), Accra, Ghana John C. Reagor = Veterinary Diagnostic Laboratory, Texas A&M University, College Station, Texas, U.S.A. Kenneth G. Renard = Southwest Watershed Research Center, Agricultural Research Service (USDA-ARS), U.S. Department of Agriculture, Tucson, Arizona, U.S.A. John A. Replogle = U.S. Water Conservation Laboratory, Agricultural Research Service (USDA-ARS), U.S. Department of Agriculture, Phoenix, Arizona, U.S.A. Marty B. Rhoades = Feedlot Research Group, West Texas A&M University, Canyon, Texas, U.S.A. Mark J. Roberson = Keller-Bleisner Engineering, LLC, Sacramento, California, U.S.A. Paul D. Robillard = Agricultural and Biological Engineering, Pennsylvania State University, University Park, Pennsylvania, U.S.A. Brett H. Robinson = Environment and Risk Management Group, Horticulture and Food Research Institute of New Zealand, Palmerston North, New Zealand Clay A. Robinson = Division of Agriculture, West Texas A&M University, Canyon, Texas, U.S.A. David A. Robinson = Geophysics, Stanford University, Stanford, California, U.S.A. Mark Robinson = Centre for Ecology and Hydrology, Oxfordshire, U.K. Renee Rockiki = Center for Modeling Hydrologic and Aquatic Systems, University of South Florida, Tampa, Florida, U.S.A. Judy A. Rogers = University of Arkansas, Fayetteville, Arkansas, U.S.A. William J. Rogers = Department of Life, Earth and Environmental Science, West Texas A&M University, Canyon, Texas, U.S.A. Wes Rosenthal = Blackland Research and Extension Center, Texas Agricultural Experiment Station, Temple, Texas, U.S.A. James F. Ruff = Engineering Research Center, Colorado State University, Fort Collins, Colorado, U.S.A. Loret M. Ruppe = Department of Civil and Environmental Engineering, University of California–Davis, Davis, California, U.S.A. John Ryan = Natural Resource Management Program, International Center for Agricultural Research in the Dry Areas (ICARDA), Aleppo, Syria D. W. Rycroft = Department of Civil and Environmental Engineering, Southampton University, Southampton, U.K. Edward John Sadler = Coastal Plains Soil, Water, and Plant Research Center, Agricultural Research Service (USDA-ARS), U.S. Department of Agriculture, Florence, South Carolina, U.S.A. Kyoji Saito = Department of Geography, Saitama University, Saitama, Japan R. Sakthivadivel = International Water Management Institute (IWMI), Colombo, Sri Lanka Hilmy Sally = International Water Management Institute (IWMI), Colombo, Sri Lanka Gary Sands = Department of Biosystems and Agricultural Engineering, University of Minnesota, St. Paul, Minnesota, U.S.A. Jan van Schilfgaarde = U.S. Department of Agriculture (USDA), Fort Collins, Colorado, U.S.A. Arland D. Schneider = Hedges Engineering and Consulting, Sumner, Washington, U.S.A., and U.S. Department of Agriculture (USDA), Bushland, Texas, U.S.A. Steven Schultz = Department of Agribusiness and Applied Economics, North Dakota State University, Fargo, North Dakota, U.S.A. Lawrence J. Schwankl = Department of Land, Air and Water Resources (LAWR), University of California–Davis, Davis, Californina, U.S.A. Linda Schweitzer = Department of Chemistry, Oakland University, Rochester, Michigan, U.S.A.

© 2008 by Taylor & Francis Group, LLC

xvii

Mark S. Seyfried = Northwest Watershed Research Center, Agricultural Research Service (USDA-ARS), U.S. Department of Agriculture, Boise, Idaho, U.S.A. Muhammad Siddique Shafique = Office for Project Services, United Nations, Lahore, Pakistan Michael C. Shannon = George E. Brown Jr. Salinity Laboratory, U.S. Department of Agriculture (USDA), Riverside, California, U.S.A. Andrew N. Sharpley = Pasture Systems and Watershed Management Research Laboratory, Agricultural Research Service (USDA-ARS), U.S. Department of Agriculture, University Park, Pennsylvania, U.S.A. Brenton S. Sharratt = Agricultural Research Service (USDA-ARS), U.S. Department of Agriculture, Washington State University, Pullman, Washington, U.S.A. Roger Shaw = University of California–Davis, Davis, California, U.S.A. John Sheehy = Crop, Soil and Water Sciences Division, International Rice Research Institute, Manila, Philippines F. Douglas Shields Jr. = National Sedimentation Laboratory, Agricultural Research Service (USDA-ARS), U.S. Department of Agriculture, Oxford, Mississippi, U.S.A. Adel Shirmohammadi = Biological Resources Engineering Department, University of Maryland, College Park, Maryland, U.S.A. Clinton C. Shock = Malheur Experiment Station, Oregon State University, Ontario, Oregon, U.S.A. Thomas R. Sinclair = Crop Genetics and Environmental Research Unit, U.S. Department of Agriculture (USDA), University of Florida, Gainesville, Florida, U.S.A. Vijay P. Singh = Department of Biological and Agricultural Engineering, Texas A&M University, College Station, Texas, U.S.A. James ‘‘Buck’’ Sisson = Idaho National Engineering and Environmental Laboratory (INEL), Idaho Falls, Idaho, U.S.A. Gaylord V. Skogerboe = Utah State University, Ogden, Utah, U.S.A. Jeffrey G. Skousen = West Virginia University, Morgantown, West Virginia, U.S.A. Douglas R. Smith = National Soil Erosion Laboratory, U.S. Department of Agriculture (USDA), Purdue University, West Lafayette, Indiana, U.S.A. James E. Smith = School of Geography and Geology, McMaster University, Hamilton, Ontario, Canada Jim T. Smith = Centre for Ecology and Hydrology, Winfrith Technology Centre, Dorchester, Dorset, U.K. Matt C. Smith = Department of Biological and Agricultural Engineering, Coastal Plain Experiment Station, University of Georgia, Tifton, Georgia, U.S.A. Sean M. Smith = Ecosystem Restoration Center, Maryland Department of Natural Resources, Annapolis, Maryland, U.S.A., and Geography and Environmental Engineering, Johns Hopkins University, Baltimore, Maryland, U.S.A. Richard L. Snyder = Department of Land, Air and Water Resources (LAWR), and Atmospheric Sciences, University of California–Davis, Davis, California, U.S.A. Robert E. Sojka = Northwest Irrigation and Soils Research Laboratory, U.S. Department of Agriculture (USDA), Kimberly, Idaho, U.S.A. Ken H. Solomon = BioResource and Agricultural Engineering Department, California Polytechnic State University, San Luis Obispo, California, U.S.A. Bretton Somers = Department of Geography and Anthropology, Louisiana State University, Baton Rouge, Louisiana, U.S.A. Marios A. Sophocleous = Kansas Geological Survey, University of Kansas, Lawrence, Kansas, U.S.A. Roy F. Spalding = Water Science Laboratory, University of Nebraska, Lincoln, Nebraska, U.S.A. James L. Starr = Environmental Chemistry Laboratory, National Resources Institute (NRI), Agricultural Research Service (USDA-ARS), U.S. Department of Agriculture, Beltsville, Maryland, U.S.A. Jean L. Steiner = Grazinglands Research Laboratory, Agricultural Research Service (USDA-ARS), U.S. Department of Agriculture, El Reno, Oklahoma, U.S.A. B. A. Stewart = Dryland Agriculture Institute, West Texas A&M University, Canyon, Texas, U.S.A.

© 2008 by Taylor & Francis Group, LLC

xviii

Richard J. Stirzaker = Commonwealth Scientific and Industrial Research Organisation (CSIRO), Canberra, Australian Capital Territory, Australia Claudio Osvaldo Stockle = Biological Systems Engineering, Washington State University, Pullman, Washington, U.S.A. Jeffry Stone = Southwest Watershed Research Center, Agricultural Research Service (USDA-ARS), U.S. Department of Agriculture, Tucson, Arizona, U.S.A. David A. Stonestrom = U.S. Geological Survey (USGS), Menlo Park, California, U.S.A. David E. Stooksbury = Biological and Agricultural Engineering Department, University of Georgia, Athens, Georgia, U.S.A. Rosemary Streatfeild = Owen Science and Engineering Library, Washington State University, Pullman, Washington, U.S.A. Scott J. Sturgul = Nutrient and Pest Management, University of Wisconsin–Madison, Madison, Wisconsin, U.S.A. I.H. (Mel) Suffet = Environmental Science and Engineering Program, University of California–Los Angeles, Los Angeles, California, U.S.A. Nicola Surian = Dipartimento di Geografia, University of Padova, Padova, Italy John M. Sweeten = Agricultural Research and Extension Center, Texas Agricultural Experiment Station (TAES), Amarillo, Texas, U.S.A. D. L. Tanaka = Northern Great Plains Research Laboratory (NGIRL), Agricultural Research Service (USDA-ARS), U.S. Department of Agriculture, Mandan, North Dakota, U.S.A. Kenneth K. Tanji = Department of Land, Air and Water Resources (LAWR), University of California–Davis, Davis, California, U.S.A. Mark Patrick Taylor = Department of Physical Geography, Macquarie University, Sydney, New South Wales, Australia Robert Teeter = Library, Santa Clara Valley Water District, San Jose, California, U.S.A. Bekele Temesgen = California Department of Water Resources, Sacramento, California, U.S.A. Daniel L. Thomas = Department of Biological and Agricultural Engineering, Coastal Plain Experiment Station, University of Georgia, Tifton, Georgia, U.S.A. Thomas L. Thurow = Department of Renewable Resources, University of Wyoming, Laramie, Wyoming, U.S.A. Jeanette Thurston-Enriquez = Soil and Water Cons Research Unit, U.S. Department of Agriculture (USDA), Lincoln, Nebraska, U.S.A. Judy A. Tolk = Agricultural Research Service (USDA-ARS), U.S. Department of Agriculture, Bushland, Texas, U.S.A. Desiree Tullos = Biological and Ecological Engineering, Oregon State University, Corvallis, Oregon, U.S.A. Neil C. Turner = Centre for Mediterranean Agricultural Research, Plant Industry, Commonwealth Scientific and Industrial Research Organization (CSIRO), Wembley, Western Australia, Australia Ronald W. Tuttle = U.S. Department of Agriculture (USDA), Chantilly, Virginia, U.S.A. Andree Tuzet = Environnement et Grandes Cultures, Institut National de la Recherche Agronomique (INRA-INAPG), Thiverval Grignon, France Melvin T. Tyree = Aiken Forestry Sciences Laboratory, Forest Service, U.S. Department of Agriculture (USDA), Burlington, Vermont, U.S.A. Darrel N. Ueckert = Texas A&M System Agriculture Research and Extension Center, San Angelo, Texas, U.S.A. Paul W. Unger = Conservation and Production Research Laboratory, Agricultural Research Service (USDA-ARS), U.S. Department of Agriculture, Bushland, Texas, U.S.A. John Van Brahana = Division of Geology, Department of Geosciences, University of Arkansas, Fayetteville, Arkansas, U.S.A. H. H. Van Horn = Department of Animal Sciences, University of Florida, Gainesville, Florida, U.S.A.

© 2008 by Taylor & Francis Group, LLC

xix

R. Scott Van Pelt = Wind Erosion and Water Conservation Research Unit, Cropping Systems Research Laboratory, Agricultural Research Service (USDA-ARS), U.S. Department of Agriculture, Big Spring, Texas, U.S.A. George F. Vance = Agricultural Experiment Station, University of Wyoming, Laramie, Wyoming, U.S.A. Curtis J. Varnell = Environmental Dynamics, University of Arkansas, Fayetteville, Arkansas, U.S.A. Oleg Voitsekhovitch = Ukrainian Hydrometeorological Institute, Kiev, Ukraine Joseph C. V. Vu = Crop Genetics and Environmental Research Unit, Agricultural Research Service (USDA-ARS), U.S. Department of Agriculture, University of Florida, Gainesville, Florida, U.S.A. H. Jesse Walker = Department of Geography and Anthropology, Louisiana State University, Baton Rouge, Louisiana, U.S.A. Wynn R. Walker = Biological and Irrigation Engineering Department, Utah State University, Logan, Utah, U.S.A. Desmond E. Walling = Department of Geography, University of Exeter, Exeter, Devon, U.K. Ivan A. Walter = Ivan’s Engineering, Inc., W.W. Wheeler, Denver, Colorado, U.S.A. Dong Wang = Department of Soil, Water and Climate, University of Minnesota, St. Paul, Minnesota, U.S.A. Pao K. Wang = Atmospheric and Oceanic Sciences, University of Wisconsin–Madison, Madison, Wisconsin, U.S.A. Ynuzhang Wang = Academy of Yellow River Conservancy Science, Zhengzhou, Henan, P.R. China Anderson L. Ward = Battelle Pacific Northwest National Laboratory, Richland, Washington, U.S.A. Andrew D. Ward = Food, Agricultural and Biological Engineering, Ohio State University, Columbus, Ohio, U.S.A. Ashley A. Webb = Research and Development Division, Forests NSW, Coffs Harbour Jetty, New South Wales, Australia Ole Wendroth = Department of Plant and Soil Sciences, University of Kentucky, Lexington, Kentucky, U.S.A. Walter W. Wenzel = University of Agricultural Sciences, Vienna, Austria Hal Werner = Agricultural and Biosystems Engineering, South Dakota State University, Brookings, South Dakota, U.S.A. Mark E. Westgate = Department of Agronomy, Iowa State University, Ames, Iowa, U.S.A. French Wetmore = French & Associates, Ltd., Park Forest, Illinosis, U.S.A. Norman K. Whittlesey = Department of Agricultural Economics, Washington State University, Pullman, Washington, U.S.A. Dennis Wichelns = Rivers Institute at Hanover College, Hanover, Indiana, U.S.A. Penny E. Widdison = Department of Geography, Science Laboratories, University of Durham, Durham, U.K. Keith D. Wiebe = Resource Economics Division, U.S. Department of Agriculture (USDA), Washington, District of Columbia, U.S.A. Bradford P. Wilcox = Department of Rangeland Ecology and Management, Texas A&M University, College Station, Texas, U.S.A. Mark Wilf = Hydranautics, Oceanside, California, U.S.A. Donald A. Wilhite = National Drought Mitigation Center, University of Nebraska, Lincoln, Nebraska, U.S.A. Lyman S. Willardson = Department of Biological and Irrigation Engineering, Utah State University, Logan, Utah, U.S.A. Dennis E. Williams = Geoscience Support Services, Inc., Claremont, California, U.S.A. Jonathan Witter = Food, Agricultural and Biological Engineering, Ohio State University, Columbus, Ohio, U.S.A. David A. Woolhiser = Colorado State University, Fort Collins, Colorado, U.S.A.

© 2008 by Taylor & Francis Group, LLC

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Stephen R. Workman = University of Kentucky, Lexington, Kentucky, U.S.A. Jon M. Wraith = Department of Land Resources and Environmental Sciences, Montana State University, Bozeman, Montana, U.S.A. Graeme C. Wright = Queensland Department of Primary Industries, Farming Systems Institute, Kingaroy, Queensland, Australia I. Pai Wu = College of Tropical Agriculture and Human Resources, University of Hawaii, Honolulu, Hawaii, U.S.A. Laosheng Wu = Department of Environmental Sciences, University of California–Riverside, Riverside, California, U.S.A. Richard T. Wynne = National Weather Service, National Oceanic and Atmospheric Administration (NOAA), Amarillo, Texas, U.S.A. Youichi Yasuda = Department of Civil Engineering, Nihon University College of Science and Technology, Tokyo, Japan C. Dean Yonts = Biological Systems Engineering, Panhandle Research and Extension Center, University of Nebraska, Scottsbluff, Nebraska, U.S.A. Michael H. Young = Division of Hydrologic Sciences, Desert Research Institute, Las Vegas, Nevada, U.S.A. Robert A. Young = Colorado State University, Estes Park, Colorado, U.S.A. Bofu Yu = School of Environmental Engineering, Griffith University, Nathan, Queensland, Australia Xinhua Zhou = School of Natural Resources, University of Nebraska, Lincoln, Nebraska, U.S.A. Yifei Zhu = Gemune LLC., Fremont, California, U.S.A. Zixi Zhu = Henan Institute of Meteorology, Zhengzhou, Henan, P.R. China

© 2008 by Taylor & Francis Group, LLC

Contents

Contributors . . . . . . . . . . Topical Table of Contents Preface to Second Edition Preface to First Edition . .

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Academic Disciplines = Joseph R. Makuch and Paul D. Robillard . . . . . . . . . . . . . . . . . . Acid Rain and Precipitation Chemistry = Pao K. Wang . . . . . . . . . . . . . . . . . . . . . . . . . . African Market Garden = Amnon Bustan and Dov Pasternak . . . . . . . . . . . . . . . . . . . . . Agricultural Runoff: Characteristics = Matt C. Smith, Daniel L. Thomas and David K. Gattie Agroforestry: Enhancing Water Use Efficiency = James R. Brandle, Xinhua Zhou and Laurie Hodges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Alluvial Fan = Takashi Oguchi and Kyoji Saito . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ancient Greece: Agricultural Hydraulic Works = Demetris Koutsoyiannis and A. N. Angelakis Ancient Greece: Hydrologic and Hydraulic Science and Technology = A. N. Angelakis and Demetris Koutsoyiannis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ancient Greece: Urban Water Engineering and Management = Demetris Koutsoyiannis and A. N. Angelakis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Antitranspirants: Film-Forming Types = Zvi Plaut . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Antitranspirants: Stomata Closing Types = Zvi Plaut . . . . . . . . . . . . . . . . . . . . . . . . . . . Aquifers = Miguel A. Marino . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Aquifers: Artificial Recharge = Steven P. Phillips . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Aquifers: Ogallala = B. A. Stewart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Aquifers: Recharge = John Nimmo, David A. Stonestrom and Richard W. Healy . . . . . . . . . Aquifers: Transmissivity = Mohamed M. Hantush . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Aral Sea Disaster = Guy Fipps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Arctic Hydrology = Bretton Somers and H. Jesse Walker . . . . . . . . . . . . . . . . . . . . . . . . Boron = Rami Keren . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bound Water in Soils = Sally D. Logsdon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Brahmaputra Basin = Vijay P. Singh and Sharad K. Jain . . . . . . . . . . . . . . . . . . . . . . . . Canal Automation = Albert J. Clemmens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chemical Measurement = Kevin Jeanes and William J. Rogers . . . . . . . . . . . . . . . . . . . . . Chemigation = William L. Kranz . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chernobyl Accident: Impacts on Water Resources = Jim T. Smith and Oleg Voitsekhovitch . . . Chesapeake Bay = Sean M. Smith . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chromium = Bruce R. James . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conservation: Tillage and No-Tillage = Paul W. Unger . . . . . . . . . . . . . . . . . . . . . . . . . . Consumptive Water Use = Freddie L. Lamm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Crop Coefficients = Richard G. Allen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Crop Development Models = Peter S. Carberry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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31 39 43 46 49 53 55 59 62 66 70 74 78 82 86 91 96 102 105 110 113 117 121

xxii

Crop Plants: Critical Developmental Stages of Water Stress = Zvi Plaut . . . . . . . . . . . . Crop Residues: Snow Capture by = Donald K. McCool, Brenton S. Sharratt and John R. Morse . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cyanobacteria in Eutrophic Freshwater Systems = Anja Gassner and Martin V. Fey . . . . Dam Removal = Desiree Tullos and Gregory Jennings . . . . . . . . . . . . . . . . . . . . . . . Darcy’s Law = Keith C. Cameron and Graeme D. Buchan . . . . . . . . . . . . . . . . . . . . Databases = Rosemary Streatfeild . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Deltas = Paul F. Hudson . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Desalination = Fawzi Karajeh and Fethi BenJemaa . . . . . . . . . . . . . . . . . . . . . . . . Drainage and Water Quality = James L. Baker . . . . . . . . . . . . . . . . . . . . . . . . . . . . Drainage Coefficient = Gary Sands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Drainage: Controlled = Robert O. Evans . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Drainage: Hydrologic Impacts = D. W. Rycroft and Mark Robinson . . . . . . . . . . . . . . Drainage: Inadequacy and Crop Response = Norman R. Fausey . . . . . . . . . . . . . . . . . . Drainage: Irrigated Land = James E. Ayars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Drainage: Land Shaping = Rodney L. Huffman . . . . . . . . . . . . . . . . . . . . . . . . . . . Drainage: Materials = James L. Fouss . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Drainage: Modeling = George M. Chescheir . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Drainage: Soil Salinity Management = Glenn J. Hoffman . . . . . . . . . . . . . . . . . . . . . Drinking Water Supply Distribution Systems = Laura J Ehlers . . . . . . . . . . . . . . . . . . Drip Lines and Emitters: Acidification for Prevention of Clogging = Andrew C. Chang . . . Drip Lines and Emitters: Chlorination for Disinfection and Prevention of Clogging = Andrew C. Chang . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Drought = Donald A. Wilhite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Drought Hardening and Pre-Sowing Seed Hardening = Neil C. Turner . . . . . . . . . . . . . Drought: Avoidance and Adaptation = J. W. Chandler and Dorothea Bartels . . . . . . . . . Drought: Management = Donald A. Wilhite and Michael J. Hayes . . . . . . . . . . . . . . . Drought: Resistance = M. B. Kirkham . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Dryland and Semiarid Regions: Research Centers = John Ryan . . . . . . . . . . . . . . . . . . Dryland Cropping Systems = William A. Payne . . . . . . . . . . . . . . . . . . . . . . . . . . . Dryland Farming = Clay A. Robinson . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Dust Bowl Era = R. Louis Baumhardt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ˜o = David E. Stooksbury . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . El Nin Energy Dissipation Structures = Iwao Ohtsu and Youichi Yasuda . . . . . . . . . . . . . . . . Erosion and Precipitation = Bofu Yu . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Erosion and Productivity = Francisco Arriaga and Birl Lowery . . . . . . . . . . . . . . . . . Erosion Control: Mechanical = Mark A. Nearing, Mary Nichols, Kenneth G. Renard and Jeffry Stone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Erosion Control: Tillage/Residue Methods = Jerry Neppel, Krisztina Eleki, John Kost and Richard Cruse . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Erosion Control: Vegetative = Seth M. Dabney . . . . . . . . . . . . . . . . . . . . . . . . . . . . Erosion Problems: Historical Review = Andrew S. Goudie . . . . . . . . . . . . . . . . . . . . . Erosion Research: History = Rattan Lal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Erosion Research: Instrumentation = Gary D. Bubenzer . . . . . . . . . . . . . . . . . . . . . . Erosion: Accelerated = Michael C. Hirschi and J. Kent Mitchell . . . . . . . . . . . . . . . . . Erosion: Bank = Steven B. Mitchell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Erosion: Prediction = Mark A. Nearing, Mary Nichols, Kenneth G. Renard and Jeffrey Stone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Erosion: Process Modeling = John M. Laflen . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

© 2008 by Taylor & Francis Group, LLC

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xxiii

Eutrophication = Kyle D. Hoagland and Thomas G. Franti . . . . . . . . . . . . . . . . . . . . . . Evaporation = Alain Perrier and Andree Tuzet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Evaporation and Eddy Correlation = Roger Shaw and Richard L. Snyder . . . . . . . . . . . . . Evaporation and Energy Balance = Matthias Langensiepen . . . . . . . . . . . . . . . . . . . . . . Evaporation from Lakes and Large Bodies of Water = John Borrelli . . . . . . . . . . . . . . . . Evaporation from Soils = Andre Chanzy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Evapotranspiration: Canopy Architecture and Climate Effects = Delphis F. Levia . . . . . . . . Evapotranspiration: Formulas = Robert D. Burman . . . . . . . . . . . . . . . . . . . . . . . . . . . Evapotranspiration: Greenhouses = Thierry Boulard . . . . . . . . . . . . . . . . . . . . . . . . . . . Evapotranspiration: Reference and Potential = Marcel Fuchs . . . . . . . . . . . . . . . . . . . . . Evapotranspiration: Remote Sensing = G. R. Diak, William P. Kustas and M. Susan Moran . Evapotranspiration: Weather Station Network Information = Simon O. Eching, Kent Frame, Bekele Temesgen and Richard L. Snyder . . . . . . . . . . . . . . . . . . . . . . . Everglades = Kenneth L. Campbell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Farm Ponds = Ronald W. Tuttle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fertilizer and Pesticide Leaching: Irrigation Management = Luciano Mateos . . . . . . . . . . . Field Water Supply and Balance = Jean L. Steiner . . . . . . . . . . . . . . . . . . . . . . . . . . . . Filtration and Particulate Removal = Lawrence J. Schwankl . . . . . . . . . . . . . . . . . . . . . Floodplain Management = French Wetmore . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Floods and Flooding = Steven Schultz and Jay A. Leitch . . . . . . . . . . . . . . . . . . . . . . . . Flouride = Judy A. Rogers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Flow Measurement: History = James F. Ruff . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fluvial Islands = Waite R. Osterkamp . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Frozen Soil: Water Movement in = John M. Baker . . . . . . . . . . . . . . . . . . . . . . . . . . . . Furrow Dikes = Ordie R. Jones and R. Louis Baumhardt . . . . . . . . . . . . . . . . . . . . . . . Ganges River = M. Monirul Qadar Mirza . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Giant Reed (Arundo donax): Effects on Streams and Water Resources = Gretchen C. Coffman Global Temperature Change and Terrestrial Ecology = Sherwood B. Idso and Keith E. Idso . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Groundwater: Contamination = Charles W. Fetter Jr. . . . . . . . . . . . . . . . . . . . . . . . . . . Groundwater: Contamination, Arsenic = Dipankar Chakraborti, Uttam Kumar Chowdhury, Kunal Paul, Mohammad Mahmudur Rahman and Quazi Quamruzzaman . . . . . . . . . . Groundwater: Mapping Levels = Marios A. Sophocleous . . . . . . . . . . . . . . . . . . . . . . . . Groundwater: Measuring Levels = Paul F. Hudak . . . . . . . . . . . . . . . . . . . . . . . . . . . . Groundwater: Mining = Hugo A. Loa
iciga . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Groundwater: Modeling = Jesu
s Carrera . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Groundwater: Modeling Using Numerical Methods = Jesu
s Carrera . . . . . . . . . . . . . . . . . Groundwater: Pollution from Mining = Jeffrey G. Skousen and George F. Vance . . . . . . . . Groundwater: Pollution from Nitrogen Fertilizers = Lloyd B. Owens . . . . . . . . . . . . . . . . . Groundwater: Pollution from Phosphorous Fertilizers = Bahman Eghball . . . . . . . . . . . . . Groundwater: Pumping and Land Subsidence = Steven P. Phillips and Devin Galloway . . . . Groundwater: Pumping Methods = Dennis E. Williams . . . . . . . . . . . . . . . . . . . . . . . . . Groundwater: Quality = Loret M. Ruppe and Timothy R. Ginn . . . . . . . . . . . . . . . . . . . Groundwater: Quality and Irrigated Agriculture = Stephen R. Grattan . . . . . . . . . . . . . . . Groundwater: Regulation = Kevin B. McCormack . . . . . . . . . . . . . . . . . . . . . . . . . . . . Groundwater: Saltwater Intrusion = Alexander H.-D. Cheng . . . . . . . . . . . . . . . . . . . . . Groundwater: Western United States Law = J. David Aiken . . . . . . . . . . . . . . . . . . . . . . Groundwater: World Resources = Lucila Candela . . . . . . . . . . . . . . . . . . . . . . . . . . . .

© 2008 by Taylor & Francis Group, LLC

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xxiv

Hydrologic Cycle = John Van Brahana . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hydrologic Management of Contaminated Sites Using Vegetation = Tessa Marie Mills and Brett H. Robinson . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hydrologic Process Modeling = Matt C. Smith and Daniel L. Thomas . . . . . . . . . . . . . . . Hydrology Research Centers = Daniel L. Thomas . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hypoxia: Gulf of Mexico = Nancy N. Rabalais . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Internet = Janet G. Norris . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Irrigated Agriculture: Economic Impacts of Investments = Robert A. Young . . . . . . . . . . . . Irrigated Agriculture: Endangered Species Policy = Norman K. Whittlesey, Ray G. Huffaker and Joel R. Hamilton . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Irrigated Agriculture: Historical View = Lyman S. Willardson . . . . . . . . . . . . . . . . . . . . Irrigated Agriculture: Managing toward Sustainability = Wayne Clyma, Muhammad Siddique Shafique and Jan van Schilfgaarde . . . . . . . . . . . . . . . . . . . Irrigated Agriculture: Social Impacts = Gaylord V. Skogerboe . . . . . . . . . . . . . . . . . . . . Irrigated Water: Market Role in Reallocating = Bonnie G. Colby . . . . . . . . . . . . . . . . . . Irrigated Water: Polymer Application = Bill Orts and Robert E. Sojka . . . . . . . . . . . . . . . Irrigation Design: Steps and Elements = Gary A. Clark . . . . . . . . . . . . . . . . . . . . . . . . . Irrigation Districts and Similar Organizations = David E. Nelson and Anisa Joy Divine . . . . Irrigation Economics: Global = Keith D. Wiebe and Noel Gollehon . . . . . . . . . . . . . . . . . Irrigation Economics: United States = Noel Gollehon . . . . . . . . . . . . . . . . . . . . . . . . . . Irrigation Management: Humid Regions = Carl R. Camp Jr., Edward John Sadler and James E. Hook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Irrigation Management: Tropics = R. Sakthivadivel and Hilmy Sally . . . . . . . . . . . . . . . . Irrigation Return Flow and Quality = Kenneth K. Tanji and Ramo
n Aragu€e
s . . . . . . . . . . Irrigation Sagacity (IS) = Charles M. Burt and Ken H. Solomon . . . . . . . . . . . . . . . . . . Irrigation Scheduling: Plant Indicators (Field Application) = David A. Goldhamer . . . . . . . . Irrigation Scheduling: Plant Indicators (Measurement) = David A. Goldhamer . . . . . . . . . . Irrigation Scheduling: Remote Sensing Technologies = Stephan J. Maas . . . . . . . . . . . . . . Irrigation Scheduling: Soil Water Status = Philip Charlesworth and Richard J. Stirzaker . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Irrigation Scheduling: Water Budgeting = Brian Leib and Claudio Osvaldo Stockle . . . . . . Irrigation Systems: Drip = I. Pai Wu, Javier Barragan and Vince Bralts . . . . . . . . . . . . . . Irrigation Systems: History = Wayne Clyma . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Irrigation Systems: Subsurface Drip = Carl R. Camp Jr. and Freddie L. Lamm . . . . . . . . . Irrigation: Deficit = M.H. Behboudian . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Irrigation: Efficiency = Terry A. Howell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Irrigation: Frost Protection and Bloom Delay = Robert W. Hill and Richard L. Snyder . . . . Irrigation: Impact on River Flows = Robert W. Hill and Ivan A. Walter . . . . . . . . . . . . . . Irrigation: Metering = John A. Replogle and Albert J. Clemmens . . . . . . . . . . . . . . . . . . Irrigation: Preplant = James E. Ayars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Irrigation: Saline Water = B. A. Stewart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Irrigation: Sewage Effluent Use = B. A. Stewart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Irrigation: Site-Specific = Dennis C. Kincaid and Gerald W. Buchleiter . . . . . . . . . . . . . . Irrigation: Sprinklers (Mechanical) = Dennis Kincaid . . . . . . . . . . . . . . . . . . . . . . . . . . Irrigation: Supplemental = Phillipe Debaeke . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Irrigation: Surface = Wynn R. Walker . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Isotopes = Michael A. Anderson . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Journals = Joseph R. Makuch and Stuart R. Gagnon . . . . . . . . . . . . . . . . . . . . . . . . . . Karst Aquifers = John Van Brahana . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

© 2008 by Taylor & Francis Group, LLC

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611 615 618 622 633 638 640 646 650 655 658 662 664 666 670 675 678 684 688 693

xxv

Karst Aquifers: Water Quality and Water Resource Problems = John Van Brahana . . . . . . . . . . ˜a = David E. Stooksbury . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . La Nin Land Drainage: Subsurface = Stephen R. Workman . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Land Drainage: Wells = Rameshwar Kanwar and Allah Bakhsh . . . . . . . . . . . . . . . . . . . . . Leaf Water Potential = Andreas J. Karamanos . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Library Resources = Robert Teeter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Livestock and Poultry Production: Water Consumption = David B. Parker and Michael S. Brown . . . Livestock Water Quality Standards = Floron C. Faries Jr., Guy H. Loneragan, John C. Reagor and John M. Sweeten . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Livestock: Water Harvesting Methods = Gary W. Frasier . . . . . . . . . . . . . . . . . . . . . . . . . . Low-Impact Development = Curtis H. Hinman and Derek B. Booth . . . . . . . . . . . . . . . . . . . . Manure Management: Beef Cattle Industry Requirements = Brent W. Auvermann and John M. Sweeten . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Manure Management: Dairy = D. R. Bray and H. H. Van Horn . . . . . . . . . . . . . . . . . . . . . . Manure Management: Poultry = Patricia K. Haan and Saqib Mukhtar . . . . . . . . . . . . . . . . . Manure Management: Swine = Frank J. Humenik . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Marketing = Lal Khan Almas and W. Arden Colette . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Matric Potential = Melvin T. Tyree . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Microbial Sampling = George Di Giovanni and Suresh D. Pillai . . . . . . . . . . . . . . . . . . . . . Mining Impact: Metals = Mark Patrick Taylor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Natural Levees = Paul F. Hudson . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Neuse River = Curtis J. Varnell and John Van Brahana . . . . . . . . . . . . . . . . . . . . . . . . . . . Nitrogen Measurement = Jacek A. Koziel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nonpoint Source Pollution (NPSP) = Ray Correll, Peter Dillon, Rai Kookana, Mallavarapu Megharaj, Ravendra Naidu and Walter W. Wenzel . . . . . . . . . . . . . . . . . . . Nutrients: Best Management Practices = Keith A. Kelling and Scott J. Sturgul . . . . . . . . . . . . Observation Wells = Phillip D. Hays and John Van Brahana . . . . . . . . . . . . . . . . . . . . . . . On-Farm Irrigation Flow Measurement = Lawrence J. Schwankl . . . . . . . . . . . . . . . . . . . . . . Open-Channel Flow Rate Measurements = John E. Costa and Stephen F. Blanchard . . . . . . . . Open-Channel Spillways = Sherry L. Britton and Gregory J. Hanson . . . . . . . . . . . . . . . . . . . Oxygen Measurement: Biological–Chemical Oxygen Demand = David B. Parker and Marty B. Rhoades . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Pathogens: General Characteristics = Jeanette Thurston-Enriquez . . . . . . . . . . . . . . . . . . . . . Pathogens: Transport by Water = Graeme D. Buchan and Markus Flury . . . . . . . . . . . . . . . . Pesticide Contamination: Groundwater = Roy F. Spalding . . . . . . . . . . . . . . . . . . . . . . . . . . Pesticide Contamination: Surface Water = Sharon A. Clay . . . . . . . . . . . . . . . . . . . . . . . . . Pfiesteria Piscicida = Henry (Hank) S. Parker . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . pH = William J. Rogers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Pharmaceuticals in Water Supplies = Klaus Ku€mmerer . . . . . . . . . . . . . . . . . . . . . . . . . . . . Phosphorus: Inputs into Freshwater from Agriculture = Richard McDowell . . . . . . . . . . . . . . . Phosphorus: Measurement = Bahman Eghball and Daniel H. Pote . . . . . . . . . . . . . . . . . . . . Phosphorus: Transport in Riverine Systems = Peter Kleinman, Andrew N. Sharpley, Tore Krogstad and Richard McDowell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Plant Water Stress: Exposure during Specific Growth Stages = Zvi Plaut . . . . . . . . . . . . . . . . Plant Water Stress: Optional Parameters for Stress Relief = Zvi Plaut . . . . . . . . . . . . . . . . . . Plant Water Use: Stomatal Control = James I.L. Morison . . . . . . . . . . . . . . . . . . . . . . . . . . Plant Yield and Water Use = M. H. Behboudian and Tessa Marie Mills . . . . . . . . . . . . . . . . . Plants: Critical Growth Periods = M. H. Behboudian and Tessa Marie Mills . . . . . . . . . . . . . . Plants: Osmotic Adjustment = James M. Morgan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

© 2008 by Taylor & Francis Group, LLC

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xxvi

Plants: Osmotic Potential = Mark E. Westgate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Plants: Salt Tolerance = Michael C. Shannon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Platte River = William L. Graf . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Pollution: Point Source (PS) = Ray Correll, Peter Dillon, Rai Kookana, Mallavarapu Megharaj, Ravendra Naidu and Walter W. Wenzel . . . . . . . . . . . . . . . . Porous Pavements = Bruce K. Ferguson . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Precipitation: Distribution Patterns = Gregory L. Johnson . . . . . . . . . . . . . . . . . . . . . . . Precipitation: Forms = Richard T. Wynne . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Precipitation: Measurement = Marshall J. McFarland . . . . . . . . . . . . . . . . . . . . . . . . . Precipitation: Modification = George W. Bomar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Precipitation: Remote Sensing Measurement = Marshall J. McFarland . . . . . . . . . . . . . . . Precipitation: Simulation Models = Timothy O. Keefer . . . . . . . . . . . . . . . . . . . . . . . . . Precipitation: Stochastic Properties = David A. Woolhiser . . . . . . . . . . . . . . . . . . . . . . . Precipitation: Storms = Clayton L. Hanson . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Precision Agriculture and Water Use = Robert J. Lascano and Hong Li . . . . . . . . . . . . . . Precision Conservation = Joseph K. Berry and Jorge A. Delgado . . . . . . . . . . . . . . . . . . Professional Societies = Faye Anderson . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Psychrometry: Accuracy, Interpretation, and Sampling = Derrick M. Oosterhuis . . . . . . . . . Psychrometry: Theory, Types, and Uses = Derrick M. Oosterhuis . . . . . . . . . . . . . . . . . . Pumps: Displacement = Dorota Z. Haman . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Pumps: Internal Combustion Engines = Hal Werner . . . . . . . . . . . . . . . . . . . . . . . . . . . Rainfall Shelters = Arland D. Schneider . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Rainfed Farming = Edward John Sadler, James R. Frederick and Philip J. Bauer . . . . . . . Rainwater Harvesting = K.F. Andrew Lo . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Rangeland Management: Enhanced Water Utilization = Darrel N. Ueckert and W. Allan McGinty . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Rangeland Water Yield: Influence of Brush Clearing = Jeffrey G. Arnold, Steven Bednarz, Tim Dybala, William A. Dugas, Ranjan S. Muttiah and Wes Rosenthal . . . . . . . . . . . . Rangelands: Water Balance = Bradford P. Wilcox, David D. Breshears and Mark S. Seyfried Research Organizations = Gabriel Eckstein . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Residential Irrigation Water Use and Control = Michael D. Dukes . . . . . . . . . . . . . . . . . Reverse Osmosis = Mark Wilf . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Richards’ Equation = Graeme D. Buchan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ring and Tension Infiltrometers = Laosheng Wu and Dong Wang . . . . . . . . . . . . . . . . . . Riparian Geomorphology = Ashley A. Webb . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . River Channelization = Nicola Surian . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . River Channels = Paul F. Hudson . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Rural Water Supply: Water Harvesting = F. A. El-Awar, Rabi H. Mohtar and W. Jabre . . . Sacramento–San Joaquin Delta = Mark J. Roberson . . . . . . . . . . . . . . . . . . . . . . . . . . Saline Seeps = Ardell D. Halvorson . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Saline Water = Khaled M. Bali . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Salton Sea = Timothy P. Krantz . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Satellite Sensing: Atmospheric Water = Vincenzo Levizzani . . . . . . . . . . . . . . . . . . . . . . Scaling Processes in Watersheds = Ole Wendroth and David A. Robinson . . . . . . . . . . . . . Seas: Dead and Dying = Andrey G. Kostianoy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sediment Budgets = Desmond E. Walling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sediment Load = Xi Xi Lu and David L. Higgit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Selenium = Dean A. Martens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

© 2008 by Taylor & Francis Group, LLC

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xxvii

Software Development = Matthias Langensiepen . . . . . . . . . . . . . . . . . . . . . . . . . . Soil Macropores: Water and Solute Movement = David E. Radcliffe . . . . . . . . . . . . . Soil Moisture Measurement by Feel and Appearance = Rick P. Leopold . . . . . . . . . . . Soil Water Measurement: Capacitance = Ioan Paltineanu and James L. Starr . . . . . . Soil Water Measurement: Granular Matrix Sensors = Clinton C. Shock . . . . . . . . . . . Soil Water Measurement: Gravimetric = Joseph L. Pikul Jr. . . . . . . . . . . . . . . . . . . Soil Water Measurement: Neutron Thermalization = Steven R. Evett . . . . . . . . . . . . . Soil Water Measurement: Soil Probes = Clay A. Robinson . . . . . . . . . . . . . . . . . . . Soil Water Measurement: Tensiometers = James ‘‘Buck’’ Sisson and Joel M. Hubbell . . Soil Water Measurement: Time Domain Reflectometry = Steven R. Evett . . . . . . . . . . Soil Water: Antecedent = Sally D. Logsdon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Soil Water: Capillary Rise = James E. Smith . . . . . . . . . . . . . . . . . . . . . . . . . . . . Soil Water: Diffusion = Laosheng Wu . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Soil Water: Energy Concepts = Sally D. Logsdon . . . . . . . . . . . . . . . . . . . . . . . . . Soil Water: Flow under Saturated Conditions = Jan W. Hopmans and Graham E. Fogg . Soil Water: Flow under Unsaturated Conditions = Jacob H. Dane and Jan W. Hopmans Soil Water: Functions in Pedostructure = Erik Braudeau and Rabi H. Mohtar . . . . . . . Soil Water: Hysteresis = Dan Jaynes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Soil Water: Plant-Available = Judy A. Tolk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Soil Water: Salinity Measurement = Dennis L. Corwin . . . . . . . . . . . . . . . . . . . . . . Soil Water: Sensor-Based Automatic Irrigation of Vegetable Crops = Michael D. Dukes and Rafael Mun~oz-Carpena . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Soils: Field Capacity = Laj R. Ahuja, Mahmood H. Nachabe and Renee Rockiki . . . . . Soils: Hydraulic Conductivity Rates = Adel Shirmohammadi and David D. Bosch . . . . Soils: Hygroscopic Water Content = Daniel G. Levitt and Michael H. Young . . . . . . . Soils: Permanent Wilting Points = Judy A. Tolk . . . . . . . . . . . . . . . . . . . . . . . . . . Soils: Water Infiltration = Sally D. Logsdon . . . . . . . . . . . . . . . . . . . . . . . . . . . . Soils: Water Percolation = Kurt D. Pennell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Soils: Waterborne Chemicals Leaching through = John Hutson . . . . . . . . . . . . . . . . . Stomatal Responses to the Environment: Quantifying = Matthias Langensiepen . . . . . . Storativity and Specific Yield = Hugo A. Loa
iciga and Paul F. Hudak . . . . . . . . . . . . Stormwater Management = Derek B. Booth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Streambank Stabilization = F. Douglas Shields Jr. . . . . . . . . . . . . . . . . . . . . . . . . . Summer Fallow = D. L. Tanaka and Randy L. Anderson . . . . . . . . . . . . . . . . . . . . Surface Water: Concentrated Animal Feeding Operations = Douglas R. Smith and Philip A. Moore Jr. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Surface Water: Nitrogen Enrichment = Tim P. Burt and Penny E. Widdison . . . . . . . . Surface Water: Pollution by Nitrogen Fertilizers = Gregory McIsaac . . . . . . . . . . . . . Surface Water: Pollution by Surface Mines = Jeffrey G. Skousen and George F. Vance . Surface Water: Quality and Phosphorous Applications = Andrew N. Sharpley, Peter Kleinman and Richard McDowell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Surface Water: Western United States Law = J. David Aiken . . . . . . . . . . . . . . . . . . Suspended-Sediment Transport Measurement = John R. Gray . . . . . . . . . . . . . . . . . . Tailwater Recovery and Reuse = C. Dean Yonts . . . . . . . . . . . . . . . . . . . . . . . . . . Tamarisk = Tom L. Dudley . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Timber Harvesting: Influence on Water Yield and Water Quality = C. Rhett Jackson . . . Time Domain Reflectometry: Salinity and Solute Measurement = Jon M. Wraith . . . . . . Total Maximum Daily Load (TMDL) = Robin K. Craig . . . . . . . . . . . . . . . . . . . . .

© 2008 by Taylor & Francis Group, LLC

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xxviii

Transpiration = Thomas R. Sinclair . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Transpiration: Carbon Dioxide and Plants = Leon Hartwell Allen Jr., Joseph C. V. Vu and John Sheehy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Transpiration: Efficiency = Graeme C. Wright and Nageswararao C. Rachaputi . . . . . . . . Transpiration: Scaling from Leaves and Canopies = Matthias Langensiepen . . . . . . . . . . . Transpiration: Water Use Efficiency = Miranda Y. Mortlock . . . . . . . . . . . . . . . . . . . . Two-Stage Channel Geometry: Active Floodplain Requirements = Andrew D. Ward, Jonathan Witter, A. D. Jayakaran, Dan E. Mecklenburg and G. Erick Powell . . . . . . Uptake by Plant Roots = S.G.K. Adiku . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Uptake by Plant Roots: Modeling Water Extraction = S. G. K. Adiku . . . . . . . . . . . . . . Urban Hydrology = C. Rhett Jackson . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Urban Water Quality = Linda Schweitzer and I.H. (Mel) Suffet . . . . . . . . . . . . . . . . . Vadose Zone and Groundwater Protection = Charalambos Papelis and Michael H. Young . Vapor Transport in Dry Soils = Anderson L. Ward and Glendon W. Gee . . . . . . . . . . . . Virtual Water = Tony Allan and Dennis Wichelns . . . . . . . . . . . . . . . . . . . . . . . . . . Virtual Water: Economic Perspective = Dennis Wichelns . . . . . . . . . . . . . . . . . . . . . . . Virtual Water: Measuring Flows around the World = Arjen Y. Hoekstra . . . . . . . . . . . . . Wastewater Use in Agriculture: Agronomic Considerations = Manzoor Qadir, Liqa Raschid-Sally and Pay Drechsel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Wastewater Use in Agriculture: Empirical Evidence = Liqa Raschid-Sally and Pay Drechsel Wastewater Use in Agriculture: Public Health Considerations = Blanca Jimenez . . . . . . . . Wastewater Use in Agriculture: Saline and Sodic Waters = Manzoor Qadir and Paramjit Singh Minhas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Water Balance Scheduling in Arid Regions = Richard L. Snyder, Simon Eching, Kent Frame and Bekele Temesgen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Water Footprints = Arjen Y. Hoekstra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Water Harvesting = Gary W. Frasier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Water Properties = James D. Oster . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Water Quality: Modeling = Richard Lowrance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Water Quality: Range and Pasture Land = Thomas L. Thurow . . . . . . . . . . . . . . . . . . . Water Quality: Sampling of Runoff from Agricultural Fields = John M. Laflen . . . . . . . . . Wellhead Protection = Babs Makinde-Odusola . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Wells: Drilling = Thomas Marek . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Wells: Hydraulics = Mohamed Hantush . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Wetlands and Petroleum = John V. Headley and Kerry M. Peru . . . . . . . . . . . . . . . . . . Wetlands as Treatment Systems = Kyle R. Mankin . . . . . . . . . . . . . . . . . . . . . . . . . . Wetlands: Ecosystems = Sherri L. DeFauw . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Wind Erosion and Water Resources = R. Scott Van Pelt . . . . . . . . . . . . . . . . . . . . . . . Yellow River = Zixi Zhu, Ynuzhang Wang and Yifei Zhu . . . . . . . . . . . . . . . . . . . . . .

© 2008 by Taylor & Francis Group, LLC

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Topical Table of Contents

Climate Drought Drought = Donald A. Wilhite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 215 Drought Hardening and Pre-Sowing Seed Hardening = Neil C. Turner . . . . . . . . . . . . . . . . . . 218 Drought: Avoidance and Adaptation = J. W. Chandler and Dorothea Bartels . . . . . . . . . . . . . . 222 Drought: Management = Donald A. Wilhite and Michael J. Hayes . . . . . . . . . . . . . . . . . . . . 225 Drought: Resistance = M. B. Kirkham . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 228

Evapotranspiration Antitranspirants: Film-Forming Types = Zvi Plaut . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

39

Antitranspirants: Stomata Closing Types = Zvi Plaut . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

43

Evaporation = Alain Perrier and Andree Tuzet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 307 Evaporation and Eddy Correlation = Roger Shaw and Richard L. Snyder . . . . . . . . . . . . . . . . 311 Evaporation and Energy Balance = Matthias Langensiepen . . . . . . . . . . . . . . . . . . . . . . . . . 314 Evaporation from Lakes and Large Bodies of Water = John Borrelli . . . . . . . . . . . . . . . . . . . 318 Evaporation from Soils = Andre Chanzy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 321 Evapotranspiration: Canopy Architecture and Climate Effects = Delphis F. Levia . . . . . . . . . . . 325 Evapotranspiration: Formulas = Robert D. Burman . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 332 Evapotranspiration: Greenhouses = Thierry Boulard . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 337 Evapotranspiration: Reference and Potential = Marcel Fuchs . . . . . . . . . . . . . . . . . . . . . . . . 343 Evapotranspiration: Remote Sensing = G. R. Diak, William P. Kustas and M. Susan Moran . . . . 346 Evapotranspiration: Weather Station Network Information = Simon O. Eching, Kent Frame, Bekele Temesgen and Richard L. Snyder . . . . . . . . . . . . . . . . . . . . . . . . . . 353 Transpiration = Thomas R. Sinclair . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1230 Transpiration: Carbon Dioxide and Plants = Leon Hartwell Allen Jr., Joseph C. V. Vu and John Sheehy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1235 Transpiration: Efficiency = Graeme C. Wright and Nageswararao C. Rachaputi . . . . . . . . . . . . 1240 Transpiration: Scaling from Leaves and Canopies = Matthias Langensiepen . . . . . . . . . . . . . . . 1247 Transpiration: Water Use Efficiency = Miranda Y. Mortlock . . . . . . . . . . . . . . . . . . . . . . . . 1250

Humidity and Precipitation Precipitation: Distribution Patterns = Gregory L. Johnson . . . . . . . . . . . . . . . . . . . . . . . . . . 885 Precipitation: Forms = Richard T. Wynne . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 891 Precipitation: Measurement = Marshall J. McFarland . . . . . . . . . . . . . . . . . . . . . . . . . . . . 893 Precipitation: Modification = George W. Bomar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 896

xxix © 2008 by Taylor & Francis Group, LLC

xxx

Climate (cont’d.) Humidity and Precipitation (cont’d.) Precipitation: Remote Sensing Measurement = Marshall J. McFarland . . . . . . . . . . . . . . . . . . 898 Precipitation: Simulation Models = Timothy O. Keefer . . . . . . . . . . . . . . . . . . . . . . . . . . . . 901 Precipitation: Stochastic Properties = David A. Woolhiser . . . . . . . . . . . . . . . . . . . . . . . . . . 906 Precipitation: Storms = Clayton L. Hanson . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 909 Psychrometry: Accuracy, Interpretation, and Sampling = Derrick M. Oosterhuis . . . . . . . . . . . . 924 Psychrometry: Theory, Types, and Uses = Derrick M. Oosterhuis . . . . . . . . . . . . . . . . . . . . . 927 Rainfall Shelters = Arland D. Schneider . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 940 Rainfed Farming = Edward John Sadler, James R. Frederick and Philip J. Bauer . . . . . . . . . . 943 Satellite Sensing: Atmospheric Water = Vincenzo Levizzani . . . . . . . . . . . . . . . . . . . . . . . . . 1018

Temperature Global Temperature Change and Terrestrial Ecology = Sherwood B. Idso and Keith E. Idso . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 415

Drainage Drainage and Water Quality = James L. Baker . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 162 Drainage Coefficient = Gary Sands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166 Drainage: Controlled = Robert O. Evans . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 169 Drainage: Hydrologic Impacts = D. W. Rycroft and Mark Robinson . . . . . . . . . . . . . . . . . . . 176 Drainage: Inadequacy and Crop Response = Norman R. Fausey . . . . . . . . . . . . . . . . . . . . . . . 180 Drainage: Irrigated Land = James E. Ayars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183 Drainage: Land Shaping = Rodney L. Huffman . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 186 Drainage: Materials = James L. Fouss . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 190 Drainage: Modeling = George M. Chescheir . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 195 Drainage: Soil Salinity Management = Glenn J. Hoffman . . . . . . . . . . . . . . . . . . . . . . . . . . 200 Land Drainage: Subsurface = Stephen R. Workman . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 702 Land Drainage: Wells = Rameshwar Kanwar and Allah Bakhsh . . . . . . . . . . . . . . . . . . . . . 705

Economics of Water and Virtual Water Marketing = Lal Khan Almas and W. Arden Colette . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 749 Virtual Water = Tony Allan and Dennis Wichelns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1285 Virtual Water: Economic Perspective = Dennis Wichelns . . . . . . . . . . . . . . . . . . . . . . . . . . . 1289 Virtual Water: Measuring Flows around the World = Arjen Y. Hoekstra . . . . . . . . . . . . . . . . . 1292

Groundwater Aquifers = Miguel A. Marino . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Aquifers: Artificial Recharge = Steven P. Phillips . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

46 49

Aquifers: Ogallala = B. A. Stewart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Aquifers: Recharge = John Nimmo, David A. Stonestrom and Richard W. Healy . . . . . . . . . . .

53 55

© 2008 by Taylor & Francis Group, LLC

xxxi

Aquifers: Transmissivity = Mohamed M. Hantush . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

59

Darcy’s Law = Keith C. Cameron and Graeme D. Buchan . . . . . . . . . . . . . . . . . . . . . . . . . 143 Groundwater: Contamination = Charles W. Fetter Jr. . . . . . . . . . . . . . . . . . . . . . . . . . Groundwater: Contamination, Arsenic = Dipankar Chakraborti, Uttam Kumar Chowdhury, Kunal Paul, Mohammad Mahmudur Rahman and Quazi Quamruzzaman . . . . . . . . . Groundwater: Mapping Levels = Marios A. Sophocleous . . . . . . . . . . . . . . . . . . . . . . . Groundwater: Measuring Levels = Paul F. Hudak . . . . . . . . . . . . . . . . . . . . . . . . . . . Groundwater: Mining = Hugo A. Loa´iciga . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Groundwater: Modeling = Jesu´s Carrera . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . 418 . . . . 421 . . . . 427 . . . . 433 . . . . 436 . . . . 440

Groundwater: Modeling Using Numerical Methods = Jesu´s Carrera . . . . . . . . . . . . . . . . . . . . 448 Groundwater: Pollution from Mining = Jeffrey G. Skousen and George F. Vance . . . . . . . . . . . 453 Groundwater: Pollution from Nitrogen Fertilizers = Lloyd B. Owens . . . . . . . . . . . . . . . . . . . . 459 Groundwater: Pollution from Phosphorous Fertilizers = Bahman Eghball . . . . . . . . . . . . . . . . 464 Groundwater: Pumping and Land Subsidence = Steven P. Phillips and Devin Galloway . . . . . . . 466 Groundwater: Pumping Methods = Dennis E. Williams . . . . . . . . . . . . . . . . . . . . . . . . . . . . 471 Groundwater: Quality = Loret M. Ruppe and Timothy R. Ginn . . . . . . . . . . . . . . . . . . . . . . 484 Groundwater: Quality and Irrigated Agriculture = Stephen R. Grattan . . . . . . . . . . . . . . . . . . 487 Groundwater: Regulation = Kevin B. McCormack . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 493 Groundwater: Saltwater Intrusion = Alexander H.-D. Cheng . . . . . . . . . . . . . . . . . . . . . . . . 499 Groundwater: Western United States Law = J. David Aiken . . . . . . . . . . . . . . . . . . . . . . . . . 502 Groundwater: World Resources = Lucila Candela . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 505 Karst Aquifers = John Van Brahana . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 693 Karst Aquifers: Water Quality and Water Resource Problems = John Van Brahana . . . . . . . . . . 698 Observation Wells = Phillip D. Hays and John Van Brahana . . . . . . . . . . . . . . . . . . . . . . . 785 Richards’ Equation = Graeme D. Buchan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 974 Vadose Zone and Groundwater Protection = Charalambos Papelis and Michael H. Young . . . . . 1276 Wellhead Protection = Babs Makinde-Odusola . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1339 Wells: Drilling = Thomas Marek . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1344 Wells: Hydraulics = Mohamed Hantush . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1347

Irrigation Management and Economics Irrigated Agriculture: Historical View = Lyman S. Willardson . . . . . . Irrigated Agriculture: Managing toward Sustainability = Wayne Clyma, Muhammad Siddique Shafique and Jan van Schilfgaarde . . . . . Irrigated Agriculture: Social Impacts = Gaylord V. Skogerboe . . . . . . Irrigated Water: Market Role in Reallocating = Bonnie G. Colby . . . .

. . . . . . . . . . . . . . . . . 539 . . . . . . . . . . . . . . . . . 542 . . . . . . . . . . . . . . . . . 548 . . . . . . . . . . . . . . . . . 551

Irrigation Districts and Similar Organizations = David E. Nelson and Anisa Joy Divine . . . . . . . 564 Irrigation Sagacity (IS) = Charles M. Burt and Ken H. Solomon . . . . . . . . . . . . . . . . . . . . . 594 Irrigation Scheduling: Plant Indicators (Field Application) = David A. Goldhamer . . . . . . . . . . . 597 Irrigation Scheduling: Plant Indicators (Measurement) = David A. Goldhamer . . . . . . . . . . . . . 602 Irrigation Scheduling: Remote Sensing Technologies = Stephan J. Maas . . . . . . . . . . . . . . . . . 606 Irrigation Scheduling: Soil Water Status = Philip Charlesworth and Richard J. Stirzaker . . . . . 611 Irrigation Scheduling: Water Budgeting = Brian Leib and Claudio Osvaldo Stockle . . . . . . . . . 615

© 2008 by Taylor & Francis Group, LLC

xxxii

Irrigation (cont’d.) Management and Economics (cont’d.) Irrigation Systems: History = Wayne Clyma . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 622 Irrigation: Frost Protection and Bloom Delay = Robert W. Hill and Richard L. Snyder . . . . . . . 646 Irrigation: Supplemental = Phillipe Debaeke . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 675 Irrigation: Surface = Wynn R. Walker . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 678 Residential Irrigation Water Use and Control = Michael D. Dukes . . . . . . . . . . . . . . . . . . . . 965 Tailwater Recovery and Reuse = C. Dean Yonts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1209 Water Balance Scheduling in Arid Regions = Richard L. Snyder, Simon Eching, Kent Frame and Bekele Temesgen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1311

Mechanical Canal Automation = Albert J. Clemmens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

82

Drip Lines and Emitters: Acidification for Prevention of Clogging = Andrew C. Chang . . . . . . . . 209 Drip Lines and Emitters: Chlorination for Disinfection and Prevention of Clogging = Andrew C. Chang . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211 Irrigated Water: Polymer Application = Bill Orts and Robert E. Sojka . . . . . . . . . . . . . . . . . . 554 Irrigation Design: Steps and Elements = Gary A. Clark . . . . . . . . . . . . . . . . . . . . . . . . . . . . 559 Irrigation Return Flow and Quality = Kenneth K. Tanji and Ramo
n Aragu€e
s . . . . . . . . . . . . . 588 Irrigation Systems: Drip = I. Pai Wu, Javier Barragan and Vince Bralts . . . . . . . . . . . . . . . . . 618 Irrigation Systems: Subsurface Drip = Carl R. Camp Jr. and Freddie L. Lamm . . . . . . . . . . . . 633 Irrigation: Metering = John A. Replogle and Albert J. Clemmens . . . . . . . . . . . . . . . . . . . . . 655 Irrigation: Preplant = James E. Ayars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 658 Irrigation: Sprinklers (Mechanical) = Dennis Kincaid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 670 On-Farm Irrigation Flow Measurement = Lawrence J. Schwankl . . . . . . . . . . . . . . . . . . . . . . 788 Pumps: Displacement = Dorota Z. Haman . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 932 Pumps: Internal Combustion Engines = Hal Werner . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 937 Time Domain Reflectometry: Salinity and Solute Measurement = Jon M. Wraith . . . . . . . . . . . . 1223

Physical and Environmental Chemigation = William L. Kranz . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91 Consumptive Water Use = Freddie L. Lamm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113 Crop Coefficients = Richard G. Allen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117 Crop Plants: Critical Developmental Stages of Water Stress = Zvi Plaut . . . . . . . . . . . . . . . . . 125 Drainage: Irrigated Land = James E. Ayars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183 Fertilizer and Pesticide Leaching: Irrigation Management = Luciano Mateos . . . . . . . . . . . . . . 363 Groundwater: Quality and Irrigated Agriculture = Stephen R. Grattan . . . . . . . . . . . . . . . . . . 487 Irrigated Agriculture: Economic Impacts of Investments = Robert A. Young . . . . . . . . . . . . . . . 533 Irrigated Agriculture: Endangered Species Policy = Norman K. Whittlesey, Ray G. Huffaker and Joel R. Hamilton . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 536 Irrigation Economics: Global = Keith D. Wiebe and Noel Gollehon . . . . . . . . . . . . . . . . . . . . 568 Irrigation Economics: United States = Noel Gollehon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 572 Irrigation Management: Humid Regions = Carl R. Camp Jr., Edward John Sadler and James E. Hook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 576

© 2008 by Taylor & Francis Group, LLC

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Irrigation Management: Tropics = R. Sakthivadivel and Hilmy Sally . . . . . . . . . . . . . . . . . . . 581 Irrigation Return Flow and Quality = Kenneth K. Tanji and Ramo´n Aragu¨e´s . . . . . . . . . . . . . 588 Irrigation Systems: Drip = I. Pai Wu, Javier Barragan and Vince Bralts . . . . . . . . . . . . . . . . . 618 Irrigation: Deficit = M.H. Behboudian . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 638 Irrigation: Efficiency = Terry A. Howell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 640 Irrigation: Impact on River Flows = Robert W. Hill and Ivan A. Walter . . . . . . . . . . . . . . . . . 650 Irrigation: Saline Water = B. A. Stewart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 662 Irrigation: Sewage Effluent Use = B. A. Stewart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 664 Irrigation: Site-Specific = Dennis C. Kincaid and Gerald W. Buchleiter . . . . . . . . . . . . . . . . . 666 Precision Agriculture and Water Use = Robert J. Lascano and Hong Li . . . . . . . . . . . . . . . . . 912 Tailwater Recovery and Reuse = C. Dean Yonts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1209 Water Balance Scheduling in Arid Regions = Richard L. Snyder, Simon Eching, Kent Frame and Bekele Temesgen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1311

Land Use and Water Mining Mining Impact: Metals = Mark Patrick Taylor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 759 Surface Water: Pollution by Surface Mines = Jeffrey G. Skousen and George F. Vance . . . . . . . 1191

Rural Cropland Agricultural Runoff: Characteristics = Matt C. Smith, Daniel L. Thomas and David K. Gattie . .

13

Conservation: Tillage and No-Tillage = Paul W. Unger . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110 Crop Coefficients = Richard G. Allen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117 Crop Development Models = Peter S. Carberry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121 Crop Residues: Snow Capture by = Donald K. McCool, Brenton S. Sharratt and John R. Morse . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131 Rural Water Supply: Water Harvesting = F. A. El-Awar, Rabi H. Mohtar and W. Jabre . . . . . . 998 Timber Harvesting: Influence on Water Yield and Water Quality = C. Rhett Jackson . . . . . . . . . 1219 Water Quality: Sampling of Runoff from Agricultural Fields = John M. Laflen . . . . . . . . . . . . . 1334 Forest Agroforestry: Enhancing Water Use Efficiency = James R. Brandle, Xinhua Zhou and Laurie Hodges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 Rural Water Supply: Water Harvesting = F. A. El-Awar, Rabi H. Mohtar and W. Jabre . . . . . . 998 Timber Harvesting: Influence on Water Yield and Water Quality = C. Rhett Jackson . . . . . . . . . 1219 Grazed Land and Livestock Issues Livestock and Poultry Production: Water Consumption = David B. Parker and Michael S. Brown . . 722 Livestock: Water Harvesting Methods = Gary W. Frasier . . . . . . . . . . . . . . . . . . . . . . . . . . 731 Manure Management: Beef Cattle Industry Requirements = Brent W. Auvermann and John M. Sweeten . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 738 Manure Management: Dairy = D. R. Bray and H. H. Van Horn . . . . . . . . . . . . . . . . . . . . . . 740 Manure Management: Poultry = Patricia K. Haan and Saqib Mukhtar . . . . . . . . . . . . . . . . . 744 Manure Management: Swine = Frank J. Humenik . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 747

© 2008 by Taylor & Francis Group, LLC

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Land Use and Water (cont’d.) Rural (cont’d.) Rangeland Management: Enhanced Water Utilization = Darrel N. Ueckert and W. Allan McGinty . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Rangeland Water Yield: Influence of Brush Clearing = Jeffrey G. Arnold, Steven Bednarz, Tim Dybala, William A. Dugas, Ranjan S. Muttiah and Wes Rosenthal . . . . . . . . . . Rangelands: Water Balance = Bradford P. Wilcox, David D. Breshears and Mark S. Seyfried . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Water Balance Scheduling in Arid Regions = Richard L. Snyder, Simon Eching, Kent Frame and Bekele Temesgen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Water Quality: Range and Pasture Land = Thomas L. Thurow . . . . . . . . . . . . . . . . . . Water Quality: Sampling of Runoff from Agricultural Fields = John M. Laflen . . . . . . . .

. . . . . 950 . . . . . 955 . . . . . 958 . . . . . 1311 . . . . . 1330 . . . . . 1334

Urban Low-Impact Development = Curtis H. Hinman and Derek B. Booth . . . . . . . . . . . . . . . . . . . . 734 Porous Pavements = Bruce K. Ferguson . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 882 Residential Irrigation Water Use and Control = Michael D. Dukes . . . . . . . . . . . . . . . . . . . . 965 Stormwater Management = Derek B. Booth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1163 Urban Hydrology = C. Rhett Jackson . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1268 Urban Water Quality = Linda Schweitzer and I.H. (Mel) Suffet . . . . . . . . . . . . . . . . . . . . . 1272

Regional and Historical Case Studies Geographical Areas African Market Garden = Amnon Bustan and Dov Pasternak . . . . . . . . . . . . . . . . . . . . . . .

7

Aral Sea Disaster = Guy Fipps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Arctic Hydrology = Bretton Somers and H. Jesse Walker . . . . . . . . . . . . . . . . . . . . . . . . . .

62 66

Brahmaputra Basin = Vijay P. Singh and Sharad K. Jain . . . . . . . . . . . . . . . . . . . . . . . . . .

78

Chernobyl Accident: Impacts on Water Resources = Jim T. Smith and Oleg Voitsekhovitch . . . . . 96 Chesapeake Bay = Sean M. Smith . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102 Dryland and Semiarid Regions: Research Centers = John Ryan . . . . . . . . . . . . . . . . . . . . . . . 232 Dryland Cropping Systems = William A. Payne . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 237 Dryland Farming = Clay A. Robinson . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 242 Dust Bowl Era = R. Louis Baumhardt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 246 ˜o = David E. Stooksbury . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 251 El Nin Everglades = Kenneth L. Campbell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 356 Ganges River = M. Monirul Qadar Mirza . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 405 Hypoxia: Gulf of Mexico = Nancy N. Rabalais . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 524 Irrigated Agriculture: Historical View = Lyman S. Willardson . . . . . . . . . . . . . . . . . . . . . . . 539 ˜a = David E. Stooksbury . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 700 La Nin Neuse River = Curtis J. Varnell and John Van Brahana . . . . . . . . . . . . . . . . . . . . . . . . . . . 768 Platte River = William L. Graf . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 873 Sacramento–San Joaquin Delta = Mark J. Roberson . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1005 Salton Sea = Timothy P. Krantz . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1014

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Seas: Dead and Dying = Andrey G. Kostianoy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1029 Yellow River = Zixi Zhu, Ynuzhang Wang and Yifei Zhu . . . . . . . . . . . . . . . . . . . . . . . . . . 1368

Historical Case Studies Ancient Greece: Agricultural Hydraulic Works = Demetris Koutsoyiannis and A. N. Angelakis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ancient Greece: Hydrologic and Hydraulic Science and Technology = A. N. Angelakis and Demetris Koutsoyiannis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ancient Greece: Urban Water Engineering and Management = Demetris Koutsoyiannis and A. N. Angelakis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Dust Bowl Era = R. Louis Baumhardt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . .

24

. . . . . . .

28

. . . . . . . 31 . . . . . . . 246

Seas: Dead and Dying = Andrey G. Kostianoy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1029 Yellow River = Zixi Zhu, Ynuzhang Wang and Yifei Zhu . . . . . . . . . . . . . . . . . . . . . . . . . . 1368

Research Archives, Data Sources and Research Centers Databases = Rosemary Streatfeild . . . . . . . . . . . . . . . . . Evapotranspiration: Weather Station Networks Information = Kent Frame, Bekele Temesgen and Richard L. Snyder . . Hydrology Research Centers = Daniel L. Thomas . . . . . . . Internet = Janet G. Norris . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . Simon O. Eching, . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . 147 . . . . . . . . . . . . 353 . . . . . . . . . . . . 521 . . . . . . . . . . . . 529

Journals = Joseph R. Makuch and Stuart R. Gagnon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 688 Library Resources = Robert Teeter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 719 Precipitation: Measurement = Marshall J. McFarland . . . . . . . . . . . . . . . . . . . . . . . . . . . . 893 Precipitation: Remote Sensing Measurement = Marshall J. McFarland . . . . . . . . . . . . . . . . . . 898 Satellite Sensing: Atmospheric Water = Vincenzo Levizzani . . . . . . . . . . . . . . . . . . . . . . . . . 1018

General Academic Disciplines = Joseph R. Makuch and Paul D. Robillard . . . . . . . . . . . . . . . . . . . .

1

Professional Societies = Faye Anderson . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 920 Research Organizations = Gabriel Eckstein . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 962

Methods Software Development = Matthias Langensiepen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1044 Soil Moisture Measurement by Feel and Appearance = Rick P. Leopold . . . . . . . . . . . . . . . . . 1051 Soil Water Measurement: Capacitance = Ioan Paltineanu and James L. Starr . . . . . . . . . . . . 1054 Soil Water Measurement: Granular Matrix Sensors = Clinton C. Shock . . . . . . . . . . . . . . . . . 1058 Soil Water Measurement: Gravimetric = Joseph L. Pikul Jr. . . . . . . . . . . . . . . . . . . . . . . . . 1063 Soil Water Measurement: Neutron Thermalization = Steven R. Evett . . . . . . . . . . . . . . . . . . . 1066 Soil Water Measurement: Soil Probes = Clay A. Robinson . . . . . . . . . . . . . . . . . . . . . . . . . 1071 Soil Water Measurement: Tensiometers = James ‘‘Buck’’ Sisson and Joel M. Hubbell . . . . . . . . 1074 Soil Water Measurement: Time Domain Reflectometry = Steven R. Evett . . . . . . . . . . . . . . . . 1078 Soil Water: Salinity Measurement = Dennis L. Corwin . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1117 Soil Water: Sensor-Based Automatic Irrigation of Vegetable Crops = Michael D. Dukes and Rafael Mu~ noz-Carpena . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1123

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Research (cont’d. ) Modelling Crop Development Models = Peter S. Carberry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121 Hydrologic Process Modeling = Matt C. Smith and Daniel L. Thomas . . . . . . . . . . . . . . . . . . 518 Water Quality: Modeling = Richard Lowrance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1327

Soil and Plant Water Management Drought Hardening and Pre-Sowing Seed Hardening = Neil C. Turner . . . . . . . . . . . . . . . . . . 218 Field Water Supply and Balance = Jean L. Steiner . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 366 Plant Yield and Water Use = M. H. Behboudian and Tessa Marie Mills . . . . . . . . . . . . . . . . . 855 Storativity and Specific Yield = Hugo A. Loa
iciga and Paul F. Hudak . . . . . . . . . . . . . . . . . . 1158 Vadose Zone and Groundwater Protection = Charalambos Papelis and Michael H. Young . . . . . 1276

Measurement Matric Potential = Melvin T. Tyree . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 752 Ring and Tension Infiltrometers = Laosheng Wu and Dong Wang . . . . . . . . . . . . . . . . . . . . . 977 Soil Moisture Measurement by Feel and Appearance = Rick P. Leopold . . . . . . . . . . . . . . . . . 1051 Soil Water Measurement: Capacitance = Ioan Paltineanu and James L. Starr . . . . . . . . . . . . 1054 Soil Water Measurement: Granular Matrix Sensors = Clinton C. Shock . . . . . . . . . . . . . . . . . 1058 Soil Water Measurement: Gravimetric = Joseph L. Pikul Jr. . . . . . . . . . . . . . . . . . . . . . . . . 1063 Soil Water Measurement: Neutron Thermalization = Steven R. Evett . . . . . . . . . . . . . . . . . . . 1066 Soil Water Measurement: Soil Probes = Clay A. Robinson . . . . . . . . . . . . . . . . . . . . . . . . . 1071 Soil Water Measurement: Tensiometers = James ‘‘Buck’’ Sisson and Joel M. Hubbell . . . . . . . . 1074 Soil Water Measurement: Time Domain Reflectometry = Steven R. Evett . . . . . . . . . . . . . . . . 1078 Soil Water: Salinity Measurement = Dennis L. Corwin . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1117

Nomenclature and Process Frozen Soil: Water Movement in = John M. Baker . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 398 Leaf Water Potential = Andreas J. Karamanos . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 710 Plants: Salt Tolerance = Michael C. Shannon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 870 Soil Macropores: Water and Solute Movement = David E. Radcliffe . . . . . . . . . . . . . . . . . . . 1048 Soil Water: Antecedent = Sally D. Logsdon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1083 Soil Water: Capillary Rise = James E. Smith . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1086 Soil Water: Diffusion = Laosheng Wu . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1090 Soil Water: Energy Concepts = Sally D. Logsdon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1093 Soil Water: Flow under Saturated Conditions = Jan W. Hopmans and Graham E. Fogg . . . . . . . 1096 Soil Water: Flow under Unsaturated Conditions = Jacob H. Dane and Jan W. Hopmans . . . . . . 1100 Soil Water: Functions in Pedostructure = Erik Braudeau and Rabi H. Mohtar . . . . . . . . . . . . . 1104 Soil Water: Hysteresis = Dan Jaynes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1110 Soil Water: Plant-Available = Judy A. Tolk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1113 Soil Water: Sensor-Based Automatic Irrigation of Vegetable Crops = Michael D. Dukes and Rafael Mun˜oz-Carpena . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1123

© 2008 by Taylor & Francis Group, LLC

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Soils: Field Capacity = Laj R. Ahuja, Mahmood H. Nachabe and Renee Rockiki . . . . . . . . . . . 1128 Soils: Hydraulic Conductivity Rates = Adel Shirmohammadi and David D. Bosch . . . . . . . . . . 1132 Soils: Hygroscopic Water Content = Daniel G. Levitt and Michael H. Young . . . . . . . . . . . . . 1136 Soils: Permanent Wilting Points = Judy A. Tolk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1140 Soils: Water Infiltration = Sally D. Logsdon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1143 Soils: Water Percolation = Kurt D. Pennell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1147 Soils: Waterborne Chemicals Leaching through = John Hutson . . . . . . . . . . . . . . . . . . . . . . . 1150 Vapor Transport in Dry Soils = Anderson L. Ward and Glendon W. Gee . . . . . . . . . . . . . . . . 1280

Transpiration Drought: Resistance = M. B. Kirkham . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 228 Giant Reed (Arundo donax): Effects on Streams and Water Resources = Gretchen C. Coffman . . 408 Leaf Water Potential = Andreas J. Karamanos . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 710 Matric Potential = Melvin T. Tyree . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 752 Plant Water Stress: Exposure during Specific Growth Stages = Zvi Plaut . . . . . . . . . . . . . . . . 843 Plant Water Stress: Optional Parameters for Stress Relief = Zvi Plaut . . . . . . . . . . . . . . . . . . 846 Plant Water Use: Stomatal Control = James I.L. Morison . . . . . . . . . . . . . . . . . . . . . . . . . . 850 Plant Yield and Water Use = M. H. Behboudian and Tessa Marie Mills . . . . . . . . . . . . . . . . . 855 Plants: Critical Growth Periods = M. H. Behboudian and Tessa Marie Mills . . . . . . . . . . . . . . 858 Plants: Osmotic Adjustment = James M. Morgan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 861 Plants: Osmotic Potential = Mark E. Westgate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 865 Stomatal Responses to the Environment: Quantifying = Matthias Langensiepen . . . . . . . . . . . . 1154 Summer Fallow = D. L. Tanaka and Randy L. Anderson . . . . . . . . . . . . . . . . . . . . . . . . . . 1172 Transpiration: Carbon Dioxide and Plants = Leon Hartwell Allen Jr., Joseph C. V. Vu and John Sheehy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1235 Transpiration: Efficiency = Graeme C. Wright and Nageswararao C. Rachaputi . . . . . . . . . . . . 1240 Transpiration: Scaling from Leaves and Canopies = Matthias Langensiepen . . . . . . . . . . . . . . . 1247 Transpiration: Water Use Efficiency = Miranda Y. Mortlock . . . . . . . . . . . . . . . . . . . . . . . . 1250 Uptake by Plant Roots = S.G.K. Adiku . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1261 Uptake by Plant Roots: Modeling Water Extraction = S. G. K. Adiku . . . . . . . . . . . . . . . . . . 1264

Soil and Water Conservation Bound Water in Soils = Sally D. Logsdon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

74

Conservation: Tillage and No-Tillage = Paul W. Unger . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110 Erosion and Precipitation = Bofu Yu . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 258 Erosion and Productivity = Francisco Arriaga and Birl Lowery . . . . . . . . Erosion Control: Mechanical = Mark A. Nearing, Mary Nichols, Kenneth G. and Jeffry Stone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Erosion Control: Tillage/Residue Methods = Jerry Neppel, Krisztina Eleki, John Kost and Richard Cruse . . . . . . . . . . . . . . . . . . . . . . . . . . . Erosion Control: Vegetative = Seth M. Dabney . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . 262 Renard . . . . . . . . . . . . . . 265 . . . . . . . . . . . . . . 268 . . . . . . . . . . . . . . 272

Erosion Problems: Historical Review = Andrew S. Goudie . . . . . . . . . . . . . . . . . . . . . . . . . . 277 Erosion Research: History = Rattan Lal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 280 Erosion Research: Instrumentation = Gary D. Bubenzer . . . . . . . . . . . . . . . . . . . . . . . . . . . 284 Erosion: Accelerated = Michael C. Hirschi and J. Kent Mitchell . . . . . . . . . . . . . . . . . . . . . . 287 Erosion: Bank = Steven B. Mitchell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 293

© 2008 by Taylor & Francis Group, LLC

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Soil and Water Conservation (cont’d.) Erosion: Prediction = Mark A. Nearing, Mary Nichols, Kenneth G. Jeffrey Stone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Erosion: Process Modeling = John M. Laflen . . . . . . . . . . . . . . Furrow Dikes = Ordie R. Jones and R. Louis Baumhardt . . . . . .

Renard and . . . . . . . . . . . . . . . . . . . . 297 . . . . . . . . . . . . . . . . . . . . 300 . . . . . . . . . . . . . . . . . . . . 401

Precision Conservation = Joseph K. Berry and Jorge A. Delgado . . . . . . . . . . . . . . . . . . . . . 916 Tamarisk = Tom L. Dudley . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1213 Transpiration: Water Use Efficiency = Miranda Y. Mortlock . . . . . . . . . . . . . . . . . . . . . . . . 1250 Wind Erosion and Water Resources = R. Scott Van Pelt . . . . . . . . . . . . . . . . . . . . . . . . . . . 1365

Streams and Channels Form and Process Alluvial Fan = Takashi Oguchi and Kyoji Saito . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

20

Dam Removal = Desiree Tullos and Gregory Jennings . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138 Deltas = Paul F. Hudson . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152 Energy Dissipation Structures = Iwao Ohtsu and Youichi Yasuda . . . . . . . . . . . . . . . . . . . . . 254 Floodplain Management = French Wetmore . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 375 Floods and Flooding = Steven Schultz and Jay A. Leitch . . . . . . . . . . . . . . . . . . . . . . . . . . . 380 Fluvial Islands = Waite R. Osterkamp . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 394 Giant Reed (Arundo donax): Effects on Streams and Water Resources = Gretchen C. Coffman . . 408 Irrigation: Impact on River Flows = Robert W. Hill and Ivan A. Walter . . . . . . . . . . . . . . . . . 650 Natural Levees = Paul F. Hudson . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 763 Open-Channel Flow Rate Measurements = John E. Costa and Stephen F. Blanchard . . . . . . . . 792 Open-Channel Spillways = Sherry L. Britton and Gregory J. Hanson . Phosphorus: Transport in Riverine Systems = Peter Kleinman, Andrew Tore Krogstad and Richard McDowell . . . . . . . . . . . . . . . . . . Riparian Geomorphology = Ashley A. Webb . . . . . . . . . . . . . . . . . River Channelization = Nicola Surian . . . . . . . . . . . . . . . . . . . . .

. . N. . . . . . .

. . . . . . . Sharpley, . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . 795 . . . . . . . . . 837 . . . . . . . . . 981 . . . . . . . . . 986

River Channels = Paul F. Hudson . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 991 Scaling Processes in Watersheds = Ole Wendroth and David A. Robinson . . . . . . . . . . . . . . . . 1024 Sediment Budgets = Desmond E. Walling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1032 Sediment Load = Xi Xi Lu and David L. Higgit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1037 Streambank Stabilization = F. Douglas Shields Jr. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1167 Suspended-Sediment Transport Measurement = John R. Gray . . . . . . . . . . . . . . . . . . . . . . . . 1204 Tamarisk = Tom L. Dudley . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1213 Total Maximum Daily Load (TMDL) = Robin K. Craig . . . . . . . . . . . . . . . . . . . . . . . . . . . 1227 Two-Stage Channel Geometry: Active Floodplain Requirements = Andrew D. Ward, Jonathan Witter, A. D. Jayakaran, Dan E. Mecklenburg and G. Erick Powell . . . . . . . . . . 1253

Geographical Entities Ganges River = M. Monirul Qadar Mirza . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 405 Neuse River = Curtis J. Varnell and John Van Brahana . . . . . . . . . . . . . . . . . . . . . . . . . . . 768 Platte River = William L. Graf . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 873

© 2008 by Taylor & Francis Group, LLC

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Sacramento–San Joaquin Delta = Mark J. Roberson . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1005 Yellow River = Zixi Zhu, Ynuzhang Wang and Yifei Zhu . . . . . . . . . . . . . . . . . . . . . . . . . . 1368

Surface Water Aral Sea Disaster = Guy Fipps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

62

Chesapeake Bay = Sean M. Smith . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102 Eutrophication = Kyle D. Hoagland and Thomas G. Franti . . . . . . . . . . . . . . . . . . . . . . . . . 304 Farm Ponds = Ronald W. Tuttle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 359 Hypoxia: Gulf of Mexico = Nancy N. Rabalais . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 524 Salton Sea = Timothy P. Krantz . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1014 Seas: Dead and Dying = Andrey G. Kostianoy . . . . . Surface Water: Concentrated Animal Feeding Operations and Philip A. Moore Jr. . . . . . . . . . . . . . . . . . Surface Water: Western United States Law = J. David

. . . . . . . . . . . . . . . . . . . . . . . . . . . . 1029 = Douglas R. Smith . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1176 Aiken . . . . . . . . . . . . . . . . . . . . . . . . 1200

Water Characteristics, Chemistry, Pollution, and Pollution Control Pollution Cyanobacteria in Eutrophic Freshwater Systems = Anja Gassner and Martin V. Fey . . . . . . . . . 135 Hypoxia: Gulf of Mexico = Nancy N. Rabalais . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 524 Nonpoint Source Pollution (NPSP) = Ray Correll, Peter Dillon, Rai Kookana, Mallavarapu Megharaj, Ravendra Naidu and Walter W. Wenzel . . . . . . . . . . . . . . . . . . . 778 Pathogens: General Characteristics = Jeanette Thurston-Enriquez . . . . . . . . . . . . . . . . . . . . . 803 Pathogens: Transport by Water = Graeme D. Buchan and Markus Flury . . . . . . . . . . . . . . . . 808 Pesticide Contamination: Groundwater = Roy F. Spalding . . . . . . . . . . . . . . . . . . . . . . . . . . 812 Pesticide Contamination: Surface Water = Sharon A. Clay . . . . . . . . . . . . . . . . . . . . . . . . . 816 Pfiesteria Piscicida = Henry (Hank) S. Parker . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 820 Pharmaceuticals in Water Supplies = Klaus Ku¨mmerer . . . . . . . . . . . . . . . . . . . . . . . . . . . . 827 Phosphorus: Inputs into Freshwater from Agriculture = Richard McDowell . . . . . . Phosphorus: Transport in Riverine Systems = Peter Kleinman, Andrew N. Sharpley, Tore Krogstad and Richard McDowell . . . . . . . . . . . . . . . . . . . . . . . . . . . Pollution: Point Source (PS) = Ray Correll, Peter Dillon, Rai Kookana, Mallavarapu Megharaj, Ravendra Naidu and Walter W. Wenzel . . . . . . . . . . Selenium = Dean A. Martens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . 831 . . . . . . . . . 837 . . . . . . . . . 876 . . . . . . . . . 1041

Surface Water: Nitrogen Enrichment = Tim P. Burt and Penny E. Widdison . . . . . . . . . . . . . . 1180 Surface Water: Pollution by Nitrogen Fertilizers = Gregory McIsaac . . . . . . . . . . . . . . . . . . . 1185 Surface Water: Pollution by Surface Mines = Jeffrey G. Skousen and George F. Vance . . . . . . . 1191 Surface Water: Quality and Phosphorous Applications = Andrew N. Sharpley, Peter Kleinman and Richard McDowell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1196 Total Maximum Daily Load (TMDL) = Robin K. Craig . . . . . . . . . . . . . . . . . . . . . . . . . . . 1227 Water Quality: Sampling of Runoff from Agricultural Fields = John M. Laflen . . . . . . . . . . . . . 1334

Pollution Control Desalination = Fawzi Karajeh and Fethi BenJemaa . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157 Fertilizer and Pesticide Leaching: Irrigation Management = Luciano Mateos . . . . . . . . . . . . . . 363

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Water Characteristics, Chemistry, Pollution, and Pollution Control (cont’d.) Pollution Control (cont’d.) Filtration and Particulate Removal = Lawrence J. Schwankl . . . . . . . . . . . . . . . . . . . . . . . . 370 Livestock Water Quality Standards = Floron C. Faries Jr., Guy H. Loneragan, John C. Reagor and John M. Sweeten . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 727 Microbial Sampling = George Di Giovanni and Suresh D. Pillai . . . . . . . . . . . . . . . . . . . . . 755 Nitrogen Measurement = Jacek A. Koziel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 772 Nutrients: Best Management Practices = Keith A. Kelling and Scott J. Sturgul . . . . . . . . . . . . 781 Phosphorus: Measurement = Bahman Eghball and Daniel H. Pote . . . . . . . . . . . . . . . . . . . . 834 Reverse Osmosis = Mark Wilf . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 968 Surface Water: Concentrated Animal Feeding Operations = Douglas R. Smith and Philip A. Moore Jr. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1176 Wastewater Use in Agriculture: Agronomic Considerations = Manzoor Qadir, Liqa Raschid-Sally and Pay Drechsel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1296 Wastewater Use in Agriculture: Empirical Evidence = Liqa Raschid-Sally and Pay Drechsel . . . . 1300 Wastewater Use in Agriculture: Public Health Considerations = Blanca Jimenez . . . . . . . . . . . . 1303 Wastewater Use in Agriculture: Saline and Sodic Waters = Manzoor Qadir and Paramjit Singh Minhas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1307

Water Characteristics and Chemistry Acid Rain and Precipitation Chemistry = Pao K. Wang . . . . . . . . . . . . . . . . . . . . . . . . . . . .

4

Boron = Rami Keren . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

70

Chemical Measurement = Kevin Jeanes and William J. Rogers . . . . . . . . . . . . . . . . . . . . . . . Chemigation = William L. Kranz . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

86 91

Chromium = Bruce R. James . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105 Drip Lines and Emitters: Acidification for Prevention of Clogging = Andrew C. Chang . . . . . . . . 209 Drip Lines and Emitters: Chlorination for Disinfection and Prevention of Clogging = Andrew C. Chang . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211 Eutrophication = Kyle D. Hoagland and Thomas G. Franti . . . . . . . . . . . . . . . . . . . . . . . . . 304 Flouride = Judy A. Rogers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 386 Giant Reed (Arundo donax): Effects on Streams and Water Resources = Gretchen C. Coffman Hydrologic Management of Contaminated Sites Using Vegetation = Tessa Marie Mills and Brett H. Robinson . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Isotopes = Michael A. Anderson . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Oxygen Measurement: Biological–Chemical Oxygen Demand = David B. Parker and Marty B. Rhoades . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . pH = William J. Rogers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Saline Seeps = Ardell D. Halvorson . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . 408 . . 513 . . 684 . . 800 . . 824 . . 1008

Saline Water = Khaled M. Bali . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1011 Water Properties = James D. Oster . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1324

Water Law Groundwater: Western United States Law = J. David Aiken . . . . . . . . . . . . . . . . . . . . . . . . . 502 Total Maximum Daily Load (TMDL) = Robin K. Craig . . . . . . . . . . . . . . . . . . . . . . . . . . . 1227

© 2008 by Taylor & Francis Group, LLC

xli

Water Supply and Distribution Drinking Water Supply Distribution Systems = Laura J. Ehlers . . . . . . . . . . . . . . . . . . . . . . 204 Flow Measurement: History = James F. Ruff . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 388 Hydrologic Cycle = John Van Brahana . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 510 Rainwater Harvesting = K.F. Andrew Lo . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 946 Reverse Osmosis = Mark Wilf . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 968 Rural Water Supply: Water Harvesting = F. A. El-Awar, Rabi H. Mohtar and W. Jabre . . . . . . 998 Water Footprints = Arjen Y. Hoekstra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1316 Water Harvesting = Gary W. Frasier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1320

Wetlands Wetlands and Petroleum = John V. Headley and Kerry M. Peru . . . . . . . . . . . . . . . . . . . . . . 1352 Wetlands as Treatment Systems = Kyle R. Mankin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1356 Wetlands: Ecosystems = Sherri L. DeFauw . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1361

© 2008 by Taylor & Francis Group, LLC

Preface to Second Edition

Continuing and updating the Encyclopedia of Water Science over the past four years has been an exciting but daunting task. It has been exciting because the subject is so critical to life and living, and daunting because the original editors set such a high standard. As ever, water is the central focus in environmental science. In addition to all the critical factors cited in the Preface to the first edition, global warming has now become more certain so that water issues will become even more important. My goals for this new edition have been to (1) expand the number of entries along the same subject areas found in the first edition, (2) revise old entries as needed and (3) expand into new subject areas. For the latter, the main thrust has been the science of streams, not only the engineering aspects, but also the natural ones including morphology and process. The latter, fluvial geomorphology, is my own field. Presumably, future editors will also insure that their own specialty is well covered too. We have also moved into entirely new fields such as virtual water. The number of entries has increased by more than 50 percent since the first edition. There are many people to thank for making the second edition possible. First and foremost is the Editorial Advisory Board. While small in number for an undertaking of this magnitude, these people have been giants in their contributions. Not only have they recommended authors and referees and even refereed themselves, but they have also contributed fine entries, several in some cases. I want to mention in particular Mary Beth Kirkham, Andrew Goudie, Dennis Wichelns and Steve Parker without whose help this second edition would have been impossible. The second group of people who have made this possible are the authors who have given generously of their expertise and time. The third group is the referees. Usually with little or no thanks or credit, they have given of themselves to improve entries, sometimes being absolutely critical to making entries most effective. The fourth group to which I owe a debt of gratitude is the editorial team at Dekker, now Taylor and Francis. They took care of countless details and helped me stay on track. In particular, Sapna Maloor, Susan Lee, and Laura Sylvest were outstanding as my editorial assistants. They saw to contacting authors and referees, all involving thousands of e-mail exchanges. And I must thank Claire Miller who oversaw the project and gave valuable direction. To all, thanks! Stanley W. Trimble Prospect TN 27 July 2007

xliii © 2008 by Taylor & Francis Group, LLC

Preface to First Edition

All living things require water. More specifically, they require it daily and often in a nearly pure state. As world population increased from 2.5 billion people in 1950 to more than 6 billion in 2000, water demand escalated. One in five people on this planet does not have access to safe and affordable drinking water, and half do not have access to sanitation. With global population predicted to reach almost 8 billion by 2025, water management and treatment will become critical. Most of the world’s water is salt water, unsuitable for most uses. Fresh water makes up only 2.5% of the water supply and two-thirds of this is in the form of glaciers and permafrost, leaving less than 1% of the world’s water available for use. Of this remaining 1%, agriculture is the biggest user of water withdrawals from groundwater and surface water supplies: Agriculture comprises 69% of water use compared with industrial and domestic users, who consume 21% and 10%, respectively. But as population growth continues and industrialization expands, there will be greater competition among all users and an increasing need for more efficient water use. Food production during the past 50 years has kept pace with population growth. The increase in per capita grain production from 247 kg in 1950 to more than 300 kg today is due largely to increased irrigation requiring large amounts of water:  Approximately 500 kg of water is required to produce 1 kg of potato, while wheat, maize, and rice require approximately 900, 1400, and 2000 kg, respectively, for each kilogram of grain.  Around 95% of all agricultural land and 83% of the cropland depend entirely on precipitation to meet plant needs.  Although the 17% of cropland that is irrigated uses an enormous amount of water, it produces almost 40% of the world’s food and fiber needs. Future food requirements will require even additional irrigated lands. However, while irrigation greatly increases food production, it can simultaneously degrade soil and water resources, leading to serious environmental problems. In recent years, there has been a dramatic change in the way meat and dairy animals are handled. Large, concentrated animal feeding operations are becoming commonplace and require large amounts of water for both livestock consumption and manure and waste handling. These facilities can also present a potential pollution hazard for surface and groundwater resources. Industrial and domestic water users often degrade water to the point that it must be treated before it can be used for other purposes or even returned to the environment. Wastewater treatment usually requires extensive facilities and expenditures to ensure environmental protection. Efficient water use and water resource protection can be accomplished only by informed producers and policymakers with access to state-of-the-art information. The Encyclopedia of Water Science is designed and compiled to meet this need. An international team of hundreds of dedicated scientists, policymakers, educators, and others involved with water use have prepared nearly 250 entries addressing important topics ranging from water composition to irrigation water application to agricultural fields. An advisory board was important in planning the scope, and topic editors reviewed entries and offered advice to the editors and authors. We thank all these individuals for

xlv © 2008 by Taylor & Francis Group, LLC

their efforts. The initial edition, to be updated quarterly online, addresses critical issues of water use. Perhaps more importantly, the authors have identified additional sources of information for readers who need further, in-depth resources. Published in both online and print formats, the encyclopedia’s features will appeal to a wide range of users. Thanks are also due to the staff of Marcel Dekker, Inc., for their efforts in handling the thousands of communications required to invite authors, review drafts, and manage other matters necessary to produce a publication of this magnitude. It was a great pleasure to work with Ellen Lichtenstein and Sapna Maloor. Their professionalism, commitment to excellence, and dedication are much appreciated and were the key to accomplishing this task. The information assembled will be a useful tool in helping humanity address and meet water use challenges of the 21st century. The Founding Editors B. A. Stewart Terry A. Howell

xlvi © 2008 by Taylor & Francis Group, LLC

Irrigation– Journals

Hydrologic– Irrigation

Global– Groundwater

Evapotrans–Giant

El Nin˜o– Evaporation

Second Edition

Volume 1

Pages 1 through 692 Academic–Journals

© 2008 by Taylor & Francis Group, LLC

Chemical– Desalination

Water Science Drainage–Dust

Academic–Canal

Encyclopedia of

Wastewater– Yellow

Tailwater–Virtual

Soils–Suspended

Scaling– Soil Water

Rainfall–Satellite

Second Edition

Volume 2

Pages 693 through 1370 Karst–Yellow

© 2008 by Taylor & Francis Group, LLC

Nitrogen–Plant

Water Science Plants–Pumps

Karst–Neuse

Encyclopedia of

Academic–Canal

Academic Disciplines Joseph R. Makuch Water Quality Information Center, National Agricultural Library, Beltsville, Maryland, U.S.A.

Paul D. Robillard Agricultural and Biological Engineering, Pennsylvania State University, University Park, Pennsylvania, U.S.A.

INTRODUCTION Several academic disciplines from the agricultural, environmental, and social sciences contribute to the field of agricultural water management. Successful agricultural water management enhances the prospects for farm profitability by efficiently meeting the water needs of crops and livestock while protecting the natural environment. Achieving these results requires a multidisciplinary approach: knowledge from many disciplines must be integrated into agricultural water management decisions.

OVERVIEW OF AGRICULTURAL WATER MANAGEMENT Agricultural water management encompasses a wide range of activities that relate to managing water efficiently to grow food and fiber, while protecting water, and other elements of the natural environment, from degradation. The focus is on managing the quantity and quality of water resources/supplies. Agricultural water management encompasses:  Crop irrigation, drainage, erosion control, nutrient and pest management.  Animal production, especially manure management.  Provision of safe drinking water for humans and animals.  Groundwater and surface water quality protection.

AGRICULTURAL WATER MANAGEMENT DISCIPLINES Academic disciplines are fields of study characterized by academic departments, scholarly journals, and professional societies. Those disciplines that address agricultural water management are as varied as

agricultural water management issues. Academic disciplines usually associated with managing agricultural water quality and quantity are described below.

Descriptions of Specific Disciplines Agricultural economics Agricultural economics contributes to agricultural water management by addressing both farm-level business decisions and broader policy decisions related to water. Agricultural economists examine issues such as farm profitability under different irrigation systems; costs and benefits of government conservation programs; and the influence of cost-share rates on the adoption of best management practices to reduce non-point-source pollution. Agricultural engineering Agricultural engineering is concerned with the design and development of systems to identify, analyze, treat, and remediate water resources, particularly as they relate to agricultural activities. Agricultural engineers are also involved in the operation of environmental quality protection and control systems. Engineering principles are applied to monitoring, design, and operation of systems to evaluate the environmental impact of agricultural practices on surface water and groundwater quantity and quality. Emphasis areas include engineering design and operation of irrigation and drainage systems; erosion and sedimentation control structures; modeling of contaminant transport processes; and design of watershed monitoring and control systems utilizing simulation models, geographic information systems (GIS), and remote-sensing data. Specialized fields in agricultural engineering include land application of wastewater and controlled environments (such as water and wastewater treatment and operation systems for greenhouses; confined livestock 1

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Academic–Canal

2

facilities; and aquaculture). In all cases, the impact of these operations on water quality is of concern. Agricultural law Agricultural law covers the legal aspects of agricultural water use, including environmental impacts. The issues may relate to individual farm enterprises or to larger public policy questions. At the farm level, legal issues might include rights to water for irrigation, or liability for degradation of a stream. Broader public policy issues could include developing, implementing, and enforcing regulations specifying manure management practices that are protective of water quality. Agricultural meteorology and climatology Agricultural meteorology relates to agricultural water management in a number of ways. The type, timing, amount, intensity, and duration of precipitation are vitally important to agriculture. For example, longterm rainfall records are used in water quality models that examine agricultural contaminants in runoff or those that leach into groundwater. Agricultural meteorology also provides a knowledge base for irrigation scheduling. The related field of climatology helps identify possible changes in precipitation patterns attributable to global climate change that could have an effect on agricultural water management. The study of climate variability also helps broaden our understanding of droughts and floods. Understanding of this variability can help agricultural water managers plan and prepare for extreme weather events. Agronomy and soil science Agronomy, and the associated disciplines of crop science, plant science, and horticulture, are all concerned with growing plants. Plant types range from alfalfa to tomatoes to turf grass, but all require water in the proper amount and at the proper time. These disciplines study plant water requirements and evaluate irrigation systems for their efficiency and effectiveness. The disciplines are also concerned with long-term effects of irrigation, particularly the buildup of salinity in soils and contaminated return flows. Land application of municipal wastewater and biosolids is also under the purview of these disciplines. Understanding the quality of irrigation water and its effects on plants and the environment is especially important in such waste management/crop production systems. Nutrients in fertilizers and manures—applied to promote plant growth—and pesticides—applied to combat diseases and kill weeds and damaging

© 2008 by Taylor & Francis Group, LLC

Academic Disciplines

insects—can harm water quality if not managed effectively. Agronomists study ways to optimize the amount, timing, and application methods of fertilizers and manures to ensure efficient plant growth while protecting groundwater and surface water quality. Similarly, agronomists help design integrated pest management programs that protect plant health while avoiding unnecessary applications of synthetic pesticides that may result in water contamination. To reduce sedimentation of water bodies by soil erosion, agronomists study cover crops and plant-related aspects of conservation tillage systems. A closely related discipline, soil science, focuses on soil characteristics, responses to management, and effects of use. With regard to agricultural water management, soil scientists study the water-holding capacities and drainage characteristics of soils, the transport of nutrients and pesticides through the soil profile, and the effectiveness of conservation tillage systems in controlling erosion. Animal science Animal science deals with raising livestock and poultry. Animal scientists study the water quality and quantity requirements of various farm animals for optimum production. Since nutrients in manures can cause water quality problems, animal scientists address this problem by finding ways for animals to use nutrients in their feed more efficiently, thereby excreting less in the manure. Aquatic biology Aquatic biology is the study of living resources—and the factors affecting them—in freshwater systems. Agricultural activities may produce unintended, negative effects on aquatic organisms. For example, expanding cropland by clearing trees from a riparian area removes shading from the stream. Without this protection, stream temperatures may rise to a level unsuitable for cold-water fish. Aquatic biologists can assess the effect agricultural practices may have on aquatic life, both on the farm and downstream. Environmental engineering Environmental engineering is concerned with the physical, chemical, and biological control of water and wastewater treatment systems. All aspects of water and wastewater treatment systems are related to environmental engineering practices including collection, storage, stabilization, advanced chemical and biological treatment, and distribution systems. Environmental engineers are typically responsible for the design and operation of water and wastewater

treatment plants. Specialized areas include engineering design and evaluation of conduit (pressurized) and open channel flow systems as well as hazardous waste treatment and disposal systems. Environmental engineers also model and evaluate the impact of municipal and industrial wastewater discharges on surface water and groundwater quality. Forestry and natural resources conservation Forestry and natural resources conservation also have relevance to agricultural water management. Specialists in these fields may study the establishment, growth, maintenance, and species composition of woody and herbaceous plants in riparian buffers; the effects of stream bank plantings to provide shade and cover and reduce stream bank erosion and other management methods to reduce sediment inputs to streams. Hydrology and hydrogeology Hydrology and hydrogeology are focused on water availability and movement as they relate to both surface and groundwater flow regimes. Hydrology is the scientific building block for scientific and engineering water management disciplines. Monitoring of precipitation, stream flows and groundwater flows, in space and time, are the basis for both short-term and longterm quantity and quality records. These records are used to evaluate agricultural and other land-use impacts on water resources, as well as to develop tools that provide predictive capabilities for impact analysis. Hydrogeology—the integrated sciences of geology and hydrology—focuses on the various geologic formations and their hydraulic characteristics, providing opportunities to develop tools for the estimation of aquifer recharge and yield under different climates, land use, and water withdrawal scenarios. Hydrogeologists are also concerned with the vulnerability of aquifers to contamination from land uses in recharge areas. Studies in this area involve monitoring and evaluation of land-use practices and how they impact the natural aquifer quality parameters; water quality changes derived from land-use impacts; and the effect these quality changes may have on water supply withdrawals and return flows to streams.

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3

Range science Range science is concerned with rangeland ecology and the use of rangelands to meet human needs, including raising livestock for food. When animals graze in rangeland riparian areas, they may increase erosion and sedimentation, deposit manure and urine in or very near streams, and reduce stream shading by damaging adjacent vegetation. Stream water quality can suffer, and aquatic habitats can be negatively altered. Range scientists investigate ways to utilize rangelands for livestock grazing in ways that protect water quality and associated natural resources.

CONCLUSION The broad spectrum of agricultural water management activities provides opportunities for many disciplines to contribute to the field. Some disciplines focus on biological, chemical, and physical aspects, while others address social and economic issues. Each discipline has tools and technologies that can be applied to improving agricultural water management, while addressing related environmental, economic, and social problems.

BIBLIOGRAPHY 1. Ag Career Database; National FFA Organization, 2000; http://209.206.185.114/careersearch.cfm (accessed May 2001). 2. Brown, J.A. Disciplines in Agriculture. Agrologist 1983, 12 (4), 12–13. 3. Careers in Agricultural Science; Florida Department of Agriculture and Consumer Services; http://www.fl-ag. com/PlanetAg/careers.htm (accessed May 2001). 4. Exploring Careers in Agronomy, Crops, Soils, and Environmental Sciences; American Society of Agronomy; Crop Science Society of America; Soil Science Society of America: Madison, WI, 1996; 27 pp. 5. Kunkel, H.O. The issue of academic disciplines within agricultural research. In Agricultural Research Policy: Selected Issues, Papers Presented to the 150th National Meeting of the American Association for the Advancement of Science, New York, NY, May 24–29, 1984; Batie, S.S.; Marshall, J.P., Eds.; Virginia Polytechnic Institute and State University: Blacksburg, VA, 1986; 19–32.

Academic–Canal

Academic Disciplines

Academic–Canal

Acid Rain and Precipitation Chemistry Pao K. Wang Atmospheric and Oceanic Sciences, University of Wisconsin–Madison, Madison, Wisconsin, U.S.A.

INTRODUCTION Acid rain is a phenomenon of serious environmental concern. By definition, acid rain refers to rainwater that is acidic. But in reality, it is more accurate to use the term acid deposition since not only rain but also snow, sleet, hail, and even fog can become acidic. In addition to the process where acids become associated with precipitation (called wet deposition), acid gases and particles can also be deposited on the earth surface directly (called dry deposition). However, since the name ‘‘acid rain’’ has become a household term and its formation is better understood than other types of acid deposition, the following discussions will focus on acid rain. Whereas an aqueous solution is acidic if its pH value is less than 7.0, acid rain refers to rainwater with pH less than 5.6. This is because, even without the presence of man-made pollutants, natural rainwater is already acidic as CO2 in the atmosphere reacts with water to produce carbonic acid: CO2 þ H2 OðlÞ , H2 CO3 ðlÞ

ð1Þ

The pH value of this solution is around 5.6. Even though the carbonic acid in rain is fairly dilute, it is sufficient to dissolve minerals in the Earth’s crust, making them available to plant and animal life, yet not acidic enough to cause damage. Other atmospheric substances from volcanic eruptions, forest fires, and similar natural phenomena also contribute to the natural sources of acidity in rain. Still, even with the enormous amounts of acids created annually by nature, normal rainfall is able to assimilate them to the point where they cause little, if any, known damage. However, large-scale human industrial activities have the potential of throwing off this acid balance, and converting natural and mildly acidic rain into precipitation with stronger acidity and far-reaching environmental effects. This is the root of the acid rain problem, which is not only of national but also international concern. This problem may have existed for more than 300 yr starting at the time when the industrial revolution demanded a large scale burning of coal in which sulfur was a natural contaminant. Several English scholars, such as Robert Boyle in the 17th century and Robert A. Smith of the 19th century, 4

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wrote about the acids in air and rain; though, there was a lack of appreciation of the magnitude of the problem at that time. Individual studies of the acid rain phenomenon in North America started in the 1920s, but the true appreciation of the problem came only in the 1970s. To address this problem, the U.S. Congress established the National Acid Precipitation Assessment Program (NAPAP) to study the causes and impacts of the acid deposition. This research established that the acid rain does cause broad environmental and health effects, the pollution causing acid deposition can travel hundreds of miles, and the electric power generation is mainly responsible for SO2 (
65%) and NOx emissions (
30%). Subsequently, Congress created the Acid Rain Program under Title IV (Acid Deposition Control) of the 1990 Clean Air Act Amendments. Electric utilities are required to reduce their emissions of SO2 and NOx significantly. By 2010, they need to lower their emissions by 8.5 million tons compared to their 1980 levels. They also need to reduce their NOx emissions by 2 million tons each year compared to the levels before the Clean Air Act Amendments. However, it may not be adequate to solve the acid emission merely at the national level. With increasing industrialization of the Third World countries in the coming century, one can expect great increase of the atmospheric loading of SO2 and NOx because many of these countries will burn fossil fuels to satisfy their energy needs. Clearly, some form of international agreements need to be forged to prevent serious environmental degradation due to acid rain.

THE CHEMISTRY OF ACID RAIN Sulfuric acid (H2SO4) and nitric acid (HNO3) are the two main acid species in the rain. The partitioning of acids in rain may be different in different places. In the United States, the partitioning is H2SO4 (
65%), HNO3 (
30%), and others (
5%). While there are many possible chemicals that may serve as the precursors of acid rain, the two main substances are SO2 and NOx (and NOx consists of NO and NO2), and both are released to the atmosphere via the industrial combustion process. While power generation is the predominant source of these precursors, industrial boilers and

5

Nitric Acid The main ingredients for the formation of nitric acid are NO and NO2 (and are often combined into one category, NOx). It is commonly thought that the main path of nitric acid found in clouds and raindrops is the formation of gas phase, HNO3, followed by its uptake by liquid water. Although there are reactions of NOx with liquid water that can lead to nitric acid, they are thought to be unimportant due to their slow reaction rates. The main reaction for HNO3 formation is NO2 þ OH þ M ! HNO3 þ M Fig. 1 A schematic of the acid rain formation process.

where M can be any neutral molecule. NO can be converted to NO2 by the following reaction: 2NO þ O2 ! 2NO2

automobiles also contribute substantially. When these precursors enter the cloud and precipitation systems, acid rain occurs. Fig. 1 shows a schematic of the acid rain formation process. Once airborne, these chemicals can be involved in milliards of chemicals reactions. The main paths that lead to acid rain formation are described as follows.

Sulfuric Acid SO2 is believed to be the main precursor for the formation of sulfuric acid drops. Its main source in the atmosphere is the combustion of fossil fuels. This is because sulfur is a natural contaminant in coal (especially the low grade ones) and oil. The following reactions are thought to occur when SO2 is absorbed by a water drop (see e.g., Refs.[1,2]): SO2 ðgÞ þ H2 OðlÞ , SO2  H2 OðlÞ

ð2Þ

SO2  H2 O , Hþ þ HSO 3

ð3Þ

þ þ SO2 HSO 3 , H 3

ð4Þ

SO32

oxidation !

SO2 4

ð7Þ

DROP-SCALE TRANSPORT PROCESSES OF ACID RAIN The chemical reactions described earlier must be considered together with the transport processes to obtain a quantitative picture of the acid rain formation. This is especially true for SO2 because absorption and reactions occur simultaneously. The convective transport influences the concentrations of different species and hence the reaction rates. Fig. 2 illustrates these processes schematically. These include the following.

ð5Þ

The oxidant of the last step can be H2O2, O3, OH, and others. There are still controversies about the identity of the oxidants. Note that the equilibrium of the above reaction system is controlled by the pH values of the drop, and the presence of ammonia is often considered together with these reactions since it affects the pH of the drop. A detailed discussion of these reactions and their rates is given in Chapter 17 of Ref.[1].

© 2008 by Taylor & Francis Group, LLC

ð6Þ

Fig. 2 A schematic of the drop-scale transport process of sulfur species involved in the acid rain.

Academic–Canal

Acid Rain and Precipitation Chemistry

Academic–Canal

6

External Transport

Acid Rain and Precipitation Chemistry

This refers to the transport of SO2 gas toward the surface of the drop. It is a convective diffusion process (both convective transport and diffusional transport occur) and is influenced by the flow fields created by the falling drop and atmospheric conditions (pressure and temperature).

energy and hence more fossil fuel used), the snow in winter can also pick up substantial amount of acids. These snow-borne acids can accumulate throughout winter (if the weather is cold enough) and then are released in large doses during the spring thaw. These large doses of acid may have more significant effects during fish spawning or seed germination than the same doses at some less critical time.

Interfacial Transport

Topography and Geology

Once SO2 is adsorbed on the surface of the drop, it must be transferred into the interior for further reactions to occur. The time for establishing phase equilibrium is controlled by Henry’s law constant and mass accommodation coefficient of SO2.

The topography and geology of an area have marked influence on acid rain effects. Research from the U.S. EPA pointed out that areas most sensitive to acid precipitation are those with hard, crystalline bedrock and very thin surface soils. Here, in the absence of buffering properties of soil, acid rains will have direct access to surface waters and their delicate ecosystem. Areas with steep topography, such as mountainous areas, generally have thin surface soils and hence are very vulnerable to acid rain. In contrast, a thick soil mantle or one with high buffering capacity, such as most flatlands, helps keep acid rain damage down. The location of water bodies is also important. Headwater lakes and streams are especially vulnerable to acidification. Lake depth, the ratio of water-shed area to lake area, and the residence time in lakes all play a part in determining the consequent threat posed by acids. The transport mode of the acid (rains or runoff) also influences the effects.

Internal Transport In the interior of the drop, reactions (2)–(5) occur. At the same time, these species are transported by both diffusion and internal circulation. The latter is caused by the motion of the liquid drop in a viscous medium and can influence the production rates of these species (see Ref.[1]).

ENVIRONMENTAL FACTORS INFLUENCING THE ACID RAIN FORMATION AND IMPACTS Like many environmental hazards, the acid rain process is not driven by a few well-controlled physical and chemical processes, but involves complicated interactions between the chemicals and the environments they exist in. While the main ingredients of acid rain come from industrial activities, many other factors may influence the formation of acid rain and its impacts. The following are some of the most important. Meteorological Factors Acid rain occurs in the atmosphere and hence is greatly influenced by meteorological factors such as wind direction and speed, amount and frequency of precipitation, pressure patterns, and temperature. For example, in drier climates, such as the western United States, wind blown alkaline dust is abundant and tends to neutralize the acidity in the rain. This is the buffering effect of the dust. In humid climates, like the Eastern Seaboard, less dust is in the air, and precipitation tends to be more acidic. Seasonality may also influence acid precipitation. For example, while it is true that rain may be more acidic in summer (because of higher demands for

© 2008 by Taylor & Francis Group, LLC

Biota Acid rain may fall on trees causing damages. The kinds of trees and plants in an area, their heights, and whether they are deciduous or evergreen may all play a part in the potential effects of acid rain. Without a dense leaf canopy, more acid may reach the earth to impact on soil and water chemistries. Stresses on the plants will also affect the balance of local ecosystem. Additionally, the rate at which different types of plants carry on their normal life processes influences an area’s ratio of precipitation to evaporation. In locales with high evaporation rates, acids will concentrate on leaf surfaces. Another factor is that leaf litter decomposition may add to the acidity of the soil due to normal biological actions. REFERENCES 1. Pruppacher, H.R.; Klett, J.D. Microphysics of Clouds and Precipitation; Kluwer Academic Publishers, 1997; 954 pp. 2. Seinfeld, J.H.; Pandis, S.N. Atmospheric Chemistry and Physics; John Wiley and Sons, 1998.

Academic–Canal

African Market Garden Amnon Bustan Institutes for Applied Research, Ben-Gurion University of the Negev, Beer-Sheva, Israel

Dov Pasternak International Crops Research Institute for the Semi-Arid Tropics, Niamey, Niger

INTRODUCTION Agricultural production in sub-Saharan Africa relies mainly on rain-fed systems.[1] In the semiarid and dry subhumid regions of Africa, these systems are neither sustainable nor profitable. Areas of monocultures of grains and legumes exhibit severe land degradation, mostly as a result of water and wind erosion.[2] Crop yields are very small, the commercial value of the common grains (millet and sorghum) is very low, and revenue is thus meager. On average, crop failures occur in two out of five years as a result of droughts. The final outcome of these processes is severe poverty. Irrigated agriculture can help alleviate poverty and reduce the stress on natural resources, especially since many countries in dry Africa are rich in non-utilized water resources. For example, the combined annual discharge of the Niger and the Senegal rivers is about 40 billion m3 yr
1, a value comparable with the 75 billion m3 yr
1 of the Nile river. In addition, rich shallow aquifers underlie much of Sahelian Africa,[3] and there are phreatic shallow aquifers in the proximity of seasonal rivers. The large-scale utilization of such water reserves for irrigation is hampered by the relatively high costs of large water projects and of inputs and by low crop yields.[4] Furthermore, there is little motivation to initiate large-scale schemes, since international prices for irrigated commodities are relatively low. In many areas of dry Africa, market gardens are the only form of irrigated agriculture.[4] These gardens are sustainable and profitable, because they supply perishable products such as fruit and vegetables that, for obvious reasons, cannot be imported from elsewhere. The importance of market gardens is growing steadily due to the rapid increase of urban population that can afford to buy fruits and vegetables in city markets. However, for the market gardens to be successful, many problems associated with climatic and social conditions and the lack of appropriate technologies have to be addressed first. One of the most important problems is that of irrigation. Most market gardens are irrigated by hand with watering cans. This activity

is obviously extremely labor demanding and inefficient. In some places, surface irrigation (mainly basin irrigation) is also practiced. The drawbacks of this latter system are the relatively high energy input required for motorized water pumps and the low water use efficiency, particularly in sandy soils, from which a large proportion of the water is lost through seepage. Yields and quality of fruits and vegetables are low because of the inefficient supply of water and nutrients, the low quality of seeds, and ineffective pest and disease control methods.

BACKGROUND INFORMATION In many countries (particularly those towards the north of the continent), market gardens operate only five months of the year (November–March). During the rainy season (July–October) there is shortage of labor for irrigation, since, at that time, all labor is directed to rain-fed fields. Furthermore, in the rainy season, it is difficult to control diseases and pests. The April–July period is very hot, and most vegetable species do not give good yields under conditions of severe heat. Furthermore, in the hot season, it is difficult to carry watering cans or to do any sort of work under the scorching sun. Drip irrigation can provide the solution to the above-described problems. Drip systems can easily deliver water to the field in daily quantities required on the basis of transpiration demands. With this system, all plants receive the same quantity of water and fertilizers, and very low soil water tension is maintained throughout the day. These advantages lead to significant increases in yield and in improved product quality as compared with other systems.[5–9] An additional advantage is that drip irrigation is particularly suitable for saline water irrigation.[10] Conventional drip-irrigation systems were designed for large surfaces and are too costly and difficult to maintain for small fields. In response to the need for systems suitable for small areas, Israeli drip-irrigation companies have recently developed low-pressure 7

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drip-irrigation (LPDI) systems for the small traditional greenhouses of China. Although the LPDI concept was first tested in Israel as long ago as the mid-1980s,[11] the system was not commercialized at that time, mainly because of problems of drip clogging. The recently developed LPDIs have large drip orifices that minimize clogging by impurities in the irrigation water. The LPDI system has been adapted by us to become the basis of an integrated production system, designated the African Market Garden (AMG) [or in French, Jardin Potager Africain (JPA)] described below.

THE AFRICAN MARKET GARDEN Technical Description and Operation of the LDPI System The hydraulic performance of three different LPDI systems manufactured in Israel (Ein Tal, Netafim, and Plastro-Gvat) has been tested in the field.[12] In all the tested systems, the variation in water discharge from the first to the last dripper in a 12.5-m long drip lateral at a water pressure of 1.3 m did not fall below 90%, and at 1.8 m of pressure there were no differences in water discharge among individual drippers along a 12.5-m drip lateral.

African Market Garden

A schematic presentation of the LPDI manufactured by Netafim is given in Fig. 1. The system consists of the following elements: a water reservoir positioned at least 1 m above the field level, a water valve, a filter, distribution lines, and drip laterals. Additional details of the system are given in Fig. 1. The operation of the system is very simple. It involves filling of the reservoir to a particular level, cleaning the filter, and opening the tap. Irrigation is completed when the reservoir is empty. Once a week, the ends of all laterals are opened; the system is flushed for about 5 min to clean it from possible accumulated impurities; and the reservoir is drained. The relationship between the reservoir size and the irrigated area is constant and depends on the daily evapotranspiration (ET) of the particular region. As an example, the average daily potential ET in Niamey for the 12 months of the year is given in Fig. 2. Two distinct seasons can be observed. A season of high ET of about 8 mm day
1 (February–May) and a season of lower ET of about 6 mm day
1 (June–February). The volume of the reservoir should be planned to accommodate the maximum daily quantity of irrigation water required in the particular region. In places with two distinct seasons, such as Niamey, two lines are drawn on the reservoir. In Niamey, the upper line indicates the volume that is needed to give the field a daily irrigation rate of 8 mm. The lower line indicates the volume that will supply 6 mm day
1.

Fig. 1 Schematic presentation of the LDPI system: 1. Water reservoir; 2. plastic ball valve 100 female thread; 3. plastic filter 100 male thread (120 mesh); 4. P.E. quick-coupling elbow—25 mm  100 , female thread; 5. P.E. quick-coupling elbow—25 mm; 6. main line—LDPE pipe 25 mm class 2.5; 7. dripline—LDPE integrated FDS dripline; 8. start connector—barbed type, for FDS dripline; 9. start connector plug; 10. insert connector for 8-mm dripline; 11. mini puncher for start connector. (Note: The illustration of the Netafim design does not infer any preference of the authors for the design of that company.)

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Fig. 2 Average monthly values of potential ET in Niamey, Niger. (From ICRISAT Sahelian Center-Niger.)

The ‘‘basic unit’’ concept was introduced to describe a system with a fixed ratio between the reservoir volume and the field size. For example, in Niamey, an area of 500 m2 requires a reservoir with a volume of 4000 L (4 m3) to supply a peak daily water requirement of 8 mm. Two systems were developed: 1) a ‘‘thrifty’’ system, which caters for farmers with limited resources (in many instances these are women who operate small backyard gardens or small plots in communal gardens); and 2) a ‘‘commercial’’ system, which caters to larger-scale producers, particularly periurban farmers. The basic unit for the thrifty system consists of an old 200-L oil drum (these are available everywhere in Africa at low prices), supplying an area of 40 m2 and giving a daily irrigation rate of 5 mm (compromises in the quantity of water have to be made for the sake of simplification). The system is very versatile. For example, the same reservoir and distribution lines can serve an area of 80 m2 or an area of 120 m2 by filling the reservoir two or three times a day, as required. Likewise, two or three (or more) barrels can be interconnected to provide a basic unit of 80 m2 or 120 m2. The same principles are applicable to the commercial version of the AMG.

Water, Salt Buildup, and Nutrition Management The problem of salt buildup in the soil usually accompanies irrigation systems in various degrees of severity. Among the parameters involved in salt buildup are the ratio between the evaporative demands and precipitations (including irrigation), water quality, soil texture, and the ability of irrigation water to leach salts. In arid and semiarid regions, evaporative demands are high (e.g., Fig. 2). To prevent rapid salt buildup it is recommended to keep the daily amount of irrigation water above (10–20%) plant water requirements. The excess amount of water is needed to leach salts away from the rhizosphere. In cases of saline

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irrigation water, leaching requirements increase. In general, salt accumulation tends to be more intensive and fast in fine- than in coarse-textured soils due to the much larger specific surface area (SSA) of the former, that provides tight interactions with soil water solution. Water, as the vector of salt in soils, determines salt distribution in the soil profile. Water moves in the soil in three directions: 1) upward, due to evaporation and capillarity; 2) horizontally, by capillary action; and 3) vertically (downward), by gravitational forces. In conventional drip irrigation, with water discharge rates above 2 L hr
1, salts are leached thus creating a gradient of salt concentration that increases from beneath the emitter to the margins of the wetted zone. It has been suspected that with the lower water discharge rates (0.3–0.7 L hr
1) of LPDI the vertical water vector is too weak to provide sufficient salt leaching. To clarify this point, we conducted a series of experiments, in which salt distribution in the soil profile was compared between water discharge rates of 0.3 L hr
1 (1.3 mm hr
1) and 2 L hr
1 (8.0 mm hr
1) in fine- and coarse-textured soils, respectively (Fig. 3). The crop was sweet corn. The field was irrigated daily based on 80% evaporation from a class A USWB evaporation pan adjusted to the leaf area index (LAI) of the crop. The electrical conductivity (EC) of irrigation water containing soluble fertilizers was 1.5 dS m
1. Soil samples were taken 75 days after sowing. There was no rain during the experimental period. Large differences occurred, as expected, between fineand coarse-textured soils. In fine-textured soil, salt accumulation at the upper layer of soil profile (0– 10 cm) was remarkably fast, with no influence of water discharge rates (Fig. 3A and B). Below that layer, the pattern of salt distribution was a reflection of water distribution. Indeed, salt leaching to the margins was less pronounced under 0.3 as compared with 2-L hr
1 emitters. Similar patterns of salt distribution, but to a much lesser extent, were observed in the coarse-textured soil (Fig. 3C and D). Two conclusions can be derived from these observations. First, to ensure sustainability, the use of the LPDI system in fine-textured soils should be restricted to regions with a considerable rainy season (above 400 mm yr
1), during which accumulated salts are discarded either by water runoff or leaching to deep soil layers. The second conclusion relates to fertilizing management. The relatively weak salt leaching under LPDI emitters implies that it is not necessary to add expensive soluble fertilizers to every irrigation event, as practiced with conventional drip irrigation. The prevailing horizontal movement of water under the low-discharge dripper can become an advantage when a heavy dose of manure (‘‘side dressing’’) is applied between the drip

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African Market Garden

Fig. 3 Salt distribution (EC [dS m
1] of saturated soil extracts) in the soil profile. Soil was sampled from 30 places at various depths and distances from the emitter. Drawings (produced using Surfer 7.0, Golden Software Ltd) represent soil profiles under 0.3-L hr
1 (A, C) and 2-L hr
1 (B, D) water discharge rates in fine- (A, B) and coarse-textured (C, D) soils, 75 day after sowing sweet corn. Amounts of irrigation water (ECI  1.5 dS m
1) were equal. The fine-textured soil was a silty clay loam containing 30% silt, 45% clay, and 25% sand. The coarse-textured soil contained 10% silt, 10% clay, and 80% sand.

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lines. The roots will move towards the buried manure and draw most of the required nutrients from this source. Nevertheless, to guarantee a minimal supply of nitrogen to the crops, it is recommended that nitrogen, in the form of urea, be applied once a week at a rate of 0.5 g m
2. The minimization of fertilizer application through water will prevent the formation of bacterial slime in the laterals, an important cause of drip clogging. In a preliminary trial on lettuce in which soluble fertilizer was replaced by an application of organic manure, there was no difference in yield between the manured plots and the plots that received daily application of fertilizer through the irrigation system. Effect of Rate of Emitter Discharge on Yield At an average discharge of 1.5 mm hr
1 and a daily ET of 6 mm, an irrigation cycle in the AMG lasts for 4 hr. In the light of reports that continuous irrigation may be beneficial to crop yields,[13–15] the effect of two different irrigation rates on sweet corn yield was tested (Table 1). It is evident from Table 1 that in both soil types prolonged irrigation has a slight positive effect on dry matter yield but does not affect ear yield. Thus, for corn (which under desert conditions suffers from water stress[15]), continuous application of water does not offer a significant advantage. Crop Management The typical market garden of Africa is characterized by the production of a mix of vegetables and fruit trees in a relatively small plot. The AMG thus also incorporates a mixture of crops. In hot dry areas, date palms are added to produce a three-layered production system (date palms, fruit trees, and annual crops). In a typical 500 m2 plot, 9 date palms are planted (1 male and 8 female) in a 9  11 m configuration. Date palms, through their high transpiration rates and

shading effects, produce a microclimate that facilitates reasonable growth of fruit trees and vegetables during the hot dry season. This date-palm-based production system is known as ‘‘oasis agriculture.’’ Date palms also improve the profitability of the system. At present-day prices, the income from 8 female plants is about $800 yr
1. To prevent competition for water between the date palms and the other crops in the AMG, the drip laterals are looped around the stems of the palms to triple the amount of water given to the palms.

CONCLUSION The AMG—a new production system conceived in 1998—incorporates all the advantages of the conventional drip-irrigation system at a fraction of its cost. It is simple to operate and to maintain, and it provides significant increases in yield as well as considerable savings of energy and labor (this aspect is particularly important for women who operate small gardens). This system is applicable to all developing countries that require small-scale irrigation schemes. The International Program for Arid Land Crops (IPALAC, which is managed by Ben-Gurion University of the Negev, Beer-Sheva, Israel) and Desert Margins Program (DMP, managed by the International Crops Research Institute for the Semi-Arid Tropics) have joined hands to disseminate the AMG system in semiarid Africa, starting in Ethiopia and in the Sahel. Recently, such systems were also installed in Rajasthan, India. The AMG can therefore serve as a platform for the improvement of the small-scale irrigated agriculture in Africa. Its introduction will facilitate year-round production of irrigated fruit and vegetables, the incorporation of quality vegetable and fruit tree varieties, and the application of modern cost-effective methods for pest and disease management. An adoption of the AMG should significantly contribute to the alleviation of poverty—the most serious problem plaguing subSaharan Africa at the beginning of the 21st Century.

Table 1 Effect of two rates of irrigation on stover and ear yield of sweet corn fine- and coarse-textured soils Irrigation intensity (mm hr
1)

Stover DM yielda (kg m
2)

Ear yield (kg m
2)

Ears (m
2)

Fine-textured soil 1.3

1.67a

2.45

6.17

8.0

1.25b

2.36

6.21

1.3

1.25

2.06

6.32

8.0

1.18

1.89

5.97

Coarse-textured soil

a

No significant difference between values denoted with same letter at P  0.05.

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REFERENCES 1. Leisinger, K.M.; Shcmitt, K. Survival in the Sahel. An Ecological and Development Challenge, International Service for National Agriculture Research (ISNAR), ISBN 9.2-9118-020-3, 1995. 2. Goorse, J.E.; Steeds, D.R. Desertification in the Sahelian and the Sudanian Zones of West Africa; World Bank Technical Paper No 61; Washington, DC, 1988. 3. Issar, S.A.; Nativ, R. Water beneath deserts; key to the past, resource for the present. Episodes 1988, 2, 256–269.

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4. Postel, S. Pillar of Sand; W. W. Norton & Company: New York, 1999; 313 pp. 5. Bernstein, L.; Francois, L.E. Comparison of drip, furrow and sprinkler irrigation. Soil Sci. 1973, 115, 73–86. 6. Kadam, J.R. Effects of irrigation methods on root shoot biomass and yield of tomatoes. J. Maharashtra Agric. Univ. 1993, 18, 493–494. 7. Bresler, E. Trickle-drip irrigation principles and applications to soil water management. Adv. Agron. 1975, 29, 343–393. 8. Shmueli, M.; Goldberg, D. Emergence, early growth and salinity of five vegetable crops germinated by sprinkle and trickle irrigation in an arid zone. Hortic. Sci. 1971, 6, 563–565. 9. Sanders, D.C.; Howell, T.A.; Hile, M.M.S.; Hodges, L.; Meek, D.; Phene, C.J. Yield and quality of processing tomatoes in response to irrigation rate and schedule. J. Am. Soc. Hortic. Sci. 1989, 114, 904–908. 10. DeMalach, Y.; Pasternak, D. Drip irrigation a better solution for crop production with brackish water in deserts. In Conference on Alternative Strategies for Desert Development and Management; Sacramento, California, 1/5–10/6, 1973.

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11. Or, U.; Rimon, D. Advanced technologies in traditional agriculture. J. Sustainable Agric. 1991, 2, 1–10. 12. Pasternak, D.; Bustan, A.; Ventura, M.; Klotz, H.; Eshetu, F.; Mpuisang, T. Use of low-pressure drip irrigation to produce dates in the market gardens of semi Arid Africa. In Proceedings of the Date Palm International Symposium, Windhoek, Namibia, 2000; 322–327. 13. Pasternak, D.; DeMalach, Y. Irrigation with brackish water under desert conditions (X). Irrigation management of tomatoes (Lycopersicon esculentum) on desert sand dunes. Agric. Water Manag. 1995, 26, 121–132. 14. Levinson, B.; Adato, I. Influence of reduced rate of water and fertilizer application using daily intermittent drip irrigation on the water requirement, root development and responses of avocado trees. J. Hortic. Sci. 1991, 66, 449–463. 15. Pasternak, D.; De Malach, Y.; Borovic, I. Irrigation with brackish water under desert conditions II. The physiological and yield response of corn (Zea mays) to continuous irrigation with brackish water and to alternating brackish–fresh–brackish water irrigation. Agric. Water Manag. 1985, 10, 47–60.

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Agricultural Runoff: Characteristics Matt C. Smith Daniel L. Thomas Department of Biological and Agricultural Engineering, Coastal Plain Experiment Station, University of Georgia, Tifton, Georgia, U.S.A.

David K. Gattie Biological and Agricultural Engineering Department, College of Agricultural and Environmental Sciences, University of Georgia, Athens, Georgia, U.S.A.

INTRODUCTION Agricultural runoff is surface water leaving farm fields as a result of receiving water in excess of the infiltration rate of the soil. Excess water is primarily due to precipitation, but it can also be due to irrigation and snowmelt on frozen soils. In the early 20th century, there was considerable concern about erosion of farm fields due to rainfall. The concern was primarily related to the loss of valuable topsoil from the fields and the resulting loss in productivity (see Erosion and Productivity). With the passage of the Federal Water Pollution Control Act Amendments of 1972, the potential for pollution of surface water features such as rivers and lakes due to agricultural runoff was officially recognized and an assessment of the nature and extent of such pollution was mandated.[1,2] Agricultural runoff is grouped into the category of non-point source pollution because the potential pollutants originate over large, diffuse areas and the exact point of entry into water bodies cannot be precisely identified (see Pollution, Point and Non-point Source). Non-point sources of pollution are particularly problematic in that it is difficult to capture and treat the polluted water before it enters a stream. Point sources of pollution such as municipal sewer systems usually enter the water body via pipes and it is comparatively easy to collect that water and run it through a treatment system prior to releasing it into the environment. Because of the non-point source nature of agricultural runoff, efforts to minimize or eliminate pollutants are, by necessity, focused on practices to be applied on or near farm fields themselves. In other words, we usually seek to prevent the pollution rather than treating the polluted water. Due to the great successes made in treating polluted water from point sources such as municipal and industrial wastewater treatment plants, the relative significance of pollution from agricultural runoff has increased. Agricultural runoff is now considered to be the primary source of pollutants to the streams and lakes in the United States. It is also the third

leading source of pollution in U.S. estuaries.[3] The water pollutants that occur in agricultural runoff include eroded soil particles (sediments), nutrients, pesticides, salts, viruses, bacteria, and organic matter.

AGRICULTURAL RUNOFF QUANTITY Agricultural runoff occurs when the precipitation rate exceeds the infiltration rate of the soil. Small soil particles that have been dislodged by the impact of raindrops can fill and block soil pores with a resulting decrease in infiltration rate throughout the duration of the storm. As the excess precipitation builds up on the soil surface it flows in thin layers from higher areas of the field towards lower areas. This diffuse surface runoff quickly starts to concentrate in small channels called rills. The concentrated flow will generally have a higher velocity than the flow in thin films over the surface. The concentrated flow velocity may become rapid enough to cause scouring of the soil that makes up the channel sides and bottom. The dislodged soil particles can then be carried by the flowing water to distant locations in the same field or be carried all the way to a receiving water body. If the quantity of flow and the velocity of flow are large enough, the rills can grow so large that they cannot be easily repaired by typical earth moving machinery. When this happens, the rill has become a gulley. The quantity of runoff from agricultural fields is not usually listed explicitly as a concern separate from the quality of the runoff. However, it should be considered because it transports the pollutants and can cause erosion of receiving streams due to excessive flows. If less runoff is allowed to leave a field, there is less flow available to transport pollutants to the stream. Also, if more water is retained on the field, there is likely to be a corresponding reduction in the amount of supplemental water that will need to be added through irrigation. Runoff quantity varies significantly due to 13

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factors such as soil type, presence of vegetation and plant residue, physical soil structures such as contoured rows and terraces, field topography, and the timing and intensity of the rainfall event. Some agricultural practices increase the infiltration capacity of the soil while other practices can result in decreases. The presence of vegetation and plant residues on a field reduce runoff due to improving and maintaining soil infiltration capacity. Actively growing plants also reduce the amount of water in the soil due to evapotranspiration, thus making more room for infiltrating water to be stored in the soil profile. Bare soils increase runoff because there is nothing except the soil surface to absorb the energy of the falling raindrops. The rain, therefore, dislodges soil particles that will tend to seal the surface and reduce infiltration.

SOIL EROSION AND ASSOCIATED POLLUTANTS One of the primary pollutants in agricultural runoff is eroded soil. In 1975, 223 million acres of cropland produced 3700 million tons of eroded sediments or an average of 17 tons of soil lost per acre of cropland per year (see various Erosion articles). It is estimated that cropland, pasture, and rangeland contributed over 50% of the sediments discharged to surface waters in 1977.[4] As noted above, the energy of raindrops can dislodge and transport soil particles. In the aquatic environment the eroded soil is called sediment. There are several concerns related to excessive sediments in aquatic systems. These include loss of field productivity, habitat destruction, reduced capacity in reservoirs, and increased dredging requirements in shipping channels. Eroded sediments represent a loss of fertile topsoil from the field, which can reduce the productivity of the field itself. Soil formation is an extremely slow process occurring over periods ranging from decades to centuries.[5] Possible results to a grower from excessive erosion of their fields include increasing fertilizer and water requirements, planting more tolerant crops, and possibly abandoning the field for agricultural production (see the article Erosion and Productivity). A second concern is that many of these sediments are heavy and will settle out in slow moving portions of streams or in reservoirs. The settled sediments can dramatically alter the ecology of the streambed. Aquatic plants, insects, and fish all have specific requirements related to composition of the streambed for them to live and reproduce.[6] Sediments in reservoirs reduce the volume of the reservoir available to store water. This may result in reduced production of hydroelectric power, reduced water availability for

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Agricultural Runoff: Characteristics

municipal supply, interference with navigation and recreation, and increased dredging requirements to maintain harbor navigability. Another concern with eroded sediments is that they can transport other pollutants into receiving waters. The plant nutrient phosphorus, for example, is most often transported from the fields where it was applied as fertilizer by chemically bonding to clay minerals. Many agricultural pesticides also bond to eroded clays and organic matter. Once these chemicals have entered the aquatic ecosystem, many processes occur that can result in the release of the pollutants from their sediment carriers. Phosphorus, when released, can contribute to the eutrophication of lakes and reservoirs (see the articles Eutrophication and Surface Water: Quality and Phosphorus Applications). Pesticides and their degradation products can be toxic to aquatic life and must be removed from municipal water supplies (see the article Pesticide Contamination: Surface Water). Erosion from animal agriculture such as feedlots and pastures can also result in the transport of sediments composed of animal manures (see the various Manure Management articles). These sediments can transport significant quantities of potential pathogens (viruses and bacteria). The animal manures are primarily organic in nature and can serve as a food source for natural bacteria in the receiving water. When these naturally occurring bacteria begin to utilize the organic matter in this way they may lower or deplete the water of dissolved oxygen as they respire and multiply. This use of oxygen by aquatic bacteria is known as biochemical oxygen demand (BOD). High levels of BOD can reduce stream oxygen level to the point that fish and other organisms that require dissolved oxygen suffer, die, or relocate, when possible, to more suitable habitats.[6]

DISSOLVED POLLUTANTS Agricultural runoff can carry with it many pollutants that are dissolved in the runoff water itself. These may include plant nutrients, pesticides, and salts. Since these pollutants are dissolved in the runoff, control measures are most often aimed at reducing the volume of runoff leaving an agricultural field, or making the pollutants less available to be dissolved into the runoff water. One of the major pollutants of concern in agricultural runoff is the plant nutrient nitrogen. Nitrogen is a relatively cheap component of most fertilizers and is necessary for plant growth. Unfortunately, nitrogen in the form of nitrate is highly soluble in water. Thus nitrate can be easily dissolved in runoff water. Just as it does in an agricultural field, nitrogen

can promote growth of aquatic vegetation. Excess nitrogen and phosphorus in runoff can lead to the eutrophication of lakes, reservoirs, and estuaries (see the articles Eutrophication and Surface Water: Pollution by Nitrogen Fertilizers). Nitrogen in the form of ammonia can be dissolved into runoff from pastures and feedlots. Ammonia is toxic to many aquatic organisms, thus it is important to minimize ammonia in runoff.[7] Many agriculturally applied pesticides are also soluble in water. They can be dissolved in runoff and transported into aquatic ecosystems where there is a potential for toxic effects. These pesticides must also be removed from drinking water supplies and, if concentrations are high or persistent, such treatment can be difficult and expensive. Stable, persistent pesticides can bioaccumulate in the food chain with the result that consumers of fish from contaminated waters might be exposed to higher concentrations than exist in the water itself.[8] Runoff from agricultural fields can contain significant concentrations of dissolved salts. These salts originate in precipitation, irrigation water, fertilizers and other agricultural chemicals, and from the soil minerals. Plants generally exclude ions of chemicals that they do not need. In this way, dissolved salts in irrigation water, for example, can be concentrated in the root zone of the growing crop. Runoff can redissolve these salts and transport them into aquatic ecosystems where some, naturally occurring selenium for example, can be toxic to fish and other wildlife.[9] Transport of fertilizers and pesticides from their point of application can result in significant environmental costs. This transport, or loss from the field, can also have significant negative economic impacts on the grower. Fertilizers lost from the field are not available to promote crop growth. Agricultural chemicals lost from the field, likewise, are not available to protect the plants from pests and diseases. In both cases the grower is paying for expensive inputs and paying to apply them. It is always in the growers’ and the environment’s best interests, therefore, to keep agricultural chemicals in the field where they are needed and where they were applied.

CONTROL OF AGRICULTURAL RUNOFF One of the most direct methods of controlling pollution by agricultural runoff is to minimize the potential for runoff to occur. Other methods can be employed to reduce the amounts of sediments and dissolved chemicals in runoff. As a whole, management practices designed to minimize the potential for environmental damage from agricultural runoff are called best management practices (BMPs), (see the

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article Nutrients: Best Management Practices). Many times, practices aimed at controlling one aspect of agricultural runoff are also effective at reducing other components. This is due to the interrelationships between runoff volume, erosion, transport, dissolution, and delivery. Maintaining good soil tilth and healthy vegetation can minimize runoff. This will promote increased infiltration and a resultant decrease in runoff. Other management practices such as terracing, contour plowing, and using vegetated waterways to convey runoff can result in decreased quantities of runoff by slowing the water leaving the field and allowing more time for infiltration to occur. Construction of farm ponds to receive runoff can result in less total runoff from the farm, lowered peak rates of runoff, and storage of runoff for use in irrigation or livestock watering.[2] Control of water pollution by the mineral and organic sediments and associated chemicals in agricultural runoff is most effectively achieved by reducing erosion from the field. The primary method of reducing erosion is by maintaining a vegetative or plant residue cover on the field at all times or minimizing areas of the field that are bare. Techniques utilized to accomplish these tasks include conservation tillage, strip tillage, and the use of cover crops (see the article Erosion Control: Tillage/Residue Methods). Additional measures that can be employed at the edge of the field, or off-site include vegetative filter strips and farm ponds (see the article Farm Ponds). Methods to control the loss of nitrogen and other plant nutrients from cropland include applying nitrogen in the quantity required by the crop and at the time the crop needs it (see the article Nutrients: Best Management Practices). This requires multiple applications and can be difficult for tall crops. For this reason, most, or all, of the nitrogen required by the crop is often applied at planting. Nitrogen fertilizers have often been applied based on general recommendations for the type of crop to be grown. Since nitrogen fertilizers are relatively inexpensive, growers have tended to over apply rather than under apply. Soil tests can tell a grower how much nitrogen is already in the soil and how much needs to be applied for a specific crop. Efforts have been made to make the nitrogen less soluble by changing the form of nitrogen applied to the field so that it becomes available to the plants (and, thus available for loss in runoff) more slowly.[10] One method of controlling the loss of agricultural chemicals is to minimize their solubility in water. Another is to minimize their use through programs such as integrated pest management (IPM) where some crop damage is allowed until it reaches a point that it becomes economically justified to apply pesticides.[11] And a third approach is to make the chemicals more easily degraded so that they do their job and then

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degrade into other, less harmful, chemicals so that they do not stay around long enough to be influenced by runoff-producing rainfall events.

Agricultural Runoff: Characteristics

2.

CONCLUSION Agricultural runoff is one of the leading causes of water quality impairment in streams, lakes, and estuaries in the United States. It can transport large quantities of sediments, plant nutrients, agricultural chemicals, and natural occurring minerals from farm fields into receiving water bodies. In many cases the loss of these substances from the field represent an economic loss to the grower as well as a potential environmental contaminants. There are many methods by which the quantity of agricultural runoff can be reduced. Many of these methods are referred to generically as BMPs. Adoption of BMPs can also improve the quality (reduce contaminant concentrations) of the runoff that does leave the farm. By reducing the quantity and improving the quality of agricultural runoff, it will be possible to improve the water quality in our streams, river, lakes, and estuaries.

3.

4. 5. 6.

7. 8.

9.

10.

REFERENCES 11. 1. U.S. Environmental Protection Agency. EPA Releases Guidelines for New Water Quality Standards; 2002;

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http://www.epa.gov/history/topics/fwpca/02.htm (accessed July 2002). Stewart, B.A.; Woolhiser, D.A.; Wischmeier, W.H.; Caro, J.H.; Frere, M.H. Control of Water Pollution from Cropland, Volume II—An Overview, EPA-600/ 2-75-026b; U.S. Environmental Protection Agency: Washington, DC, 1976. U.S. Environmental Protection Agency. Nonpoint Source Pollution: The Nation’s Largest Water Quality Problem; 2002; http://www.epa.gov/OWOW/NPS/ facts/point1.htm (accessed July 2002). Leeden, Van der. The Water Encyclopedia; Lewis Publishers: Chelsea, MI, 1990. Foth, H.D. Fundamentals of Soil Science, 8th Ed.; John Wiley & Sons, Inc.: New York, NY, 1990. Gordon, N.D.; McMahon, T.A.; Finlayson, B.L. Stream Ecology: An Introduction for Ecologists; John Wiley & Sons Inc.: New York, NY, 1992. Abel, P.D. Water Pollution Biology, 2nd Ed.; Taylor & Francis, Inc.: Bristol, 1996. U.S. Environmental Protection Agency. The Persistent Bioaccumulators Project; 2002; http://www.epa.gov/ chemrtk/persbioa.htm (accessed 15 July 2002). U.S. Geological Survey. Public Health and Safety: Element Maps of Soils; http://minerals.cr.usgs.gov/ gips/na/0elemap.htm#elemap (accessed 15 July 2002). Owens, L.B. Impacts of soil N management on the quality of surface and subsurface water. In Soil Process and Water Quality; Lal, R., Stewart, B.A., Eds.; Lewis Publishers, Inc.: Boca Raton, FL, 1994. U.S. Department of Agriculture. National Integrated Pest Management Network; 2002; http://www.reeusda. gov/agsys/nipmn/ (accessed 15 July 2002).

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Agroforestry: Enhancing Water Use Efficiency James R. Brandle Xinhua Zhou School of Natural Resources, University of Nebraska, Lincoln, Nebraska, U.S.A.

Laurie Hodges Department of Agronomy and Horticulture, University of Nebraska, Lincoln, Nebraska, U.S.A.

INTRODUCTION Agroforestry is the intentional integration of trees and shrubs into agricultural systems. Windbreaks, riparian forest buffers, alley-cropping, silvopastoral grazing systems, and forest farming are the primary agroforestry practices found in temperate regions of North America.[1] Placing trees and shrubs on the landscape changes the surface energy balance, influences the surrounding microclimate, and has the potential to alter water use and productivity of adjacent crops.[2,3] In agricultural systems, water is often the major factor limiting growth. When water availability is limited as a result of limited supply or high cost, its efficient use becomes critical to successful production systems. For example, proper irrigation at the appropriate stage of crop development minimizes pumping costs and increases yield; reducing soil tillage conserves soil water and may enhance yield, and reducing surface runoff or trapping snow improves soil water storage for future crop use. These water conservation efforts contribute to the efficient use of available water and are determined primarily by management practices. In contrast, Tanner and Sinclair[4] distinguish between the efficient use of water and water use efficiency (WUE). WUE is primarily a function of physiological responses of plants to environmental conditions. This review focuses on WUE defined as the amount of biomass (or grain) produced per unit of land area for each unit of water consumed.[4] Soil water may be consumed by evaporation from the soil surface or by the transport of water through the plant and subsequent evaporation from the leaf surface. The rate of water consumption is determined by the microclimate of the crop. Because agroforestry practices alter the microclimate of adjacent fields, they affect WUE of plants growing in those fields.

DISCUSSION Windbreaks, riparian forest buffers or alley-cropping systems are the practices most likely to be integrated

into crop production systems. In all three practices, trees and shrubs tend to be arranged in narrow barriers adjacent to the crop field. Microclimate responses downwind of any of these types of barriers are similar and the following discussion applies to all three types of barriers. As wind approaches these barriers, it is diverted up and over the barrier creating two zones of protection, a larger zone to the lee of the barrier (the side away from the wind) and a smaller zone on the windward side of the barrier. In these zones, wind speed is reduced and turbulence and eddy structure in the vicinity of the barrier are altered. As a result of these changes, the transfer coefficients for heat and mass between the crop and the atmosphere are altered; the gradients of temperature, humidity, and carbon dioxide concentration above the soil and canopy are changed;[5] and the plant processes of transpiration and photosynthesis are altered.[6] McNaughton[5] defined two regions within the leeward zone of protection: the quiet zone, extending from the top of the barrier down to a point in the field located approximately 8H leeward (H is the height of the barrier) and a wake zone, lying beyond the quiet zone and extending from approximately 8H to a distance of 20H to 25H from the barrier. Within the quiet zone where turbulence is reduced, we expect conditions to be such that the canopy is ‘‘uncoupled’’ from the atmospheric conditions above the sheltered zone, while in the wake zone where turbulence is increased, we expect the canopy to become more strongly ‘‘coupled’’ to the atmosphere above. In both locations we would expect the rates of photosynthesis and transpiration to be altered and WUE to change. The magnitude of change in wind speed, as well as the extent of microclimate modifications within the quiet and wake zones, are largely determined by the structure of the windbreak or barrier and the underlying meteorological conditions. Structure refers to the amounts of solid material and open space and their arrangement within the barrier. Dense barriers, for example, multiple rows of conifers, generally result in greater wind speed reduction but more turbulence. 17

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More porous barriers, for example, single rows of deciduous species, result in less wind speed reduction but also less turbulence. The downwind extent of the protected area is generally greater for more porous barriers. As a result, narrow, less dense barriers (40– 60% density) are typically used to protect crop fields. The overall influence of wind protection on plant water relations is complex and linked to temperature, humidity, wind speed, and other meteorological conditions found in the protected zone, the amount of available soil water, crop size, and stage of development.[2,3,7] Until recently, the major effect of wind protection and its influence on crop growth and yield were assumed to be due primarily to soil water conservation and reduced water stress of sheltered plants.[8,9] There is little question that the evaporation rate from bare soil is reduced in the protected zone.[3] However, the effect of reduced wind speed on transpiration, evaporation from the plant canopy, and overall plant water status is less clear.[2,3,7] According to Grace,[9] transpiration rates may increase, decrease, or remain unaffected by wind protection depending on wind speed, atmospheric resistance, and saturation vapor pressure deficit. Cleugh[3] suggests that as stomatal resistance increases, evaporation from the canopy may actually be increased with a reduction in wind speed. When stomatal resistance is high and water is limited, stomatal resistance controls the rate of evaporation from the leaf surface, not the amount of turbulence. Under these conditions a decrease in wind speed and turbulent mixing may increase the potential for evaporation from the leaf surface.[3] Evaporation from the leaf surface consists of two phases, an energy driven phase and a diffusion driven phase. Movement of water through the plant and out the stomata is driven by the water potential gradient within the plant. This gradient is influenced by the plant’s energy balance. On the lee side of the buffer, reduced wind speed and turbulent mixing lead to increases in leaf temperature and transpiration to meet the increased energy load on the plant. If adequate water is available, it is moved through the plant to the leaf surface and the potential for evaporation from the leaf surface is increased. If water is limited, the stomata partially or completely close, transpiration is reduced, and evaporation from the leaf surface declines. In contrast, movement of water vapor across the leaf boundary layer is controlled by the vapor pressure gradient and the thickness of the leaf boundary layer. As wind speed decreases, the thickness of this boundary layer increases, the vapor pressure gradient decreases, and the rate of evaporation from the leaf surface decreases. The relative magnitude of the two processes determines whether or not transpiration and subsequent evaporation from the canopy are increased, decreased, or remain unchanged.[7,9,10]

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Agroforestry: Enhancing Water Use Efficiency

While these theoretical considerations are important in understanding the process, several studies[11–13] have demonstrated a good correlation between wind protection, conservation of soil water, and enhanced crop yield. Even so, the effect of wind protection on WUE is neither constant throughout the growing period[7] nor is it consistent over varying meteorological conditions. Agroforestry practices impact the water relations of the crop by affecting the loss of water through damaged leaves. On soils subject to wind erosion, windbreaks or other agroforestry buffers provide significant reductions in the amount of wind blown soil and subsequent abrasion of plant parts and cuticular damage.[9,14] Loss of cuticular integrity or direct tearing of the leaves[15] reduces the ability of the plant to control water loss. Agroforestry buffers have a direct effect on the distribution of precipitation, both rain and snow. In the case of snow, a porous barrier will result in a more uniform distribution of snow across the field, providing additional soil water for the crop.[16] In the case of rain, the barrier has minimal influence on the distribution of precipitation across the field; however, in the area immediately adjacent to the barrier a rain shadow may occur on the leeward side. On the windward side, the barrier may lead to slightly higher levels of measured precipitation at or near the base of the trees due to increased stem flow or dripping from the canopy. Trees and shrubs used in agroforestry practices also consume a portion of the available water. In the area immediately adjacent to the barrier, competition for water between the crop and the barrier has a negative impact on yield. These same areas are also subject to some degree of shading depending on the orientation of the barrier. These changes in radiation load influence the energy balance and thus the growth and development of the crop and the utilization of water.[2]

SUMMARY In summary, agroforestry practices such as windbreaks, riparian forest buffers and alley-cropping systems generally improve both the efficient use of water by the agricultural system and the WUE of the individual crop. In the case of efficient water use, the evidence is clear. In the case of crop WUE, the evidence leaves some unanswered questions. How do we account for the varied crop yield responses reported in the literature? In many cases yields are increased but no clear relationship to crop water budget is shown. In other cases crop yield response is minimal. Under what meteorological conditions are the effects of agroforestry practices most valuable to water balance questions? Final crop yield is a integration of the environmental

conditions over the entire growing season. Many different combinations of environmental conditions may result in similar plant responses. How do we address the numerous combinations of plant stress and plant growth to determine ‘‘a response’’ to wind protection? To answer many of these questions it will be necessary to intensify the numerical modeling methods developed by Wilson[17] and Wang and Takle.[18] With a better model to describe the turbulence fields and the transport of water, heat, and carbon dioxide as influenced by agroforestry practices, it should be possible to assess the numerous combinations of environmental factors influencing crop growth in these systems.

REFERENCES 1. Lassoie, J.P.; Buck, L.E. Development of agroforestry as an integrated land use management strategy. In North American Agroforestry: An Integrated Science and Practice; Garrett, H.E., Rietveld, W.J., Fisher, R.F., Eds.; American Society of Agronomy, Inc.: Madison, WI, 2000; 1–29. 2. Brandle, J.R.; Hodges, L.; Wight, B. Windbreak practices. In North American Agroforestry: An Integrated Science and Practice; Garrett, H.E., Rietveld, W.J., Fisher, R.F., Eds.; American Society of Agronomy, Inc.: Madison, WI, 2000; 79–118. 3. Cleugh, H.A. Effects of windbreaks on airflow, microclimates and crop yields. Agrofor. Syst. 1998, 41, 55–84. 4. Tanner, C.B.; Sinclair, T.R. Efficient water use in crop production: research or re-search? In Limitations to Efficient Water Use in Crop Production; Taylor, H.M., Jordan, W.R., Sinclair, T.R., Eds.; American Society of Agronomy, Inc.: Madison, WI, 1983; 1–27. 5. McNaughton, K.G. Effects of windbreaks on turbulent transport and microclimate. Agric. Ecosyst. Environ. 1988, 22/23, 17–39. 6. Grace, J. Some effects of wind on plants. In Plants and Their Atmospheric Environment; Grace, J., Ford, E.D., Jarvis, P.G., Eds.; Blackwell Scientific Publications: Oxford, 1981; 31–56.

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7. Nuberg, I.K. Effect of shelter on temperate crops: a review to define research for Australian conditions. Agrofor. Syst. 1998, 41, 3–34. 8. Caborn, J.M. Shelterbelts and microclimate, Forestry Commission Bulletin No. 29; Her Majesty’s Stationery Office: Edinburgh, 1957; 135 pp. 9. Grace, J. Plant response to wind. Agric. Ecosyst. Environ. 1988, 22/23, 71–88. 10. Thornley, J.H.M.; Johnson, I.R. Plant and Crop Modeling: A Mathematical Approach to Plant and Crop Physiology; Clarendon Press: New York, 1990; 669 pp. 11. Song, Z.M.; Wei, L. The correlation between windbreak influenced climate and crop yield. In Agroforestry Systems in China; Zhu, Z.H., Cai, M.T., Wang, S.J., Jiang, Y.X., Eds.; International Development Research Centre (IDRC, Canada), Regional Office for Southeast and East Asia, published jointly with the Chinese Academy of Forestry: Singapore, 1991; 21–115. 12. Wu, Y.Y.; Dalmacio, R.V. Energy balance, water use and wheat yield in a Paulownia-wheat intercropped field. In Agroforestry Systems in China; Zhu, Z.H., Cai, M.T., Wang, S.J., Jiang, Y.X., Eds.; International Development Research Centre (IDRC, Canada), Regional Office for Southeast and East Asia, published jointly with the Chinese Academy of Forestry: Singapore, 1991; 54–65. 13. Huxley, P.A.; Pinney, A.; Akunda, E.; Muraya, P. A tree/crop interface orientation experiment with a Grevillea robusta hedgerow and maize. Agrofor. Syst. 1994, 26, 23–45. 14. Kort, J. Benefits of windbreaks to field and forage crops. Agric. Ecosyst. Environ. 1988, 22/23, 165–190. 15. Miller, J.M.; Bo¨hm, M.; Cleugh, H.A. Direct Mechanical Effects of Wind on Selected Crops: A Review, Technical Report Number 67; CSIRO Center for Environmental Mechanics: Canberra, Australia, 1995; 68 pp. 16. Scholten, H. Snow distribution on crop fields. Agric. Ecosyst. Environ. 1988, 22/23, 363–380. 17. Wilson, J.D. Numerical studies of flow through a windbreak. J. Wind Eng. Ind. Aerodyn. 1985, 21, 119–154. 18. Wang, H.; Takle, E.S. A numerical simulation of boundary-layer flows near shelterbelts. Boundary-Layer Meteorol. 1995, 75, 141–173.

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Agroforestry: Enhancing Water Use Efficiency

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Alluvial Fan Takashi Oguchi Center for Spatial Information Science, Division of Spatial Information Analysis, University of Tokyo, Tokyo, Japan

Kyoji Saito Department of Geography, Saitama University, Saitama, Japan

INTRODUCTION The term ‘‘alluvial fan’’ is used in both geomorphology and geology. An alluvial fan in the geomorphological sense is a semiconical depositional landform produced by fluvial and/or debris-flow processes. Fans have concave-up to straight long profiles but convex-up cross profiles. A body of deposits forming such a landform is also called an alluvial fan. Deltas and delta deposits may also have semiconical shapes but are mainly produced by marine processes and so are not alluvial fans. When alluvial fans enter a standing water body, they are described as fan deltas. The apex of a fan corresponds to a point where a stream leaves a mountainous area and enters a flatter area. This point also corresponds to the boundary between the upper erosional area and the lower depositional area in a fluvial system. Fans may also occur where small tributary streams enter a larger river valley. The occurrence of fans reflects a relatively abundant sediment supply due to erosion in source basins, broad lowlands that allow the sedimentation to spread out, and sharp topographic boundaries between the mountains and lowlands. Such sharp boundaries often result from faults and tight folds and hence fans are characteristic of tectonically active regions. A series of coalescing fans along a relatively linear boundary between the mountain and the piedmont provides an apron-type topography, which has been called a bajada in arid regions and an alluvial slope in humid regions. The deposition that causes fan formation is ascribable to the sudden increase in channel width and concurrent decrease in water depth and velocity when a stream leaves a narrow bedrock gorge for an unconfined flatter area. High permeability of clastic fan sediment facilitates further deposition since it reduces surface runoff. This effect seems to be more enhanced in dry regions where fans form under conditions of ephemeral flow. Downstream reduction in river gradient at a fan apex is usually too limited to induce fan deposition. Fans occur under various environmental conditions: in arid regions such as the American Southwest and 20

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Spain; in humid regions such as Japan and northern India; and in high-latitude regions such as Alaska and Canada. Fans have been studied in the field and also in the laboratory using map measurements, analysis of digital data, and flume experiments.

FAN SIZE The size of alluvial fans is highly variable (Fig. 1). Small fans with radii of tens to hundreds of meters can be observed along small steep drainages in almost any environment. The radius of a large fan can exceed 100 km but such large fans are rare because of space limitations in depositional lowlands. Fan area tends to correlate positively with source basin area reflecting the fact that larger source basins generally supply greater amounts of sediment and water to transport the sediment a greater distance. For a given source area, fan area tends to increase with increasing source basin slope as steep basins produce more sediment. Relationships between fan sizes and source basin topography also vary from region to region, depending on the age of the fan, and other environmental conditions such as the lithology of the source basins, precipitation, and the rate of tectonic deformation.

FAN SLOPE The slope of alluvial fans is generally inversely proportional to fan area, source basin area and discharge, but positively proportional to source basin slope and particle sizes. There are two different opinions concerning the lower limit of fan slope that affect the definition of alluvial fans. Studies in arid regions suggest that fans are relatively steep with gradients larger than 1 . In humid regions, however, much gentler depositional landforms have been identified as fans (with gradients as low as 0.013 ).[1] Blair and McPherson[2] indicated that alluvial fans have average slopes between 1.5 and 25 , whereas river gradients in flat sedimentary basins are significantly lower, rarely

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Fig. 1 A comparison of fan systems. Source: From Ref.[1].

exceeding 0.5 , resulting in a natural depositional slope gap of 0.5–1.5 . Thus, they renamed piedmont depositional landforms gentler than 0.5 (formerly classified as alluvial fans or fan deltas) as rivers or river deltas. Abrupt change in the depositional slope, associated with sudden change in riverbed material from gravel to sand, also occurs at the toes of humid-region fans,[3] but the river gradient is significantly lower than 0.5 . In addition, semiconical piedmont depositional landforms with gradients of 0.5–1.5 frequently occur in humid regions such as Japan, Taiwan, and the Philippines.[4] Therefore, it seems more appropriate to define alluvial fans based on their semiconical shapes rather than on their slopes.

characteristics are intermediate between fluvial processes and debris flows have also been observed on fans, including transitional debris flows, hyperconcentrated flows, sheet flows, and intermediate flows associated with sieve deposition. Various types of fan deposits are often observed within a single fan or adjacent fans reflecting spatial and temporal variations in dominant geomorphic processes. In some fans, for instance, debris-flow deposits tend to predominate near the fan apex, while fluvial sediments often occur in the distal zone. Ancient fan sediments are also found in the geologic column.

CHANNEL FORMS AND FAN SEGMENTATION FAN DEPOSITS AND SEDIMENTARY PROCESSES The processes of sediment transport and deposition on alluvial fans are varied depending on the topographical and lithological properties of the source areas as well as the climatic conditions. Mass flows such as debris flows and mud flows tend to create small and steep fans, whereas fluvial processes play a major role in forming some large and gentle fans (Fig. 1). Debris flows provide poorly sorted fan deposits with boulders supported by a fine matrix and depositional lobes having well-defined margins. Fluvial processes provide more sorted and bedded fan deposits without distinct margins, and their gravel diameters tend to decrease downstream. Sediment transport processes whose

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Channels on non-entrenched active alluvial fans are generally wide, shallow, and unconfined. Thus, they are typically braided and tend to move laterally, migrating across the fan surface. The lateral shift of channels on arid fans is more intermittent and random than on humid fans. Meandering rivers may also form very gentle fans.[1] Some alluvial fans are dissected with entrenched trunk channels. A common style of entrenchment is fan-head trenching associated with a reduced riverbed gradient. Deep fan-head trenching usually reflects reduced sediment load relative to discharge caused by climatic change or land-cover change in the source areas. Shallow fan-head trenching may be temporarily a part of alternate entrenchment and backfilling

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Alluvial Fan

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Alluvial Fan

Fig. 2 A three-stage model showing responses of Japanese alluvial fan/source basin systems to increased rainfall at Pleistocene– Holocene transition. Source: From Ref.[6].

processes reflecting natural temporal variation in flood discharge or intrinsic slope thresholds.[5] Non-temporary fan-head trenching leaves behind older inactive surfaces in the upper part of the fan, while an active fan surface occurs in the lower part. Inactive fan surfaces are generally more dissected, more vegetated, and covered with a thicker soil or weathered material. In desert regions, thick gravel coating by desert vanish, composed dominantly of fine-grained clay minerals including black manganese oxide and red iron oxide, is also indicative of older fan surfaces. The apex of the active fan surface immediately below the fan-head entrenched channel is called an intersection point. Long-term erosional and depositional changes may produce more than two intersection points, resulting in long profiles of fans with several segments having different slopes. A decline in base level below fans may lead to deep incision of fan surfaces. In this case, incision begins from the fan toe and propagates toward the apex. A series of base-level drops may form ‘‘fan-terraces’’ along the trunk stream flowing through a fan. The balance between river processes and tectonic movement also affects the presence or absence of fan trenching. Relationships between Quaternary climatic change and the development of segmented alluvial fans have received special attention. Climatic conditions that favored fan deposition and entrenchment vary significantly depending on local conditions. For instance, a drier climate may lead to fan deposition because of increased hillslope sediment supply under less vegetated conditions, while it may also lead to fan entrenchment because of decreased discharge available to transport sediment to a fan. In addition, the mode of fan development, including channel trenching and

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backfilling, may change progressively while fluvial systems are responding to rapid climatic change (Fig. 2).[6]

ECONOMIC SIGNIFICANCE AND HAZARD Alluvial fans can be important sources of both floodwater and ground water. Water flowing through permeable fan deposits is useful for irrigation and water supply, especially in arid regions. The ground water table tends to be low in the mid-fan, but high in the fan toe where abundant pure water is available from wells or springs. In addition, concentrations of heavy minerals, facilitated by repeated trenching and backfilling in the fan head, may provide placer deposits with economic values. Floods on fans may pose serious problems for settlement and development.[7] Uncertainty as to the flood path on a fan especially in arid regions, high flow velocities with debris and active erosion, and deposition during flood events can make alluvial-fan flooding disastrous. Geomorphological analysis to separate active fan surfaces from inactive ones provides a basis for flood-hazard zoning on a fan.

CONCLUSION Alluvial fans have attracted the attention of many geomorphologists and geologists, because fans respond to intrinsic and extrinsic change in various ways and provide a key to understand links between erosional and depositional systems. Although the characteristics of fans vary depending on regional environments, previous studies tended to focus on fans in arid regions and rarely dealt with a large dataset of humid

and high-latitude fans. More research on non-arid fans is needed. REFERENCES 1. Stanistreet, I.G.; McCarthy, T.S. The Okavango fan and the classification of subaerial fan systems. Sediment. Geol. 1993, 85, 115–133. 2. Blair, T.C.; McPherson, J.G. Alluvial fans and their natural distinction from rivers based on morphology, hydraulic processes, sedimentary processes, and facies assemblages. J. Sediment. Res. Sect. A Sediment Petrol Processes 1994, 64, 450–489.

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3. Yatsu, E. On the longitudinal profile of the graded river. Trans. Am. Geophys. Union 1955, 36, 655–663. 4. Saito, K. Model of Alluvial Fan Development Based on Channel Pattern and Gravel Size, Report of Research Project; Grant-In-Aid for Scientific Research, Saitama University: Urawa, Japan, 2003. 5. Schumm, S.A. The Fluvial System; John Wiley & Sons: New York, 1977. 6. Oguchi, T. Relaxation time of geomorphic responses to Pleistocene–Holocene climatic change. Trans. Jpn. Geomorphol. Union 1996, 17, 309–321. 7. Committee on alluvial fan flooding. Alluvial Fan Flooding; National Academy Press: Washington, DC, 1996.

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Alluvial Fan

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Ancient Greece: Agricultural Hydraulic Works Demetris Koutsoyiannis Department of Water Resources, School of Civil Engineering, National Technical University of Athens, Zographou, Greece

A. N. Angelakis Regional Foundation for Agricultural Research, National Foundation of Agricultural Research, Heracleion, Greece

INTRODUCTION Agricultural development requires hydraulic works including flood protection of agricultural areas, land reclamation, and drainage. In addition, in a Mediterranean climate, irrigation of crops is necessary to sustain agricultural production and, at the same time, water storage projects are necessary to remedy the scarcity of water resources during the irrigation period. In modern Greece, irrigation is responsible for more than 85% of the water consumption and, to provide this quantity, several large hydraulic works have been built. Similarly, in ancient times, Greeks had to develop technological means to capture, store, and convey water and simultaneously to make agricultural areas productive and protect them from flooding. Agricultural developments in Greece, traced to the Minoan and Mycenaean states,[1,2] were responsible for the increase of agricultural productivity, the growth of large populations, and the economic progress that led to the creation of classical civilization. Some examples of agricultural hydraulic projects of the ancient times chronologically extending from the Mycenaean to the Hellenistic period are discussed in this article.

THE EARLY DEVELOPMENTS AND THE MYTH OF HERACLES Urban water projects such as water supply aqueducts and sewer systems have been common in many ancient civilizations. Archeological and historical evidence suggests that several such projects were constructed in ancient Greece, some of which are astonishing. Obviously, agricultural projects, in comparison to urban ones, are rougher and also exposed to damages and decay and thus can hardly be preserved for millennia. However, there is convincing evidence at several places of Greece and in several stages of the Greek civilization that important agricultural hydraulic works have been built. This evidence comes from 24

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mythology, scripts including epigraphs, and remnants of certain works. The first actions of hydraulic engineering in mainland Greece are traced to around 1600 B.C.[3]; there is no written information about these actions, which, however, survived in the mythic folklore in the legend of the hero Heracles (also known with the Latin name Hercules). Even from the ancient times, several authors such as the historian Diodoros Siculus (90–20 B.C.), the geographer Strabo (67 B.C.–23 A.D.), and the traveler Pausanias (2nd century AD) explained Heracles in a historic way demystifying him from a mythic hero into a hydraulic engineer; this continues today.[3–5] The myth of Heracles fighting against Acheloos indicates the struggle of the early Greeks against the destructive power of floods. Acheloos, the river with the highest mean flow rate in Greece, was then worshipped as a god. As depicted on Greek vessels, Acheloos was metamorphosed into a snake and then a bull, but finally was defeated by Heracles who won Deianira as his wife. According to the historian Diodoros Siculus (IV 35) and the geographer Strabo (X 458–459), the meaning of the victory is related to channel excavation and construction of dikes to confine the shifting bed of Acheloos. There are no technical descriptions of these works; only some presumed remnants of dikes.[3] From Strabo (IX 440) and Diodoros (IV 18), it is also known that similar structures had been built on another large river located at the Thessaly plain, Peneios at Larissa, which are again attributed to Heracles. Other labors of Heracles such as those of the Lernaean Hydra and the Augean stables also symbolize hydraulic works. Lernaean Hydra was a legendary creature in the form of a water snake with nine heads that lived in the Lerna swamp near Argos. Hydra possibly symbolizes the karstic springs of the area or the Lerna swamp itself, and its annihilation by Heracles has been interpreted as the drying up of the swamp. The Augean stables were cleaned by Heracles who diverted two rivers to run through the stables (a more sanitary-environmental labor).

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THE MYCENAEAN GREECE AND THE LAND RECLAMATION SYSTEM IN KOPAIS Archeological evidence traces the earliest significant hydraulic works in Greece to the Minoan civilization at Crete. These, however, were related to urban water developments and no traces of agricultural hydraulic projects have been found to date in Crete. Nevertheless, Platon[6] believed that the Minoans had practiced irrigation and developed irrigation and land reclamation projects. In addition, according to Marinatos,[7] many agricultural crops of the present day such as vegetables, cereals, olives, grapes, and aromatic species were grown in Minoan Crete. After the decline of the Minoan civilization (ca. 15th century B.C.), the Mycenaean civilization in mainland Greece achieved supremacy. The great Mycenaean cities (Mycenae, Tiryns, and Pylos in Peloponnese and Thebes and Orchomenos in Boeotia, north of Athens) were noted for their heavy fortifications with their massive, cyclopean masonry, while Minoan cities were totally unfortified. Close to Thebes and Orchomenos, there was a large shallow lake, named Kopais, where the Boeoticos Kephisos River discharged. Natural karstic sinkholes (katabothres) discharged some of the water, above a certain level, toward the sea. At the end of the 19th century AD, the lake was permanently drained and converted into an irrigated plain, one of central Greece’s most fertile agricultural areas. The modern drainage of Kopais has also revealed massive hydraulic engineering works that most probably drained it in late Mycenaean times (ca. 1450– 1300 B.C.). According to Strabo (IX 406–407, 414–415), the draining of Kopais was achieved by the Minyae people who lived there. Huge earthen dykes furnished with cyclopean walls were built in Kopais. Three main canals with length 40–50 km, width 40–80 m, and parallel walls up to 2–3 m thick traverse the former lake area.[5,8] The whole project included the construction of polders (Fig. 1) and artificial reservoirs for floodwater retention and storage and the improvement of the drainage capacity of the natural sinkholes. The scale of this vast project, which includes the construction of the enormous citadel at Gla, another Mycenaean palatial site on a low limestone island rising up from the floor of the basin, dwarfs any other Mycenaean building project. According to Knauss,[9] the sophisticated hydraulic system in the Kopais and its advantages in developing the country and especially the agricultural production allow the hypothesis that Kopais was the ‘‘fat province’’ of Boeotia mentioned by Homer in Iliad, Book 7 (219–224). Knauss is so much impressed by the system as to write ‘‘As an hydraulic engineer of today, always advised to look for the best economic and ecologic solution of a given hydrotechnical problem, I admire my early colleagues in what they could do and what they did, with

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Fig. 1 Cross section of the polder dyke in Kopais made of loam-sealed stone. Source: From Ref.[10].

simple tools and materials, but with an intensive and sensitive observation of natural processes, some thousands years before modern hydraulic engineering could reach a similar standard.’’ According to Strabo and newer evidence, the area became reflooded sometime later, probably because of earthquakes (ca. 1100 B.C.). Interestingly, in the case of Kopais, the myth relates Heracles with the destruction, rather than construction, of the project and the reflooding of the area, thus indicating that war actions (related to the intra-Mycenaean rivalry) probably contributed to the collapse of the project. Another important project of the same Mycenaean period (ca. 1250–1200 B.C.) is the Tiryns dam. It seems that, during a flood, a stream south of Tiryns abandoned its bed and shifted to the north of the Tiryns. To protect the lower town from future floods, the inhabitants of Tiryns installed an artificial river diversion consisting of a 10-m-high and 300-m-long dam and a 1.5-km-long canal.[11] The dam is a huge earthen embankment lined with cyclopean masonry across the earlier streambed. Yet another massive hydraulic project of the same period has been found in Olympia. This includes a dyke in Cladeios river with length 800 m, width 3 m, and height 3 m and a dam in Alpheios river with length 1000 m, height 2 m at least, and width 30 m.[10] The two rivers may be those related to the Augean stables myth mentioned above, and it has been conjectured[10] that the project is related to an initial stage of the Olympic Games.

THE DRAINING OF THE LAKE IN PTECHAE AND THE GREEK INSTITUTIONS FOR CONSTRUCTING PUBLIC WORKS The hydrotechnical skill achieved by the Mycenaean engineers on their land reclamation activities was lost

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in the centuries after the decline and finally the collapse of the Mycenaean world (ca. 1100–900 B.C.). Later, in the classical Greece civilization, the progress in construction of hydraulic projects is accompanied by improvement in the understanding of water-related natural phenomena. However, most findings of hydraulic works of that period are related to the urban rather than agricultural water use. There is evidence, however, that the draining of the Kopais plain was also attempted at later times. Thus another salient work, a tunnel 2.5 km long, 1.8 m high, and 1.5 m wide leading from Kopais to the sea has been revealed. This would provide discharge capacity, additional to that of the natural sinkholes, for draining the lake. Shafts up to 60 m high were lowered at distances 40–200 m that helped excavating the tunnel, allowed some daylight, and made orientation easy. This tunnel has not been explored to date, and it is not known whether the project was completed and operated until it went damaged some time later or was never completed. Papademos[5] maintains that this tunnel was not built at Mycenaean times and it was never completed. Strabo (IX 406) mentions that draining works were executed in the Kopais Lake by Crates, engineer of Alexander the Great in 336–323 B.C.. At the same period, the draining of another lake in Ptechae, which is probably identified with the Dystos Lake in Southern Euboea, was performed. To validate the fact that scripta manent, the contract of this project was revealed in excavations in Chalkis in 1860.[5] The contract for draining and exploitation of the lake is between the Eretrians and the engineer-contractor Chairephanes. The project is what we call today BOOT (build, own, operate, transfer). The rather wordy (such as those of today) contract is written on a Pentelian marble stele (87  47  9 cm). On the surface, relief sculptures show the gods that were worshiped in the region, Apollo, Artemis, and Leto. A carved scripture in 66 verses signed by more than 150 people contains the construction contract, starting as KaTa
Tade XairefanZB epaggelleTai ’EreTrieusin e
xaxein kai
xZra
n poi Zsein TZn limnZn TZ
n en PTewaiB . . .  (In this, Chairephanes promises to the Eretrians that he will drive away the lake in Ptechae and make it land . . . ). The first 35 verses are the main cotract. In the continuation, two resolutions of the parliament and the Demos are given. With the first one (verses 36–42), asylum is granted to Chairephanes and his collaborators for the whole duration of the contract, and in the second resolution (verses 42– 60), the keeping of the contract is confirmed by oath to Apollo and Artemis. Moral and material sanctions (penalty for breach of contract) such as the confiscation of their property and the dedication of it to Artemis are foreseen against misdemeanors.

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Ancient Greece: Agricultural Hydraulic Works

A summary of the main contract is as follows (adapted from Ref.[5]): 1. Between the city of the Eretrians representing the 31 municipalities of the Eretrian region and the contractor Chairephanes, a contract is signed concerning the draining of the lake in Ptechae. 2. The draining works include the construction of drainage canals, sewers, and wells for the drainage of water to natural underground holes or cracks and miscellaneous protection works, including wooden or metallic railings. 3. Irrigation works, such as the construction of a reservoir with side length up to 2 stadia (360 m) for storing irrigation water, and sluice gates, are included in the project. 4. A 4-year construction period is agreed, which can be extended in case of war. 5. The contractor is granted the right to exploit the dried fields for 10 years (extended in case of war), commencing by the finishing of the drying works. 6. The contractor is granted the privilege of customs-free import of materials (stones and wood). 7. The contractor is obliged 1) to pay all labor costs without any charge for the people of Eretria; 2) to pay the amount of 30 talents in monthly installments as a rental for the exploit of the project for 10 years; 3) to maintain all works for the exploitation period to be in good condition after the finishing of the contract; 4) to compensate the land owners by 1 drachma per foot of land area that is to be expropriated for the construction of works; and 5) to avoid harm on private property as much as possible by locating the works in non-cultivating land. 8. In case of death of the contractor, his heirs and collaborators will substitute him in the relations to the city. 9. Penalties are enforced against any person trying to annul the contract. 10. The contractor is obliged to submit a good construction guarantee up to the amount of 30 talents. Interestingly, the epigraph mentions that it would be erected in the temple of Apollo at Eretria and copies of it would be deposited at Megara and Andros. It appears that it was common practice in ancient Greece that detailed competition announcements, project specifications, and project contracts written on marble steles were erected in public sites so that everyone would have known all project details and, simultaneously, the breach of contract would be difficult;

as Tassios[12] puts it, if someone wished to avoid some terms, he would ‘‘stumble on them.’’

ARTICLES OF FURTHER INTEREST Ancient Greece: Hydrologic and Hydraulic Science and Technology, p. 24. Ancient Greece: Urban Water Engineering and Management, p. 31.

REFERENCES 1. Sallares, J.R. The Ecology of the Ancient Greek World; Cornell University Press: Ithaca, NY, 1991. 2. Lembesi, A. To iero´ tou ErmZ kai tZB AjroditZB stZ SumZ Biavvou (The Sanctuary of Hermes and Afrodite in Symi, Viannos) (in Greek); The Athens Archaeological Society: Athens, 1985. 3. Constantinidis, D. Ta udraulika erga stZv Ellada (Hydraulic works in Greece). Notes of Two Lectures at the National Technical University of Athens (in Greek); National Technical University of Athens: Athens, 1993.

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4. Schoo, J.H. Hercules’ Labors; Argonaut Inc.: Chicago, 1969. 5. Papademos, D.L. Ta Ydraulika E´rga Para toiB ArwaioiB (The Hydraulic Works in Ancient Greece); TEE, Vol. B (in Greek), Athens, 1975; 444–458, 490–498. 6. Platon, N. ZakroB, To Neov Mivoi€ko´v Avaktorov (Zakros, The New Minoan Palace) (in Greek); The Athens Archaeological Society: Athens, 1974. 7. Marinatos, S. O ArwaioB KrZtiko´B Politismo´B (The Ancient Cretan Civilization) (in Greek); Stergiadou: Athens, 1927. 8. Lazos, C.D. MZwavikZ kai Tewvologia stZv Arwaia Ellada (Mechanics and Technology in Ancient Greece) (in Greek); Aeolos: Athens, 1993; 33 pp. 9. Knauss, J. The prehistoric water management and land reclamation system in the Kopais-Basin, Boiotia, middle Greece. ICID J. 2000, 49 (4), 39–48. 10. Knauss, J. Olympische Studien, Herakles und der Stall des Augias; Technnische Universita¨t Mu¨nchen, 1998. 11. Zangger, E. Landscape changes around tiryns during the bronze age. Am. J. Archaeol. 1994, 98 (2). 12. Tassios, T.P. H diajaveia kai oi arwaioi (Transparency and the ancients) (in Greek); 5 May, 2002. To Vima.

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Ancient Greece: Hydrologic and Hydraulic Science and Technology A. N. Angelakis Regional Foundation for Agricultural Research, National Foundation of Agricultural Research, Heracleion, Greece

Demetris Koutsoyiannis Department of Water Resources, School of Civil Engineering, National Technical University of Athens, Zographou, Greece

INTRODUCTION The approach typically followed in problem solving today is represented by the sequence in the order: Understanding—data—application. However, historical evolution in the development of water science and technology (and other scientific and technological fields) followed the reverse order: application preceded understanding.[1] Thus, technological application in water resources started in Greece as early as ca. 2000 B.C.. Specifically, in the Minoan civilization and later in the Mycenaean civilization several remarkably advanced technologies have been applied for groundwater exploitation, water transportation, water supply, stormwater and wastewater sewerage systems, flood protection, drainage, and irrigation of agricultural lands. Much later, around 600 B.C., Greek philosophers developed the scientific views of natural phenomena for the first time ever. In these, hydrologic and meteorological phenomena had a major role, given that water was considered by the Ionic school of philosophy (founded by Thales of Miletus; ca. 624 –545 B.C.) as the primary substance from which all things were derived. Even later, during the Hellenistic period, significant developments were done in hydraulics, which along with progress in mathematics allowed the invention of advanced instruments and devices, like Archimedes’ water screw pump.

SCIENTIFIC VIEWS OF HYDROLOGIC PHENOMENA AND HYDRAULICS It has been believed by many contemporary water scientists that ancient Greeks did not have understanding of water related phenomena, and had a wrong conception of the hydrologic cycle. This belief is mainly based on views of Plato (ca. 429–347 B.C.), who in his dialogue Phaedo (14.112) expresses an erroneous theory (based on Homer’s poetical view) of hydrologic cycle; notably, his wrong theory was adopted by many 28

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thinkers and scientists from Seneca (ca. 4 B.C.–65 A.D.) to Descartes (1596–1650). However, long before Plato, as well as much later, several Greek philosophers had developed correct explanations of hydrologic cycle, revealing good understanding of the related phenomena. In fact, as Koutsoyiannis and Xanthopoulos[2] note, the first civilization in which these phenomena were approached in an organized theoretical manner, through reasoning combined with observation, and without involving divine and other hyperphysical interventions, was the Greek civilization. The same authors catalog a number of ancient Greek contributions revealing correct understanding of water related phenomena. Thus, the Ionic philosopher, Anaximenes (585–525 B.C.) studied the meteorological phenomena and presented reasonable explanations for the formation of clouds, hail and snow, and the cause of winds and rainbow. The Pythagorean philosopher Hippon (5th century B.C.) recognizes that all waters originate from sea. Anaxagoras, who lived in Athens (500–428 B.C.) to Empedocles (ca. 493– 433 B.C.) and is recognized equally as the founder of experimental research, clarified the concept of hydrologic cycle: the sun raises water from the sea into the atmosphere, from where it falls as rain; then it is collected underground and feeds the flow of rivers. He also studied several meteorological phenomena, generally supporting and complementing Anaximenes’ theories; his theory about thunders, which was against the belief that they are thrown by Zeus, probably cost him imprisonment (ca. 430 B.C.). In particular, he correctly assumed that winds are caused by differences in the air density: the air, heated by the sun, moves towards the North pole leaving gaps that cause air currents. He also studied Nile’s floods and attributed them to snowmelt in Ethiopia. The ‘‘enigma’’ of Nile’s floods (which, contrary to the regime of Mediterranean rivers, occur in summer) was also thoroughly studied by Herodotus (480–430 B.C.), who seemed to have clear knowledge of hydrologic cycle and its mechanisms.

Aristotle (384–323 B.C.), in his treatise Meteorologica clearly states the principles of hydrologic cycle, clarifying that water evaporates by the action of sun and forms vapor, whose condensation forms clouds; he also recognizes indirectly the principle of mass conservation through hydrologic cycle. Theophrastus (372–287 B.C.) adopts and completes the theories of Anaximenes and Aristotle for formation of precipitation from vapor condensation and freezing; his contribution to the understanding of the relationship between wind and evaporation was significant. Epicurus (341–270 B.C.) contributed to physical explanations of meteorological phenomena, contravening the superstitions of his era. Archimedes (287–212 B.C.), the famous Syracusan scientist and engineer considered by many as the greatest mathematician of antiquity or even of the entire history, was also the founder hydrostatics. He introduced the principle, named after him, that a body immersed in a fluid is subject to an upward force (buoyancy) equal in magnitude to the weight of fluid it displaces. Hero (Heron) of Alexandria, who lived after 150 B.C., in his treatise Pneumatica studied the air pressure, in connection to water pressure, recognizing that air is not void but a substance with mass consisting of small particles. He is recognized[3] as the first person who formulated the discharge concept in a water flow and made flow measurements. Unfortunately, many of these correct explanations and theories were ignored or forgotten for many centuries, only to be re-invented during Renaissance or later. This was not restricted to water related phenomena. For example, the heliocentric model of the solar system was first formulated by the astronomer Aristarchus of Samos (310–230 B.C.), 1800 yr before Copernicus (who admits this in a note). Aristarchus also figured out how to measure the distances to the Sun and the Moon and their sizes. In addition, not only did ancient Greeks know that Earth is spherical, but also Eratosthenes (276–194 B.C.) calculated, 1700 yr before Columbus, the circumference of the earth, with an error of only 3%, by measuring the angle of the sun’s rays at different places at the same time; in addition, the geographer Strabo (67 B.C.–23 A.D.) had defined the five zones or belts of Earth’s surface (torrid, two temperate, and two frigid) that we use even today.

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the characteristics of a pump with the modern meaning is Archimede’s helix or water screw. The invention of the water screw is based on the study of the spiral, for which Archimedes wrote a treatise entitled On Spirals, in 225 B.C.. This invention of Archimedes was first mentioned by Diodorus Siculus (first century B.C.; Bibliotheke, I 34.2, V 37.3) and Athenaeus of Naucratis (ca. 200 B.C.; Deipnosophistae, V) who transferred an earlier text (of the late 3rd century B.C.) by Moschion, describing a giant ship named Syracusia. This pump is an ingenious device which functions in a simple and elegant manner by rotating an inclined cylinder bearing helical blades around its axis whose bottom is immersed in the water to be pumped. As the screw turns, water is trapped between the helical blades and the walls, and thus rises up to the length of the screw and drains out at the top (Fig. 1). As mentioned by Athenaeus of Naucratis, the first use of the water screw must have been by Archimedes himself to remove the large amount of bilge water that would accumulate on the large ship Syracusia. There is historical and archaeological evidence that in past the use of the water screw was propagated to all Mediterranean countries as well as to the east up to India. It was rotated by a man or a draft animal. Its uses range from irrigation (e.g., in Egypt) to draining of water in mines (e.g., in Spain). In its original form, the screw of Archimedes is used even today in some parts of the world. For example, farmers in Egypt and other countries in Africa use it to raise irrigation water from the banks of rivers. A modern version of the screw that is in industrial use today has two main differences from its original one: it is powered by a motor and the screw rotates

HYDRAULIC MECHANISMS AND DEVICES The foundation of hydraulics after Archimedes led to the invention of several hydraulic mechanisms and devices with significant contribution to diverse applications from lifting of water to musical instruments. Although in past several devices were in use to lift water to a higher elevation, the first device that had

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Fig. 1 Archimedes’ water screw in its original form as depicted in an Italian stamp (not quite correctly from a technical point of view) along with a bust probably representing Archimedes (from http://www.mcs.drexel.edu/~crorres/ Archimedes/Stamps/).

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Ancient Greece: Hydrologic and Hydraulic Science and Technology

pump has been described by Philon Byzantius (Pneumatica), Hero of Alexandria (Pneumatica, I 28), and Vitruvius (X 7, 1–3). This pump is composed of two cylinders with pistons that were moved by means of connecting rods attached to opposite ends of a single lever. The force pump was used in many applications, such as in wells for pumping water, boats for bilgewater pump, basement pump, mining apparatus, fire extinguisher, and water jets. Yet another pumping device, the chain pump was invented in Alexandria by an engineer Philon Byzantius (260–180 B.C.). This comprised a set of pots attached to a chain or belt that was moved by a rotating wheel. Several pneumatic devices and mechanisms including a steam boiler, a reactive motor, the organ (harmonium), and several jet springs have been invented by Hero of Alexandria.[4–6] Most of them were based on the siphon principle, or more generally, the combined action of air and water pressure. Ctesibius, Philon Byzantius, and Hero were the three most famous engineers of Hellenistic Alexandria, whose studies mark a significant progress in hydraulics. This progress allowed installation of advanced water supply systems like that of the citadel at Pergamon, in which pressure pipes (probably made of metal) were implemented. It also led to the great advances in the art of aqueducts during the Roman period. Fig. 2 Archimedes’ water screw in its modern form, as implemented in the wastewater treatment plant of Athens (one of nine screws that pump 1 million m3 per day).

REFERENCES

inside the cylinder rather than the entire cylinder being rotated; the latter modification allows the top-half of the cylinder to be removed, which facilitates cleaning and maintenance. The modern screw is the best choice for pumping installations when water contains large sediments or debris, and when the discharge is large and the height small. Thus, the screw is used today mainly for pumping wastewater and stormwater runoff (Fig. 2). It has been also used in other types of applications such as pumping of oil and supporting blood circulation during surgical procedures. Another pumping mechanism, the force pump was invented by engineer (initially barber) Ctesibius of Alexandria (ca. 285–222 B.C.) who was also the inventor of other instruments such as the hydraulic clock and hydraulis—a hydraulic musical instrument. The force

1. Dooge, J.C.I. Hydrology, past and present. J. Hydraulic Res. 1988, 26 (1), 5–26. 2. Koutsoyiannis, D.; Xanthopoulos, T. TEwnikZ Ydroloia (Engineering Hydrology), 3rd Ed.; National Technical University of Athens: Athens, 1999; 1–418 (in Greek). 3. U.S. Committee on Opportunities in the Hydrological Sciences. Opportunities in the Hydrologic Sciences; National Academy Press: Washington, DC, 1991. 4. Lazos, C.D. MZwanikZ kai TEwnologia stZn Arwaia Ellada (Mechanics and Technology in Ancient Greece); Aeolos: Athens, 1993 (in Greek). 5. Lazos, C.D. Ydraulika O´rgana kai MZwanismoi stZn Aigupto ton PtolEmaion (Hydraulic Instruments and Mechanisms in Egypt of Ptolemies); Aeolos: Athens, 1999 (in Greek). 6. Tokaty, G.A. A History and Philosophy of Fluid Mechanics; Henley on Thames: Foulis, 1971 (reprinted by Dover: New York, 1994).

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Ancient Greece: Urban Water Engineering and Management Demetris Koutsoyiannis Department of Water Resources, School of Civil Engineering, National Technical University of Athens, Zographou, Greece

A. N. Angelakis Regional Foundation for Agricultural Research, National Foundation of Agricultural Research, Heracleion, Greece

INTRODUCTION Ancient Greek civilization has been thoroughly studied, focusing on mental and artistic achievements like poetry, philosophy, science, politics, and sculpture. On the other hand, most of technological exploits are still relatively unknown. However, recent research reveals that ancient Greeks established critical foundations for many modern technological achievements, including water resources. Their approaches, remarkably advanced, encompass various fields of water resources, especially for urban use, such as groundwater exploitation, water transportation, even from long distances, water supply, stormwater and wastewater sewerage systems, flood protection and drainage, construction and use of fountains, baths and other sanitary and purgatory facilities, and even recreational uses of water. The scope of this chapter is not the exhaustive presentation of what is known today about hydraulic works, related technologies and their uses in ancient Greece but, rather, the discussion of a few characteristic examples in selected urban water fields that chronologically extend from the early Minoan civilization to the classical Greek period. Agricultural hydraulic works like flood protection, drainage and irrigation of agricultural lands, and drainage of lakes were also in use in ancient Greece starting from the Mycenaean times, but are not covered in this chapter. Scientific advances in water resources as well as invention of hydraulic mechanisms and devices are presented in the entry Ancient Greece: Hydrologic and Hydraulic Science and Technology. CLIMATIC AND HYDROLOGIC CONDITIONS Unlike preceding civilizations such as those in Mesopotamia and Egypt, which were based on the exploitation of water of the large rivers such as Tigres, Euphrates, and Nile, the Greek civilization has been characterized by limited and often inadequate natural water resources. The rainfall regime and consequently the water availability over Greece vary substantially

in space. Thus, the mean annual rainfall exceeds 1800 mm in the mountainous areas of western Greece whereas in eastern regions of the country may be as low as 300 mm. Interestingly, the most advanced cultural activities in ancient Greece appeared in semiarid areas with the lowest rainfall and thus the poorest water resources; for example, Knossos in Crete, Cyclades islands, and Athens have annual rainfall about 500 mm, 300–400 mm, and 400 mm, respectively. The potential evapotranspiration exceeds 1000 mm all over Greece, with the highest rates appearing in summer months. Thus, irrigation of cultivated areas during summer is absolutely necessary and becomes the most demanding water use in Greece. Under these climatic and hydrological conditions, Greeks had to develop technological means to capture, store, and convey water even from long distances, as well as legislation and institutions to more effectively manage water. THE WATER SUPPLY IN MINOAN CIVILIZATION Cultural advancements in the Minoan civilization can be observed throughout the third and second millennia B.C., which indicate that the main technical operations of water resources have been practiced in varying forms since ca. 3000 B.C.. During the Middle Bronze Age (ca. 2100–1600 B.C.) Crete’s population in its central and south regions increased, towns were developed and the first palaces were built. At that time, a ‘‘cultural explosion’’ occurred on the island. A striking indication of this is manifested, inter alia, in the advanced water resources management technologies applied in Crete at that time. The sanitary life style developed at this civilization can be paralleled to the modern standards. It is evident that in Minoan civilization extensive systems and elaborate structures for water supply, sewerage systems, irrigation, and drainage were planned, designed, and built to supply the growing population with water for the cities and for irrigated agriculture.[1] In the early phases of the Late Bronze Age (ca. 1600–1400 B.C.), Crete appears to have prospered 31

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even more, as the larger houses and more luxurious palaces of this period indicate.[2] At this time, the flourishing arts, improvements in metal-work along with the construction of better-equipped palaces and an excellent road system, reveal a wealthy, highly cultured, well-organized society, and government in Crete, before the island’s power collapsed following the destruction of the Minoan palaces.[3] The geological catastrophe through the eruption of the Santorini volcano in 1450 B.C. halted the Minoan civilization. Our knowledge of how Minoan cities were supplied with potable water is mainly acquired from the Palace of Knossos. A few cisterns, fountains, and wells were also found at other archeological sites like Zakros, Mallia, Gortys, and other Minoan palaces and cities. At Phaistos some cisterns have been discovered too, but owing to the nature of the ground, no wells or springs have been found there.[1] Even at Knossos, the sources of water and the methods used for supplying it are only partially understood. Several wells have been discovered in the Palace area, and a single well slightly to the northwest of the Little Palace. The latter, restored to its original depth of about 12.5 m and 1.0 m diameter, continues to furnish an excellent supply of potable water.[4] In the Protopalatial stage (ca. 1900–1700 B.C.), several wells were used for drawing drinking water. Their depth did not exceed 20 m and their diameter was not more than 5 m.[5] At least six such wells have been reported.[4] The most important and best known is the one found in the north-west of the Palace in the basement of the House A, which belongs to the first stage of the Middle Minoan period. According to Evans,[4] its upper circuit was mostly a patchwork of rubble masonry, recalling the construction of Roman wells in the site. However, below its crudely built upper ‘‘collar,’’ the well was found to be cased in a series of terracotta cylinders of fine clay and of material so hard that it was initially mistaken for some kind of close-grained stone (Fig. 1). The inhabitants of the Knossos Palace, however, did not depend on the water of the wells alone. There are indications that the water supply system of the Palace of Minos at Knossos was initially dependent on the spring water of Mavrokolybos and later on the Fundana, and other springs. Mavrokolybos, a pure limestone spring, is located at a distance of 700 m south of the palace and an elevation of about 115 m, whereas Knossos lies at an elevation of 85 m from sea level; Fundana, a typical karstic spring with excellent quality of water even today, is at a distance of about 5 km from the palace and at an elevation of about 220 m. Water supply in the Palace was provided through a network of terracotta piping located beneath the palace floors. The pipes were constructed in sections of about 60–75 cm each. These pipes with their expertly shaped, tightly interlocked sections, date from the

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Ancient Greece: Urban Water Engineering and Management

Fig. 1 Perspective view of well below House A, NW of Knossos Palace. Source: From Ref.[4].

earliest days of the building and are quite up to modern standards (Fig. 2). The sections of the clay pipes resemble those used in Greece in classical times, though Evans considered the Minoan to have been designed more efficiently; each section was rather strongly tapped toward one end with the objective of increasing the rate of water flow, thus helping to flush any sediment through the pipe.[5] On the basis of their accomplishments, it can be assumed that Minoan hydraulic engineers were, in a sense, aware of the basic hydrostatic law, known today as the principle of communicating vessels. It is manifested in the water supply of the Knossos Palace through pipes and conduits fed by springs; this is supported by the discovery of the Minoan conduit heading towards the Knossos Palace from Mavrokolybos which suggests a descending and subsequently ascending channel.[4,6] However, it appears that Minoans had only a vague understanding of the relationship between flow and friction. In the Zakros Palace the water supply system depended on groundwater. Here the potable water came from the Main Spring. In the southwest corner of the Cistern Hall an opening leads into a small chamber where the water was collected and channeled into a square underground fountain built on the south; this was thought to correspond with the celebrated man made fountain of the Odyssey known as ‘‘Tukt
Z’’ fountain.[7] The fountain was built of regular limestone, and there is a descending staircase with fourteen steps (Fig. 3). The room may also have served as a shrine. The water of the fountain is brackish today, of about 13.00 dS/m electric conductivity (EC), due to intrusion of seawater. However, this may well be an indication that some reduction in the distance of the palace from the coast has occurred. Another comparable chamber in Zakros is a well– spring located near the southeast corner of the Central

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Fig. 2 Minoan water supply pipes (terracotta pipe sections): (A) overview, and (B) with real dimensions. Source: From Ref.[5].

Court; here again steps lead down into the chamber. The wood of the windlass was found in the water, along with an offering cup containing olives; this is a unique, remarkable find, since the olives were perfectly preserved, as though they had just been picked from the trees; unfortunately they maintained their relative freshness for only a few minutes after they were taken out of the water.[7] A view of this well-spring is given in Fig. 4. In contrast to Knossos, where water was conveyed mainly from springs, and Zakros dependent entirely on groundwater, in Phaistos the water supply system was dependent directly on precipitation: here, the rainwater was collected from the roofs and yards of buildings in cisterns. Special care was given to securing clean surfaces in order to maintain the purity of water. Also, coarse sandy filters were used to treat the rainfall water before it flowed into the cisterns.

THE WATER SUPPLY OF SAMOS AND THE AWESOME FEAT OF EUPALINOS The most famous hydraulic work of ancient Greece was the aqueduct of ancient Samos (located where

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Pythagoreio or Tigani village in the Samos island is currently present), which was admired both in antiquity (as recognized by Herodotus) and in modern times.[8–14] The most amazing part of the aqueduct is the 1036 m long ‘‘Enpal
ineion
orngma’’ or ‘‘Eupalinean digging,’’ more widely known as Tunnel of Eupalinos. The aqueduct includes two additional parts (Fig. 5) so that its total length exceeds 2800 m. The aqueduct was the work of Eupalinos, an engineer from Megara. Its construction was commenced in ca. 530 B.C., during the tyranny of Polycrates and lasted for 10 yr. It was in operation until the 5th century A.D. and then it was abandoned and forgotten. Owing to the text of Herodotus, Guerin[8] uncovered the entrance of the aqueduct. The inhabitants of the island attempted to reuse the aqueduct in 1882 without success. Only 90 yr later, between 1971 and 1973, the German Archaeological Institute of Athens undertook the task to finally uncover the tunnel. Herodotus (History, G, 60) called the tunnel ‘‘amj
istomon’’ or ‘‘bi-mouthed,’’ a characterization that caused curiosity to the readers (any tunnel has two openings or mouths). Only when the tunnel was totally explored was it understood that Herodotus

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Ancient Greece: Urban Water Engineering and Management

Fig. 3 Views of the ‘‘TuktZ’’ fountain: (A) overall view and (B) scheme. Source: From Ref.[7].

meant that the construction of the tunnel was started from two openings. Today, it is very common that water transportation tunnels are constructed from two openings to reduce construction time; high-tech geodetic means and techniques like global positioning systems and laser rays are used to ensure that the two fronts will meet each other. The great achievement of Eupalinos is that he did this using the simple means available at that time; apparently, however, he had good knowledge of geometry and geodesy. Later, in the 1st century B.C., his achievement inspired the mathematician and engineer Hero (Heron) of Alexandria

Fig. 4 Well-spring located in the eastern wing of the Zakros Palace. Source: From Ref.[7].

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(Dioptra, III) who in his geometrical Problem #15 studied how ‘‘to dig a mountain on a straight line from two given mouths.’’ His method is based on walking around the mountain measuring out in one direction, then turning at a right angle, measuring again, etc., and finally using geometrical constructions with similar triangles. Moreover, in modern times, it inspired many mathematicians, engineers, and archeologists who attempted to reconstruct the methods used by Eupalinos to build the tunnel, as, apart from the mention by Herodotus, no written document was found from that time about the project. Today, most of the questions have been answered but not all. For example, there is evidence that Eupalinos did not follow Hero’s method, which would produce a large error. Most probably, Eupalinos walked over the mountain and put poles up along the path in a straight line. When the workers were digging they could try to line themselves up with these poles. This also leaves room for error; as shown in Fig. 5, there was a small departure in the two axes that Eupalinos implemented (NA and SF), which is now estimated to 7 m. Another question is: what led Eupalinos to leave the straight line NA at point A and follow the direction AB? A plausible explanation is given by Tsimpourakis:[14] Eupalinos found a natural fracture or rift and broadening this rift, he was able to proceed much faster. At the end of the rift, he attempted to correct the departure from the initial axis, following the route BC, but C was past this axis. Again according to

35

Fig. 5 Sketch of the Tunnel of Eupalinos (above: vertical section; below: horizontal plan).

Tsimpourakis,[14] when the two teams of workers (each consisting of two people) were simultaneously at points C and F, they realized (hearing the sounds of the opposite team’s excavating tools) that there were close to each other. Then, guided by the sounds of tools they managed to meet at point E. Hermann Kienast of the German Archaeological Institute of Athens proposed a different explanation: the last meters of the two routes of the tunnel (sections CDE and FE) were ingeniously designed rather than coincidentally followed: both teams were directed at points C and F to change direction to the right and then at D the northern one turned to the left on purpose; with this trick it is mathematically sure that the two lines would intersect. Interestingly, the floor of the tunnel was done virtually horizontal, as observed from the elevations shown in Fig. 5; one would expect that it should have some slope for the water to flow. The choice of a horizontal tunnel is related to the excavation from both sides. In a sloping tunnel, the front of the upper section would be inundated (mostly from groundwater), so that the workers could not dig. Another reason is the fact that the horizontal tunnel was easier to control and build with the simple instruments and tools of that time and facilitated the meeting of the two fronts (indeed, the difference in the elevation of the two sections at point F is only 0.60 m). However, this horizontal tunnel could not operate as an aqueduct, simply because the water would not flow horizontally. Therefore, Eupalinos excavated a slopping duct below the floor of the tunnel, shown in the photo of Fig. 6. Its bottom, where clay pipes were arranged, is located 3.5 m and 8.5 m in the inlet (N) in the outlet (S), respectively, below the floor of the tunnel; the large depth at the inlet is another question mark of the project, whose discussion is out of the scope of this chapter. At points where the depth

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becomes too large (in about two-thirds of the tunnel length) Eupalinos preferred to make a second tunnel, the water tunnel, below the main tunnel, the access tunnel. The water tunnel is about 0.60 m wide whereas the access tunnel is about 1.80 m  1.80 m. The

Fig. 6 The Tunnel of Eupalinos. The duct is shown to the left.

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construction of the water tunnel was easy and fast, provided that the access tunnel was completed; 28 vertical shafts were constructed for easy access to the water tunnel and many teams of workers must have been worked simultaneously to dig it. The outer parts of the aqueduct were constructed in a similar manner. Thus, section PQ of the north duct (Fig. 5) was constructed as an open channel whereas section QN was a tunnel with five shafts. What Eupalinos did was not the only solution to the problem of conveying water to Samos. A simple alternative solution was to continue the simple and fast method of section PQN constructing a chain of open channels and tunnels at small depths with shafts. In this solution, the route from point N to S would be around the mountain. Not only is this alternative solution technically feasible, but also it is technically easier, less expensive, and faster. Why Eupalinos preferred his unorthodox and breakthrough solution? How did he persuade the tyrant Polycrates to support this solution? These are unanswered questions. Probably he wished to build a monument of technology rather than simply solving a specific water transportation problem.

THE SUSTAINABLE URBAN WATER MANAGEMENT IN ATHENS Water management in ancient Athens, the most important city of antiquity with a population of more than 200,000 during the golden age (5th century B.C.), is of great interest. Athenians put great efforts into the water supply of their anhydrous city. The first inhabitants of the city chose the hill of Acropolis for their settlement due to the natural protection it offered

Ancient Greece: Urban Water Engineering and Management

and the presence of three natural springs,[15] the most famous being ‘‘Clepsydra.’’ However, natural springs in Acropolis and elsewhere were not enough to meet water demand. Therefore, Athenians used both groundwater, by practicing the art of drilling of wells, and stormwater, by constructing cisterns. In addition, the water from the two main streams of the area, Kephisos and Ilissos, whose flow was very limited in summer, was mainly used for irrigation. Archeological evidence reveals that the city had developed an important system of public water supply consisting of wells, fountains, and springs and there were also a number of private springs and wells. There are indications that a primitive distribution system was in place underneath the city, consisting of underground connections of wells;[15] this expanded all around the city to the outskirts.[16] The most important public work was the Peisistratean aqueduct, built in the time of the tyrant Peisistratos and his descendants (ca. 510 B.C.). The exact location and route of the aqueduct is not well known to date. It is known, however, that it carried water from the foothill of the Hymettos mountain, probably from east of the Holargos suburb at a distance around 7.5 km,[17] to the center of the city near Acropolis. The greater part of it was carved as a tunnel at a depth reaching 14 m. In other parts it was constructed as a channel, either carved in rock or made of stone masonry, with depth 1.30 m–1.50 m and width 0.65 m.[18] In the bottom of the tunnel or channel, a pipe made of ceramic sections was placed (Fig. 7). The pipe sections had elliptic openings with ceramic covers in their upper part for their cleaning and maintenance; the ends of the sections were appropriately shaped, so that they could be tightly interlocked, and were joined with lead. In the recent excavations for the construction of the metro, the widespread use of such ceramic pipes

Fig. 7 Part of the Peisistratean aqueduct (top) and detail of the pipe sections and their connection (bottom). (Photos reproduced from newspaper Kathimerini.)

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was revealed. Similar pipes were also used for sewers. Sewers of large cross section, most probably storm sewers, were built of stone masonry; some of them were natural streams, like Heridanos, that were covered (Fig. 8). Apart from the structural solutions for water supply and sewerage, the Athenian civilization developed a legislation and institutional framework for water management. The first known laws are due to Solon, the Athenian statesman and poet of the late 7th and early 6th century B.C., who was elected archon in 594 and shaped a legal system by which he reformed the economy and politics of Athens. Most of his laws have been later described by Plutarch (47–127 A.D.), from whom it could be learnt that: Since the area is not sufficiently supplied with water, either from continuous flow rivers, or lakes or rich springs, but most people used artificial wells, Solon made a law, that, where there was a public well within a hippicon, that is, four stadia (4 furlongs, 710 m), all

37

should use that; but when it was farther off, they should try and procure water of their own; and if they had dug ten fathoms (18.3 m) deep and could find no water, they had liberty to fetch a hydria (pitcher) of six choae (20 L) twice a day from their neighbors; for he thought it prudent to make provision against need, but not to supply laziness (Plutarch, Solon, 23).

MacDowell[19] conjectures that these laws have been kept unchanged through the classical period. As the city’s public system grew and aqueducts transferred water to public fountains, private installations like wells and cisterns tended to be abandoned. But, the latter would be necessary in times of war because the public water system would be exposed; therefore, the owners were forced by decree to maintain their private facilities in good condition and ready to use.[20] Other regulations protected surface waters from pollution.[19] An epigraph of ca. 440 B.C. contains the ‘‘law for tanners,’’ who are enforced not to dispose their wastes to Ilissos river.[15]

Fig. 8 The Heridanos stream converted into a sewer at Ceramicos (up) and two tributary sewers (down) at Ceramicos (left) and Agora (right). Source: From Ref. [18].

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A distinguished public administrator, called ‘‘kroun^ onEpimElZt
ZB’’, that is, officer of fountains, was appointed to operate and maintain the city’s water system, and to ensure keeping of regulations and fair distribution of water. In addition, a number of guards were responsible for the proper daily use of the public springs and fountains. From Aristotle (Athenaion Politeia, 43.1) it is learnt that the officer of fountains was one of the few that were elected by vote whereas most other officers were chosen by lot; so important was this position within the governance system of classical Athens.[17] Themistocles himself had served in this position. In 333 B.C. the Athenians awarded a gold wreath to the officer of fountains Pytheus because he restored and maintained several fountains and aqueducts. The entire regulatory and management system of water in Athens must have worked exceptionally well and approached what today we call sustainable water management. For example, modern water resource policymakers and hydraulic engineers emphasize the non-structural measures in urban water management and the importance of small-scale structural measures like domestic cisterns, which reduce the amount of stormwater to be discharged and provide a source of water for private use. The importance of water in Athens was not only related to the basic uses like drinking, cooking, and cleaning. Water was also related to the beauty of the city; this is revealed from the many fountains that Athenians constructed and the depictions thereof on vessels. Given that vessels were used to export goods, they can be regarded as sort of advertisement of the city’s beauty. Another important water use in Athens was in public baths, cool or warm, called ‘‘balanE
ia’’ (later passed in Latin as balineae or balneae), which, interestingly, at times were common for men and women (what we call today bains mixtes), and were related to enjoyment, health, socialization, and culture.[21] Later, the Romans took up and extended the Greek water technology including, of course public fountains and balneae, which became a matter of luxury and prestige. As a sort of requital, the Roman emperor Hadrian (117–138 A.D.) showed particular interest for Athens; at his time the famous Hadrianic aqueduct was commenced, which conveyed water from mountains, Parnes and Pentele, to Athens covering a distance of 25 km. This aqueduct was in operation until the middle of the 20th century.

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2. 3.

4.

5. 6. 7.

8. 9.

10.

11. 12.

13.

14.

15.

16.

17.

18.

19.

REFERENCES 1. Angelakis, A.N.; Spyridakis, S.V. The status of water resources in minoan times: a preliminary study. In Diachronic Climatic Impacts on Water Resources with Emphasis on Mediterranean Region; Angelakis,

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20.

21.

A.N., Issar, A.S., Eds.; Springer-Verlag: Heidelberg, Germany, 1996; 161–191; Chap. 8. History of the Greek Nation (in Greek); Ekdotiki Athinon: Athens, 1970. Angelakis, A.N.; Spyridakis, S.V. In Wastewater Management in Minoan Times, Intern. Proc. of the Meeting on Protection and Restoration of Environment, Chania, Greece, August 28–30, 1996; 549–558. Evans, S.A. The Palace of Minos at Knossos: A Comparative Account of the Successive Stages of the Early Cretan Civilization as Illustrated by the Discoveries; Macmillan and Co.: London, 1921–1935; Vol. I–IV (reprinted by Biblo and Tannen: New York, 1964). Buffet, B.; Evrard, R. L’Eau Potable a Travers Les Ages; Editions Soledi: Liege, Belgium, 1950. Hutchinson, R.W. Prehistoric town planning in crete. Town Plan. Rev. 1950, 21, 199–220. Platon, N. Za
kroB To NE
on Minoi€ko
n Ana
ktoron (Zakros, The New Minoan Palace); The Athens Archaeological Society: Athens, 1974. Guerin, V. Description de l’Ile Patmos et de l’Ile Samos; Paris, 1865. Fabricius, E. Alterthuemer auf der insel samos. Mitt. Dtsch. Archaologischen Inst. Athen 1884, 9, 165–192. Stamatiades, E. PEri
tou Oru
gmatoB tou Eupali
nou En Sa
mo (About the Digging of Eupalinos in Samos); Hegemonion Press: Samos, 1884 (in Greek). Goodfield, S.T. Toulmin. How was the tunnel of eupalinos of samos aligned? Isis 1965, LVI, 46. Kienast, H.J. Der tunnel des eupalinos auf samos. Architectura, Zeitschrift fu¨r Geschichte der Architektur 1977, 97–116. Lazos, C.D. MZwanikZ
kai TEwnologi
a stZn Arwai
a Ella
da (Mechanics and Technology in Ancient Greece); Aeolos: Athens, 1993 (in Greek). Tsimpourakis, D. 530 p.X., To O´rugma tou Eupali
nou stZn Arwai
a Sa
mo (530 B.C., The Digging of Eupalinos in Ancient Samos); Editions Arithmos: Athens, 1997 (in Greek). Pappas, A. H’YdrEusiB ton Arwai
on AyZno
n (The Water Supply of Ancient Athens); Eleuphtheri Skepsis: Athens, 1999 (in Greek). Kallis, G.; Coccossis, H. Metropolitan Areas and Sustainable Use of Water: The Case of Athens, Report 2000; University of Aegean: Mytilene, Greece, 2000. Tassios, T.P. Apo
to ‘‘PEisistra
tEio’’ ston Eu
Zno (From ‘‘Peisistratean’’ to Evinos); Kathimerini, March 24, 2002; (in Greek) 2002. Papademos, D.L. Ta Udraulika
E´rga Para
toiB Arwai
oiB (The Hydraulic Works in Ancient Greece); TEE: Athens, 1975; Vol. B (in Greek). MacDowell, D.M. The Law in the Classical Athens; Thames and Hudson: London, 1978. Korres, M. H u
drEusZ tZB AyZ
naB kata
tZn arwaio
tZta (Water supply of athens in antiquity). Workshop: ‘‘Water and Environment’’; EYDAP: Athens, 2000 (in Greek). Koromilas, L.G. To AyZnai€ko
KEla
rusma (The Athenian Gurgle); EEY: Athens, 1977.

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Antitranspirants: Film-Forming Types Zvi Plaut Volcani Center, Institute of Soil, Water and Environmental Sciences, Agricultural Research Organization (ARO), Bet-Dagan, Israel

INTRODUCTION As most of the water absorbed by plants is lost by transpiration, reducing plant transpiration could conserve irrigation water and minimize plant water stress. The term antitranspirants refers to a series of compounds intended for this purpose. A decrease in transpiration rate can be achieved either by covering the canopy with film-forming polymers, or by regulating stomatal aperture. Substantial research has investigated the effects of various antitranspirants on transpiration, other physiological activities, and growth. As it was assumed that polymers were less permeable to CO2 than to water, and that the chemicals that close the stomata inhibit photosynthesis as much as transpiration, research on antitranspirants became limited. Thus, it is surprising that a large number of studies on antitranspirants were conducted thereafter. Many (but not all) of those that were conducted during the last two decades will be assessed herein. The present entry will deal with film-forming polymers, whereas compounds regulating stomatal aperture will be delineated in the next. Table 1 presents a list of polymers, giving commercial and chemical names or nature and references.

TRANSPIRATION, PLANT WATER STATUS, AND YIELD The main objective of antitranspirants is to reduce transpiration rate while maintaining or even increasing productivity and yield. If less water is transpired, the storage of water in the soil will be higher, and its availability will be increased and prolonged. Therefore the effect of film-forming polymers on productivity will be more distinctive in crops that are sensitive even to moderate water stress (e.g., potatoes). In fact, potatoes responded very positively to antitranspirants such as Vapor Gard, Folicote, or other wax emulsions.[1–4] The decrease in transpiration reduced water uptake by up to 40%.[1] The increase in yield was mainly due to larger tubers, rather than to an increase in their number, suggesting that soil water was more abundant and available for extension

growth of the tubers. Moreover, the gradient in water potential between tubers and leaves was smaller than in untreated plants,[2] allowing better transport of assimilates from leaves to tubers. In some cases, an increase was found only in tuber size, not in total yield.[5] Similarly, an increase in the yield of onion was found to be due to larger bulbs.[6] An increase in yield connected to a decrease in transpiration was also outlined for other crops treated with film-forming polymers,[7,8] and can be attributed to improved plant water status and foliar hydration.[9] The decrease in transpiration may also induce earlier fruit ripening and improve quality (e.g., decline blossom-end rot in peppers).[10] Furthermore, using antitranspirants after harvest of fruit trees drastically reduced the rate of transpiration, without affecting growth and fruit yield the following year.[11] According to a simulation model for tomatoes, antitranspirants enhanced vegetative growth more than it increased fruit yield.[12] Under dryland farming, or with very limited availability of irrigation water, the quantity of saved water is of less importance than under conditions of full irrigation, whereas the response of crop yield to antitranspirants is significant. Increases in yield using film-forming antitranspirants were demonstrated by several investigators, who did not refer to the actual decrease in transpiration. An increase was found with Folicote in corn,[13] with Vapor Gard in sweet corn,[14] with pinolene in snap beans,[15] and with Vapor Gard in pepper.[10,16] There are conditions under which the shape and appearance of the plant are major factors in determining its quality, whereas dry matter production is only of secondary importance (e.g., in flower crops). A reduction in the rate of transpiration, which can be achieved by antitranspirants, may increase leaf water content and produce a more vigorous plant. Growth and dry matter production have been neglected in floricultural studies. A good example for this is the effect of several film-forming antitranspirants on producing higher-quality hydrangea florets[17,18] and Cineraria flowers.[19] The studies showed a reduced transpiration rate, which also resulted in a smaller drop in xylem water potential and less water stress than in other crops when irrigation was terminated.[20] 39

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Antitranspirants: Film-Forming Types

Table 1 A list of film-forming antitranspirants used by the investigators cited in this review (arranged alphabetically) Commercial name

Chemical name or nature

User

All-safe

Di-1-p-menthene

[18]

Anti-stress 550

Latex of acrylic polymer

[8,27]

Aquawiltless

Wax emulsion

[26,28]

Clear Spray

Latex of acrylic polymer

[19,26,28]

Cloud Cover

Latex of acrylic polymer

[18,23]

DC-200

Silicone (dimethyl–siloxane polymer)

Folicote

Hydrocarbon wax emulsion

[26,28] [1,4,5,6,10,13,17–20,25,26,28,34]

Elvanol 71-30

Hot H2O-soluble polyvinyl alcohol

[20]

Envy

Hydrophylic polymer

[18]

Exhalt 4-10

Polyenpenes and polyethylenes

Linseed Oil

Linseed oil

[9]

Magen 2001

Acrylic polymer

[22]

Moisturin-R

Latex emulsion

[24]

Pinolene

Di-1-p-menthene

[15,16]

Protec

Carboxylated hydrophilic polymer

[26,28]

[17,26,28]

Vapor Gard

Di-1-p-menthene

[1,3,6,7,10–12,14,17,19,20,23,26,28,30,31,34]

Wilt Pruf

Di-1-p-menthene

[17,20,23,26,28,32]

Unidentified materials are not listed.

WATER VAPOR CONDUCTANCE AND CO2 FIXATION

IMPROVEMENT OF GROWTH AFTER TRANSPLANTING

The mechanism of the effects of antitranspirants on yield and transpiration cannot be concluded from most of the reported studies. It is possible that mesophyll conductance was a rate-limiting factor in photosynthesis equal or greater than stomatal conductance, so the decrease in stomatal conductance would reduce transpiration more than photosynthesis.[21] However, even if the film reduces conductance of CO2 more than H2O, there is probably a marked improvement of plant water status that could promote growth and yield, regardless of gas exchange rates. We tried in a recent study to clarify this dilemma by simultaneous determination of water vapor conductances and CO2 fixation rates in the absence of any water stress.[22] The study, which was conducted on several crop plants treated with a film-forming material, ruled out the possibility that improved water status was the reason for higher yields (Fig. 1). Although the curvature was not the same for the three plants, the change in water vapor conductance was large at fully open stomata, when the interference for transpiration was minimal but the changes in CO2 fixation were much smaller. This can serve as evidence, rather than speculation, that mesophyll conductance was involved in the determination of CO2 fixation, and was responsible for the lower rates of CO2 fixation.

Plants are subject to water stress when the rate of water loss exceeds the rate of uptake. This may easily occur when the root system is sparse or inactive (i.e., when seedlings of trees or herbaceous plants are transplanted). Restricting the rate of water loss shortly after transplanting can be achieved by film-type antitranspirants, which may lead to better survival and establishment of the seedlings. This was found for Chinese elm, white spruce, and white pine;[23] scarlet oak, green ash, and Turkish hazelnut;[24] and seedlings of annual plants.[25] Film-forming antitranspirants were also used for tissue-cultured plantlets before their transfer to growth media in a greenhouse.[26,27] The transfer of such plantlets from in vitro conditions to the greenhouse may result in rapid desiccation, and the use of films was found to improve survival rates. However, Pospisilova et al.[28] claimed that most film-forming antitranspirants were ineffective in retaining plant vigor, whereas abscisic acid (ABA) alleviated transplantation shock.

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IMPROVED PRESERVATION AND QUALITY DURING STORAGE Whole plants as well as detached organs (mainly fruits) are subjected to dehydration during storage.

41

blight, and Ulocladium leaf spot of cucurbit plants.[33] This effect was attributed to interference with the adhesion and penetration of the germ into the leaf. Similarly, the infestation of Puccinia recondite could be successfully suppressed in wheat seedlings by several film-forming antitranspirants.[34]

CONCLUSION The potential of film-forming antitranspirants to reduce transpiration was reviewed in detail by Gale and Hagan[35] in 1966. Although it was claimed that these compounds are of marginal value for improving plant water status simultaneously with improving productivity,[36] it was shown that they can be effective for several purposes. Water status of crop plants, which are sensitive to water shortage, was improved and their yield was increased; vigor and vitality of flower crops was increased. Crops grown under dryland conditions and limited available water responded by better growth. Additional potential uses of filmforming materials are better establishment of young seedlings and improved preservation of cut plants and harvested fruits.

REFERENCES

Fig. 1 Ratio between water vapor conductance and CO2 fixation rates of pepper, tomato, and apple leaves sprayed with a film-forming antitranspirant. Source: From Ref.[22].

Film-forming antitranspirants have been recommended for decreasing dehydration, extending the period of storage and maintaining quality during shipment. This was shown for rose plants packed after harvesting, which lost less weight during storage and resumed better growth than untreated plants.[29] Vapor Gard was found to delay ripening, extend shelf life, reduce weight loss, and improve flavor in mango and avocado fruits.[30,31]

CONTROL OF FOLIAR DISEASES Film-forming antitranspirants can also control foliar diseases in several plants, mainly ornamentals. Vapor Gard effectively controlled powdery mildew on Hydrangea macrophylla and Lagerstroemia indica.[32] Other film-forming polymers were found to control powdery mildew, gummy stem blight, Alternaria leaf

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1. Lipe, W.N. Effect of antitranspirants as water use regulators on norgold russet potatoes. Am. Potato J. 1980, 57 (10), 486. 2. Win, K.; Berkowitz, G.A.; Henninger, M. Antitranspirantinduced increased in leaf water potential increase tuber calcium and decrease tuber necrosis in water stressed potato plants. Plant Physiol. 1991, 96 (1), 116–120. 3. Georgiev, G.L.; Velichkov, D. Effect of antitranspirants ‘‘vapor gard’’ and ‘‘chlorotex’’ on the leaf gas exchange activity, water relations and productivity of potato plants under different conditions of soil moisture supply. Fiziol. Rasteniyata (Sofia) 1992, 17 (4), 59–68. 4. Pavlista, A.D. Paraffin enhanced yield and quality of potato cultivar Atlantic. J. Prod. Agric. 1995, 8 (1), 40–42. 5. Bender, D.A.; Lipe, W.N. Potato response to antitranspirants on the texas high plains. HortScience 1986, 21 (4), 941. 6. Lipe, W.N.; Hodnett, K.; Gerst, M.; Wendt, C.W. Effect of antitranspirants on water use and yield of greenhouse and field grown onions. HortScience 1982, 17 (2), 242–244. 7. Shekour, G.M.; McDavid, C.R.; Brathwaite, R.A.I. Response of sweet corn to different rates and frequencies of application of PMA and vapor gard. Trop. Agric. 1991, 68 (3), 225–230. 8. Makus, D.I. Effect of anti-transpirant on cotton grown under conventional and conservation tillage systems,

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9.

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Proceedings Beltwide Cotton Conference, New Orleans, LA, Jan 6–10, 1997; Vol. 1, 642–644. Barek, M.; Moncef, S. Improvement of water use efficiency in wheat through the use of antitranspirants. Medit. 2000, 11 (1), 58–62. Schon, M.K. Effect of foliar antitranspirant or calcium nitrate application on yield and blossom-end rot occurrence in greenhouse-grown peppers. J. Plant Nutr. 1993, 16 (6), 1137–1149. Steinberg, S.L.; McFarland, M.J.; Worthington, J.W. Antitranspirant reduces water use by peach trees following harvest. J. Am. Soc. Hort. Sci. 1990, 115 (1), 20–24. Irmak, A.; Jones, J.W. Use of crop simulation to evaluate antitranspirant effects on tomato growth and yield. Trans. ASAE 2000, 43 (5), 1281–1289. Fuehring, H.D.; Finkler, M.D. Effect of folicote antitranspirant application on field grain yield of moisturestressed corn. Agron. J. 1983, 75 (4), 579–582. Shekour, G.M.; Brathwaite, R.A.I.; McDavid, C.R. Dry season sweet corn response to mulching and antitranspirants. Agron. J. 1987, 79 (4), 629–631. El-Sayad, S.F. Growth and yield of snap bean under cold conditions as affected by growth regulators and pinolene. Sci. Horticult. 1991, 47 (3–4), 193–200. Martinez, P.F.; Tertoura, S.A.A.; Roca, D.; Fernandez, J.A.; Martinez, P.F.; Castilla, N. Air humidity, transpiration and blossom-end rot in soilless sweet pepper culture. Acta Horticult. 2001, 559, 425–429. Tracy, T.E.; Lewis, A.J. Effect of antitranspirants on Hydrangea. HortScience 1981, 16 (1), 87–89. McDaniel, G.L. Transpiration in hydrangea as affected by antitranspirants and chloromequat. HortScience 1985, 20 (2), 293–296. McDaniel, G.L.; Bresenham, G.L. Use of antitranspirants to improve water relations of Cineraria. HortScience 1978, 13 (4), 466–467. Hummel, R.L. Water relations of container grown woody and herbaceous plants following antitranspirant spray. HortScience 1990, 25 (7), 772–775. Solarova, J.; Pospisilova, J.; Slavik, B. Gas exchange regulation by changing of epidermal conductance with antitranspirants. Photosynthetica 1981, 15 (3), 365–400. Plaut, Z.; Magril, Y.; Kedem, U. A new film forming material, which reduces water vapor conductance more than CO2 fixation in several horticultural crops. J. Hort. Sci. Biotechnol. 2004, 79 (4), 528–532.

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23. Williams, P.A.; Gordon, A.M.; Moeller, A.W. Effect of five antitranspirants on white spruce and white pine seedlings subjected to greenhouse drought. Tree Plant. Notes 1990, 41 (1), 34–38. 24. Harris, J.R.; Bassuk, N.L. Effect of defoliation and antitranspirant treatments on transplant response of scarlet oak, green ash and Turkish hazelnut. J. Arboricult. 1995, 21 (1), 33–36. 25. Nitzsche, P.; Berkowitz, G.A.; Rabin, J. Development of a seedling—applied antitranspirant formulation to enhance water status, growth, and yield of transplanted bell pepper. J. Am. Soc. Hort. Sci. 1991, 116 (3), 405–411. 26. Sutter, E.G.; Hutzell, M. Use of humidity tents and antitranspirants in the acclimatization of tissue-cultured plants to the greenhouse. Sci. Horticult. 1984, 23 (4), 303–312. 27. Voyiatzis, D.G.; McGranaham, G.H. An improved method for acclimatizing tissue-cultured walnut plantlets using antitranspirants. HortScience 1994, 29 (1), 42. 28. Pospisilova, J.; Ticha, I.; Kadlecek, P.; Haisel, D.; Plzakova, S. Acclimatization of micropropagated plants to ex vitro conditions. Biol. Plant. 1999, 42 (4), 481–497. 29. Schuch, U.K.; Karlik, J.F.; Harwood, C. Antidesiccants applied to packaged rose plants after growth and field performance. HortScience 1995, 30 (1), 106–108. 30. Lazan, H.; Ali, Z.M.; Sani, H.A. Effect of vapor gard on polygalacruronase, malic enzyme and ripening of Harumanis mango. Acta Horticult. 1990, 269, 359–366. 31. Awad, S.M. Effect of some postharvest treatments on physical properties of avocado fruits during storage. Ann. Agric. Sci. Cairo 1992, 37 (2), 511–521. 32. Ziv, O.; Hagiladi, A. Control of powdery mildew on Hydrangea and Crapemyrtle with antitranspirants. HortScience 1984, 19 (5), 708–709. 33. Ziv, O.; Zitter, T.A. Effects of bicarbonates and filmforming polymers on cucurbit foliar diseases. Plant Dis. 1992, 76 (5), 513–517. 34. Zekaria-Oren, J.; Eyal, Z.; Ziv, O. Effect of film-forming compounds on the development of leaf rust on wheat seedlings. Plant Dis. 1991, 75 (3), 231–234. 35. Gale, J.; Hagan, R.M. Plant antitranspirants. Annu. Rev. Plant Physiol. 1966, 17, 269–282. 36. Kramer, P.J.; Boyer, J.S. Water Relations of Plants; Academic Press: San Diego, CA , 1995; 400–402.

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Antitranspirants: Stomata Closing Types Zvi Plaut Volcani Center, Institute of Soil, Water and Environmental Sciences, Agricultural Research Organization (ARO), Bet-Dagan, Israel

INTRODUCTION It was shown that changes in stomatal conductance can affect transpiration rate more than the rate of assimilation (see the chapter ‘‘Antitranspirants: FilmForming Types’’). Compounds that artificially decrease stomatal conductance, by inducing partial closure, may also be used to reduce transpiration. Whereas films are always located outside the leaf tissue, these compounds must be inside the leaf. They are either taken up by roots, transported to the leaves, and finally reach the epidermis, or absorbed directly by the leaf. It is expected that these compounds will affect metabolic reactions responsible for changes in the osmotic potential of guard cells or change the permeability of guard cell membranes or rigidity of cell walls. It is also expected that these compounds will have no significant side effects, mainly harmful effects; natural compounds such as plant hormones were thus more acceptable as antitranspirants.

In the equations, T and A represent the rates of transpiration and assimilation, el and ea the water vapor pressure of leaf and air, respectively, at their temperatures. Similarly, ra and rG represent the atmospheric and leaf internal partial pressure of CO2, respectively, and ra, rs, and rm0 the air boundary layer, stomatal, and mesophyll resistances, respectively (without prime referring to water vapor and with prime to CO2). If constant environmental conditions are assumed, then the ratio between T and A will be as follows: T=A  ðra þ rs Þ=ðra0 þ rs0 þ rm0 Þ

ð3Þ

Because the denominator, representing CO2 assimilation, contains an extra term, it may be expected that under high stomatal resistance (equivalent to lower conductance, which is its reciprocal) assimilation rate will be less reduced than transpiration rate. This is the basis for decreasing stomatal conductance with antitranspirants, as water-use efficiency (WUE) is expected to increase.

THEORETICAL BACKGROUND TYPES OF ANTITRANSPIRANTS The possibility of reducing transpiration from plants was very attractive in the past, and many studies were conducted in this field during the 1950s and 1960s (e.g., the reviews by Gale and Hagan[1] and Waggoner[2]). This can be achieved either by thin transparent films covering the canopy, as described in the abovementioned chapter, or by materials that will induce stomatal closure and thus decrease leaf water vapor conductance, which are described in the present article. The theoretical background for using such antitranspirants is based on the possibility of reducing water vapor conductance more than conductance of CO2 into the leaf. Both transpiration, namely, diffusion of H2O vapor from leaf to atmosphere and diffusion of CO2 from atmosphere to leaf can be described by the following two equations, as shown by Jones.[3] T ¼ ðel
ea Þ=Paðra þ rs Þ

ð1Þ

A ¼ ðra
rG Þ=Paðra0 þ rs0 þ rm0 Þ

ð2Þ

Antitranspirants can be classified according to their chemical structure, mode of action, function, or purpose of application and the treated crop. The primary classification used in the present article is according to the function of the material. Table 1 gives commercial and chemical names or nature and references. The most common ‘‘stomata-closing materials’’ are phenyl mercuric acetate (PMA) and abscisic acid (ABA). Although PMA was considered toxic in many cases and the effect of artificially added ABA was short,[4] numerous studies were conducted on their use as antitranspirants. Reflecting materials such as kaolin, which are used to reduce the energy load of the foliage and may thus indirectly reduce transpiration, cannot be considered antitranspirants, because they have no direct effect on stomatal aperture or on leaf conductance. Several investigators documented a decrease in transpiration in canopies sprayed with kaolin.[5–7] However, the increase in yield and in WUE[8,9] can be attributed to the prevention of photoinhibition 43

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Antitranspirants: Stomata Closing Types

Table 1 List of materials regulating stomatal aperture used by the investigators cited in this review Commercial name

Chemical name or nature

User (see list of references)

ABA

Abscisic acid

[11,14,20,21]

Alachlor

2-Chloro-2,6-diethyl-N-(methoxy methyl)acetanilide

Ambiol

Derivative of 5-hydroxybenzimizole

[20]

Aspirin

Acetylsalicylic acid

[11]

Atrazine

6-Chloro-N2-ethyl-N4-isopropyl-1,3,5 triazine-2,4-diamine

[11]

B-9

Diaminozide

[13]

Chitosan

Waste product from shells of shrimp, crabs, and lobsters

[15]

CCC (Cycocel)

Chlormequat (2-chloro-N,N,N-trimethylethanaminium)

8-HQ

8-Hydroxyquinoline

Paraquat

1,10 -Dimethyl-4,40 -bipyridinium dichloride

PMA

Phenyl mercuric acetate

[10,13]

[11,13] [5,13,14,20,22,23] [24] [5,10,12–14,17,19,22,23]

Unidentified materials are not listed.

and enhanced photosynthesis (probably photosystem II) rather than being an effect on transpiration.

TRANSPIRATION, PLANT WATER STATUS, AND YIELD The effect of stomata-closing materials on transpiration and growth is not unequivocal. The rate of transpiration was reduced, plant water status was improved, and growth of sweet corn was enhanced by PMA,[10] ABA, chlormequat, and atrazine.[11] Coudret et al.[12] on the other hand indicated that dry matter production was decreased by PMA in two species of Plantago. Amaregouda et al.[13] found no effect of PMA on the yield of groundnuts, although plant water status improved. These inconsistent results may be due to toxic effects of these materials on some crops or at specific growth stages, and damage to leaves.[14,6] The use of this group of antitranspirants will thus need a further painstaking evaluation of each compound for every crop and growth stage. Recently a natural compound, chitosan, was shown to induce stomatal closure of pepper, decrease transpiration, and reduce water use, while maintaining biomass production and yield.[15] Another compound, GLK-8923, was claimed to serve as an antitranspirant when added to the growing medium.[16] However, as reduced transpiration was obtained by decreasing the osmotic potential of the medium, plant water status and growth were worsened, similar to the effect of salinity or restricted water supply; thus it cannot be considered an antitranspirant. The response of crop yield to antitranspirants is, in some cases, of more importance than the saving of water. An increase in yield with stomata-closing

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materials can be shown for several crops: tomatoes with PMA and 8-HQ, sweet corn with PMA and alachlor,[10] chickpeas with PMA,[17] soybeans with silica, which accumulated in the leaf epidermal cells,[18] and green grams with PMA.[19] These increases in yield can be mainly attributed to milder water stress as compared with untreated plants and is reflected in higher water and osmotic potentials.[5] In plants exposed to drought and treated with several growth regulators (ABA, polyamines, and an antioxidant, Ambiol) higher rates of photosynthesis and lower Ci/Ca ratios, as compared to untreated plants, were outlined. This will also increase WUE.[20,21] An improvement in plant water status and relative water content was also found with stomata-closing chemicals such as PMA, 8-hydroxyquinoline in tomato and onion plants,[22,23] and paraquat in rice.[24]

CONCLUSION Stomatal closing antitranspirants were believed to act as superior transpiration inhibitors, as they are taken up by the plant, and transported to new growing and expanding organs, in contrast to films, which have to be reapplied. Moreover, they are supposed to be more stable and not damaged like external films by winds or other environmental factors. In fact, stomatal closing materials were, in many cases, effective in reducing transpiration, improving plant water status and increasing yield, as was shown. However, their use was more limited than those of film-forming materials, due to side effects in many plants, which may cause damage to plants at various developmental stages. Moreover, the lifetime of several such materials may be limited, which may also limit their effectivity.

45

ARTICLE OF FURTHER INTEREST Antitranspirants: Film-Forming Types, p. 39.

13.

REFERENCES 14. 1. Gale, J.; Hagan, R.M. Plant antitranspirants. Annu. Rev. Plant Physiol. 1966, 17, 269–282. 2. Waggoner, P.E. Decreasing transpiration and the effect upon growth. In Plant Environment and Efficient Water Use; Pierre, W.H., Kirkham, D., Pesek, J., Shaw, R., Eds.; Am. Soc. Agronomy and Soil Sci. Soc. Am.: Madison, WI, 1966; 49–72. 3. Jones, H.G. Drought tolerance and water-use efficiency. In Water Deficit—Plant Responses from Cell to Community; Smith, J.A.C., Griffiths, H., Eds.; Bios Scientific Publishers: Oxford, UK, 1993; 193–203. 4. Solarova, J.; Pospisilova, J.; Slavik, B. Gas exchange regulation by changing of epidermal conductance with antitranspirants. Photosynthetica 1981, 15 (3), 365–400. 5. Rao, N.K.S. The effect of antitranspirants on leaf water status, stomatal resistance and yield in tomato. J. Hortic. Sci. 1985, 60 (1), 89–92. 6. Joshi, N.L.; Singh, D.V. Water use efficiency in relation to crop production in arid and semi arid regions. Ann. Arid Zone 1994, 33 (3), 169–189. 7. Anandacoomaraswamy, A.; De Costa, W.A.J.M.; Shyamalie, H.W.; Campbell, G.S. Factors controlling transpiration of mature field-grown tea and its relationship with yield. Agric. For. Meteorol. 2000, 103 (4), 375–376. 8. Naveen, P.; Daniel, K.V.; Subramanian, P.; Kumar, P.S. Response of irrigated groundnut (Arachis hypogea) to moisture stress and its management. Indian J. Agron. 1992, 37 (1), 82–85. 9. Jifon, J.L.; Syvertsen, J.P. Kaolin particle film application can increase photosynthesis and water use efficiency of ‘‘Ruby red’’ grapefruit leaves. J. Am. Soc. Hortic. Sci. 2003, 128 (1), 107–108. 10. Shekour, G.M.; McDavid, C.R.; Brathwaite, R.A.I. Response of sweet corn to different trates and frequencies of application of PMA and vapor gard. Trop. Agric. 1991, 68 (3), 225–230. 11. Yadav, R.S.; Anil-Kumar; Kumar, A. Effect of some antitranspirants on water relations, NR-activity and seed yield of rabi maize (Zea mays L.) under limited irrigation. Indian J. Agric. Res. 1998, 32 (1), 57–60. 12. Coudret, A.; Ferron, F.; Gaudillere, J.P.; Costes, C. Comparative action of antitranspirants upon stomatal movement, CO2 exchanges and dry-matter production

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15.

16.

17.

18.

19.

20.

21.

22.

23.

24.

in Plantago lanceolata L and Plantago maritime L. Physiol. Veg. 1980, 18 (4), 631–643. Amaregouda, A.; Chetti, M.B.; Salimath, P.M.; Kulkarni, S.S. Effect of antitranspirants on stomatal resistance and frequency, relative water content and pod yield in summer groundnuts (Arachis hypogea L.). Ann. Plant Physiol. 1994, 8 (1), 18–23. Rajapakse, N.C.; Kelly, J.W.; Reed, D.Wm. Use of antitranspirants under low-light environments to control transpiration of Epipremnum aurum leaves. Sci. Hortic. 1990, 43 (3–4), 307–312. Bittelli, M.; Flury, M.; Campbell, G.S.; Nichols, E.J. Reduction of transpiration through foliar application of chitosan. Agric. For. Meteorol. 2001, 107, 167–175. Gu, S.; Fuchigami, L.h.; Cheng, L.; Guak, S.H. Effect of antitranspirant and leaching on medium solution osmotic potential, leaf stomatal status, transpiration, abscisic acid content and plant growth in ‘‘early spring’’ tomato plants (Lycopersicon esculentum). J. Hortic. Sci. Biotechnol. 1998, 73 (4), 473–477. Dalvi, D.G.; More, P.R.; Nayeem, K.A. Effect of antitranspirants on transpiration rate, relative water content, water saturation deficit and yield of ‘‘Chafa’’ chickpea (Cicer arientinum). Indian J. Agric. Sci. 1991, 61 (3), 204–206. Kupfer, C.; Kahnt, G. Effect of the application of amorphous silica on transpiration and photosynthesis of soybean plants under varied soil and relative air humidity conditions. J. Agron. Crop Sci. 1992, 168 (5), 318–325. Arya, R.L.; Sharma, J.P. Effect of irrigation and antitranspirant and growth regulators on summer greengram (Phaseolus radiatus). Indian J. Agron. 1994, 39 (1), 31–33. Rajasekaran, L.R.; Blake, T.J. New plant growth regulators protect photosynthesis and enhance growth under drought of jack pine seedlings. J. Plant Growth Regul. 1999, 18 (4), 175–181. Fuchs, E.E.; Livingston, N.J.; Abrams, S.R.; Rose, P.A. Structure–activity relationships of ABA analogs based on their effect on the gas exchange of clonal white spruce (Picea glauca) emblings. Physiol. Plant. 1999, 105 (2), 246–256. Rao, N.K.S. The effect of antitranspirants on stomatal opening, and the proline and relative water content in the tomato. J. Hortic. Sci. 1986, 61 (3), 369–372. Rao, N.K.S.; Bhatt, R.N. Response of onion to antitranspirants—plant water balance. Plant Physiol. Biochem. 1990, 17 (2), 69–74. Sarma, S.; Dey, S.C. Paraquat an efficient antitranspirant. Indian J. Plant Physiol. 1992, 35 (4), 371–376.

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Antitranspirants: Stomata Closing Types

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Aquifers Miguel A. Marino Hydrology, Civil and Environmental Engineering, and Biological & Agricultural Engineering, University of California–Davis, Davis, California, U.S.A.

INTRODUCTION An aquifer is a geologic formation that can transmit, store, and yield significant quantities of water. An aquifer can be confined (one bounded above and below by impervious formations), unconfined (one with a water table serving as its upper boundary), or leaky (one that can gain or lose water through adjacent semipervious formations). In addition to the aforementioned porous-media aquifers, there are karst aquifers in which water flow is concentrated along fractures, fissures, conduits, and other interconnected openings. The hydraulic properties of aquifers (e.g., transmissivity and storativity) are best determined by testing the aquifers in place. Water contained in an aquifer is called ground water and is commonly extracted by means of a well. The quality of ground water determines, to a large extent, its suitability for a particular use, such as irrigation, public water supply, etc. Human activity poses a threat to ground-water quality and already has resulted in incidents of ground-water pollution or contamination. Because ground water tends to move slowly, it may take many years after the start of pollution before contaminated water shows up in a well. The best way to protect the quality of ground water is to prevent its contamination.

AQUIFERS An aquifer is a geologic formation that yields significant quantities of water (e.g., coarse sand and gravel formation). In contrast, an aquiclude is a formation that may contain water but cannot transmit it in significant quantities. A clay stratum is an example. For all practical purposes, an aquiclude can be considered an impervious formation. An aquitard is a semipervious formation, transmitting water very slowly compared with an aquifer. It can, however, permit the passage of large quantities of water over a large (horizontal) area. An aquitard is often called a semipervious layer or a leaky formation. An aquifuge is an impervious formation that neither contains nor transmits water. Solid granite is an example. An aquifer can be regarded as an undergroundstorage reservoir. Water enters the aquifer naturally 46

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through precipitation or influent streams—and artificially through wells or other recharge methods. Water leaves the aquifer naturally through springs or effluent streams—and artificially through pumping wells. Fig. 1 is a schematic representation of several aquifers and observation wells.[1] A confined aquifer, also called artesian aquifer or pressure aquifer, is bounded above and below by impervious formations. Water in a well penetrating such an aquifer will rise above the base of the upper confining formation; it may or may not reach the ground surface. A well that penetrates a confined aquifer is called an artesian well—it is called a flowing well if the water level in the well reaches, or exceeds the elevation of, the ground surface. The water levels in a number of wells penetrating a confined aquifer are the hydrostatic-pressure levels of the water in the aquifer at the well sites. The water levels define an imaginary surface called the piezometric or potentiometric surface. An unconfined aquifer, also called phreatic aquifer or water-table aquifer, is one with a water table (phreatic surface or surface of atmospheric pressure) serving as its upper boundary. Actually, above the water table is a capillary fringe often neglected in ground-water studies. A special case of an unconfined aquifer is the perched aquifer. It occurs wherever an impervious (or relatively impervious) stratum of limited horizontal area supports a ground-water body that is above the main water table. A well that taps an unconfined aquifer is called a water-table (or gravity) well. The water level in such a well corresponds approximately to the position of the water table at that location. Ground-water levels can be measured to estimate the piezometric-surface or water-table distribution in an aquifer or to determine fluctuations in hydraulic head over time. Maps of ground-water levels are used to estimate ground-water flow direction and velocity, to assess ground-water vulnerability, to locate landfills and wastewater disposal sites, etc. Aquifers, whether confined or unconfined, that can gain or lose water through adjacent aquitards or semipervious formations are called leaky aquifers. A confined aquifer that has at least one semipervious confining bed is called a leaky-confined aquifer. An unconfined aquifer that rests on a semipervious

47

Fig. 1 Schematic of several aquifers and observation wells. Source: From Ref.[1].

stratum is called a leaky-unconfined aquifer. Leakage across semipervious formations can be significant. Fig. 1 shows an unconfined aquifer underlain by a confined one. In the recharge area, the confined aquifer becomes unconfined. A portion of the confined and unconfined aquifers is leaky, with the amount and direction of leakage governed by the difference in piezometric head across the semipervious stratum. In addition to the aforementioned types of aquifers, there are karst aquifers made up of soluble rock strata at or near the earth’s surface in which water flow is concentrated along fractures, fissures, conduits, and other interconnected openings. Aquifer elasticity is the main mechanism responsible for volumes of water released from or added to storage in aquifers. In confined aquifers, water is derived from storage primarily by the elastic properties of both the aquifer matrix and the water. In unconfined aquifers, water released from storage is due mainly to gravity drainage (drainage of the pore space above the lowered water table) and partly to elastic storage (as in confined aquifers). In leaky aquifers, water is derived from storage in the main (confined) aquifer, gravity drainage if the aquifer is unconfined, and elastic storage in aquitards, and induced vertical leakage across these units. The general properties of aquifers to transmit, store, and yield water (e.g., transmissivity, storativity, and leakage factor) are usually referred to as hydraulic properties of aquifers, or simply aquifer parameters. Because of the many factors on which these parameters depend, numerical values must depend on experimental determination. Although various laboratory techniques are available,[2,3] more reliable results are obtained from field tests[1–6] of the aquifers in place. The transmissivity of the aquifer determines the ability of the aquifer to transmit water through its entire thickness. In confined aquifers, the transmissivity is represented as the product of hydraulic

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conductivity and aquifer thickness, in the direction normal to the base of the aquifer (the hydraulic conductivity expresses the ease with which a fluid is transported through a porous medium). Because in unconfined aquifers the saturated thickness extends from the water table to the base of the aquifer, the transmissivity varies in time as the water table often fluctuates in response to recharge and pumping. The storage capacity of an aquifer is quantified by its storativity, also called the storage coefficient. The storativity indicates the relationship between changes in the volume of water stored in an aquifer and corresponding changes in the elevations of the piezometric surface, or the water table. It can be defined as the volume of water that a column of the aquifer, of unit crosssection, releases from or adds to storage per unit decline or rise of piezometric surface (confined aquifers) or water table (unconfined aquifers). In a confined aquifer, the storativity is caused by the compressibility of the water and the elastic properties of the aquifer. In an unconfined aquifer, the storativity is due mostly to dewatering or refilling the zone through which the water table moves (e.g., water removed by gravity drainage) and partly to water and aquifer compressibility in the saturated zone. A certain amount of water, however, is held in place against gravity in the pores between grains under molecular and surface-tension forces. Thus, the storativity of an unconfined aquifer is less than the porosity by a factor called specific retention (the ratio between the volume of water that a soil will retain against gravity and the total volume of the soil). Reflecting this phenomenon, the storativity of an unconfined aquifer is often called specific yield (the ratio between the volume of water that a soil will yield by gravity and the total volume of the soil). Also often used in this context is the term, effective porosity. A parameter characterizing a leaky aquifer is the leakance, or coefficient of leakage, of the semipervious formation. It is a measure of the ability of this

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Aquifers

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formation to transmit vertical leakage and is defined by the ratio of the hydraulic conductivity of this formation to its thickness. The reciprocal of the leakance can be thought of as the resistance of the semipervious formation. Another parameter, the leakage factor, is the root of the ratio of the transmissivity of the aquifer to the leakance of the semipervious formation. It determines the areal distribution of the leakage. The quality of water contained in an aquifer determines, to a large extent, the suitability of the water for a particular use, such as irrigation, public water supply, etc. The quality of ground water is a consequence of all processes and reactions that have acted on the water from the moment it condensed in the atmosphere to the time it is discharged by a well.[2] Human activity poses a threat to ground-water quality and already has resulted in incidents of ground-water pollution or contamination.[2,6] The latter refers to the presence of a chemical or biological agent in the ground water in such a concentration that it renders

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Aquifers

the water unfit for a certain use. Because ground water tends to move slowly, it may take many years after the start of pollution before contaminated water shows up in a well. The best way to protect the quality of ground water is to prevent its contamination.

REFERENCES 1. Marin˜o, M.A.; Luthin, J.N. Seepage and Groundwater; Elsevier Scientific Publishing Company: New York, 1982. 2. Bouwer, H. Groundwater Hydrology; McGraw-Hill: New York, 1978. 3. Todd, D.K. Groundwater Hydrology, 2nd Ed.; John Wiley: New York, 1980. 4. Freeze, R.A.; Cherry, J.A. Groundwater; Prentice Hall: Englewood Cliffs, NJ, 1979. 5. Domenico, P.A.; Schwartz, F.W. Physical and Chemical Hydrogeology, 2nd Ed.; John Wiley: New York, 1998. 6. Fetter, C.W. Applied Hydrogeology, 4th Ed.; Prentice Hall: Upper Saddle River, NJ, 2000.

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Aquifers: Artificial Recharge Steven P. Phillips U.S. Geological Survey (USGS), Sacramento, California, U.S.A.

INTRODUCTION Limited freshwater resources in many parts of the world have led to the development of artificial recharge techniques for conveying surface water and reclaimed wastewater to groundwater reservoirs for later use and for other applications. Other applications include using artificial recharge to create a barrier to saltwater intrusion, reduce land subsidence, raise water levels, and improve water quality by using the natural filtering capabilities of aquifer systems. Subsurface storage of water has many advantages over surface storage, and often is the more physically and economically viable alternative. The worldwide use of artificial recharge likely will increase in the future with continued growth in population and associated competition for finite freshwater resources.

Europe. It was recognized early on that storing water in the groundwater system held certain advantages over traditional surface storage, including proximity to water sources and points of use, limited engineering and construction costs, and little or no evaporative losses. Prior to the mid-20th century, the primary application of these early programs was enhancement of groundwater resources for drinking and agricultural purposes. Surface water generally was captured when it was available and stored in the groundwater system for later use during high-demand periods. An annotated bibliography of early artificial recharge work is provided by Todd.[1] Research during the early period of artificial recharge spawned a number of modern applications.[3] These applications include purification of wastewater or poor-quality surface water, creation of barriers against the intrusion of saltwater and other contaminants, abatement of land subsidence, and other environmentally or economically driven applications.

DEFINITION Artificial recharge has been defined in many ways, varying with points of view and the evolution of applications and methods. It is generally defined by Todd[1] as ‘‘the practice of increasing by artificial means the amount of water that enters a groundwater reservoir.’’ These artificial means include various forms of surface infiltration and direct well injection. For this discussion, forms of enhanced, induced, and incidental recharge, as defined by Bouwer,[2] will be excluded. Enhanced recharge is the increased infiltration of precipitation; it is controlled primarily through vegetation management. Induced recharge is the increased flow of surface water into the aquifer system caused by the placement of wells or other collectors near surface-water bodies. Incidental recharge is caused by leakage of water and sewer pipes, excess irrigation, and other human activities not designed to cause groundwater recharge. Also excluded from this discussion is a recharge from the injection of saltwater used to enhance petroleum recovery, and from deep disposal of wastes.

APPLICATIONS Artificial recharge programs began in the late 19th century in the United States, and well before that in

METHODS Surface Infiltration The most common form of artificial recharge is surface infiltration, whereby engineered systems allow an increased infiltration of water through subsurface materials to the water table. Although surface infiltration systems are subject to losses from evaporation and may unintentionally attract insects and waterfowl, they often are an efficient and economical means of artificial recharge. Common surface infiltration systems are shown in Fig. 1. Surface infiltration systems can be divided into two categories: in-channel and off-channel.[2] In-channel systems use temporary or permanent dams or levees designed to impede flow and raise the water surface in existing surface-water channels. The raised water surface increases in-channel storage and the area of the streambed through which infiltration occurs, thereby increasing recharge. Often, in-channel systems are self-cleaning, as fine particles that impede infiltration are removed during high flows, but associated dams and levees require maintenance. Off-channel systems involve the use of water from any source to fill existing or constructed basins, pits, 49

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Aquifers: Artificial Recharge

Fig. 1 Diagram showing examples of artificial recharge systems.

ponds, and other structures for surface infiltration. Favorable site conditions for off-channel systems include available and affordable land, an unconfined aquifer that has sufficient transmissivity to keep the water table below the infiltration surface, permeable surface soils for sufficient infiltration rates, a lack of poorly permeable subsurface units that impede downward flow to the water table, and a source of water that has low suspended solids and compatible chemical and biological characteristics. The site conditions for off-channel systems, if unfavorable, can be altered in some circumstances. Poorly permeable surface soils and (or) deeper fine-grained units above the water table can be penetrated with trenches or holes backfilled with sand or gravel to enhance their capacity to transmit water vertically. These trenches or holes can be used alone, with water delivered through perforated pipes, or in combination with basins or other surface impoundments. If sediment concentrations or other suspended solids in the source water are too high, rapid clogging of infiltration basins can occur. Pretreatment of the source water in a reservoir or dedicated basin allows solids to settle, sometimes with the help of coagulants. Some chemical and biological processes can also cause clogging, which often is addressed by filtering and disinfecting during pretreatment. Post-treatment clogging generally is controlled by maintenance of infiltration basins, which involves periodic cleaning of the basin bottom.[2]

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Additional considerations for surface infiltration systems include the presence of soluble or dissolved potential contaminants in the subsurface, and desirable soil types for various applications. Knowledge of the presence and distribution of natural and anthropogenic contaminants in the vicinity of a proposed artificial recharge site is required to avoid introducing contaminants to the groundwater system and (or) transporting them to undesired locations. Specific soil types are preferable for some applications. For example, a coarse-grained homogeneous soil is preferable for achieving maximum recharge. A finer-grained soil that has higher capacity for sorption may be preferable for sites that use poor-quality source water.

Direct Well Injection Direct well injection is becoming a common method of artificial recharge. For this method, water is pumped or gravity-fed through wells into confined and unconfined aquifers, and sometimes into the unsaturated zone.[2,4] The same wells used for injection often are used to recover the water. Direct well injection generally is more expensive than surface infiltration because of costs of well construction and water treatment requirements. However, injection allows water to bypass poorly permeable soils and subsurface units to allow rapid recharge of deep aquifers through thick unsaturated zones, avoids transport of near-surface

contaminants to the saturated zone, and requires little land, enabling strategic well placement. These attributes make direct well injection particularly useful in urban settings and in areas where the unconfined aquifer is too contaminated to use surface infiltration methods. Clogging is a key design and operational consideration for direct well injection because recharge water must pass through a very small area, the well screen and borehole wall, to enter the aquifer system. Clogging is one of the reasons that injection rates typically are about one-third to one-half of the extraction rate, though this ratio varies widely from site to site.[5] Suspended solids are a common cause of clogged injection wells. Most suspended solids can be removed in reservoirs or basins as described previously and (or) through specially designed piping systems and filters. However, the small amount that typically remains in the recharge water often is the primary clogging agent. The management strategy most often employed is periodic extraction from the well, which removes much of the caked solids. Clogging can also be caused by biological growth, mineral precipitates, and air or gas bubbles. Growth of existing or introduced microorganisms can rapidly clog an injection well, and generally is managed by using continual low-level disinfection and periodic shock treatments with chlorine.[4] Mixing of water types during injection can cause precipitation of minerals on the well screen and within the gravel pack and aquifer materials. Commonly, geochemical modeling is done during the design phase to determine the potential for adverse geochemical reactions; adjustments of pH or other properties of the recharge water are sometimes made during operations to avoid mineral precipitation. Air introduced into the well through free-falling water or cavitation in pipes can enter the aquifer system and lodge in pore spaces, effectively reducing the hydraulic conductivity of the aquifer materials and thus the rate of injection. Dissolved gases coming out of solution have a similar effect. These reductions in hydraulic conductivity can be avoided through proper system design and pretreatment of source water.

ISSUES Public Health Public health issues associated with artificial recharge are most often raised in connection with the use of treated wastewater. There is much interest in expanding the use of wastewater for artificial recharge, because it is a continuous and increasing source of water and more stringent regulations have increased disposal costs.[5,6] Artificial recharge can improve

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the quality of treated wastewater through microbial degradation and sorption of some organic constituents; however, there are well-understood and emerging pathogens and other potential toxicants for which diligent monitoring and active research are required to protect public health.[7] However, defined, data from ongoing studies and active projects, which include potable and non-potable uses, suggest that wastewater is a viable source of water for artificial recharge.[5,6] Environmental Artificial recharge can have environmental effects that may be important to predict prior to implementation.[5] Flow in streams and other water bodies can increase with a rise in the water table, or decrease with diversions to recharge facilities. The quality of surface water and groundwater can be improved, degraded, or changed in some way that affects the end use of the water. Land subsidence can be reduced by slowing or reversing water-level declines. Development of a shallow water table can cause waterlogging and increased salinity. Reduced pumping lifts saves energy, reducing the environmental effects of energy production. A good understanding of these and other potential environmental effects of artificial recharge projects is a key to their long-term viability.

THE ROLE OF SCIENCE Artificial recharge involves complex hydraulic, chemical, and biological responses and interactions in a groundwater system.[7] The role of science is to generate an understanding of these complexities through monitoring and analysis, and to develop tools to aid in the planning and management of artificial recharge projects. Monitoring and Analysis A set of methods has evolved over the years for monitoring and analyzing the effects of artificial recharge projects.[2,4,8] However, shortcomings in these methods and a number of existing and emerging issues continue to drive research efforts. Microgravity surveying is a geophysical technique that has been used recently to better define water-table changes associated with artificial recharge.[9] New tracer methods using existing and introduced chemicals, isotopes, and heat are improving our ability to track recharged water.[7] Recent research shows that the introduction of organic matter through artificial recharge and the composition of the organic matter may affect the evolution of groundwater chemistry and the formation of disinfection

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byproducts. A large body of research is focused on addressing the fate of compounds introduced through artificial recharge of treated wastewater, which is needed to safeguard public health. Other research efforts include development of more sophisticated modeling techniques for predicting complex biochemical processes and microbial transport; improving methods for detecting microbial pathogens; and determining the potential for mobilization of arsenic and other trace elements.[7]

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4. 5.

6. 7.

Planning and Management Tools 8.

Successful planning and management of an artificial recharge project often requires consideration of many water-management objectives, water-routing capabilities, economics, and hydraulic effects. Simultaneous consideration of these diverse factors can be accomplished by using optimization techniques designed to identify an efficient way to meet an objective, given a set of constraints. The linkage of a predictive groundwater flow model with optimization techniques (a simulation/optimization model) allows for simultaneous consideration of the flow system and physical and (or) economic constraints determined by waterresource managers. Simulation/optimization models have been applied to groundwater problems for decades[10,11] and have been used in the planning and management of artificial recharge projects (e.g., Refs.[12,13]).

9.

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REFERENCES 1. Todd, D.K. Artificial Recharge of Ground Water Through 1954; Water-Supply Paper 1477; U.S. Geological Survey: Washington, DC, 1954; 115 pp. 2. Bouwer, H. Artificial recharge of groundwater: hydrogeology and engineering. Hydrogeol. J. 2002, 10 (1), 121–142. 3. Signor, D.C.; Growitz, D.J.; Kam, W. Annotated Bibliography on Artificial Recharge of Ground Water,

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13.

(1955–67); Water-Supply Paper 1990; U.S. Geological Survey: Washington, DC, 1970; 141 pp. Pyne, R.D.G. Groundwater Recharge and Wells; Lewis Publishers: Boca Raton, FL, 1995; 376 pp. National Research Council, Ground Water Recharge Using Waters of Impaired Quality; National Academy Press: Washington, DC, 1994; 283 pp. Asano, T. Artificial Recharge of Groundwater; Butterworth Publishers: Stoneham, MA, 1985; 767 pp. Aiken, G.R.; Kuniensky, E.L.; Eds. U.S. Geological Survey Artificial Recharge Workshop Proceedings Sacramento, CA, April 2–4, 2002; Open-ile Report 02US Geological Survey: Washington, DC, 2002; 85 p. Fowler, L.C.; Ed. Operation and Maintenance of Ground Water Facilities; ASCE Manuals and Reports on Engineering Practice No. 86; American Society of Civil Engineers: New York, 1996. Howle, J.F.; Phillips, S.P.; Ikehara, M.E. Estimating water-table change using microgravity surveys during an ASR program in Lancaster, Antelope Valley, California. In Management of Aquifer Recharge for Sustainability, Proceedings of the Fourth International Symposium on Artificial Recharge of Groundwater, Adelaide, Australia, Sept 22–26, 2002; Dillon, P.J., Ed.; Balkema, 2002; 567 pp. Gorelick, S.M. A review of distributed parameter groundwater management modeling methods. Water Resour. Res. 1983, 19 (2), 305–319. Ahlfeld, D.P.; Mulligan, A.E. Optimal Management of Flow in Groundwater Systems; Academic Press: San Diego, 2000; 185 pp. Phillips, S.P.; Carlson, C.S.; Metzger, L.F.; Sneed, M.; Galloway, D.L.; Ikehara, M.E.; Hudnut, K.W. Optimal management of an ASR program to control land subsidence in Lancaster, Antelope Valley, California. In Management of Aquifer Recharge for Sustainability, Proceedings of the Fourth International Symposium on Artificial Recharge of Groundwater, Adelaide, Australia, Sept 22–26, 2002; Dillon, P.J., Ed.; Balkema, 2002, 567 pp. Reichard, E.G. Groundwater–surface water management with stochastic surface water supplies: a simulationoptimization approach. Water Resour. Res. 1995, 31 (11), 2845–2865.

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Aquifers: Ogallala B. A. Stewart Dryland Agriculture Institute, West Texas A&M University, Canyon, Texas, U.S.A.

INTRODUCTION The Ogallala aquifer underlies about 174,000 mi2 of land in the U.S. Great Plains. States with the largest land areas above the aquifer are Nebraska, Texas, Kansas and Colorado with smaller amounts in New Mexico, Oklahoma, South Dakota, and Wyoming (Fig. 1). The aquifer extends approximately 800 mi north to south and 400 mi east to west. The aquifer consists of sand and gravel beds that generally lie from 50 to 300 ft beneath the surface and are 150–300 ft thick. In some areas, however, the saturated thickness of the aquifer exceeds 1000 ft.[1] The water contained in the Ogallala aquifer is essentially fossil water taken 10,000–25,000 yr ago from the glacier-laden Rocky Mountains before it was geologically cut off by the Pecos River and the Rio Grande.[2] More than 3 billion acre-feet (an acre-foot is a foot of water on 1 acre, or 325,851 gal) are stored in the aquifer. The amount of water that can be extracted from an aquifer is primarily a function of two characteristics— hydraulic conductivity and specific yield. Hydraulic conductivity is the rate of water flow in m3/sec through a cross-sectional area of 1 m2 under a hydraulic gradient of 1 m/m at a temperature of about 15 C. Specific yield is the ratio of the volume of water that the saturated aquifer will yield by gravity drainage to the total volume of the aquifer. The hydraulic conductivity governs how fast water can be pumped from an aquifer and the specific yield indicates how much water can be pumped. Gutentag et al.[1] reported that 76% of the aquifer had specific yields of 10–20%, although the values ranged from less than 5 to 30% and averaged 15.1%. Thus, as an average, pumping enough to drop the water level of the aquifer 100 cm will yield 15.1 cm of water at the surface.

USE OF WATER FROM THE AQUIFER The area underlain by the Ogallala aquifer is semiarid with average annual precipitation ranging from about 375 in. to 625 in. There is also wide variability both within and between years. The annual precipitation ranges for any given year from about 50% of average to 200% of average. Because of the low and varied

precipitation, much of the area has no year around water supply from surface supplies. The Ogallala aquifer, therefore, became the lifeblood for development of the Great Plains. Opie[2] tells of Gropp homesteading 20 mi northwest of Garden City, Kansas in 1887. For 2 yr, Gropp rolled a large barrel of water three-fourths of a mile from a neighbor’s well until he hand-dug his own 70-m-deep well. Windmills became widely used through the Great Plains to supply water for livestock and homesteads. The Great Plains was largely developed by livestock producers and dryland farmers. Crop production, however, was highly variable because of the erratic precipitation. As technologies for pumping water improved, irrigation became common and now uses more than 90% of the water pumped from the Ogallala aquifer. Although some irrigation from the Ogallala aquifer using windmills occurred in the late 1800s, it was limited and sporadic. The drought of the 1930s spurred irrigation development because the precipitation was so low that crops could not be produced otherwise. The major expansion occurred during the 1950s when another drought occurred at the same time that there were technological advances in well drilling and pumping plants, readily available nitrogen fertilizer, inexpensive energy, profitable crop prices, development of hybrid grain sorghum, and available financing. Irrigation from the Ogallala aquifer increased from about 800,000 ha in 1949 to 5.2 million ha in 1978. During this period, approximately 60 cm of water was pumped annually from the aquifer for every irrigated hectare.[1] Irrigation developed most rapidly in the beginning in the southern part of the Ogallala aquifer area because this area was more drought prone, the depth to the water table was less, and the land was generally flat making it relatively easy to apply water and run it down the rows. Many of the soils in the northern plains were too rolling and too sandy to irrigate by running water down furrows so irrigation development was slowed until reliable center-pivot sprinkler systems became available. The major problem facing the Ogallala aquifer region today is not knowing how long the water supply will last. Also, the increase in energy prices has also had major impacts on irrigation at different times. In some cases and in some years, the cost of pumping the water is greater than the benefits derived. 53

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Aquifers: Ogallala

Fig. 1 The area underlain by the Ogallala Aquifer.

The distribution of the water in the Ogallala is very different among the states. Nebraska has 36% of the land (but most of it unsuitable for cultivation) above the aquifer and in 1980 had 66% of the drainable water in storage. Texas had 20% of the land but only 12% of the drainable water comparing closely with Kansas that had 17% of the land and 10% of the drainable water. The water rights that govern the pumping of water from the aquifer vary for different states. In Nebraska, for example, the water is public property. However, the owner of land is entitled to appropriate the ground waters found under the land, but the owner cannot extract and appropriate them in excess of a reasonable and beneficial use upon the land owned, especially if such is injurious to others who have substantial rights to the waters, and if the natural underground supply is insufficient for all users, each is entitled to a reasonable proportion of the whole. In contrast, Texas law gives the overlying landowner the right to capture and use ground water beneath the land, regardless of the impact on adjoining or more distant users of the supply.[3] The future of irrigation from the Ogallala aquifer is not clear. While only a relatively small percentage of the total water in the aquifer has been removed, essentially all of the recoverable water has been removed in some portions of the aquifer. The ratio of the volume of drainable water remaining to the volume of water

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depleted by 1980 was about 19 for the entire aquifer area. Even in Texas, the State with the greatest depletion, 3.4 times as much water remains as has been removed. In some areas, however, the volume of water remaining in the aquifer was less than the volume that had been removed.[1] Depletion since 1980 has continued, although at a somewhat slower rate in most areas because of declining well capacities, higher energy prices, and lower grain prices. It is generally believed that irrigation will decline in much of the Ogallala aquifer region both in areas and in the amount of water applied per hectare. Irrigation in the Texas High Plains has already declined from more than 2.4 million ha to about 1.8 million ha. The efficiency of water use, however, is increasing as a result of improved irrigation systems and crop management practices. This will enable irrigation to continue longer than otherwise would be feasible and irrigation will continue for many portions of the Ogallala aquifer area even in Texas nearly indefinitely. Many cities, towns, and rural homes depend on water from the Ogallala aquifer. For the most part, there will be sufficient water for domestic uses even when there is no longer sufficient water for irrigation. Irrigation requires huge amounts of water and pumping for irrigation usually stops well before all the water that can be pumped is removed. Thus, there is usually adequate water remaining for domestic use even in areas where irrigation is no longer feasible. The future of the Ogallala aquifer is unknown. Although some of the early settlers in the area thought there were underground rivers flowing beneath the land, it is clearly understood today that the water stored in the aquifer accumulated over eons of time. The rate of recharge is almost negligible in relation to the rate that water has been pumped since the 1950s. The way that the aquifer is used in the future will be shaped by many factors, but most importantly by the ability and willingness of the people to manage the water in the Ogallala as a non-renewable resource.

REFERENCES 1. Gutentag, E.D.; Heimes, F.J.; Krothe, N.C.; Luckey, R.R.; Weeks, J.B. Geohydrology of the High Plains Aquifer in Parts of Colorado, Kansas, Nebraska, New Mexico, Oklahoma, South Dakota, Texas and Wyoming, U.S. Geological Survey Professional Paper 1400-B; U.S. Government Printing Office: Washington, DC, 1984. 2. Opie, J.O. Water for a Dry Land; University of Nebraska Press: Lincoln, NE, 1993. 3. Templer, O.W. The legal context for ground water use. In Groundwater Exploitation in the High Plains; Kromm, D.E., White, S.E., Eds.; University Press of Kansas: Lawrence, 1992; 64–87.

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Aquifers: Recharge John Nimmo David A. Stonestrom U.S. Geological Survey (USGS), Menlo Park, California, U.S.A.

Richard W. Healy U.S. Geological Survey (USGS), Lakewood, Colorado, U.S.A.

INTRODUCTION Aquifer recharge was defined by Meinzer[1] and Heath[2] as water that moves from the land surface or the unsaturated zone into the saturated zone. This definition excludes saturated flow between aquifers, which avoids double-accounting in large-scale studies, so it might be more precisely called ‘‘aquifer-system’’ or ‘‘saturated-zone’’ recharge. Recharge rate designates either a flux [L3/T] into a specified portion of aquifer, or a flux density [L/T] into an aquifer at a point. Sources of water for recharge include precipitation that infiltrates, permanent or ephemeral surface water, irrigation, and artificial recharge ponds. Recharge may reach the aquifer directly from portions of rivers, canals, or lakes,[3] though usually it first travels by various means through the unsaturated zone. Recharge varies considerably with time and location. Temporal variation occurs, for example, with seasonal or short-term variations in precipitation and evapotranspiration (ET). This variability is especially evident in thin unsaturated zones, where recharge may occur within a short time of infiltration. In deep unsaturated zones, recharge may be homogenized over several years so that it may occur with essentially constant flux even though fluxes at shallow depths are erratic. Spatial variation occurs with climate, topography, soils, geology, and vegetation. For example, a decrease of slope or increase of soil permeability may lead to greater infiltration and greater recharge. Many applications use a concept of recharge that is timeaveraged or areally averaged. Both the amount of infiltration and the fraction of it that becomes recharge tend to be greater with more abundant water, so the recharge process is most efficient if infiltration is concentrated in space and time. Because ET may extract most or all of the water that infiltrates, water is more likely to become recharge if it moves rapidly below the root zone. Temporal concentration occurs during storms, floods, and snowmelt, when ongoing processes such as ET are overwhelmed. Spatial concentration typically occurs in depressions and channels, where higher water contents promote

rapid movement by increasing the hydraulic conductivity (K), the amount of preferential flow, and the downward driving force at a wetting front. Quantitative estimation of recharge rate contributes to the understanding of large-scale hydrologic processes. It is important for evaluating the sustainability of ground water supplies, though it does not equate with a sustainable rate of extraction.[4] Because it represents a first approximation to the rate of solute transport to the aquifer, the recharge rate is also important to estimate contaminant fluxes and travel times from sources near the land surface. Methods for obtaining a quantitative estimate of recharge mostly require a combination of various types of data which themselves may be hard to estimate, so in general it is wise to apply multiple methods and compare their results.

WATER BUDGET METHODS The water balance for a basin can be stated as gw sw þ ETuz þ ETgw þ Qsw P þ Qsw on þ Qon ¼ ET off snow þ Qgw þ Q þ DS bf off þ DSsw þ DSuz þ DSgw ð1Þ

where P is precipitation and irrigation; Qon and Qoff are water flow on and off of the site, respectively; Qsw off is runoff; Qbf is baseflow (ground water discharge to streams or springs); and DS is change in water storage. Superscripts refer to surface water, ground water, unsaturated zone, or snow, and all parameters are in units of L/T (or volume per unit surface area per unit time). For the saturated zone only, a water balance can be written for a defined area as R ¼ DQgw þ Qbf þ ETgw þ DSgw

ð2Þ

where R is recharge and DQgw is the difference between ground water flow off of and onto the basin. This equation implies that water arriving at the water table: 1) flows out of the basin as ground water flow; 55

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2) discharges to the surface; 3) is evapotranspired; or 4) goes into storage. Substitution in Eq. (1) produces a simpler form of the water balance: sw sw
ETuz
DSsnow R ¼ P þ Qsw on
Qoff
ET


DSsw
DSuz

ð3Þ

Water budget methods include all techniques based, in one form or another, on one of these water balance equations. The most common water budget method is the ‘‘residual’’ approach: all other components in the water budget are measured or estimated and R is set equal to the residual. Water budget methods can be applied over a wide range of space and time scales. The major limitation of the residual approach is that the accuracy of the recharge estimate depends on the accuracy with which other components can be measured. This limitation can become significant when the magnitude of R is small relative to other variables. The time scale for applying water budget methods is important, with more frequent tabulations likely to improve accuracy. If the water budget is calculated daily, P can greatly exceed ET on a single day, even in arid settings. Averaging over longer time periods tends to dampen out extreme precipitation events and hence underestimate recharge. Annual recharge estimated with water budgets range from 23 mm in a region of India[5] to 400 mm at a site in the eastern United States.[6] Watershed, surface water flow, and ground water flow models constitute an important class of water budget methods that have been used to estimate of recharge (e.g., Ref.[7]). An attractive feature of models is their predictive capability. They can be used to gauge the effects of future climate or land-use changes on recharge rates.

METHODS BASED ON SURFACE WATER OR GROUND WATER DATA Fluctuations in ground water levels can be used to estimate recharge to unconfined aquifers according to R ¼ Sy dh=dt ¼ Sy Dh=Dt

ð4Þ

where Sy is specific yield, h is water table height, and t is time. The method is best applied over short time periods in regions with shallow water tables that display sharp water-level rises and declines. Analysis of water-level fluctuations can also, however, be useful for determining the magnitude of long-term change in recharge rates caused by climate or land-use change. The method is only appropriate for estimating recharge

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for transient events; recharge occurring under steady flow conditions cannot be estimated. Difficulties lie in determining Sy and ensuring that fluctuations are due to recharge, not to changes in pumping rates or atmospheric pressure or other phenomena. Recharge rates estimated by this technique range from 11 mm over a 26-month period in Saudi Arabia[8] to 541 mm yr
1 over 1 yr for a small basin in the United States.[9] Ground water levels can also be used to estimate flow, Q, through a cross-section of an aquifer that is aligned with an equipotential line. Multiplying K by the hydraulic gradient normal to the section times the area of the section calculates Q. Recharge is determined by dividing Q by the surface area of the aquifer upgradient from the section. Methods of estimating recharge based on surfacewater data include the Channel Water Balance Method (CWBM) and determination of baseflow by hydrograph separation. The CWBM involves measuring discharge at two gauges on a stream; the difference in discharge between the upstream and downstream gages is the transmission loss. This loss may become recharge, ET, or bank storage. Hydrograph separation involves identifying what portion of gauged stream flow is derived from ground water discharge. Rutledge and Daniel[10] developed an automated technique for this purpose and applied the method to estimate recharge at 15 sites. Drainage areas for the sites ranged from less than 52 km2 to more than 5200 km2; estimated annual recharge was between about 13 cm and 64 cm.

DARCIAN METHODS Applied in the unsaturated zone, Darcy’s law gives a flux density equal to K times the driving force, which equals the recharge rate if certain conditions apply. Matric-pressure gradients must be measured or demonstrated to be negligible. Some types of preferential flow are inherently non-darcian and if important would need to be determined separately. Accurate measurements are necessary to know K adequately under field conditions at the point of interest. For purposes requiring areal rather than point estimates, additional interpretation and calculation are necessary. In the simplest cases, in a region of constant downward flow in a deep unsaturated zone, gravity alone drives the flow. With a core sample from this zone, laboratory K measurements at the original field water content directly indicate the long-term average recharge rate.[11] In the general case, transient water contents and matric pressures must be measured in addition to K.[12] Transient recharge computed with Darcy’s law

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can relate to storms or other short-term events, or provide data for integration into temporal averages.

TRACER METHODS Increasing availability and precision of physical and chemical analytical techniques have led to a proliferation in applications of tracer methods for recharge estimation. Isotopic and chemical tracers include tritium, deuterium, oxygen-18, bromide, chloride, chlorine-36, carbon-14, agricultural chemicals, dyes, chlorofluorocarbons, and noble gases. In practice, concentrations measured in pore water are related to recharge by applying chemical mass-balance equations, by matching patterns inherited from infiltrating water, or by determining the age of the water. Tracer methods provide point and areal estimates of recharge. Multiple tracers used together can test assumptions and constrain estimates. The most common tracer for estimating recharge is chloride. Chloride continually arrives at the land surface in precipitation and dust. Chloride is conservative in many environments and is non-volatile. Under suitable conditions, R ¼ P½Clp =½Clr 

ð6Þ

where y is volumetric water content, A1 and A2 are ages at two points, and L is separation length. One point is often located at the water table. Preindustrial water can be dated by decay of predominately cosmogenic radioisotopes, including carbon-14 and chlorine-36. The abundance of tritium and other radioisotopes increased greatly during atmospheric weapons testing, labeling recent precipitation. Additional compounds

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OTHER METHODS Additional geophysical techniques provide recharge estimates based on the water-content dependence of gravitational, seismic, and electromagnetic properties of earth materials. Repeated high-precision gravity surveys can indicate changes in the quantity of subsurface water from recharge events.[20] Similarly, repeated surveys using seismic or ground-penetrating-radar equipment can resolve significant changes in watertable elevation associated with transient recharge.[21] In addition to surface-based techniques, cross-bore tomographic imaging can provide detailed threedimensional reconstructions of water distribution and movement during periods of recharge.[22]

ð5Þ

where P is precipitation and [Clp] and [Clr] are chloride concentrations in precipitation and pore water, respectively. Chloride mass-balance methods can be applied to unsaturated profiles[13] and entire basins.[14] Isotopic composition of water provides a useful tracer of the hydrologic cycle. The isotopic makeup of precipitation varies with altitude, season, storm track, and other factors. Recharge estimates using isotopic varieties of water usually employ temporal or geographic trends in infiltrating water. Non-conservative tracers can indicate the length of time that water is isolated from the atmosphere, that is, its ‘‘age.’’ Recharge rates can be inferred from water ages if mixing is small. If ages are known along a flow line, R ¼ yL=ðA2
A1 Þ

for dating modern recharge include chlorofluorocarbons, krypton-85, and agricultural chemicals.[15,16] Heat is yet another tracer of ground water recharge. Daily, seasonal, and other temperature fluctuations at the land surface produce thermal signals that can be traced through shallow profiles.[17,18] Water moving through deeper profiles alters geothermal gradients, which can be used in inverse modeling to obtain recharge rates.[19]

REFERENCES 1. Meinzer, O.E. The Occurrence of Ground Water in the United States, with a Discussion of Principles. U.S. Geological Survey Water-Supply Paper 489, 1923; 321 pp. 2. Heath, R.C. Ground-Water Hydrology. U.S. Geological Survey Water-Supply Paper 2220, 1983; 84 pp. 3. Winter, T.C.; Harvey, J.W.; Franke, L.O.; Alley, W.M. Ground Water and Surface Water a Single Resource. U.S. Geological Survey Circular 1139, 1998; 79 pp. 4. Bredehoeft, J.D.; Papadopoulos, S.S.; Cooper, H.H., Jr. Groundwater: The Water-Budget Myth, Scientific Basis of Water-Resource Management: Studies in Geophysics; National Academy Press: Washington, DC, 1982; 51–57. 5. Narayanpethkar, A.B.; Rao, V.V.S.G.; Mallick, K. Estimation of groundwater recharge in a basaltic aquifer. Hydrol. Proc. 1994, 8, 211–220. 6. Steenhuis, T.S.; Jackson, C.D.; Kung, S.K.; Brutsaert, W. Measurement of groundwater recharge in eastern long Island, New York, U.S.A.. J. Hydrol. 1985, 79, 145–169. 7. Bauer, H.H.; Mastin, M.C. Recharge from precipitation in three small glacial-till-mantled catchments in the puget sound Lowland, Washington. U.S. Geological Survey Water-Resources Inv. Rep. 96-4219, 1997; 119 pp. 8. Abdulrazzak, M.J.; Sorman, A.U.; Alhames, A.S. Water balance approach under extreme arid conditions—a

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case study of Tabalah Basin, Saudi Arabia. Hydrol. Proc. 1989, 3, 107–122. Rasmussen, W.C.; Andreasen, G.E. Hydrologic budget of the beaverdam creek basin, Maryland. U.S. Geol. Survey. Water-Supply Paper 1472, 1959; 106 pp. Rutledge, A.T.; Daniel, C.C. Testing an automated method to estimate ground-water recharge from streamflow records. Ground Water 1994, 32 (No. 2), 180–189. Nimmo, J.R.; Stonestrom, D.A.; Akstin, K.C. The feasibility of recharge rate determinations using the steadystate centrifuge method. Soil Sci. Soc. Am. J. 1994, 58, 49–56. Freeze, A.R.; Banner, J. The mechanism of natural ground-water recharge and discharge 2. Laboratory column experiments and field measurements. Water Resour. Res. 1970, 6 (No. 1), 138–155. Bromley, J.; Edmunds, W.M.; Fellman, E.; Brouwer, J.; Gaze, S.R.; Sudlow, J.; Taupin, J.-D. Estimation of rainfall inputs and direct recharge to the deep unsaturated zone of southern Niger using the chloride profile method. J. Hydrol. 1997, 188–189 (No. 1–4), 139–154. Anderholm, S.K. Mountain-Front Recharge Along the Eastern Side of the Middle Rio-Grande Basin, Central New Mexico. U.S. Geological Survey Water-Resources Investigations Report 00-4010, 2000; 36 pp. Ekwurzel, B.S.P.; Smethie, W.M., Jr.; Plummer, L.N.; Busenberg, E.; Michel, R.L.; Weppernig, R.; Stute, M. Dating of shallow groundwater: comparison of the transient tracers 3H/3He, chlorofluorocarbons, and 85Kr. Water Resour. Res. 1994, 30 (No. 6), 1693–1708.

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16. Davisson, M.L.; Criss, R.E. Stable isotope and groundwater flow dynamics of agricultural irrigation recharge into groundwater resources of the central valley, California. In Proc. Isotopes in Water Resources Management, Vienna (Austria), Mar 20–24, 1995; International Atomic Energy Agency: Vienna, 1996; 405–418. 17. Lapham, W.W. Use of Temperature Profiles Beneath Streams to Determine Rates of Vertical Ground-Water Flow and Vertical Hydraulic Conductivity. U.S. Geological Survey Water-Supply Paper 2337, 1989; 34 pp. 18. Constantz, J.; Thomas, C.L.; Zellweger, G. Influence of diurnal variations in stream temperature on streamflow loss and groundwater recharge. Water Resour. Res. 1994, 30, 3253–3264. 19. Rousseau, J.P.; Kwicklis, E.M.; Gillies, D.C.; Eds. Hydrogeology of the Unsaturated Zone, North Ramp Area of the Exploratory Studies Facility, Yucca Mountain, Nevada. U.S. Geological Survey Water Resources Investigations Report 98-4050, 1999. 20. Pool, D.R.; Schmidt, W. Measurement of GroundWater Storage Change and Specific Yield Using the Temporal-Gravity Method Near Rillito Creek, Tucson, Arizona. U.S. Geological Survey Water-Resources Investigations Report 97-4125, 1997; 30 pp. 21. Haeni, F.P. Application of Seismic-Refraction Techniques to Hydrologic Studies. U.S. Geological Survey Open File Report 84-746, 1986; 144 pp. 22. Daily, W.; Ramirez, A.; LaBrecque, D.; Nitao, J. Electrical resistivity tomography of vadose water movement. Water Resour. Res. 1992, 28 (No. 5), 1429–1442.

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Aquifers: Transmissivity Mohamed M. Hantush National Risk Management Research Laboratory, U.S. Environmental Protection Agency (USEPA), Cincinnati, Ohio, U.S.A.

INTRODUCTION

of the hydraulic conductivity over the saturated thickness[1]

Evaluation of groundwater resources requires the knowledge of the capacity of aquifers to store and transmit ground water. This requires estimates of key hydraulic parameters, such as the transmissivity, among others. The transmissivity T (m2/sec) is a hydraulic property, which measures the ability of the aquifer to transmit ground water throughout its entire saturated thickness. It is defined as the product of the hydraulic conductivity K (m/sec) and the saturated thickness B (m), in the direction normal to the base of the aquifer: T ¼ KB

ð1Þ

Tðx; yÞ ¼

Z

Bðx;yÞ

where x denotes the Cartesian coordinates (x,y,z). As an illustration of the use of Eq. (2), for a layered confined aquifer composed of N distinct layers, each with thickness bi and constant hydraulic conductivity Ki, the transmissivity at any given point in the horizontal plane is given by T ¼ BKA ; B ¼

N X

bi ; KA ¼ ð1=BÞ

i¼1

CONCEPTS Fig. 1 illustrates a confined unit, or permeable unit sandwiched between impervious or semipervious layers. The hydraulic (or piezometric) head gradient in the two piezometers tapping the aquifer and separated by a unit distance is unity, since they measure a drop in the hydraulic head of magnitude one. The flow through the shaded window of height B and unit width normal to the flow direction is the aquifer transmissivity T. This follows from Darcy’s law, which requires that groundwater flow rate per unit area normal to the flow direction is equal to the hydraulic conductivity, if the hydraulic head gradient is unity. In unconfined aquifers, however, the transmissivity is not as well defined as in confined units. The saturated thickness h (m) extends from the water table vertically down to the aquifer bed in an unconfined aquifer. The transmissivity varies in time in unconfined aquifers, since the water table often fluctuates in response to recharge from the overlying vadose zone or dewatering of the aquifer by pumping. It decreases during pumping and increases during recharge. In stratified formations the hydraulic conductivity distribution is also stratified and can actually vary from one location to another by orders of magnitude. With this variability and that of the saturated thickness, B(x,y), the transmissivity is given by the integral

ð2Þ

KðxÞdz 0

N X

bi Ki

ð3Þ

i¼1

where KA is the arithmetic mean of the hydraulic conductivity.

TRANSMISSIVITY OF AQUIFERS Table 1 shows range of values of T which may be encountered in common aquifers of thicknesses in the range of 5–100 m. In general, transmissivities greater than 0.015 m2/sec represent good aquifers for waterwell exploitation.[2] Karstic limestones, in which sizable proportions of the original rock has been dissolved and removed, are highly transmissive aquifers. Alluvial valleys, which are predominantly unconsolidated sand and gravel, are among the most productive aquifers in the United States.[3] Permeable basalts and fractured igneous and metamorphic rocks have a relatively large transmissivity and serve as good aquifers. Non-karstic limestones, silt, glacial till, and solid igneous and metamorphic rocks are the least transmissive and make poor aquifers. Sandstone aquifers have a low transmissivity, but they are significant sources of potable water.

RELATIONSHIP TO GROUNDWATER FLOW The concept of aquifer transmissivity is widely used in the analysis of hydraulics of water wells. It is introduced when the groundwater flow in aquifers is 59

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Aquifers: Transmissivity

ANISOTROPY AND PRINCIPAL DIRECTIONS Implicit in Eq. (4) is that the transmissivity is invariant to the orientation in the (x,y) plane. If the transmissivity is dependent on the direction of flow in an aquifer, the aquifer is said to be anisotropic. This anisotropy stems from the anisotropy of the hydraulic conductivity, and when the latter is the same in all directions, the aquifer is said to be isotropic. In natural aquifers the transmissivity is anisotropic, and groundwater flow rate, in this case, is given by the following general form:
Q ¼
T  r j;

Fig. 1 Illustration of transmissivity in a confined aquifer.

essentially horizontal. This is commonly the case in aquifers whose lateral extensions are much greater than their thicknesses and where the equipotential lines are nearly vertical. The groundwater flow rate Q (m2/sec) integrated over the saturated thickness of the aquifer, per unit aquifer width, is related to the transmissivity through the Darcy relationship: Q ¼
T r j

ð4Þ

where Q ¼ Qxi þ Qyj and r j ¼ (@j/@x)i þ (@j/ @y)j. Qx and Qy are the flow rates per unit width in the x and y directions, respectively, and i and j are the unit length orthogonal vectors in the x and y directions, respectively. Eq. (4) in combination with the groundwater flow balance equation describe essentially horizontal groundwater flow in aquifers. Table 1 Representative values of transmissivity for aquifers of thicknesses 5–100 m Material

Transmissivity (m2/sec)a

Unconsolidated Gravel Sand Silt Glacial till

5 5 5 5

Rocks Karst limestone Permeable basalt Fractured igneous and metamorphic rocks Limestone, dolomite Sandstone

   

T ¼

Txx T yx

Txy Tyy



in which T is the second order symmetric tensor of transmissivity of an anisotropic aquifer.[1] It is equal to the product of the hydraulic conductivity tensor and the aquifer saturated thickness. As an example, the component Txy gives the contribution of a unit hydraulic gradient in the y-direction to the flow rate in the x-direction Qx, and the component Txx gives the contribution of a unit hydraulic gradient in the x-direction to Qx. The four elements appearing in Eq. (5) depend on the chosen coordinate system. The principal directions of anisotropy are defined as the orientation y from the original x–y coordinates to the new x–Z coordinate system (Fig. 2), such that the off diagonal elements in the transformed system are zero,
 Txx 0 ð6Þ T ¼ 0 TZZ where Txx and TZZ are the principal transmissivities. Both are related to the transmissivities in the original x–y coordinates (Txx, Tyy, and Txy) and the orientation y by simple algebraic relationships.[1] The transmissivity in the major direction of anisotropy Txx is greater than in the minor direction TZZ. Fig. 2 illustrates the

10
3–100 10
7–1 10
9–2  10
3 10
12–2  10
4

5  10
6–2 1  10
6–2 4  10
8–3  10
2 5  10
9–2  10
4 5  10
10–6  10
4

a

Values are estimated from representative values of hydraulic conductivity. Source: Adapted from Refs.[2,3].

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ð5Þ

Fig. 2 Ellipse of directional transmissivity.

transmissivity ellipse in the principal direction coordinates and the relationship between the directional transmissivity Tr and the principal components Txx and TZZ. Anisotropy with respect to transmissivity can be estimated using aquifer test analysis.[4]

METHODS OF ESTIMATION Estimation of aquifer transmissivity through analysis of aquifer test data is a standard practice in the evaluation of groundwater resources. Aquifer test is an insitu method for estimating field scale transmissivity in which the water bearing materials are tested under natural conditions. In this test, the well is pumped at a given (usually constant) rate and drawdown (i.e., drop in elevation of the water level in the well from its initial static position) is observed and recorded in time in the pumping well itself and possibly in at least one observation well in the vicinity. In this method, a logarithmic plot of the applicable well-flow equation, called a ‘‘type curve,’’ is superimposed on a logarithmic drawdown-time plot, called a ‘‘data curve,’’ and the transmissivity is then estimated using a graphical matching technique.[5,6,7] This technique assumes the aquifer is homogeneous and requires experience and judgment of qualified persons conducting these tests, because observed drawdown variations with time and distance from the pumped well may be interpreted in several ways and therefore subject to uncertainty. The performance of graphical matching techniques depends largely on the selection of the well-flow equation that most accurately resembles the flow system under consideration. Evaluation of groundwater resources in regional aquifers requires estimates of the transmissivity at locations where it is not available. This can be achieved by solving the ‘‘inverse problem’’ for the unknown parameters. In this method, the flow domain is overlain by a discrete mesh of nodal points, and the groundwater flow equation is approximated by a set of algebraic equations, one for each nodal point. Unknown T values at the nodal points are identified by trial and error or automatically, by a gradient-based search technique.[8] The optimal set of transmissivities is the one that produces the best match, such as minimizing the sum of weighted least square errors between the observed hydraulic heads and those obtained from the solution of the algebraic equations at the measurement points. Many of the restrictive assumptions often made in aquifer test analysis are relaxed in the numerical methods for solving the inverse problem. However, these techniques may suffer from non-uniqueness and instability of the solutions, and can result in unrealistic estimates for T, e.g., negative values or solutions fluctuating between imposed lower and upper bounds of the

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transmissivity.[8] Current trends in hydrology account for local-scale spatial variation of the transmissivity using statistical methods in which this property can be idealized as a space random function. Field evidence support that transmissivity is lognormally distributed.[9] The geostatistical approach[10] combines process understanding of groundwater flow in aquifers with statistical estimation methods to provide an effective tool for mapping transmissivities over regional aquifers, by making use of their in-situ estimates inferred from aquifer tests and hydraulic head measurements. The literature on aquifer properties estimation is rich in innovative approaches for estimating both the transmissivity spatial structure and its values at locations where measurements are not available.[8,10,11] Notice: The U.S. Environmental Protection Agency through its Office of Research and Development funded and managed the research described here through in-house effort. It has been subjected to Agency review and approved for publication.

REFERENCES 1. Bear, J. The equation of motion of a homogeneous fluid. Dynamics of Fluids in Porous Media; American Elsevier: New York, 1972; 764 pp. 2. Freeze, R.A.; Cherry, J.A. Physical properties and principles. Groundwater; Prentice Hall: Englewood Cliffs, NJ, 1979; 15–79. 3. Domenico, P.A.; Scwartz, F.W. Ground water movement. Physical and Chemical Hydrogeology; John Wiley & Sons: New York, 1990; 824 pp. 4. Hantush, M.S. Analysis of data from pumping tests in anisotropic aquifers. J. Geophys. Res. 1966, 71, 421–426. 5. Hantush, M.S. Hydraulics of wells. In Advances in Hydroscience; Ven Te Chow, Ed.; Academic Press: New York, 1964; 281–442. 6. Neuman, S.P. Theory of flow in unconfined aquifers considering delayed response of the water table. Water Resour. Res. 1972, 9, 1031–1045. 7. Theis, C.V. The relation between lowering of the piezometric surface and the rate and duration of discharge of a well using ground-water storage. Am. Geophys. Union Trans. 1935, 16, 519–524. 8. Willis, R.; Yeh, W.W.-G. The inverse problem in groundwater systems. Groundwater Systems Planning & Management; Prentice-Hall: Englewood Cliffs, NJ, 1987; 347–412. 9. Delhomme, J.P. Spatial variability and uncertainty in groundwater flow parameters: a geostatistical approach. Water Resour. Res. 1979, 15, 269–280. 10. Kitanidis, P.K. Introduction to Geostatistics; Cambridge University Press: New York, 1997; 249 pp. 11. Sun, N.-Z. Inverse Problems in Groundwater Modeling; Kluwer: Norwell, MA, 1994; 352 pp.

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Aquifers: Transmissivity

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Aral Sea Disaster Guy Fipps Agricultural Engineering Department, Texas A&M University, College Station, Texas, U.S.A.

INTRODUCTION The Aral Sea is one of the worst ecological disasters on our planet. What was once the world’s fourth largest inlet sea, the Aral Sea has lost over 60% of its surface area, two-third of its volume, declined 40 m in depth, and has fallen to the eighth largest inland body of water in the world. The cause is attributed to a vast expansion of irrigation in the Central Asian Republics beginning in the 1950s, which greatly reduced inflows to the Sea. The diversion of water for massive irrigation development was done deliberately by Soviet Union officials, unconcerned about the consequences of their actions. The environmental, social, and economic damage has been immense. Winds pick up dust from the dry seabed and deposit it over a large populated area. The dust likely contains pesticide and chemical residues that are blamed for the serious rise in mortality and health problems in the region. The Sea, and the now exposed dry seabed, may also be contaminated by runoff from a former Soviet military base and a biological weapons lab. The ecosystem of the Aral Sea has collapsed, and climate changes in the Aral Sea Basin have been documented. Hundreds of agreements have been signed since 1980s on programs designed to address the ‘‘Aral Sea Problem’’ which, to date, have not been effective at preventing the continuing shrinking of the sea.

reservoirs and 60 major irrigation schemes have been constructed. About 82% of river diversions are for agricultural use and 14% is for municipal and industrial use (Table 1). Water demand due to population growth and industrial expansion continues to increase (Table 2). Since 1960, the population of the Central Asian republics has increased 140% and totals over 50 million. Likewise, industrial production using large amounts of water has also increased. Examples include steel production which rose 200%, cement production by 170%, and electricity generation by a factor of 12. The total inflows to the Aral Sea began decreasing rapidly in the 1960s, and by 1990 the storage volume of the sea has decreased by 600 km3 (Table 3). As the water level fell, salinity levels have tripled, rising from about 1000 ppm to just under 3000 ppm today. By the 1980s, as the Aral Sea problem became well known in the Soviet Union, government officials proposed ambitious projects to divert water from other rivers, including ones in South Russia and Siberia, to be transported to the Aral Sea in massive canals. However, these plans died with the breakup of the Soviet Union. The decrease in sea level has now split the Aral Sea into two separate water bodies: the Small and Large Aral Seas (Maloe More and Bol’shiye More) each separately fed by the Syr Darya and the Amu Darya, respectively. The once vast Amu Darya delta which once covered 550,000 ha has now shrunk to less than 20,000 ha.

THE ARAL SEA BASIN The Aral Sea is located in Central Asia and lies between Uzbekistan and Kazakhstan in a vast geological depression, the Turan lowlands, in the Kyzylkum and Karakum Deserts. In the 1950s, the sea covered 66,000 km2, contained about 1090 km3 of water, and had a maximum depth of about 70 m. The Aral Sea supported vast fisheries and shipping industries. At that time the sea was fed by two rivers, the Amu Darya (2540 km) and the Syr Darya (2200 km), which originate in the mountain ranges of central Asia and flow through the five republics of Uzbekistan, Kazakhstan, Kyrgyzstan, Tajikistan, and Turkmenistan. The two rivers provide most of the fresh water used in Central Asia. In the last 50 years, about 20 dams and 62

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IRRIGATION AND COTTON For thousands of years, Central Asian farmers diverted water from the Amu Darya and Syr Darya Rivers, transforming desert into green oases and supporting great civilizations. Historically, irrigation water use was conducted at a sustainable level. The creation of the Soviet Union and the collectivization of farmlands resulted in the end for traditional agricultural practices. Beginning as early as 1918, Soviet leaders began expanding irrigated land in Central Asia for export and hard currency. Cotton was known as ‘‘white gold.’’ The USSR became a net exporter of cotton

63

Table 1 Average water supply and demand in the Aral Sea Basin Total water available

km3

%

Amu Darya Basin Syr Darya Basin Total

84.3 47.8 132.1

64 36 100

Table 3 Decline of the Aral Sea during the 1980s and total estimated inflows from the Amu Darya and Syr Darya rivers Aral Sea Inflows (km3)

Volume (km3)

Surface area (km2)

56.0

1064

66,100

1981

6.0

618

50,500

1982

0.04

583

49,300

1983

2.3

539

47,700

1984

7.9

501

46,100

1986

0.0

424

41,100

1987

9.0

1988

23.0

Year

Water Demand

1911–1960

Agriculture Amu Darya Basin Syr Darya Basin Municipal Water Amu Darya Syr Darya

44.8 34.6

81.6

3 3.3

6.5

Industry Amu Darya Syr Darya

3 5

8.2

Livestock Amu Darya Syr Darya

0.2 0

0.2

Fishery Amu Darya Syr Darya

2.6 0.8

3.5

Total

1989

97.3

41,000 300

30,000

to inefficient irrigation practices used on-farm. It is believed that over the past decade, adequate maintenance, repair, and renovation of the irrigation infrastructure were not performed at a meaningful level, and water losses from deteriorating canals, gates, and other facilities have increased. Most land is under furrow irrigation, with drip irrigation accounting for about 5% of the irrigated cropland (used primarily on orchard crops), and sprinkler irrigation accounts for about 3%. Even though the water saving benefits of gated pipe are well known in the region, less than one-sixth of the farms use this technology. Reasons may include costs and product availability. Most farms follow the centuries’ old practice of cutting earthen canals with shovels in order to divert water into the field. The volume of water available at these farm ditches is not sufficient to provide an even distribution of water over the field. As a result, water logging and soil salinity now affects about 40% of all the cultivated land in the region.

100

by the 1930s, and by the 1980s, was ranked fourth in the world in cotton production. The policy of emphasizing cotton production was accelerated in the 1950s as Central Asia’s irrigated agriculture was expanded and mechanized. In 1956, the Kara Kum Canal was opened, diverting one-third of the flow in the Amu Darya to new cultivated areas in the deserts of Uzbekistan and Turkmenistan. The year 1960 represents the critical junction when the Aral Sea began to drop. Irrigated cotton production and water diversions continued to be expanded until the break-up of the Soviet Union (Table 4). Estimates are that upwards of 80% of the workforce is employed in agriculture. The main agricultural crops in the basin are cotton (6.4 million ha), forage (1.7 million ha), rice (0.4 million ha), and tree crops (0.4 million ha). Some Central Asian irrigation experts estimate that only 20–25% of the water diverted from the rivers is actually used by the crops, the rest being lost in the canals that transport the water to the fields and due

MUYNAK AND ARALSK Of all the villages affected by the drying of the Aral Sea, Muynak is the best known. Historically, Muynak was located on an island of the vast Aral

Table 2 General statistics of the Aral Sea Basin countries in 1995 Area, km2 2

Irrigated land, km Population

Population growth rate, %

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Kazakhstan

Uzbekistan

2,717,300

447,400

Turkmenistan 488,100

Kyrgyzstan 198,500

Tajikistan 143,100

23,080

41,500

12,450

10,320

6.940

17,376,615

23,089,261

4,075,316

4,769,877

6,155,474

0.62

2.08

2.5

1.5

2.6

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Aral Sea Disaster

Table 4 Cultivated land along the Amu Darya and Syr Darya rivers Before 1917

1960

1980

1992

5.2

10

15

18.3

Millions of hectares

Sea delta at the convergence of the Amu Darya River in Karakalpakstan (a semi-autonomist republic in Uzbekistan). In 1962, the island became a peninsula. By 1970 the former seaport was 10 km from the sea. The retreat of the sea accelerated and the town was 40 km from the sea by 1980, 70 km in 1995, and close to 100 km today. Over 3000 fishermen once worked the abundant waters around Muynak which supported 22 different commercial species of fish. In 1957, Muynak fishermen harvested 26,000 tons of fish, about half of the total catch that year taken from the Aral Sea. Muynak also produced 1.1 million farmed muskrat skins which were used to produce coats and hats. The Kazakhstan city of Aralsk, was once located on the northern edge of the Aral Sea, and like Muynak, had major fisheries and commerce industries. A major shipping and transport industry existed between these two cities. As the Aral Sea skunk, Aralsk found itself farther and farther from the shore which had retreated nearly 129 km by the 1980s. In the early 1990s, a dam was built just to the south of the mouth of the Syr Darya, to protect the northern part of the Aral Sea, letting the southern portion of the Aral Sea evaporate. Although only 10% of the water in the Syr Darya River reaches the northern part of the Aral Sea, the Little Aral has risen 3 m since the construction of the dam, and the shoreline has crept to within 16 km of the town.

located on the Ust-Jurt Plateau (north shore), and was closed in the mid-1980s. Renaissance Island (Vorzrozhdeniya Island), located in the central Aral Sea, was the site of the former USSR Government’s Microbiological Warfare Group which produced the deadly Anthrax virus. Some scientists believe that some containers holding the virus were not properly stored or destroyed. As the Aral Sea continues to dry and water levels recede, the ever-expanding island will soon connect to the surrounding land. Scientists fear that reptiles, including snakes that have been exposed to the various viruses, will move onto the surrounding land and possibly infect the humans living around the shores of the Aral Sea. The Area Sea once supported a complex ecosystem, an oasis in the vast desert. Over 20 species of fish are now extinct. Karakalpakstan scientists believe that a total of about 100 species of fish and animals that once flourished in the region are now extinct, as are many unique plants. Residents believe that there is a direct correlation between the drying of the sea and changes in climate of the Aral Sea Basin. The moderating effect of the

ENVIRONMENTAL PROBLEMS The Aral Sea is an unfortunate example of an old Uzbek proverb: ‘‘at the beginning you drink water, at the end you drink poison.’’ As the rivers flow through cultivated areas, they pick up fertilizers, pesticides, and salts from runoff, drainage water and groundwater flow. In the 1960s, it was common for about 550 kg ha 1 of chemicals to be applied to cotton fields in Central Asia, compared to an average of 25 kg used for other crops in the Soviet Union. Residues of these chemicals are now found on the dry seabed. Estimates are that millions of tons of dust are picked up from the seabed and distributed over the Aral Sea region. The Sea may have been contaminated from runoff from by two former USSR military installations in the area. A chemical weapons testing facility was

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Fig. 1 This NASA photograph (STS085-503-119) was taken in August 1997 and looks toward the southeast. The Amu Darya River is visible to the right and the Syr Darya on the left. The Aral Sea is now separated into the Small Aral to the north and the Large Aral to the south. Shown are the approximate extent of the Aral Sea in 1957 before a massive expansion of irrigation diversions from the rivers.

sea has diminished and temperatures are now about 2.5 C higher in the summer and lower in the winter. Rainfall in the already arid basin has decreased by about 20 mm.

THE HUMAN TRAGEDY Over the last 50 years, there has been a large increase in mortality, illnesses, and poor health in the region. Some estimate that 70–90% of the population of Karakalpakstan suffer some an environmentally induced malady. Tuberculosis is rampant. Hardest hit are women and children. Common health problems include kidney diseases, thyroid dysfunctions, anemia, bronchitis, and cancers.

CONCLUSION Some accounts are that since 1984, hundreds of international agreements have been signed to address Aral Sea problems. The early agreements had the goals of

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first stabilizing the Sea, then slowly increasing flows to restore its ecosystem. In 1992, the Interstate Commission for Water Coordination was formed by the five central Asian republics, which also accepted, in principle, to adhere to the limits on water diversions as set during the Soviet era in 1984 and 1987. To date, however, no progress has been made on stabilizing or reversing the declining inflows. With no water reaching the Aral Sea from the Amu Darya, scientists predict that this portion of the sea (the Large Aral Sea) will disappear by 2020 (Fig. 1).

BIBLIOGRAPHY 1. The Aral Sea Homepage: http://www.dfd.dlr.de/app/ land/aralsee/ 2. Requiem for a dying sea: http://www.oneworld.org/ patp/pap_aral.html/ 3. Disappearance of the Aral Sea: http://www.grida.no/ aral/maps/aral.htm 4. Earth from Space: http://earth.jsc.nasa.gov/categories. html

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Arctic Hydrology Bretton Somers H. Jesse Walker Department of Geography and Anthropology, Louisiana State University, Baton Rouge, Louisiana, U.S.A.

INTRODUCTION After a discussion about the boundaries of the Arctic, its hydrological elements are analyzed. It is noted that, in the Arctic, the solid phase of water dominates the landscape during most of the year. In the solid phase, water occurs as snow, river and lake ice, sea ice, glacial ice, and ground ice. Ground ice is integral to permafrost, which is nearly continuous. Although all of the hydrologic elements can be considered separately as though each is in storage, they are in fact interconnected and should be considered as an integral part of a cycle that progresses through all three phases among the atmosphere, lithosphere, and cryosphere. Recent research has shown that many of the Arctic’s hydrologic elements are changing rapidly.

THE ARCTIC Although no one denies the existence of the region known as the Arctic, few actually agree on its southern boundary. Astronomers usually use the Arctic Circle, oceanographers use the distribution of sea ice, biologists use the tree line, and cryosphere specialists use a permafrost boundary.[1] In one way or another, all of these boundaries are of significance to hydrologists, because they must contend with distributions that often spread well beyond the Arctic. For example, many of the rivers that drain into the Arctic Ocean originate in temperate latitudes (Fig. 1), the Arctic Ocean is impacted by flow from both the Atlantic and Pacific Oceans, and the atmosphere can be affected by non-arctic weather patterns. Nonetheless, the main hydrologic factor that distinguishes the Arctic from other climatic zones is the long period of time during which snow and ice dominate the landscape. From a regional standpoint, the Arctic may best be considered a moderatelysized ocean almost completely surrounded by a fringe of land, i.e., a configuration that is the opposite of its counterpart in the Southern Hemisphere, the Antarctic.

into it, its lakes and ponds, and the numerous islands (many with glaciers) that occur north of the continents (Fig. 1). Some scientists contend that the Arctic Ocean should be classified as a Mediterranean sea rather than an ocean because in addition to being nearly surrounded by land, it occupies less than 4% of the Earth’s ocean area and only about 1% of the Earth’s ocean volume. The Arctic Ocean is also unique in that it is only 40% as large as the continental area that drains into it. The rivers that enter the ocean vary greatly in size and discharge. Included among them are four of the Earth’s 12 longest—the Yenisey (5870 km), the Ob (5400 km), the Lena (4400 km), and the Mackenzie (4180 km). They contribute nearly 60% of the fresh water that drains from the continents.[2] This total equals some 11% of the Earth’s runoff from the continents. Because of its unique relationship between land and sea, the Arctic Ocean is impacted more by water draining into it than any other ocean.[3] In addition to its seas, rivers, and islands, the Arctic also possesses numerous lakes and ponds. Most Arctic lakes are small. One of the most studied lake types is the oriented lake. Generally elliptical in shape, oriented lakes originate as thaw lakes and range in length up to several kilometers. However, by far, the most common bodies of water dotting the land surface are small ponds whose existence is partly the result of the poor drainage and limited infiltration that characterizes permafrost environments. Glaciers in the Arctic, except for the Greenland ice cap, are generally limited to the Canadian and Russian arctic islands. Arctic glaciers are major contributors of water and ice bergs to the ocean. SNOW, ICE, AND PERMAFROST One of the unique characteristics of water is that it is the only chemical compound that occurs on Earth in three phases, i.e., as a gas (water vapor), a liquid, and a solid. In its solid phase, water appears as snow, surface ice (river, lake, and sea ice), and ground ice.

THE OCEAN, RIVERS, LAKES, AND GLACIERS

Snow

The major features of the Arctic are the Arctic Ocean with its numerous bordering seas, the rivers that drain

Although snow is not limited to high latitudes, it is there that it is nearly ubiquitous. Over most of the

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Fig. 1 Map of the Arctic showing the major rivers, permafrost distribution, September sea-ice locations, and bordering seas in the Arctic Ocean. Source: From Ref.[9].

Arctic, it covers the surface for nine or more months. Snow fall is related to the volume of water vapor in the air, a volume that is temperature dependent. Therefore, the actual amount of snow that falls is limited in quantity. Once it falls, it tends to retain its solid form until melt season. However, because most of the Arctic has low-lying vegetation (tundra), snow drifting is extensive. On flat surfaces, such as lake ice, the snowpack is thin or even missing. Uneven surfaces, such as river banks, trap snow into sizable drifts, some of which may last through most of the summer. Snow is important in several major ways in the Arctic. Because it is highly reflective, much of the sun’s energy that reaches it is returned to space. The reflected energy, or albedo, of fresh snow is more than 75%, whereas that from water is usually less than 30% and from tundra less than 20%.[4] Further, snow, because of its low thermal conductivity, is a good insulator— affecting the occurrence and maintenance of permafrost and in helping keep vegetation and fauna from freezing. Snow, when it melts, provides the bulk of the water that feeds the rivers and streams that drain into the Arctic Ocean. Because the melt season

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is relatively short and the melt rate is rapid, most rivers in the Arctic have peaks of discharge that correlate closely with river breakup (Fig. 2). River and Lake Ice In the Arctic, the temperature regime is such that surface ice, whether on rivers, lakes, or the sea, is present for most of the year. On fresh-water bodies, the ice reaches thicknesses of about 2 meters. Because most of the ponds and many of the streams have depths less than that, water freezes to the bottom. In deeper lakes and rivers, water will remain in the liquid state beneath the ice. In some rivers, especially those originating in lower latitudes (Fig. 1), flow continues throughout the winter beneath the ice. Ice in the deeper lakes lasts longer than ice on rivers, where breakup follows after snow-melt water enters the river channels. Sea Ice Some authors maintain that sea ice is the most dramatic feature of the Arctic. Formed by the freezing

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Arctic Hydrology

Fig. 2 The mouth of the Colville River, Alaska showing the distribution of snow, water, river ice and sea ice during breakup. (A) sea ice, (B) floodwater on top of sea ice, (C) sea ice floating on top of river floodwater, (D) river ice floating on top of floodwater, (E) flooded mudflats, (F) flooded island with remnant snow patches, (G) snow and ice covering ice-wedge polygons. Photograph by Donald Nemeth.

Fig. 3 Conceptual model of the arctic hydrologic cycle. Source: From Ref.[3].

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of sea water, sea ice covers nearly all of the Arctic Ocean during winter when it is attached to much of the continental coastline of Siberia and North America. Except for a narrow band of fast ice, the bulk of the sea ice is in motion, steered by ocean currents and winds. Its predominant direction of flow is clockwise. Because its motion is erratic, pressure ridges many meters high and open bodies of water, known as leads, develop. First year sea ice averages about 2 meters in thickness whereas ice that survives through the summer becomes thicker during the following winter. The minimum extent of sea ice in the Arctic is in September (Fig. 1). Sea ice suppresses the energy exchange between the ocean and the atmosphere, affects the circulation of ocean currents, and dampens wave action.[5] Permafrost and Ground Ice Permafrost is defined as earth material in which the temperature has been below 0 C for two or more years. Because it is defined only by its temperature, water is not necessary for its existence. However, most permafrost, which underlies more than 20% of the Earth’s land area, does contain ice in various amounts and forms. Ground ice occurs in the pores of the soil as lenses or veins and in large forms such as ice wedges.[6] By volume, pore ice is the largest, although ice wedges are more conspicuous. Where ice wedges are well developed, they may occupy as much as 30% of the upper 2 or 3 meters of the land surface. Their surface expression is distinctive and takes the form of icewedge polygons. In the Arctic, permafrost is continuous (Fig. 1) except beneath those water bodies that are more than 2 meters deep and do not freeze to the bottom during winter. Permafrost is also present beneath near-shore waters off Siberia and North America. Associated with permafrost is the active layer—the portion of terrain that thaws and freezes seasonally. The active layer can vary in thickness from a few centimeters to several meters, depending mainly on vegetation cover and soil texture. THE HYDROLOGIC CYCLE IN THE ARCTIC In the above discussion, each hydrologic element was considered individually, as if it was a pool of water in storage as a gas, a liquid, or a solid. However, in reality, the hydrologic elements are interconnected and mobile, moving from one phase to another through time. The examination of water from this standpoint is best done through the concept of the hydrologic or water cycle, a cycle that is considered the most fundamental principle of hydrology.[7] The conceptual model (Fig. 3) illustrates hydrologic links among the atmosphere, the land, and the ocean.[3]

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As a concept, it can be applied to small units within the hydrosphere or large units such as the Arctic. In the Arctic, the movement of water from one phase or location is, to a large extent, dependent on freeze-thaw cycles of snow, ice, and permafrost.

ARCTIC HYDROLOGY AND THE FUTURE In recent years, research on climate change has increased dramatically. Much of it has been conducted in and about the Arctic and especially about arctic hydrology. Numerous changes in the hydrologic elements of the Arctic have been recently documented. Included are a shortening of the snow-cover season, later freeze-up and earlier breakup of river, lake, and sea ice, increased fresh-water runoff, melting glaciers, the degradation of permafrost and an increase in active-layer thickness, increased groundwater flow, decreased sea-ice extent (Fig. 1), and decreased albedo, among others. Recently (2005), Dan Endres of the National Oceanic and Atmospheric Administration, in a discussion on climate change, stated that: ‘‘Whatever is going to happen is going to happen first in the Arctic and at the fastest rate.’’[8] This is a prophecy that is especially applicable to virtually every hydrologic and ecological element in the Arctic.

REFERENCES 1. Walker, H.J. Global change and the arctic. Curr. Top. Wetland Biogeochem. 1998, 3, 44–60. 2. http://www.awi-bremerhaven.dc/GEO/APARD/Backgr. html (accessed November 2005). 3. Vo¨ro¨smarty, C.J.; Hinzman, L.D.; Peterson, P.J. et al. The Hydrologic Cycle and its Role in Arctic and Global Environmental Change: A Rationale and Strategy for Synthesis Study; Arctic Research Consortium of the U.S.: Fairbanks, Alaska, 2001. 4. Collier, C.G. Snow. In Encyclopedia of Hydrology and Water Resources; Herschy, R.W., Fairbridge, R.W., Eds.; Kluwer Academic Publishers: Dordrecht, 1998; 621–624. 5. Untersteiner, N. Structure and dynamics of the Arctic ocean ice cover. In The Arctic Region; Grantz, A., Johnson, L., Sweeney, J.F., Eds.; The Geological Society of America, Inc.: Boulder, Colorado, 1990; 37–51. 6. Carter, L.D.; Heginbottom, J.A.; Woo, M.-K. Arctic lowlands. In Geomorphic Systems of North America; Graf, W.L., Ed.; Geological Society of America: Boulder, CO, 1987; Centennial Special; Vol. 2, 583–628. 7. Maidment, D.R.; Ed. Handbook of Hydrology; McGraw-Hill, Inc.: New York, 1992. 8. Glick, D. Degrees of change. Nat. Conservancy 2005, 55, 40–50. 9. Walker, H.J.; Hudson, P.F. Hydrologic and geomorphic processes in the Colville river Delta, Alaska. Geomorphology 2002, 56, 291–303.

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Boron Rami Keren Volcani Center, Institute of Soil, Water and Environmental Sciences, Agricultural Research Organization (ARO), Bet-Dagan, Israel

INTRODUCTION Boric acid is moderately soluble in water. Its solubility increases markedly with temperature due to the large negative heat of dissolution. Boron is considered as a typical metalloid having properties intermediate between the metals and the electronegative non-metals. Boron has a tendency to form anionic rather than cationic complexes. Boron chemistry is of covalent B compounds and not of B3þ ions because of its very high ionization potentials. Boron has five electrons, two in the inner spherical shell (1s2), two in the outer spherical shell (2s2), and one in the dumbbell shaped shell (2p1x ).[1] In the hybrid orbital state, the three electrons in the 2s and 2p orbitals form a hybrid orbital state (2s1 2p1x 2p1y ), where each electron is alone in an orbit whose shape has both spherical and dumbbell characteristics. Each of these three orbits can hold one electron from another element to form a covalent bond between the element and B (BX3). This leaves one 2p electron orbit that can hold two electrons, which if filled would completely fill the eight electron positions (octet) associated with the second electron shell around B. BX3 compounds behave as acceptor Lewis acids toward many Lewis bases such as amines and phosphines. The acceptance of two electrons from a Lewis base completes the octet of electrons around B. Boron also completes its octet by forming both anionic and cationic complexes.[1] Therefore, tri-coordinate B compounds have strong electron-acceptor properties and may form tetra-coordinate B structures. The charge in tetra-coordinate derivatives may range from negative to neutral and positive, depending upon the nature of the ligands. For the unshared oxygen atoms bound to B, they are, probably, always OH groups. Thus, in accordance with the electron configuration of B, boric acid acts as a weak Lewis acid: BðOHÞ3 þ 2H2 O ¼ BðOHÞ
4 þ H3 Oþ

ð1Þ

The formation of borate ion is spontaneous. The first hydrolysis constant of B(OH)3, Kh1, is 5.8  10
10 at 20 C,[2] and the other Kh2 and Kh3 values are 5.0  10
13 and 5.0  10
14, respectively.[3] A dissociation beyond B(OH)
4 is not necessary to explain 70

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the experimental data, at least below pH 13.[4,5] Boron species other than B(OH)3 and B(OH)
4 , however, can be ignored in soils for most practical purposes. The first hydrolysis constant of B(OH)3 varies with temperature from 3.646  10
10 at 178 K to 7.865  10
10 at 318 K.[6] Both B(OH)3 and BðOHÞ
4 ion species are essentially monomeric in aqueous media at low B concentration (0.025 mol L
1). However, at high B concentration, polyborate ions exist in appreciable amount.[7] The equilibria between boric acid, monoborate ions, and polyborate ions in aqueous solution are rapidly reversible. In aqueous solution, most of the polyanions are unstable relative to their monomeric [8] Results of nuclear forms B(OH)3 and B(OH)
4. [9] magnetic resonance and Raman spectrometry[10] lead to the conclusion that B(OH)3 has a trigonal-planar structure, whereas the B(OH)
4 ion in aqueous solution has a tetrahedral structure. This difference in structure can lead to differences in the affinity of clay for these two B species.

BORON–SOIL INTERACTION The elemental form of boron (B) is unstable in nature and found combined with oxygen in a wide variety of hydrated alkali and alkaline earth-borate salts and borosilicates as tourmaline. The total B content in soils, however, has little bearing on the status of available B to plants. Boron can be specifically adsorbed by different clay minerals, hydroxy oxides of Al, Fe, and Mg, and organic matter.[11] Boron is adsorbed mainly on the particle edges of the clay minerals rather than the planar surfaces. The most reactive surface functional group on the edge surface is the hydroxyl exposed on the outer periphery of the clay mineral. This functional group is associated with two types of sites that are available for adsorption: Al(III) and Si(IV), which are located on the octahedral and tetrahedral sheets, respectively. The hydroxyl group associated with this site can form an inner sphere surface complex with a proton at low pH values or with a hydroxyl at high pH values. The B adsorption process can be explained by the surface complexation approach, in which the

surface is considered as a ligand.[12] Such specific adsorption, which occurs irrespective of the sign of the net surface charge, can occur theoretically for any species capable of coordination with the surface metal ions. However, because oxygen is the ligand commonly coordinated to the metal ions in clay minerals, the B species B(OH)3 and B(OH)
4 are particularly involved in such reactions. Possible surface complex configurations for B—broken edges of clay minerals— were suggested by Keren, Grossl, and Sparks.[12] Keren and Bingham[11] reviewed the factors that affect the adsorption and desorption of B by soil constituents and the mechanisms of adsorption. Soil pH is one of the most important factors affecting B adsorption. Increasing pH enhances B adsorption on clay minerals, hydroxy-Al and soils, showing a maximum in the alkaline pH range (Fig. 1). The response of B adsorption on clays to variations in pH can be explained as follows. Below pH 7, B(OH)3 predominates and since the affinity of the clay for this species is relatively low, the amount of adsorp
tion is small. Both B(OH)
4 and OH concentrations are low at this pH; thus, their contribution to total B adsorption is small despite their relatively strong affinity for the clay. As the pH is increased to about 9, the B(OH)
4 concentration increases rapidly. Since the OH
concentration is still low relative to the B concentration, the amount of adsorbed B increases rapidly. Further increases in pH result in an enhanced OH
concentration relative to B(OH)
4 , and B adsorption decreases rapidly due to the competition by OH
at the adsorption sites. Adsorption models for soils, clays, aluminum oxide, and iron oxide minerals have been derived by various workers.[13–17] In assessing B concentration in irrigation water, however, the physicochemical characteristics of the

Fig. 1 Boron adsorption isotherms for a soil as a function of solution B concentration and pH. Bold lines—calculated values. Source: From Ref.[28].

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soil must be taken into consideration because of the interaction between B and soil. Boron sorption and desorption from soil adsorption sites regulate the B concentration in soil solution depending on the changes in solution B concentration and the affinity of soil for B. Thus, adsorbed B may buffer fluctuations in solution B concentration, and B concentration in soil solution may change insignificantly by changing the soil-water content (Fig. 2). When irrigation with water high in B is planned, special attention should be paid to this interaction because of the narrow difference between levels causing deficiency and toxicity symptoms in plants.

BORON–PLANT INTERACTION Boron is an essential micronutrient element required for growth and development of plants. Many of the experimental data suggest that B uptake in plants is probably a passive process. There are clear evidences, however, that B uptake differs among species.[18] Several mechanisms have been postulated to explain this apparent paradox.[18–20] Boron deficiency in plants initially affects meristematic tissues, reducing or terminating growth of root and

Fig. 2 Boron concentration in soil solution as a function of solution-to-soil ratio for a given total amount of B. (A) No interaction between B and soil, (B) Boron adsorption account for. Source: From Ref.[28].

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shoot apices, sugar transport, cell-wall synthesis and structure, carbohydrate metabolism and many biochemical reactions.[21,22] Tissue B concentrations associated with the appearance of vegetative deficiency symptoms have been identified in many crop species. It is essential to remember that for B, as for phosphorus and several other plant nutrient elements, deficiency may be present long before visual deficiency symptoms occur. Excess and toxicity of boron in soils of semi-arid and arid areas are more of a problem than deficiency. Boron toxicity occurs in these areas either due to high levels of B in soils or due to additions of B in irrigation water. A summary of B tolerance data based upon plant response to soluble B is given by Maas.[23] Bingham et al.[24] showed that yield decrease of some crops (wheat, barley, and sorghum) due to B toxicity could be estimated by using a model for salinity response, suggested by Maas and Hoffman.[25] There is a relatively small difference between the B concentration in soil solution causing deficiency and that resulting in toxicity symptoms in plants.[11] A consequence of this narrow difference is the difficulty posed in management of appropriate B levels in soil solution. The suitability of irrigation water has been evaluated on the basis of criteria that determine the potential of the water to cause plant injury and yield reduction. In assessing the B in irrigation water, however, the physicochemical characteristics of the soil must be taken into consideration because the uptake by plants is dependent only on B activity in soil

Fig. 3 Relationship between B content in wheat shoot and the amount of B added to soil, for three ratios of soil–sand mixtures. Source: From Ref.[26].

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Boron

solution.[26,27] Boron uptake by plants grown in a soil of low-clay content is significantly greater than that of plants grown in a soil of high-clay content at the same given level of added B (Fig. 3). This knowledge may improve the efficacy of using water of different qualities, whereby water with relatively high B levels could be used to irrigate B-sensitive crops in soils that show a high affinity to B. Such water can be used for irrigation as long as the equilibrium B concentration in soil solution is below the toxic concentration threshold (the maximum permissible concentration for a given crop species that does not reduce yield or lead to injury symptoms) for the irrigated crop. The existing criteria for irrigation water, however, make no reference to differences in soil type.

REFERENCES 1. Cotton, F.A.; Wilkinson, G. Advanced Inorganic Chemistry, 5th Ed.; Wiley & Sons: New York, NY, 1988. 2. Owen, B.B. The dissociation constant of boric acid from 10 to 50 . J. Am. Chem. Soc. 1934, 56, 1695–1697. 3. Konopik, N.; Leberl, O. Colorimetric determination of PH in the range of 10 to 15. Monatsh 1949, 80, 420–429. 4. Ingri, N. Equilibrium studies of the polyanions containing BIII, SiIV, GeIV and VV. Svensk. Kem. Tidskr. 1963, 75, 199–230. 5. Mesmer, R.E.; Baes, C.F., Jr.; Sweeton, F.H. Acidity measurements at elevated temperature. VI. Boric acid equilibria. Inorg. Chem. 1972, 11, 537–543. 6. Owen, B.B.; King, E.J. The effect of sodium chloride upon the ionozation of boric acid at various temperatures. J. Am. Chem. Soc. 1943, 65, 1612–1620. 7. Adams, R.M. Boron, Metallo-Boron Compounds and Boranes; John Wiley & Sons: New York, 1964. 8. Onak, T.P.; Landesman, H.; Williams, R.E.; Shapiro, I. The BII nuclear magnetic resonance chemical shifts and spin coupling values for various compounds. J. Phys. Chem. 1959, 63, 1533–1535. 9. Good, C.D.; Ritter, D.M. Alkenylboranes: II. Improved preperative methods and new observations on methylvinylboranes. J. Am. Chem. Soc. 1962, 84, 1162–1166. 10. Servoss, R.R.; Clark, H.M. Vibrational spectra of normal and isotopically labeled boric acid. J. Chem. Phys. 1957, 26, 1175–1178. 11. Keren, R.; Bingham, F.T. Boron in water, soil and plants. Adv. Soil Sci. 1985, 1, 229–276. 12. Keren, R.; Grossl, P.R.; Sparks, D.L. Equilibrium and kinetics of borate adsorption–desorption on pyrophyllite in aqueous suspensions. Soil Sci. Soc. Am. J. 1994, 58, 1116–1122. 13. Keren, R.; Gast, R.G.; Bar-Yosef, B. pH-dependent boron adsorption by na-montmorillonite. Soil Sci. Soc. Am. J. 1981, 45, 45–48. 14. Keren, R.; Gast, R.G. pH dependent boron adsorption by montmorillonite hydroxy-aluminum complexes. Soil Sci. Soc. Am. J. 1983, 47, 1116–1121.

15. Goldberg, S.; Glaubig, R.A. Boron adsorption on aluminum and iron oxide minerals. Soil Sci. Soc. Am. J. 1985, 49, 1374–1379. 16. Goldberg, S.; Glaubig, R.A. Boron adsorption on California soils. Soil Sci. Soc. Am. J. 1986, 50, 1173–1176. 17. Goldberg, S.; Forster, H.S.; Heick, E.L. Boron adsorption mechanisms on oxides, clay minerals and soils inferred from ionic strength effects. Soil Sci. Soc. Am. J. 1993, 57, 704–708. 18. Nable, R.O. Effects of B toxicity amongst several barley wheat cultivars: a preliminary examination of the resistance mechanism. Plant Soil 1988, 112, 45–52. 19. Nable, R.O.; Lance, R.C.M.; Cartwright, B. Uptake of boron and silicon by barley genotypes with differing susceptibilities to boron toxicity. Ann. Bot. 1990, 66, 83–90. 20. Brown, P.H.; Hu, H. Boron uptake by sunflower, squash and cultured tobacco cells. Physiol. Plant 1994, 91, 435–441. 21. Loomis, W.D.; Durst, R.W. Chemistry and biology of boron. BioFactors 1992, 3, 229–239.

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22. Marschner, H. Mineral Nutrition of Higher Plants, 2nd Ed.; Academic Press: London, 1995. 23. Maas, E.V. Salt tolerance of plants. In Handbook of Plant Science in Agriculture; Christie, B.R., Ed.; CRC Press, Inc.: Cleveland, Ohio, 1984. 24. Bingham, F.T.; Strong, J.E.; Rhoades, J.D.; Keren, R. An application of the Maas–Hoffman salinity response model for boron toxicity. Soil Sci. Soc. Am. J. 1985, 49, 672–674. 25. Maas, E.V.; Hoffman, G.J. Crop salt tolerance—current assessment. ASCE J. Irrig. Drainage Div. 1977, 103, 115–134. 26. Keren, R.; Bingham, F.T.; Rhoades, J.D. Effect of clay content on soil boron uptake and yield of wheat. Soil Sci. Soc. Am. J. 1985, 49, 1466–1470. 27. Keren, R.; Bingham, F.T.; Rhoades, J.D. Plant uptake of boron as affected by boron distribution between liquid and solid phases in soil. Soil Sci. Soc. Am. J. 1985, 49, 297–302. 28. Mezuman, U.; Keren, R. Boron adsorption by soils using a phenomenological adsorption equation. Soil Sci. Soc. Am. J. 1981, 45, 722–726.

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Bound Water in Soils Sally D. Logsdon National Soil Tilth Laboratory, U.S. Department of Agriculture (USDA), Ames, Iowa, U.S.A.

INTRODUCTION General reviews on bound water discuss the historical development, and show that water associated with hydrophobic surfaces is more rigid than water associated with hydrophilic surfaces. These reviews show that bound water has higher heat capacity, higher relative viscosity, reduced self-diffusion, reduced dielectric constant, and lower density than free water. This entry will emphasize the latest information on bound water in soils and clay, and what knowledge is incomplete.

BODY OF TEXT Properties of free water are discussed by Oster. Bound water has been designated by terms such as vicinal water,[1] hygroscopic water,[2] or simply water associated with interfaces[3,4] or colloids such as clays.[5] Bound water cannot be isolated to study its properties because bound water only exists as part of the system to which it is bound. Early Work on Clays Clays are the inorganic colloids in soils. Low[5] summarized earlier studies by considering the clay–water system to be near equilibrium. Then he assumed that slight changes in the partial specific property because of small additions of water would not significantly upset the equilibrium; that is, changes would be due only to the water, not to the whole system. This colloid-associated water has lower density, higher apparent mean heat capacity, and higher viscosity, higher expandability, and lower specific entropy than free water. Activation energies for ion migration were higher in bound water than free water.[6] Ion mobility was decreased when there was only one or two layers of sorbed water.[7] McBride and Baveye[8] have experimentally confirmed higher viscosity close to clay surfaces. As an example, Fig. 1 shows the decrease in change of specific heat capacity as water is added to a Na-montmorillonite.[9] Sposito and Prost[10] question the assumption of near equilibrium even at high water contents and indicate that the assumption definitely does not apply to the first layers of adsorbed water. 74

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Fig. 2 diagrams bound water clustering around the countercations associated with the 2:1 clay. Note that the cations (small circles in Fig. 2 in center of cluster) and their associated water clusters occupy sites on internal and external surfaces of the clay. The time dependence of bound water relates to the mechanism affecting motion of the water molecules. Sposito and Prost[10] discuss a V structure of water which shows vibrational motion around 10 10– 10 11 sec and a D structure of diffusional motion for timescales greater that 10 4 sec. Between these two extremes, water, as a polar molecule, can be subject to movement that orients the molecule back and forth in response to an alternating electrical field. Free water can continue to oscillate back and forth even at timescales down to around 10 12 sec, but ice only down to around 10 7 sec. The various forms of bound water can continue to oscillate between these extremes. These timescales are related to the inverse of the frequency. Relation to Dielectric Properties The frequency-dependent dielectric spectra are used to examine the frequency-dependent oscillations. The time or frequency when half of the molecules are no longer able to oscillate is called the relaxation time or frequency. At frequencies lower than the relaxation frequency, the relative dielectric constant (relative to that of a vacuum) is high, and at frequencies higher than the relaxation frequency, the dielectric constant is low. At high frequencies, free water can continue to oscillate and still has a high dielectric constant. At these same high frequencies, bound water cannot oscillate and has a lower dielectric constant. Many studies have examined the effect of bound water in lowering the dielectric constant of the system at high frequencies, but have failed to realize that the same mechanism would not extend to lower frequencies when bound water can also oscillate with a larger dielectric constant. Dielectric Determination of Soil Water Content Dielectric properties are used to determine soil water content, assuming that all of the water has a dielectric

Fig. 1 The change in specific heat capacity as water is added to the system for a sodium montmorillonite. Source: From Ref.[9].

constant around 80.[11] Many studies have data that do not fit assumed relations unless some of the water is assumed to be bound water with a lower dielectric constant than free water. Volumetric mixing models are often used to explain such data, using measured or assumed volume fractions and dielectric properties for each component of the system. Dobson, Ulaby, and Hallikainen[12] assumed that soil was made of four components: air, soil solids, free water, and bound water. They used two mixing model approaches to explain the reduction in bulk dielectric constant for soils with much bound water. Sihvola[13] discusses a variety of mixing models and their assumptions. One class are the power law mixing models, and the other class are based on modifications of the Maxwell– Wagner approach for two component systems in which one component is continuous and the other component is discontinuous. The equations can be altered depending on shape of the inclusions, multiple inclusions,

Fig. 2 Diagram of a 2:1 clay showing water molecules clustered around the countercations in interlayers and on external surfaces.

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layers around inclusions, orientation of anisotropic inclusions, which (or both) of the components have electrical conductivity, and multiple inclusions. Difficulties arise because dielectric properties vary with frequency of the alternating current and are complex numbers with real and imaginary components. Another difficulty is that soil is a multicomponent system (air, soil solids, and water with varying degrees of binding), in which more than one component may be continuous. Many have assumed that bound water has the same dielectric constant as ice, i.e., 4,[2,11,14] but Dirksen and Dasberg[15] could not fit their data unless a large value was chosen, around 30 for a Vertisol soil–water mixture or 50 for smectite clay–water mixture. Laird[16] assumed a logistic curve to describe the dielectric of bound water between the first adsorbed layer (dielectric constant of 4) and bulk or free water (dielectric constant of 80). Assuming a lower dielectric constant for bound water has not adequately explained the data in which the dielectric constant is larger than expected in soils with a lot of bound water. This has mainly been a problem at high water contents and high temperatures[15,17–19] or when the measurement frequency is lower than that from measured dielectric constants in the field,[20] or with long cables,[19] or for a capacitance or other system that used a lower measurement frequency.[11,21]

Dielectric Spectra of Clay–Water Systems Although several studies have examined dielectric spectra in a qualitative sense, one of the first to mechanistically examine the imaginary dielectric temperature spectra was Calvet.[22] He examined humidified smectites saturated with various divalent cations. He divided the spectra into two electrical conductivity components and two relaxation components, each of which had a temperature dependence. Ishida and Makino[23,24] examined Na-saturated clays at a range of frequencies, but only one temperature. For the slurries and gels of smectites, they identified three relaxation components, after subtracting the electrical conductivity component. Dudley et al.[25] likewise measured slurries of smectite but included three temperatures. They subtracted the electrode polarization component and observed an electrical conductivity component and a relaxation component. One of the difficulties in these studies is attributing all the observed relaxations to bound water alone. Bound water is part of a system in which polarization can be induced in macromolecules and colloids because of migration of charge carriers either in the double layer of dispersed colloids[26] or to proton hopping on external or internal surfaces of humidified soil.[27]

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The frequency range for the polarization as a result of migration of charge carriers may overlap with the frequency range for relaxation as a result of oscillation of bound water.

CONCLUSION Water bound to colloids has been shown to be more rigid than free water but not as rigid as ice. The properties of bound water are determined only indirectly because bound water can only be measured as part of the system to which it is bound. Early work emphasized properties of water bound to the inorganic colloids in soils, i.e., clays. Later work on bound water developed because the bulk dielectric constant measured at high frequencies showed reduced dielectric constants for soils high in montmorillonite clays. The reduced dielectric constants for bound water occur because of the increased rigidity, which decreases the ability of the water molecules to oscillate in an alternating electrical field. Currently, there is increased interest in examining soil dielectric properties across a range of frequencies. These dielectric spectra show the soil–water system shift from freely oscillating at low frequencies to no oscillations at high frequencies. Although this shift is sharp for free water, the dielectric spectra for soils and clays show a broad frequency range of gradual change. This suggests that multiple mechanisms are operating. Not only is water oscillating, but also polarities develop in the bound water–colloid system at lower frequencies. The dielectric properties of bound water are difficult to separate from the induced polarities of the bound water–colloid system. Careful experimental determination of dielectric spectra combined with model development will enable us to separate these components and utilize the dielectric spectra as a characterization tool for soil–water systems.

ARTICLES OF FURTHER INTEREST Water Properties, p. 1324. Soils: Hygroscopic Water Content, p. 1136.

REFERENCES 1. Etzler, F.M. A statistical thermodynamic model for water near solid interfaces. J. Colloid Interface Sci. 1983, 92 (1), 43–56. 2. Hilhorst, M.A.; Dirksen, C.; Kampers, F.W.H.; Feddes, R.A. Dielectric relaxation of bound water versus soil matric pressure. Soil Sci. Soc. Am. J. 2001, 65 (2), 311–314.

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3. Derjaguin, B.V.; Churaev, N.V. Properties of water layers adjacent to interfaces. In Fluid Inter-facial Phenomena; Croxton, C.A., Ed.; John Wiley and Sons: Chichester, 1986; 663–738. 4. Vogler, E.A. Structure and reactivity of water at biomaterial surfaces. Adv. Colloid Interface Sci. 1998, 74, 69–117. 5. Low, P.F. Nature and properties of water in montmorillonite–water systems. Soil Sci. Soc. Am. J. 1979, 43 (65), 648–651. 6. Low, P.F. The apparent mobilities of exchangeable alkali metal cations in bentonite–water systems. Soil Sci. Soc. Am. Proc. 1958, 22, 395–398. 7. Kemper, W.D.; Maasland, D.E.L.; Porter, L.K. Mobility of water adjacent to mineral surfaces. Soil Sci. Soc. Am. Proc. 1964, 28, 164–167. 8. McBride, M.B.; Baveye, P. Mobility of anion spin probes in hectorite gels: viscosity of surficial water. Soil Sci. Soc. Am. J. 1995, 59 (2), 388–394. 9. Oster, J.D.; Low, P.F. Heat capacities of clay and clay–water mixtures. Soil Sci. Am. Proc. 1964, 28, 605–609. 10. Sposito, G.; Prost, R. Structure of water adsorbed on smectites. Chem. Rev. 1982, 82 (6), 553–573. 11. Topp, G.C.; Davis, J.L.; Annan, A.P. Electromagnetic determination of soil water content: measurements in coaxial transmission lines. Water Resour. Res. 1980, 16 (3), 574–582. 12. Dobson, M.C.; Ulaby, F.T.; Hallikainen, M.T. Microwave dielectric behavior of wet soil. Part II. Dielectric mixing models. IEEE Trans. Geosci. Remote Sens. 1985, GE-23 (1), 35–46. 13. Sihvola, A. Electromagnetic mixing formulas and applications; The Institute of Electrical Engineers: London, 1999. 14. Wang, J.R.; Schmugge, T.J. An empirical model for the complex dielectric permittivity of soils as a function of water content. IEEE Trans. Geosci. Remote Sens. 1980, GE-18, 288–295. 15. Dirksen, C.; Dasberg, S. Improved calibration of time domain reflectometry. Soil Sci. Soc. Am. J. 1993, 57 (3), 660–667. 16. Laird, D.A. Model for crystalline swelling of 2:1 phyllosilicates. Clays Clay Miner. 1996, 44 (4), 553–559. 17. Roth, C.H.; Malicki, M.A.; Plagge, R. Empirical evaluation of the relationship between soil dielectric constant and volumetric water content as the basis for calibrating soil moisture measurements. J. Soil Sci. 1992, 43 (1), 1–13. 18. Wraith, J.M.; Or, D. Temperature effects on soil bulk dielectric permittivity measured by time domain reflectometry: experimental evidence. Water Resour. Res. 1999, 35 (2), 361–369. 19. Logsdon, S.D. Effect of cable length on time domain reflectometry calibration for high surface area soils. Soil Sci. Soc. Am. J. 2000, 64 (1), 54–61. 20. Bridge, B.J.; Sabburg, J.; Habash, K.O.; Ball, J.A.R.; Hancock, N.H. The dielectric behaviour of clay soils and its application to time domain reflectometry. Aust. J. Soil Res. 1996, 34, 825–835.

21. West, L.J.; Handley, K.; Huang, Y.; Pokar, M. Radar frequency dielectric dispersion in sandstone: implications for determination of moisture and clay content. Water Resour. Res. 2003, 39 (2), 1026, doi:10.1029/ 2001WR000923. 22. Calvet, R. Dielectric properties of montmorillonites saturated by divalent cations. Clays Clay Miner. 1975, 23, 257–265. 23. Ishida, T.; Makino, T. Effects of pH on dielectric relaxation of montmorillonite, allophane, and imogolite suspensions. J. Colloid Interface Sci. 1999, 212, 152–161. 24. Ishida, T.; Makino, T.; Wang, C. Dielectric-relaxation spectroscopy of kaolinite, montmorillonite, allophane,

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and imogolite under moist conditions. Clays Clay Miner. 2000, 48 (1), 75–84. 25. Dudley, L.M.; Bialkowski, S.; Or, D.; Junkermeier, C. Low frequency impedance behavior of montmorillonite suspensions: polarization mechanisms in the low frequency domain. Soil Sci. Soc. Am. J. 2003, 67 (2), 518–526. 26. Schwartz, G. A theory of the low-frequency dielectric dispersion of colloid particles in electrolyte solution. J. Phys. Chem. 1962, 66, 2636–2642. 27. Anis, M.K.; Jonscher, A.K. Frequency and time-domain measurements on humid sand and soil. J. Mater. Sci. 1993, 28, 3626–3634.

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Brahmaputra Basin Vijay P. Singh Department of Biological and Agricultural Engineering, Texas A&M University, College Station, Texas, U.S.A.

Sharad K. Jain National Institute of Hydrology, Roorkee, Uttarakhand, India

INTRODUCTION According to the Hindu belief, Brahmaputra means ‘‘son of the Creator, Lord Brahma.’’ The source of this river lies in the Kanglung Kang Glacier in the southwestern part of the Tibetan plateau at an elevation of 5300 m (82
100 E, 30
300 N) near Konggyu Tso Lake. The Brahmaputra River then traverses a distance of 2880 km before joining the Bay of Bengal (see Fig. 1). The basin is of irregular shape: the maximum eastwest length is 1540 km and the maximum north-south width is 682 km. The basin lies between 23
N and 32
N latitude and 82
E and 97
500 E longitude. The characteristics of the Brahmaputra River in different parts of the basin are given in Table 1.

TOPOGRAPHY The course of the Brahmaputra River can be divided into three reaches: upper, middle, and lower. In the upper reach, it flows for 1625 km mainly to the east, almost parallel and north to the Himalayas. Here, the river is known as Tsangpo, or ‘‘the purifier.’’ Near the eastern end of the basin, Tsangpo takes a hairpin bend and enters India at Kobo in Arunachal Pradesh. Here, the river is first known as Siang and then Dihang. At Pasighat, Dihang’s maximum discharge is 29,643 m3/s. From Pasighat up to the Indo-Bangladesh border (640 km), the river passes through alluvial plains. At Kobo in Assam, Dihang meets two major tributaries, Dibong and Lohit, and the combined river is known as Brahmaputra. This reach contains Majuli, which is the biggest river island (area 900 km2) in the world. In the lower reach from the confluence of the Tista River near Bahadurabad to Goalundo, Brahmaputra gets a new name: Jamuna. At Goalundo, Jamuna joins another major river, Ganga-Padma, and the combined river flows as Ganga-Padma for 80 km. Near Rajabari, another major tributary, Meghna, joins and the combined river flows for 32 km as Meghna River. A little downstream, Meghna trifurcates and outfalls into the Bay of Bengal forming broad estuaries. The main tributaries of Brahmaputra are: Nayang Chu, Yarling 78

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Chu, Subansiri, Manas, Jia-Bharali, Sankosh, Teesta, and Meghna.

HYDROMETEOROLOGY OF THE BASIN The Himalayas divide the basin into two distinct climatic zones (Fig. 2). The northern part, being on the rain shadow side of the monsoonal system, in the Tibetan plateau is cold, dry, and arid. The southern part, being on the ‘‘wet’’ side of the monsoonal system, is relatively warm and humid, and experiences high rainfall. The western part of the Brahmaputra valley experiences hot summer in April and May. The Monsoon is the most important factor responsible for seasonal weather variation. In summer, warm moist air moves northward bringing rain, especially on the south slopes of the Himalayas. About 65–80% of the average annual rainfall takes place in this period. Rainfall during July and August is the highest. The wettest place on the Earth, Mausynram, which receives 11,872 mm of rainfall in an average year, is located in the Khasi hills. The Tibetan plateau and higher reaches of mountain ranges above 3000 m receive snowfall during winters. In winter, cold dry airflow from central Eurasia flows southward and is warmed as it passes over the Himalayas, bringing generally pleasant conditions. The average annual rainfall in the basin varies from less than 400 mm on the north side of the Himalayas to more than 6000 mm on the south side of the Himalayas. The mean annual rainfall over the catchment, excluding the Tibetan part, is around 2300 mm. The variability of annual rainfall is 20%. The highest recorded 1-hr rainfall is 97.5 mm at Saralpara, Tikpai sub-basin.

DISCHARGE Brahmaputra is a perennial river. Typical annual discharge hydrographs of the river for five stations are shown in Fig. 3. The maximum and minimum discharges of the Brahmaputra River at different

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Fig. 1 The Brahmaputra Basin. Source: From Ref.[1].

sites are furnished in Table 2. The recorded maximum discharge of major tributaries is given in Table 3. At Bahadurabad in Bangladesh, the average discharge during the monsoon period is 31,850 m3/s. Flooding is an acute problem in the Brahmaputra valley. On average, flooding affects an area of 1 million ha in Assam. The main reasons for frequent floods are the narrowness of the valley, high rainfall, and encroachment of flood plains. Floods have frequently created havoc in the region; the floods of 1987, 1988, 1992, and 1995 were particularly severe. The maximum observed flood peak at Pandu was 72794 m3/s in 1962. In the basin, flood-prone area is 3.15 million ha. The flood management works so far adopted have been mainly embankments, drainage channels, town protection works, and erosion control works. The length of embankments was increased from 6000 km in 1954 to 15,675 km in 1990 besides improvement of 30,857 km of drainage channels. These measures have

afforded a reasonable flood protection to an area of 1.635 million ha.

CROPPING PATTERN The major crops in the basin are rice, wheat, maize, and other cereals, pulses, oilseeds, jute, sugarcane, and potato. The horticultural crops are fruit crops, plantation crops, tuber crops, and spices. Commercial cultivation of ornamentals has begun recently. Rice is by far the most important crop of Assam. Winter or Kharif rice (known as ‘‘Sali’’ ) occupies 70% of the area under rice and is cultivated from June to December. ‘‘Ahu’’ or autumn rice occupies 25% of the area under rice, the crop season is from March to July. Pulses are cultivated primarily during the Rabi season. Jute is the primary fiber crop. Important horticultural and plantation crops include orange, banana,

Table 1 Salient features of the Brahmaputra Basin Location

Catchment area in that part (km2), % of total area

Upper Reach: Source to Indo-China border, Tibet (China)

293,000 (50.52%)

Middle Reach:

195,000 (33.62%)

(i) Indo-China border to Kobo, Arunachal Pradesh (AP) (ii) Kobo to Indo-Bangladesh border, Assam Bhutan Lower Reach in Bangladesh: (i) (ii) (iii) (iv)

First 60 km from India Border Next 100 km Next 90 km Rest up to sea

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Average gradient

Topography

1:385

High Tibetan Plateau

1:515

Himalayan mountain region Brahmaputra Valley

1:690 45,000 (7.76%)

—

47,000 [up to confluence with Ganga (8.10%)]

Himalayas Plains including coastal belt

1:11,40 1:12,60 1:27,00 1:37,00

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Brahmaputra Basin

Fig. 2 Climatic zones. Source: From Ref.[1].

pineapple, papaya, lemons, ginger, turmeric, sweet potatoes, areca nut, coconut, and vegetables.

are: Brahmaputra is a geologically young river, the catchment area falls in a seismic belt, rainfall intensity is high and slopes are steep. Besides, ‘‘Jhumming’’ (shifting cultivation) greatly increases sediment load.

SEDIMENT TRANSPORT The sediment yield of the Brahmaputra River is very high; the yield at Guwahati is 755 m3 per km2 of catchment area; the maximum and minimum sediment concentrations are 679 and 36 ppm, respectively. The average annual sediment load at Bahadurabad in Bangladesh is 735 million metric tons. Here, 12.5% sediment is coarse, 14.2% medium, and 73.3% fine. Downstream of Pasighat, Brahmaputra is highly braided. The width of the river from Kobo to the Indo-Bangladesh border varies from 6 to 18 km except in nine places where it traverses through deep and narrow throats. The main reasons of high sediment load

HYDROPOWER The hydropower potential of the Brahmaputra basin in India is 34,920 MW at 60% load factor. But so far, less than 2% has been developed. Completed projects include Loktak, Kyrdem Kulai, Umtru-Umaim Stage I, II, and III. The dams under construction include Kapili Stage I and II, Lower Kapili, Thoubal, and Gumti. Three reasons for poor development of hydropower in the region are: difficult terrain, lack of demand, and non-availability of bulk transmission corridors. Major identified multipurpose projects are

Fig. 3 Discharge hydrographs of the Brahmaputra River at different sites. Source: From Ref.[1].

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flora, fauna, and ecosystem. The Great Indian One Horn Rhino is found here.

Table 2 Recorded maximum discharge of the Brahmaputra River at some sites Catchment area (km2)

Recorded maximum discharge (m3/s)

88,620

3,380

186,730

10,200

Pasighat (AP, India)

249,000

29,640

Guwahati (Assam, India)

424,100

72,794

Site (State, Country) Shigatse (Tibet, China) Tseladzong (Tibet, China)

Table 3 Observed maximum discharge of major tributaries of Brahmaputra Tributary

Gauging site

Maximum discharge (m3/s)

Subansiri

Chawidhowghat

18,799

Jia-Bharali

N.T. Road

9,939

Manas

Mathanguri

10,842

Dibong

Jiagaon

11,205

Lohit

Digarughat

12,350

WATER QUALITY Brahmaputra flow contains very high amount of sediments. Surface suspended sediments range from fine sand to clay, the size fraction greater than 12 mm constituting an important size population. The groundwater is generally mildly alkaline with a pH value ranging from 6.5 to 8.5; total dissolved solids are low. The chloride (10–40 ppm) and bicarbonate (50– 350 ppm) values are quite low. The iron content ranges from a fraction to as high as 50 ppm. At greater depths, groundwater is free from much of iron. The total hardness varies as CaCO3 generally varies from 50 to 300 ppm. In some places, good quality water is available at 14 –30 m depth.

CONCLUSIONS Dihang group of dams (13,250 MW), Subansiri group (6000 MW) and Tipaimukh (1500 MW). These projects can also provide other significant benefits.

The Brahmaputra Basin is unique with a wide variety of climate, topography, and ecology. It is hoped that its immense water resources will help in overall development of the basin.

FLORA AND FAUNA REFERENCE The forests of Brahmaputra Basin contain a great diversity of flora and fauna. Five major groups of forests in Assam are: (1) Tropical Wet Evergreen; (2) Tropical Semi-evergreen; (3) Tropical Moist Deciduous; (4) Littoral and Swamp; and (5) Tropical Dry Deciduous. The Brahmaputra valley harbors five big mammals—Rhino, Tiger, Wild Buffalo, Gangetic Dolphin, and Elephant. Famous national parks are: Kaziranga, Manas, Nameri, Dibru-saikhowa, and Orang. The Kaziranga National Park (KNP) is a place of international importance with its mega-diversity in

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1. Singh, V.P.; Sharma, N.; Ojha, C.S.P.; Eds. The Brahmaputra Basin Water Resources; Kluwer Academic Publishers: Dordrecht, 2004.

BIBLIOGRAPHY 1. Goswami, D.C. Brahmaputra River, Assam, India: physiography, basin denuadation, and channel aggradation. Water Resour. Res. 1985, 21 (7), 959–978.

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Canal Automation Albert J. Clemmens U.S. Water Conservation Laboratory, Agricultural Research Service (USDA-ARS), U.S. Department of Agriculture, Phoenix, Arizona, U.S.A.

INTRODUCTION Canal automation refers to a wide variety of hydraulic structures, mechanical and electronic hardware, communications, and software used to improve the operation of canals that transmit water and deliver it to users. Early canal automation consisted of hydro/ mechanical devices used to adjust a single canal gate, with the intent of controlling the adjacent water level or flow rate. These local devices evolved over time to include mechanical/electric controllers and finally to electronic control, although many hydro/mechanical gates are still used successfully. A major shift in canal automation resulted from the use of radio or hardwire communication to control all canal structures from a single location. Today, commercially available supervisory control and data acquisition (SCADA) systems are used for remote, manual supervisory control of canals. The use of a centralized control station (particularly SCADA systems) has also led to the use of computers for the automatic remote control of entire canals. A variety of devices, methods, and control algorithms have been developed for canal automation. These are summarized in Refs.[1,2]. BACKGROUND The objective of canal operations is to deliver a certain rate or volume of water to a particular location, for example to a reservoir, to a farm canal, etc. Canals differ radically in operation from pressurized pipelines, where users simply open an outlet to receive water. If an outlet to a canal is opened, the flow generally causes the water level (pressure or head) to drop, but only in the vicinity of the outlet. That pressure drop moves upstream only gradually and may never reach the upstream source of the canal. The increase in demand can literally empty the canal. A canal can operate as a demand system only if there is sufficient storage within the canal to handle immediate changes in demand. Even so, if an increase in demand is not matched by an increase in the canal inflow, canal volume and water levels will drop. More often, demand changes are prearranged or scheduled. Check structures are used in canals to provide a higher head on outlet structures (e.g., users’ delivery 82

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gates), and a head that is independent of canal flow rate. These check structures usually consist of a series of gates and/or weirs (Fig. 1). Methods to control the flow rates through a canal outlet usually consist of either an automatic flow-rate-controlled outlet (discussed later) or a manually controlled outlet with (manual or automatic) control of the water level upstream from the outlet structure, which if held constant usually provides constant flow (Fig. 2). The later method is the most common approach. The canal section between two check structures is often called a canal pool. Automatic control methods differ in where within the canal pool the water level is to be controlled, at the upstream end, at the downstream end, or some average value (and associated pool volume). Even when outlet devices have some automatic controls, they generally only maintain constant (or near constant) flow within a range of upstream water levels.

FLOW-RATE CONTROL Flow-rate control is most frequently applied to the head of a canal. (The outlet of one canal is the head of another.) Automatic control of flow rate at a canal outlet (or headgate) has been accomplished primarily by two methods; hydro/mechanical flow-rate control devices and mechanical/electric or electronic feedback control from a flow-measurement device, where the gate itself can serve as the measuring device. If the measurement device is a weir or flume, then a constant water-level device (discussed in the next section) can be used to adjust the outlet to maintain constant flow. Flow-rate control at canal check structures is also used with some volume and downstream-water-level controllers. However, this assumes that mismatches between inflow and outflow will be adjusted by other control actions upstream. Otherwise, flow-rate control is not sustainable.

UPSTREAM WATER-LEVEL CONTROL The most common method of manual canal control is upstream water-level control, where check gates are

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Fig. 1 Check structure with motorized check gates and a manual outlet gate shown as part of SCADA automatic control screen.

adjusted to maintain a constant water level on the upstream side of each check structure. Canal inflow is set to match the demand, usually manually. Upstream flow changes are automatically passed through each check structure as it maintains its upstream level. However, upstream water-level control will cause all

Fig. 2 Profile drawing of canal with check gates and outlets.

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errors in canal inflow and outflow to move to the downstream end of the canal, causing either shortages or surpluses there, regardless of the type of automatic control. In addition, a series of automatic upstream water-level controllers can, if not properly set or adjusted, cause the flow rate at the downstream end of the canal to oscillate. A variety of automatic methods have also been developed for upstream control. The simplest is a duckbill weir, which is a very long fixed (no moving parts) weir, where the change in water level for a large change in flow is very small (a decrement). Neyrpic gates use a float on the upstream side of the gate to adjust gate position to maintain a constant upstream level, again with a decrement (Fig. 3). Several other hydro/mechanical gates have also been used, e.g., controlled leak gates. More details and references can be found in Ref.[2]. Electrical/mechanical devices have also been used to maintain constant upstream water levels.[3] In general, these have not proven to be reliable and have been essentially replaced with electronic

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Canal Automation

Fig. 3 Amil gate for constant upstream level control.

devices. Programmable logic controllers (PLCs) and remote terminal units (RTUs) have been used to maintain constant upstream levels locally at the structure with PI or PID type logic (proportional, integral, derivative). Such control can usually be conveniently programmed into SCADA software for remote operation.

DOWNSTREAM WATER-LEVEL CONTROL Under downstream control, the controller adjusts the check gate to maintain the water-level downstream from the check structure. It is far easier to control the level immediately below the check structure; however, in general outlets are located at the downstream end of each canal pool, making the level to be controlled far from the check structure. This complicates the control and makes downstream control difficult to apply without a thorough control-engineering approach. Early attempts at downstream water-level control adjusted each check structure based on one downstream water level[3]—as a series of local controllers (Fig. 4). This has proven not to be very effective,

and a more centralized approach can dramatically improve control.[4]

CONSTANT VOLUME CONTROL Canal water levels and volumes are related. If precise control of water levels is not critical, it is sometimes easier to control the canal pool volumes. Volume control methods measure one or more water levels and convert this to a pool volume. When the pool volume deviates from a target, a volume error is determined, and an adjustment is made to the pool inflow rate. This change in pool inflow can be computed pool by pool with simple logic (such as Bival[1]) or can be determined from a centralized perspective (such as Dynamic Regulation[1]). The rate of change of pool volume can also be used to determine the difference in pool inflow and outflow, and pool inflow volume can then be adjusted to bring inflow into balance with outflow. Target pool volumes can also be varied to provide more balanced control of the canal, e.g., all pools reduced in volume in the response to canal inflow limitations. Volume control is very effective for the control of pools with pumped outlets that are not very sensitive to level. This control method is not as effective for gravity outlets, unless the outlets themselves have automatic controls.

ROUTING DEMAND CHANGES (GATE STROKING)

Fig. 4 Schematic drawing of downstream water-level control for a single canal pool.

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As discussed above, demand changes in a canal cannot be handled strictly by feedback because downstream water-level response may never reach the upstream end of the canal. Thus, most canal flow changes are prescheduled. Flow changes made at the head of a canal arrive at downstream outlets at some later time.

Knowledge of this time delay allows operators to schedule a change in flow at the canal headgate so that the flow change will arrive at the outlet gate at the desired time. Unfortunately, the sudden flow change made upstream arrives only gradually at the downstream checks, making the exact timing difficult to predict. A variety of schemes have been developed to compute these flow change schedules automatically. Some use numerical methods to solve the governing equations of flow. These have proven to be unreliable and difficult to implement. More simple, volume-based procedures have proven more effective.

CENTRALIZED CONTROL Remote monitoring of a canal, or a canal network, from a central location allows an operator to provide more timely control at check gates than if that control were done by the same individual locally, traveling from check to check. Supervisory control and data acquisition systems provide automatic data collection (including communication with remote sites), display, and archiving. In addition, all of the other automatic

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control feature discussed earlier can be implemented from a central control station. This has some advantages in terms of reliability, accountability, and safety. Some functions can be performed locally, but in general control is improved if they are based on operations from a centralized perspective.

REFERENCES 1. Malaterre, P.-O.; Rogers, D.C.; Schuurmans, J. Classification of canal control algorithms. J. Irrig. Drain. Eng. 1998, 124 (1), 3–10. 2. Rogers, D.C.; Goussard, J. Canal control algorithms currently in use. J. Irrig. Drain. Eng. 1998, 124 (1), 11–15. 3. Rogers, D.C.; Ehler, D.G.; Falvey, H.T.; Serfozo, E.A.; Voorheis, P.; Johansen, R.P.; Arrington, R.M.; Rossi, L.J. Canal Systems Automation Manual; U.S. Bureau of Reclamation: Denver, CO, 1995; Vol. 2. 4. Clemmens, A.J.; Anderson, S.S.; Eds. Modernization of Irrigation Water Delivery Systems, Proc. USCID Workshop, Scottsdale, AZ, Oct 17–21, 1999; USCID: Denver, CO, 1999; 714 pp.

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Chemical Measurement Kevin Jeanes ASK Laboratories, Inc., Amarillo, Texas, U.S.A.

Chemical– Desalination

William J. Rogers Department of Life, Earth and Environmental Science, West Texas A&M University, Canyon, Texas, U.S.A.

INTRODUCTION The analysis of water has become a common and important task for industry, municipalities, and agriculture in response to today’s increased public awareness and participation in conserving, protecting, and improving the quality of water resources. Analytical chemistry methods are used for the identification of one or more analytes or constituents and properties in a sample and the determination of the concentration of these components. The identification of components in water is called a qualitative analysis while the determination of relative amounts is a quantitative analysis. Quantitative analyses provide data describing the quantity of the analyte in a measured amount of sample. The results of environmental analysis for water are commonly expressed in relative terms as parts of analytes per unit of sample, such as percent (%), parts per million (ppm), milligrams per liter (mg L 1) or parts per billion (ppb), and micrograms per liter (mg L 1). With recent improved technology, parts per trillion detections can be obtained for some constituents. Special methods allow detections of individual molecules in some cases.

DATA QUALITY To produce quality data the sample must accurately reflect the matrix or media from where it was collected. Sampling is more efficient in liquid than solid matrices, such as soil, due to the chemical properties of water and the homogeneity of solutes. However, the increased risk to biological systems and public health associated with contaminated water complicates the decision of which methods will provide the data necessary to make informed decisions about the quality of the water, the protection of human health, and the protection of the environment. Data Quality Objectives (DQO) must be formulated that outline considerations such as regulatory compliance and guidance, accuracy, target analytes, required analytical method performance, availability and reliability of 86

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field measurement, number of samples needed to be analyzed, and cost of analysis. In summary, DQOs can be defined as those sampling and analytical objectives that provide the number of samples and the quality of results needed to satisfy the decision making process. Because the success or failure of an analysis is often critically dependent on the proper method selection, the decision of which analytical method to request is difficult and is only made easier with experience. A good working relationship with a qualified laboratory can prove an invaluable asset to the data collector. The analysis of water can be divided into common groupings, organic and inorganic analytes, which require varied analytical methods. A clear establishment of DQOs to support the decision making process requires an understanding of those methods and the methods which meet the regulatory requirements. The U.S. Environmental Protection Agency has developed a series of documents that outline the DQO process which can be accessed on the internet.[1–3] Gilbert[4] has published a very useful handbook, ‘‘Statistical Methods for Environmental Pollution’’ which addresses statistical implications of sampling and data quality analysis. Before developing a sampling plan, due to the large number of methods and variability of techniques, care must be taken to determine the accuracy and sensitivity requirements for the application and then determine holding times, preservatives, sample size, and containers required by the method chosen to meet the DQO. A listing of approved methods for National Pollutant Discharge Elimination System (NPDES) permits including sample containers, preservatives, and holding times is available in 40 CFR part 136 and can be easily downloaded from the web by following the links from the site www.access.gpo.gov/nara/cfr. The regulation, which governs the activity that is being analyzed or monitored, will provide guidance and identify the method required to meet regulatory compliance. It is important to carefully research the project requirements, because several analytes have no holding time (such as pH; see the entry pH) and must be analyzed

Chemical Measurement

ANALYTICAL METHODS AND PROTOCOLS It is important to understand that the various regulations require specific analytical methods. Within these regulations, different methods maybe required based on the media type (soil, water, sewage, etc.) Specific methods[5–8] have been established by the EPA, professional organizations, and even the state agencies for these various matrices (i.e., soil, water, and sewage). EPAs SW846 methods[5] for the evaluation of solid waste are used for the evaluation of solid waste which includes water and are used in this discussion because they are commonly used in water analysis and the methods are easily downloaded from the web at (http://www.epa.gov/epaoswer/hazwaste/test/main. htm). EPA also published EPA-600[6] series for water and EPA-500[7] series for drinking water, but the method’s advantages and disadvantages remain the same for each series of methods. It is important to note that subtle differences do exist for method procedures and quality control and the method series prescribed in the regulations should be utilized.

determinations. These methods have varying degrees of sensitivity and accuracy.

METALS Techniques for the analysis of trace-metal concentrations include direct-aspiration or flame atomic absorption spectrometry (FLAA), graphite-furnace atomic absorption spectrometry (GFAA), inductively coupled argon plasma-atomic emission spectrometry (ICPAES), inductively coupled argon mass spectrometry (ICP-MS), and cold-vapor atomic absorption spectrometry (CVAA). Each of these methods has advantages and disadvantages that should be addressed before selection of an analytical procedure.

FLAA FLAA is the most common method utilized by small commercial laboratories, municipalities, and industrial laboratories because of the affordability and ease of operation. FLAA commonly uses an acetylene/air flame as an energy source for dissociating the aspirated sample into the elemental state enabling the analyte to absorb light from a specific wavelength. Since each element has to be analyzed separately using that metal’s specific wavelength, there is reduced risk for matrix interference. The sensitivity is usually acceptable for most applications, but currently technological improvements are lowering allowable analyte limits that will strain the FLAA capabilities. SW846-7000[5] outlines the general method and associated digestions, interferences, and sensitivity.

ANALYTICAL METHODS FOR INORGANIC CONSTITUENTS GFAA The analytical methods for inorganic constituents in water for environmental monitoring are commonly subdivided into wet chemistry and metals. These methods are used for analyzing nutrients and elemental analytes, and are associated with water quality parameters and metal contamination. These methods are usually requested for waters that are accepting treated municipal and industrial effluents or that have suspected impacts from sewage, agriculture, and industry.

WET CHEMISTRY Wet chemistry methods are the classical bench methods and include common water quality parameters [i.e., pH, biochemical oxygen demand (BOD), hardness, and alkalinity] utilizing colorimetric, potentiometric, gravimetric, titration, and chromatographic

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GFAA replaces the flame with a heated graphite furnace that allows the experienced analyst to remove matrix interferences and concentrate the analyte of concern by using temperature profiles and matrix modifiers. This method requires more exacting analyst intervention and interpretation making it more difficult than FLAA. The method does increase sensitivity and when coupled with ZEEMAN has a very low background interference. GFAA greatly enhances the ability to analyze Selenium and Arsenic that can prove difficult to analyze with other methods. The method has the potential of positive interferences from memory effect, smoke producing matrices, organic materials, and carbide formation and is extremely dependent on the skill of the analyst. GFAA requires a stringent QC program to ensure that matrix interferences have no adverse effect on the analyte of concern.

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in the field. Field methods offer flexibility and reduced cost; however, accuracy is still dependent on the equipment, personnel, and the quality assurance plan that outlines training, calibration, maintenance, and procedure. Due to the large number of methods and complexity of their quality assurance requirements, a qualified laboratory should be consulted to determine which method will yield data of sufficient quantity and quality to meet the DQOs.

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SW846-7000[5] outlines the general method and associated digestions, interferences, and sensitivity.

ICP Chemical– Desalination

ICP allows rapid simultaneous or sequential analysis of many metals making it the major method utilized by large commercial laboratories. ICP instruments are expensive and complicated analytical instruments requiring skilled analysts and exacting quality control procedures. ICP-AES methods are susceptible to high single element interferences in matrices high in salts or other elements making trace analysis of other elements in these matrices problematic. Arsenic and selenium lack sensitivity due to physical properties but can be enhanced using a hydride aspiration system. Lead, antimony, and thallium also have sensitivity problems on the ICP-AES but can be analyzed at low levels using ICP-MS or GFAA. ICP-MS greatly enhances sensitivity for metals making it the preferred method when the analyses of very low concentrations are required. The main interference is selenium that has mass interference with the argon dimer. SW8466010[5] outlines the general method and associated digestions, interferences, and sensitivity.

CVAA CVAA is the technique used for the analysis of mercury by using a selective digestion method. Although this method is extremely sensitive, it is subject to interferences from sulfide, chlorine, and organic compounds. There are several models of instruments for mercury but all require a thorough quality assurance plan to ensure the accuracy of the results. SW8467470[5] outlines the general method and associated digestions, interferences, and sensitivity.

ANALYTICAL METHODS FOR ORGANIC CONSTITUENTS The analysis of organics in water offers the data collector a variety of challenges due to the sheer number of methods and analytes. The decision of which method to utilize is determined by regulations, desired data quality, and cost. It should be noted that there are usually multiple methods that can quantify a compound requiring an experienced data collector to determine which method meets the requirements in the DQOs. Organic chemical methods can be divided into those that determine total organic matter present and individual organic compounds or groups of compounds.

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Chemical Measurement

Total Organic Matter Present Total organic methods measure such parameters as BOD,[6] chemical oxygen demand (COD),[6] total organic carbon (TOC),[8] oil and grease,[5,6] total recoverable petroleum hydrocarbons (TRPH),[5,6] oil and grease in sludge,[5] total phenols,[5,6] and surfactants.[6] A detailed discussion is included on BOD and COD in the article titled Oxygen Measurement: Biological– Chemical Oxygen Demand.

Individual Organic Constituents or Groups A very detailed discussion would be needed to address the methods used to analyze the vast numbers of naturally occurring and man-made organic compounds found in water. EPA SW-846 provides a good listing of methods. Internet services, such as Toxnet (http:// toxnet.nlm.nih.gov/), can also be useful tools. Toxnet is a cluster of databases on toxicology, hazardous chemicals, and related areas. One database, the Hazardous Substances Data Base, provides detailed descriptions of specific compounds as well as analytical methods. Numerous methods are typically presented in the HSDB (http://toxnet.nlm.nih.gov/cgi-bin/sis/ htmlgen?HSDB). If the data are to be used to satisfy a regulatory requirement, the investigator must select the method that is approved by the appropriate regulatory agency. Organic chemicals are typically analyzed using one or more of the following technologies: gas chromatographic technique (GC), halogen sensitive detector (HALL), photoionization detector (PID), flame ionization detector (FID), electron capture detector (ECD), nitrogen phosphorous detector (NPD), flame photometric detector (FPD), high performance liquid chromatography (HPLC), and various extraction methods such as purge and trap (P and T), separatory funnel extraction, and continuous liquid–liquid extraction. The environmental field separates organic contaminants by their physical properties into volatile and semivolatile components. If the compound can be purged from an aqueous sample using an inert gas, the compound is considered volatile, and if the compound requires extraction using a solvent, it is considered a semivolatile compound. The separation of compounds into these groups greatly affects the sampling techniques utilized. Volatile components require sampling with zero headspace in 40 mL vials[2] with a Teflon septum. It is important that there is no air in these samples because it can significantly alter the results. Semivolatiles can be collected in liter glass jars with Teflon liners and are not as sensitive to air as volatiles. A qualified laboratory should be consulted to obtain proper sample containers, volumes, and

preservatives and can also assist in the determination of which method best matches the project requirements.

CHROMATOGRAPHY Although there are numerous detectors utilized in the quantitation of organic compounds, the methods introduced in this article will all use GC to separate and isolate individual components. In GC the vaporized components of a sample are separated as a result of partition between the mobile and stationary phases in the column. An analyst controls the separation of individual components by choice of column, method of injection, method of extraction, volume of injection, temperature program, and carrier gas flow. This separation allows the analyst to assign a retention time that a particular compound will elute off the column and identify that compound on the chromatogram. This proves to be an effective qualitative procedure if the matrix contains a small number of analytes. The identification of compounds from a highly contaminated or complex matrix tests the ability of the chromatography and requires intervention of the analyst to interpret the chromatogram or to manipulate the sample to produce more highly resolved chromatography. The effects of analyst intervention will usually reduce the limit of quantification by introducing dilution factors to achieve good baseline separations of the individual for identification. The actions of the analyst must be strictly controlled by experience and the laboratory’s quality assurance plan to maintain the integrity of the data. Chromatography is the method of separation for identification, but the eluted sample is then passed onto a detector to quantify the amount of an analyte present in the original sample. The ability to meet the required analytical limits for organic compounds is extremely dependent on the effects of the matrix and the experience of the laboratory performing the analysis.

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interferences. Some commonly used detectors include FID which measures all hydrocarbons, PID which measures aromatic hydrocarbons, and ECDs which are sensitive to halogens, peroxides, and nitro groups. These types of detectors provide sensitivity for quantitation but rely on the chromatography for identification increasing the risk of miss identifying compounds which coelute or miss quantifying compounds in highly contaminated matrices.

THREE-DIMENSIONAL DETECTORS The mass selective detector (MS) is a 3-D detector that utilizes retention time, response, and mass spectrum to identify and quantify the analyte of concern. Like the 2-D detectors, MS detectors rely on the chromatography to separate the compounds and the response to quantify the compounds but then utilized the mass spectral data to confirm identification. This technique is not as sensitive as the 2-D detectors but is much more accurate removing the possibility of miss identification. Utilizing the MS is preferred when analyzing a complex matrix, but usually increases the cost of analysis. As in all analytical methods, the quality of the data is heavily dependent on the quality of the analyst and the laboratory. SW846-8000 series outlines the general methods and associated extractions, interferences, and sensitivity of organic analysis. When analyzing an aqueous sample the decision of which analytical method to utilize is difficult and critical to the success of the project. The data collector must meet the requirements established in DQOs to ensure that the data collected is of sufficient quality and quantity to make supportable decisions. The quality of data is determined by sampling, analytical methods, and the quality plans of all the parties involved in collection of the data. The manager must make informed decisions when selecting an analytical method that will fulfill the requirements of the project while balancing cost, time, and risk.

TWO-DIMENSIONAL DETECTORS The choice of detectors is varied and is determined by the regulatory requirements and the cost of the analysis. Two-dimensional detectors utilize the retention time and response of a component to identify and quantify the compound and are extremely sensitive if operated by an experienced analyst. There are multiple types of detectors to consider when making a decision, but the regulatory requirements usually identify the detector required for compliance. These detectors all have unique qualities that aid in isolating the types of compounds that are being analyzed and the proper detector can increase sensitivity and decrease

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CONCLUSION With increased industrialization and agriculture, the quality of our water resources will become a critical issue. The need to monitor water resources such as agricultural effluents, groundwater resources, and surface water resources will increase dramatically. The existing and emerging technology will allow analysis of chemicals in water at levels approaching molecular levels. These developments, however, can be very costly. The challenge in developing a plan for water analysis sampling is to develop a plan that satisfies

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the data quality needed to satisfy the decision making process. This includes balancing both the quality and quantity of samples to characterize the water to address protection of human health and the environment and regulatory standards.

4.

REFERENCES

6.

1. USEPA. Guidance for the Data Quality Objectives Process (G-4); EPA/600/R-96/055; Office of Environmental Information: Washington, DC, August 2000 (http:// www.epa.gov/quality/qs-docs/g4-final.pdf). 2. USEPA. EPA Guidance for Quality Assurance Project Plans (G-5); EPA/600/R-98/018; Office of Research and Development: Washington, DC, February 1998 (http://www.epa.gov/quality/qs-docs/g5-final.pdf). 3. USEPA. EPA Guidance for Data Quality Assessment (G-9); EPA/600/R-96/084; Office of Environmental

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5.

7.

8.

Information: Washington, DC, July 2000 (http://erb.nfesc. navy.mil/erb_a/restoration/methodologies/g9-final.pdf). Gilbert, R.O. Statistical Methods for Environmental Pollution; John Wiley and Sons, Inc.: New York, 1987. USEPA. SW-846 Test Methods for Evaluating Solid Waste, Physical/Chemical Methods; Office of Solid Waste: Washington, DC, 2002 (http://www.epa.gov/ epaoswer/hazwaste/test/sw846.htm). USEPA. Methods for Chemical Analysis of Water and Wastes; EPA 600/4-79-020; Office of Environmental Information: Washington, DC, 2002 (http://www.epa. gov/OGWDW/methods/epachem.html). USEPA. Methods for the Determination of Organic compounds in Drinking Water; EPA 600/4-88-039; Office of Environmental Information: Washington, DC, 2002 (http://www.epa.gov/OGWDW/methods/epachem.html). American Public Health Association, American Water Works Association, Water Environmental Federation. Standard Methods for Examination of Water and Wastewater, 20th Ed.; Washington, DC, 1999.

Chemigation William L. Kranz

INTRODUCTION Chemigation is the practice of distributing approved agricultural chemicals such as fertilizers, herbicides, insecticides, fungicides, nematicides, and growth regulators by injecting them into water flowing through a properly designed and managed irrigation system. The term chemigation was originally coined to describe the concept of applying commercial fertilizers that were needed for crop production. Field research, and advances in sprinkler and chemical injection technology have stimulated the use of chemigation as a major crop production tool. Today chemigation is one of the more efficient, economical, and environmentally safe methods of applying chemicals needed for successful crop, orchard, turf, greenhouse, and landscape operations. Chemigation began with the application of commercial fertilizers through irrigation systems in the late 1950s.[1] Later tests were initiated on sprinkler application of herbicides to selectively control weeds in field crops, fruit and nut orchards, rice, and potatoes.[2,3] These research efforts led the way for what has become a major research topic to identify management and equipment required for chemical application in agricultural and non-agricultural production settings. The primary use of chemigation is to apply chemical directly to the soil using a range of irrigation water distribution systems. For example, drip/trickle, sprinklers, and some surface irrigation systems are commonly used to apply commercial fertilizers. However, federal regulations limit application of restricted use pesticides to systems that can safely and uniformly apply a chemical to a specific site at a rate specified on a chemical label. Though estimates vary greatly, chemigation is used to apply fertilizers on nearly four million hectares in the United States.[4] Specialists in Florida, Texas, and Wyoming report that more than 50% of their irrigated land received at least one chemigation application.[5]

ADVANTAGES OF CHEMIGATION Chemigation offers producers of food and fiber many advantages that result from using existing equipment

and timeliness of chemical applications. Advantages of chemigation include the following:[6,7]  Uniformity of chemical application is equal to or greater than other means of application.  Timeliness and flexibility of application are greater.  Improved efficacy of some chemicals.  Potential for reduced environmental risks.  Lower application costs in some cases.  Less mechanical damage to plants.  Less soil compaction.  Potential reduction in chemical applications.  Reduced operator hazards.  Application cost savings for multiple applications.

DISADVANTAGES OF CHEMIGATION Chemigation also requires additional equipment and management to obtain successful results. Some of the disadvantages of chemigation include:[6,7]  Chemical application accuracy depends on water application uniformity.  Longer time of application than other methods.  Some pesticide labels prohibit chemigation as a means of application.  Potential for source water contamination.  Additional capital costs for equipment.  Potential for increased legal requirements in some states.  Increased management requirements by the operator.

CHEMIGATION EQUIPMENT Safe and efficient chemigation requires that the irrigation equipment, injection device, and safety equipment be properly installed and maintained. Fig. 1 provides an overview of equipment necessary for chemigation systems using groundwater. State and federal regulations specify the type of irrigation water distribution system that can be used and the required safety equipment. It is up to the irrigator to ensure the use of appropriate equipment and procedures. 91

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Northeast Research and Extension Center, University of Nebraska, Norfolk, Nebraska, U.S.A.

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Chemigation

Chemical– Desalination Fig. 1 Chemigation injection and safety equipment commonly required when pumping groundwater. (Drawing courtesy of Midwest Plan Service, Ames, IA.)

Irrigation Equipment Chemigation requires equipment capable of applying chemicals uniformly and with differing amounts of water, accurate and dependable injection equipment, and safety equipment for source water and worker protection. Appropriate sprinkler design and high application volumes can solve problems associated with canopy penetration and deposition that impact some aerial applications. Uniform water application can precisely place and incorporate chemicals in the soil and limit leaching of soluble chemicals from the zone of application. Several different types of irrigation equipment can and are being used to distribute chemicals via chemigation. Most chemigation is conducted using either sprinkler or drip/trickle irrigation systems. Center pivot and linear-move systems are most commonly used for chemigation since prescription applications can be made with a high degree of uniformity. Drip/ trickle systems are commonly used to place precise amounts of plant nutrients near the zone of plant uptake thus increasing chemical use efficiency. In general, surface irrigation systems have limited potential for chemigation. Water distribution in furrow

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systems is typically non-uniform along the row and among rows. Thus, in-field variation in water infiltration results in chemical application uniformity that is below levels desired for chemigation. Development of surge-flow systems can improve distribution uniformity, however, the question remains whether consistent results are possible and whether producers have sufficient experience to make equipment adjustments when necessary. Level basin irrigation systems offer improved uniformity of water application, but water quality concerns have limited the use of chemigation.

Injection Equipment Chemical injection can occur using either active or passive devices. Active devices use an external energy supply to create pressures at the injector outlet that exceed the irrigation pipeline pressure. Injection pumps are often powered by constant speed or variable speed electric motors. Typical examples include piston, diaphragm, rotary, and gear pumps. However, most new installations use either piston or diaphragm pumps (Fig. 2). These injection devices are relatively expensive. Component selection allows the injection

of commercial fertilizers, acids, or pesticides. Intermittent end guns and corner systems can lead to variable chemical application by constant rate injectors due to changes in the irrigation rate per hour.[8] Application errors of approximately 20% are possible when corner systems are used with a constant rate injection device. When the irrigation rate will change during a chemigation event, it is preferable to use a variable rate injection device. Passive devices take advantage of pressure differentials that result from using a throttling valve or pitot tube unit to add chemical to water flowing through a pipeline. Chemicals are metered into the system using a venturi meter or orifice plate. These systems have low capital cost requirements. However, pumping cost may be greater since irrigation pump outlet pressure must be equal to the water distribution system pressure plus the friction loss associated with the throttling valve. In addition, changes in pumping pressure directly impact chemical injection rates which can lead to non-uniform chemical applications. Selection criteria for injection devices include potential injection rates, available power supply, and the type of chemical to be injected. A single injection device is typically not capable of covering the range of injection rates and chemical types that could conceivably be applied via chemigation. Hence, if plant nutrients and pesticides are to be applied, two injection devices

Fig. 2 Typical portable injection equipment for center pivot installations. (Photo courtesy of Agri-Inject, Inc., Yuma, CO.)

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are desirable. Diaphragm injection devices offer greater chemical compatibility, ease of calibration, and precise injection rates which make them good choices for pesticide injection. Commercial fertilizers are less caustic and require relatively high injection rates which make high capacity piston and diaphragm injection devices good options. Research has noted that injection equipment calibration was necessary for each injection device and operating pressure.[9] Manufacturing tolerances and pipeline pressure impacted the rate of chemical injection. Further, performance tests conducted on new and used diaphragm pumps found that proper maintenance is required to ensure long-term accuracy of chemical injection rates.[10]

Safety Equipment State and federal regulations differ regarding safety equipment that is required for chemigation. For example, the Nebraska Chemigation Act requires the safety equipment also found in many state regulations.[11] Most requirements are met through installation of a backflow protection device. Requirements typically include (Fig. 3): 1. A mainline check valve to prevent concentrated chemical and/or dilute chemical solution from flowing back into the water source. 2. A chemical injection line check valve to prevent flow of chemical from the chemical supply tank into the irrigation pipeline and to prevent flow of water through the injection system into the chemical supply tank. 3. Vacuum relief valve to prevent back siphoning of concentrated chemical and/or dilute chemical solution into the water source. 4. Low pressure drain to prevent back flow of chemical and/or dilute chemical solution into the water source should the mainline check valve fail. 5. An inspection port to ensure that the mainline check valve and low pressure drain are functioning properly. 6. An interlock between the injection system and the irrigation pumping plant to prevent injection of concentrated chemical into the irrigation pipeline should there be an unexpected shutdown of the irrigation pump. American Society of Agricultural Engineers have published EP409.1 Safety Devices for Chemigation[12] which recommends the addition of a two-way interlock between the injection system and irrigation pumping plant and a normally-closed solenoid valve on the outlet of the chemical supply tank to prevent chemical

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Chemical– Desalination Fig. 3 Mainline check valve used to prevent flow of chemicals into a water source. (Drawing courtesy of Midwest Plan Service, Ames, IA.)

spills attributed to chemical injection line or injection device failures. The engineering practice also encourages the positioning of a fresh water source near the chemical supply tank for washing chemicals that may contact skin, the use of a strainer on the chemical tank outlet to prevent fouling of injection equipment, the grading of the soil surface to direct flow away from the water supply, the location of mixing tanks and injection equipment safely away from potential sources of electrical sparks to prevent explosions, and the use of components that are well suited to a range of chemical formulations.

MANAGEMENT PRACTICES Management flexibility based on chemical placement, application rate and mobility in the soil, water quality, application cost, and weather factors make chemigation a unique and effective production tool. Chemigation provides the opportunity to synchronize fertilizer applications to match plant needs and incorporate and, if needed, activate pesticides to increase efficacy. Equally important, chemigation provides the opportunity to reduce chemical applications by eliminating the need for insurance-type applications. Fields can be scouted for disease or pests and chemical applied only if damage or pest numbers exceed economic thresholds. Soil and plants can be monitored to determine fertilizer needs, making near real-time adjustments in the time of application and chemical formulation possible. Individual nozzle controls make

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site-specific applications well within reach.[13] However, a considerable amount of work remains to ascertain if site-specific applications are economical and to incorporate management tools into system controls.

CONCLUSION Chemigation has gradually become one of the most effective means of chemical application available for crop production and landscape systems. Advantages of highly uniform prescription applications outweigh the potential disadvantages in most cases. Effective chemigation hinges on the selection of appropriate irrigation systems, chemical injection devices, and safety equipment. Through proper management, chemigation is poised to be a production practice that can help increase the quality and quantity of food produced worldwide.

REFERENCES 1. Bryan, B.B.; Thomas, E.L., Jr. Distribution of Fertilizer Materials Applied with Sprinkler Irrigation Systems, Agricultural Experiment Station Research Bulletin 598; University of Arkansas: Fayetteville, AK, 1958; 12 pp. 2. Ogg, A.G., Jr. Applying herbicides in irrigation water— a review. Crop Prod. 1986, 5 (1), 53–65. 3. Cary, P.J. Applying herbicides and other chemicals through sprinkler systems. Proceedings of the 21st Annual Conference of the Washington State Weed Association, Yakima, WA, 1971.

4. USDA. Application of chemicals in irrigation and times irrigated by selected crop: 1998 and 1994. In 1998 Farm and Ranch Irrigation Survey: Table 24; U.S. National Agricultural Statistics Service: Washington, DC, 1999; 102–124. 5. Adams business media; 2000 annual irrigation survey continues steady growth. Irrig. J. 2001, 51 (1), 12–40. 6. Scherer, T.F.; Kranz, W.L.; Pfost, D.; Werner, H.D.; Wright, J.A.; Yonts, C.D. Chemigation. In MWPS-30 Sprinkler Irrigation Systems; Midwest Plan Service: Ames, IA, 1999; 145–166. 7. Threadgill, E.D. Introduction to chemigation: history, development, and current status. In Proceedings of the Chemigation Safety Conference, Lincoln, NE, April 17–18, 1985; Vitzthum, E.F., Hay, D.R., Eds.; University of Nebraska Cooperative Extension: Lincoln, NE, 1985. 8. Eisenhauer, D.E. Irrigation system characteristics affecting chemigation. In Proceedings of the Chemigation Safety Conference, Lincoln, NE, April 17–18,

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9.

10.

11.

12.

13.

1985; Vitzthum, E.F., Hay, D.R., Eds.; University of Nebraska Cooperative Extension: Lincoln, NE, 1985. Kranz, W.L.; Eisenhauer, D.E.; Parkhurst, A.M. Calibration accuracy of chemical injection devices. Appl. Engr. Agric. 1996, 12 (2), 189–196. Cochran, D.L.; Threadgill, E.D. Injection Devices for Chemigation: Characteristics and Comparisons, National Meeting of the American Society of Agricultural Engineers, Paper No. 86-2587; ASAE: St. Joseph, MI, 49805, 1986. Vitzthum, E.F. Using Chemigation Safely and Effectively; University of Nebraska Cooperative Extension Division: Lincoln, NE, 2000; 1–59. ASAE. EP409.1: Safety Devices for Chemigation; American Society of Agricultural Engineers: St. Joseph, MI, 2001. Evans, R.G.; Buchleiter, G.W.; Sadler, E.J.; King, B.A.; Harting, G.H. Controls for Precision Irrigation with Self-propelled Systems. Proceedings of the 4th Decennial National Irrigation Symposium, Phoenix, AZ, 2000; 322–331.

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Chemigation

Chernobyl Accident: Impacts on Water Resources Jim T. Smith Centre for Ecology and Hydrology, Winfrith Technology Centre, Dorchester, Dorset, U.K.

Chemical– Desalination

Oleg Voitsekhovitch Ukrainian Hydrometeorological Institute, Kiev, Ukraine

INTRODUCTION The Chernobyl nuclear power plant is situated next to the river Pripyat, which is an important component of the Dnieper river–reservoir system, one of the largest surface water systems in Europe (Fig. 1). After the Chernobyl accident in April 1986, radioactive fallout on the Pripyat and Dnieper catchments threatened to wash downriver into the Kiev Reservoir, a major source of drinking water for the city of Kiev, and to other areas downstream where the river–reservoir system is also used for significant fisheries and irrigation. The radioactive contamination of aquatic systems, therefore, became a major issue in the immediate aftermath of Chernobyl.[1–3] In this entry, we will outline the major impacts of the accident on water resources, covering contamination of rivers, lakes, and reservoirs as well as uptake to fish and impacts on ground- and irrigation waters.

RADIONUCLIDES IN RIVERS, LAKES, AND RESERVOIRS Initial radioactivity concentrations in river water were relatively high as a result of direct fallout onto the river surfaces and washoff of contamination from the surrounding catchment. During the first few weeks after the accident, however, activity concentrations in river waters rapidly declined because of physical decay of short-lived isotopes and as radionuclide deposits became absorbed to catchment soils. In the longer term, relatively long-lived radiocesium and radiostrontium formed the major component of river water contamination. To our knowledge, there are few data of radionuclide concentrations in small streams in the Chernobyl area during the early phase of the accident. Most available data are for large rivers. Table 1 shows a summary of available measurements of radionuclide activity concentrations in a large river, the Pripyat, at Chernobyl at various times after the accident. Comparison with Generalised Derived Limits for radionuclides in drinking water shows that radionuclide 96

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concentrations in drinking water were a cause for concern in the weeks–months after the accident, but on longer timescales, activity concentrations in rivers were significantly below drinking water limits. Though longterm levels of radiocesium and radiostrontium in rivers were generally lower than drinking water standards, temporary increases in activity concentrations during flooding of the River Pripyat caused serious concern in Kiev and other towns over the safety of the drinking water supply and other types of water use. Lakes and reservoirs around Europe were contaminated by fallout to lake surfaces and transfers of radionuclides from their surrounding catchments. Radioactivity concentrations in water declined relatively rapidly in reservoirs and in those lakes with significant inflows and outflow of water, as radionuclides were ‘‘flushed’’ out of the system. In the areas around Chernobyl, however, there are many lakes with no inflowing and outflowing streams (‘‘closed’’ lake systems). Cycling of radiocesium in these closed systems led to much higher activity concentrations in water and aquatic biota than were seen in open lakes and rivers. Bed sediments of lakes and reservoirs are an important long-term sink for radionuclides. In the long term, approximately 99% of the radiocesium in a lake is typically found in the bed sediment. In Lake Kozhanovskoe, Russia, approximately 90% of the radiostrontium was found in the bed sediments during 1993–1994.[4]

RADIONUCLIDES IN FISH Bio-accumulation of radionuclides (particularly radiocesium) in fish resulted in activity concentrations (both in Western Europe and in the former Soviet Union, fSU), which were in many cases significantly above guideline maximum levels for consumption (guideline levels vary from country to country but are approximately 1000 Bq kg
1 or 1 kBq kg
1 in the EU). In the Chernobyl Cooling Pond, 137Cs levels in carp (Cyprinus carpio), silver bream (Blicca bjoerkna), perch (Perca fluviatilis), and pike (Esox lucius) were of order 100 kBq kg
1 w.w. in 1986, declining to a

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Fig. 1 Pripyat–Dnieper river–reservoir system showing Chernobyl and Kiev with the Kiev Reservoir in between. Source: From Ref.[1].

few tens of kBq kg
1 in 1990.[5,6] In the Kiev Reservoir, activity concentrations in fish were in the range 0.6– 1.6 kBq kg
1 wet weight (in 1987) and 0.2–0.8 kBq kg
1 w.w. (from 1990 to 1995) for adult non-predatory fish and 1–7 kBq kg
1 (in 1987) and 0.2–1.2 kBq kg
1 (from 1990 to 1995) for predatory fish species. In small lakes in Belarus and the Bryansk region of Russia, activity concentrations in a number of fish species

© 2008 by Taylor & Francis Group, LLC

varied within the range 0.1–60 kBq. kg
1 w.w. during the period 1990–1992.[7,8] It was estimated that about 14,000 lakes in Sweden had fish with 137Cs concentrations above 1500 Bq kg
1 (the Swedish guideline value) in 1987.[9] In a small lake in Germany, levels in pike were up to 5 kBq kg
1 shortly after the Chernobyl accident[10] (Fig. 2). In Devoke Water in the English Lake District, perch and brown trout (Salmo trutta)

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Chernobyl Accident: Impacts on Water Resources

Table 1 Radionuclide levels (dissolved phase) in the R. Pripyat at Chernobyl RN 137

Cs

Chemical– Desalination

134

Cs

131

I

90

Ba

100

01/05/86

02/05/86

250

555

90

130

289

8.1 day

20

2100

4440

28 yr

50

3 day

550

814

106

Ru

365 day

80

183

271

144

Ce

284 day

380

141

Ce

33 day

400

95

Zr

65 day

400

95

Nb

35 day Pu

2.4  104 yr 6.6  103 yr

0

670 800

239 þ 240

0.941 1.5

40 day

13 yr

1.8

30

Ru

Pu

1987

1400

103

241

9/8/86

1

2.1 yr

12.8 day

Mo

Radionuclide concentration in water (Bq L
1)

Guideline limit (Bq L )

30.2 yr

Sr

140 99

Half-life


1

1554

420 300

33

0.6

7

0.4

0.0074

The guideline limit is the UK Generalised Derived Limit for drinking water and shows the level of each radionuclide, which would result in a 1 mSv dose to consumers. The accident occurred on April 26, 1986 and radionuclides continued to be emitted for a 10 day period. Source: From Ref.[5,20–23].

contained around 1 kBq kg
1 in 1988 declining slowly to a few hundreds of Bequerels per kg in 1993.[11,12] The contamination of fish following the Chernobyl accident was a cause for concern in the short term (months) for less contaminated areas (for example, parts of the UK and Germany) and in the long term (years–decades) in the Chernobyl affected areas of Ukraine, Belarus, and Russia and parts of Scandinavia.

RADIONUCLIDES IN GROUNDWATER AND IRRIGATION WATER Transfers of radionuclides to groundwaters have occurred from waste disposal sites in the 30 km exclusion zone around Chernobyl. Health risks from groundwaters to hypothetical residents of these areas, however, were shown to be low in comparison with external radiation and internal doses from foodstuffs.[13] Although there is a potential for off-site (i.e. out of the 30-km zone) transfer of radionuclides from the disposal sites, these workers concluded that this will not be significant in comparison with washout of surface deposited radioactivity. Radionuclides could potentially contaminate groundwater by migration of radioactivity deposited on the surface soils. It is known, however, that longlived radionuclides such as 137Cs and 90Sr are relatively immobile in surface soils and transfers from surface fallout to deep groundwaters are expected to be very

© 2008 by Taylor & Francis Group, LLC

low in comparison with transfers from surface runoff to rivers and lakes. After fallout from nuclear weapons testing in the 1960s, it was observed that 90Sr in Danish groundwater was approximately 10 times lower than in surface streams.[14] These authors also observed that after Chernobyl, despite measurable quantities of 137 Cs in surface streams, activity concentrations were below detection limits in groundwater. Short-lived radionuclides are not expected to affect groundwater supplies since groundwater residence times are much longer than their physical decay time. Even in the Chernobyl exclusion zone, the groundwater contamination has not occurred on a large scale. In the majority of cases, significant contamination of the groundwater took place locally only as a result of local dispersion of radionuclides from the shallow underground radioactive storage facilities and from the temporary waste disposal sites. Because of retardation, there was a very low rate of transport through geological media. According to a number of studies, radionuclide groundwater fluxes did not pose a significant risk of secondary contamination of the surface water in the Pripyat River (and will not do so in the future), because of the high efficiency of natural attenuation factors in the area around Chernobyl. A large amount (about 1.8 million hectares) of agricultural land in the lower Dnieper basin is irrigated. Accumulation of radionuclides in plants on irrigated fields can take place because of root uptake of radionuclides introduced with irrigation water and owing to direct incorporation through leaves after sprinkling.

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Chernobyl Accident: Impacts on Water Resources

Fig. 2 Change in the 137Cs activity concentration in water and fish of (A) a small shallow lake in Germany, Lake Vorsee, and (B) the large, deep Lake Constance. Source: Adapted From Ref.[10,19] using data kindly supplied by Gregor Zibold, Fachhochschule Weingarten, Germany.

However, recent studies[3] have shown that in the case of irrigated lands of southern Ukraine, radioactivity in irrigation water did not add significant radioactivity to crops in comparison with that which had been initially deposited in atmospheric fallout and subsequently taken up in situ from the soil.

© 2008 by Taylor & Francis Group, LLC

RADIATION EXPOSURES VIA THE AQUATIC PATHWAY Doses from 137Cs and 90Sr contamination of waterbodies in the most affected countries (Ukraine, Russia, and Belarus) are difficult to quantify. Doses from the

100

Chemical– Desalination

freshwater pathway (including fish and irrigation water) to the people of Kiev were relatively low, being around 2–5% of doses via terrestrial foodstuffs in the most contaminated areas and up to 10% of terrestrial doses in relatively much less contaminated downstream areas of the Dnieper.[3,15] Radionuclides in the Pripyat River could potentially have led to significant doses in the first months after the accident through consumption of drinking water (Table 1). It is quite likely, though, that there was significant reduction in activity concentrations within the water supply system, so these doses are most likely to be over-estimates. Measures taken to reduce radioactivity in drinking water and fish are reviewed in Refs.[3,16]. In rural parts of the Chernobyl contaminated areas of the former Soviet Union during 1994–1995, it was found that the so-called ‘‘wild foods’’ (mushrooms, berries, freshwater fish, game animals) had radiocesium contents, which were around one order of magnitude higher than agricultural products (e.g., milk or meat). Whole body monitoring of people living close to Lake Kozhanovskoe, Bryansk, showed[17] that 137 Cs intake by the population was strongly correlated with levels of consumption of freshwater fish. In rare situations like this, where people consume fish from the (few) highly contaminated ‘‘closed’’ lakes, the ingestion dose can be dominated by 137Cs from fish. In Western Europe, consumption of freshwater fish does not form an important part of the diet, but sports and commercial fisheries may be of economic importance in some areas. In Norway, where fallout levels were among the highest in Western Europe, consumption of freshwater fish declined by up to 50% in the more contaminated areas, and the sale of freshwater fish to the general public was prohibited in these areas.[18] These authors also reported that the sale of fishing licences in parts of Norway declined by 25% after Chernobyl.

CONCLUSIONS The Chernobyl accident had a major impact on water bodies in Ukraine. Though, in the long term, contamination of surface water systems was generally below drinking water limits, major remediation works had to be put in place to demonstrate that water supplies were being protected. Bio-accumulation of radiocesium in freshwater fish meant that guideline levels were exceeded in areas close to Chernobyl and in some parts of Western Europe.

REFERENCES 1. Smith, J.T.; Beresford, N.A. Chernobyl–Catastrophe and Consequences; Springer-Praxis: Berlin, 2005; 335 pp.

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Chernobyl Accident: Impacts on Water Resources

2. Voitsekhovitch, O.V.; Smith, J.T. Radioactivity in aquatic systems. In Chernobyl Forum Report; International Atomic Energy Agency: Vienna, 2006. 3. Onishi, Y.; Voitsekhovitch, O.A.; Zheleznyak, M. Chernobyl–What Have We Learned? The Successes and Failures to Mitigate Water Contamination over 20 Years; Springer: Berlin, 2007; 290 pp. 4. Sansone, U.; Voitsekhovitch, O.V. Modelling and Study of the Mechanisms of the Transfer of Radionuclides from the Terrestrial Ecosystem To and In Water Bodies Around Chernobyl: EUR 16529 EN; European Commission: Luxembourg, 1996. 5. Kryshev, I.I. Radioactive contamination of aquatic ecosystems following the Chernobyl accident. J. Environ. Radioactivity 1995, 27, 207–219. 6. Kryshev, I.I.; Ryabov, I.N. About the efficiency of trophic levels in the accumulation of Cs-137 in fish of the Chernobyl NPP cooling pond. In Biological and Radioecological Aspects of the Consequences of the Chernobyl Accident; Ryabov, I.N., Ryabtsev, I.A., Eds.; USSR Academy of Sciences: Moscow, 1990. 7. Fleishman, D.G.; Nikiforov, V.A.; Saulus, A.A.; Komov, V.T. 137Cs in fish of some lakes and rivers of the Bryansk region and North-West Russia in 19901992. J. Environ. Radioactivity 1994, 24, 145–158. 8. Smith, J.T.; Kudelsky, A.V.; Ryabov, I.N.; Hadderingh, R.H. Radiocaesium concentration factors of Chernobylcontaminated fish: a study of the influence of potassium, and ‘‘blind’’ testing of a previously developed model. J. Environ. Radioactivity 2000, 48 (3), 359–369. ˚ . Radioactive 9. Ha˚kanson, L.; Andersson, T.; Nilsson, A caesium in fish from Swedish lakes 1986-1988–General pattern related to fallout and lake characteristics. J. Environ. Radioactivity 1992, 15, 207–229. 10. Klemt, E.; Drissner, J.; Kaminski, S.; Miller, R.; Zibold, G. Time dependency of the bioavailability of radiocaesium in lakes and forests. In Contaminated Forests; Linkov, I., Schell, W.R., Eds.; Kluwer: Dordrecht, 1998; 95–101. 11. Camplin, W.C.; Leonard, D.R.P.; Tipple, J.R.; Duckett, L. Radioactivity in freshwater systems in Cumbria (UK) following the Chernobyl accident. MAFF Fisheries Research Data Report No. 18; MAFF: Lowestoft, 1989; 90 pp. 12. Smith, J.T.; Comans, R.N.J.; Beresford, N.A.; Wright, S.M.; Howard, B.J.; Camplin, W.C. Pollution– Chernobyl’s legacy in food and water. Nature 2000, 405 (6783), 141. 13. Bugai, D.A.; Waters, R.D.; Dzhepo, S.P.; Skal’skij, A.S. Risks from radionuclide migration to groundwater in the Chernobyl 30-km zone. Health Phys. 1996, 71, 9–18. 14. Hansen, H.J.M.; Aarkrog, A. A different surface geology in Denmark, The Faroe Islands and Greenland influences the radiological contamination of drinking water. Water Res. 1990, 24, 1137–1141. 15. Konoplev, A.V.; Comans, R.N.J.; Hilton, J.; Madruga, M.J.; Bulgakov, A.A.; Voitsekhovitch, O.V.; Sansone, U.; Smith, J.T.; Kudelsky, A.V. Physico-chemical and hydraulic mechanisms of radionuclide mobilization in aquatic systems. In The Radiological Consequences of

16.

17.

18.

19.

20.

the Chernobyl Accident: EUR 16544 EN; Karaoglou, A., Desmet, G., Kelly, G.N., Menzel, H.G., Eds.; European Commission: Luxembourg, 1996; 121–135. Smith, J.T.; Voitsekhovitch, O.V.; Ha˚kanson, L.; Hilton, J. A critical review of measures to reduce radioactive doses from drinking water and consumption of freshwater foodstuffs. J. Environ. Radioactivity 2001, 56 (1–2), 11–32. Travnikova, I.G., et al. Lake fish as the main contributor of internal dose to lakeshore residents in the Chernobyl contaminated area. J. Environ. Radioactivity 2004, 77, 63–75. Brittain, J.E.; Storruste, A.; Larsen, E. Radiocaesium in Brown Trout (Salmo trutta) from a subalpine lake ecosystem after the Chernobyl reactor accident. J. Environ. Radioactivity 1991, 14, 181–191. Zibold, G.; Kaminski, S.; Klemt, E.; Smith, J.T. Time-dependency of the 137Cs activity concentration in freshwater lakes, measurement and prediction. Radioprotection–Colloques 2002, 37, 75–80. Vakulovsky, S.M.; Voitsekhovitch, O.V.; Katrich, I.Yu; Medinets, V.I.; Nikitin, A.I.; Chumichev, V.B. Radioactive contamination of water systems in the area affected by releases from the Chernobyl nuclear power plant

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accident. Proceedings of an International Symposium on Environmental Contamination Following a Major Nuclear Accident; International Atomic Energy Agency: Vienna, 1990; 231–246. 21. Voitsekhovitch, O.V.; Borzilov, V.A.; Konoplev, A.V. Hydrological aspects of radionuclide migration in water bodies following the Chernobyl accident. Proceedings of Seminar on Comparative Assessment of the Environmental Impact of Radionuclides Released During Three Major Nuclear Accident: Kyshtym, Windscale, Chernobyl: EUR 13574; European Commission: Luxembourg, 1991. 22. Vakulovsky, S.M.; Nikitin, A.I.; Chumichev, V.B.; Katrich, I.Yu; Voitsekhovitch, O.A.; Medinets, V.I.; Pisarev, V.V.; Bovkum, L.A.; Khersonsky, E.S. 137Cs and 90Sr contamination of water bodies in the areas affected by releases from the Chernobyl Nuclear Power Plant accident: an overview. J. Environ. Radioactivity 1994, 23, 103–122. 23. NRPB. Revised generalised derived limits for radioisotopes of strontium, ruthenium, iodine, caesium, plutonium, americium and curium. Documents of the NRPB Vol. 9; National Radiological Protection Board: Chilton, 1998.

Chemical– Desalination

Chernobyl Accident: Impacts on Water Resources

Chesapeake Bay Sean M. Smith Chemical– Desalination

Ecosystem Restoration Center, Maryland Department of Natural Resources, Annapolis, and Geography and Environmental Engineering, Johns Hopkins University, Baltimore, Maryland, U.S.A.

INTRODUCTION The Chesapeake Bay is the largest estuary within the U.S.A.[1] Fresh water flows to the Chesapeake Bay from a watershed that covers an estimated 166,709 km2, including portions of Delaware, Maryland, New York, Pennsylvania, Virginia, Washington, DC, and West Virginia (Fig. 1). The ecological productivity of the estuary has made it an important resource for Native Americans, European immigrants, and current residents in the region.[2] The population in the contributing watershed in recent years has swelled to over 15 million people, resulting in extensive direct and indirect impacts that are now a focus of a large-scale restoration effort led by the US Environmental Protection Agency.[1]

DISCUSSION Located along the Mid-Atlantic coast of the U.S.A. within the limits of Maryland and Virginia, the Bay is approximately 304-km long, has an estimated surface area of 11,603 km2, a width that ranges from 5.5 to 56 km, and an average depth of approximately 6.4 m.[1] Salinity in the tidal portions of the estuary transition from ‘‘fresh’’ conditions (i.e., 0–5 parts per thousand (ppt) salt concentration) at the northernmost end to ‘‘marine’’ conditions (30–35 ppt salt concentration) at the southern boundary with the Atlantic Ocean. Evidence indicates that the modern Chesapeake Bay began forming approximately 35 million years ago with a meteorite impact in the proximity of what is now the confluence of the Bay with the Atlantic Ocean.[2,3] The impact created a topographic depression that influenced the location and alignments of several large river valleys, including those associated with the present day Susquehanna, Rappahannock, and James Rivers. Since then, the river valleys have been periodically exposed and flooded in response to cycles of global glaciation and associated fluctuations in sea level. The most recent, the Wisconsin glaciation, began retreating 102

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approximately 18,000 years ago. The retreat resulted in a rise in sea level by almost ninety meters, drowning the river valleys and forming the current Bay. Eleven large rivers drain the Bay’s watershed, the Susquehanna River from Pennsylvania and New York providing the largest contribution with an average of 98 million m3/day flowing into the northern end of the estuary. The rivers drain one or more of five different physiographic provinces within the watershed, including the Appalachian Plateau, Ridge and Valley, Blue Ridge, Piedmont, and Coastal Plain (Fig. 1).[4] Each province’s geologic composition and history creates dramatically different landscape settings from the western to eastern sides of the drainage basin. The Appalachian, Ridge and Valley, and Blue Ridge are characterized by mountainous terrain, a dominance of sandstone along ridge tops, and several carbonate valleys. The Piedmont has less relief, is dominated by metamorphic rocks, and is characterized by a surface that has been dissected by dendritic stream channel networks. Further to the east, the Coastal Plain is characterized by thick layers of unconsolidated geologic materials overlying bedrock deep beneath the surface. Waterways that flow from the Piedmont into the Coastal Plain traverse the ‘‘Fall Zone,’’ a region that is easily distinguished by waterfalls coincident with an abrupt drop in the underlying bedrock elevations. Major ports and cities were developed along the Fall Zone, including Washington, DC, Baltimore, Maryland, and Richmond, Virginia, because of their locations at the upstream terminus of navigation from tidal waters and proximity to hydropower sources. The Chesapeake Bay estuary is naturally dynamic and characterized by physical conditions that can be stressful to aquatic organisms. The salinity gradient broadly governs the spatial distribution of aquatic habitat types. Alterations in currents, wind, and freshwater inputs can cause salinity conditions to vary over time. The shallow depths also cause colder winter and warmer summer water temperatures compared to the open ocean. These spatial and temporal fluctuations can create physiologically challenging conditions. However, many organisms have adapted and use the abundant nutrients and physical habitat in different

Fig. 1 The Chesapeake Bay estuary and its watershed, including physiographic provinces and state boundaries. (Courtesy of M. Herrmann, Maryland Department of Natural Resources.)

portions of the estuary for specific periods of their life cycles or seasons of the year. As a result, the Bay supports an estimated 3600 species of plants, fish, and animals, including 348 finfish and 173 shellfish species. Some of the most notable of these include striped bass, American shad, blueback herring, blue crab, and the American oyster.[5] The name ‘‘Chesapeake’’ itself was coined from the Algonquin American–Indian word ‘‘Chesepiooc’’ meaning ‘‘great shellfish bay.’’[5] Archaeologists estimate that Native American inhabitants first arrived in the Bay region from the south or west approximately 12,000 years ago as the ice sheets associated with the Wisconsin glaciation began to retreat and temperatures increased.[2] The first inhabitants are presumed to have been nomadic; however, archeological evidence suggests that selective food production started as early as 5000 years ago and settled towns began to be formed approximately 1300 years ago as the population density in the region increased. Recovered artifacts provide evidence of the extensive use of the Bay by the early inhabitants for travel, communication, tools, and food. The first recorded European contact with the Chesapeake Bay region was by the Italian captain, Giovanni da Verrazano in 1524.[2] The English established one of the most well known early settlements at Jamestown, Virginia in 1607. English colonization

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103

expanded through expeditions to the north in the Bay, partly led by the famed Captain John Smith. Immigration to the region increased throughout the 1600s and much of the area was settled by the mid1700s. The colonists made extensive use of the resources provided by the estuary, its wetlands, and tributaries. Shellfish, including oysters, blue crabs, and hard and soft clams, were harvested from shallow water areas.[2] The numerous piles of oyster shells that can be found near Coastal Plain tidal areas provide support for written claims of the extensive oyster beds that existed in the Bay when the European colonists arrived. Traps and nets were used to harvest finfish, including herring, striped bass, and shad. Migratory waterfowl, such as ducks and geese, were also plentiful food sources. The rapid growth in the human population since European colonization of the Chesapeake Bay region dramatically increased the harvest of finfish, shellfish, waterfowl, and mammals naturally supported by the estuary. Extensive landscape alterations also caused direct and indirect physical changes to the Bay and its tributaries. The combination of overharvesting, pollution, and physical alterations has severely impacted the ecosystem and many of the species that historically flourished in the estuary.[1] Dramatic declines have been documented by the harvest records of popular commercial fisheries such as shad and striped bass. Records indicate a decline in the catch of blue crabs per unit of effort since the 1940s. The oyster harvest is currently at less than 1% of historic levels, although this reduction is partly attributed to disease. Many other species not harvested commercially have also been affected by the alterations in the Bay ecology that have accompanied European settlement and population growth. One of the most important impacts to the Chesapeake Bay has been the increased erosion rates and downstream sedimentation caused by extensive deforestation of the watershed.[6] The influx of sediment into the tidal estuary has reduced the water depths in many embayments that once served as navigable ports.[7] Elevated suspended sediment inputs during storm events also increase turbidity in the tidal water column.[6] The resulting decrease in water clarity, which has been exacerbated by algal blooms associated with nutrient runoff pollution, reduces submerged aquatic vegetation (SAV) growth in shallow areas (i.e., depths less than 2 m). SAV coverage on the Bay bottom is estimated to have declined from approximately 80,900 hectares in 1937 to 15,400 hectares in 1984.[1] The loss has negative implications for a variety of species that use the vegetation for habitat, including blue crabs and juvenile finfish. An extensive effort to restore the Bay has been undertaken by the US federal government in

Chemical– Desalination

Chesapeake Bay

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Chemical– Desalination

coordination with states in the watershed.[1,8] A large part of this effort has been focused on the recovery of the historic SAV distributions, as well as reversal of the abnormally low oxygen levels that now occur in the main stem of the Bay and its major tidal tributaries during summer months.[1,9] As with the water clarity problems, the low dissolved oxygen is related to excess nutrient inputs, mainly nitrogen and phosphorous, which stimulate algal production. The oxygen depletion occurs because of algal decomposition, resulting in, estuarine habitat degradation. Substantial reductions in nutrients from watershed runoff have been concluded to be necessary to achieve restoration goals related to both SAV and low dissolved oxygen.[1,9]

CONCLUSIONS The Chesapeake Bay is a large and historically productive estuary on the Mid-Atlantic coast of the U.S.A., with extensive fisheries and wildlife resources. The Bay ecosystem has been impaired by watershed alterations and overharvesting accompanying human population growth in the region, thereby inspiring an extensive government-supported restoration effort. A large part of the restoration focuses on sediment and nutrient pollution associated with runoff from the contributing watershed.

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Chesapeake Bay

REFERENCES 1. http://chesapeakebay.net (accessed July 11, 2005). 2. Grumet, R.S. Bay, Plain, and Piedmont: A landscape history of the Chesapeake Heartland From 1.3 Billion Years Ago to 2000; The Chesapeake Bay Heritage Context Project; U.S. Department of the Interior: Annapolis, MD, 2000; 183 pp. 3. Powars, D.S.; Bruce, T.S. The Effects of the Chesapeake Bay Impact Crater on the Geological Framework and Correlation of Hydrogeologic Units of the Lower York-James Peninsula, Virginia; U.S. Geological Survey Professional Paper 1612; United States Geological Survey: Reston, VA, 1999. 4. Smith, S.; Gutuierrez, L.; Gagnon, A. Streams of Maryland-Take a Closer Look; Maryland Department of Natural Resources: Annapolis, MD, 2003; 65 pp. 5. White, C.P. Chesapeake Bay: Nature of the Estuary, A Field Guide; Tidewater Publishers: Centreville, MD, 1989. 6. Langland, M.; Cronin, T. A Summary Report of Sediment Processes in Chesapeake Bay and Watershed; United States Geological Survey Water Resources Investigations Report 03-4123; United States Geological Survey: Reston, VA, 2003. 7. Gottschalk, L.C. Effects of soil erosion on navigation in upper Chesapeake Bay. Geogr. Rev. 1945, 35, 219–238. 8. Ernst, H.R. Chesapeake Bay Blues; Rowman and Littleford Publishers, Inc.: Lanham, MD, 2003. 9. Macalaster, E.G., Barker, D.A., Kasper, M., Eds.; Chesapeake Bay Program Technical Studies: A Synthesis; United States Environmental Protection Agency: Washington, DC, 1983.

Chromium Bruce R. James Chemical– Desalination

Natural Resource Sciences, University of Maryland, College Park, Maryland, U.S.A.

INTRODUCTION Chromium is a heavy metal that is essential for human health in its trivalent form [Cr(III)], but may cause cancer if inhaled in the hexavalent state [Cr(VI)]. Trivalent Cr is only sparingly-soluble in neutral to alkaline natural waters, but it can be oxidized to Cr(VI) by manganese (III,IV) (hydr)oxides, hydrogen peroxide, ozone, chlorine gas, hypochlorite, and other electron acceptors. Hexavalent Cr can be reduced to Cr(III) by elemental iron and iron(II), sulfides, easily-oxidized organic compounds, and other electron donors. Both oxidation and reduction reactions of chromium are governed by redox potential (Eh) and acidity of natural waters (pH).

OCCURRENCE OF CHROMIUM IN NATURAL WATERS AND WATER SUPPLIES Concerns surrounding the presence of chromium (Cr) in natural waters and drinking water supplies must address a paradox of this heavy metal related to the contrasting solubilities and toxicities of its common oxidation states in natural environments: Cr(III) and Cr(VI). Chromium(III) is essential for human health in trace amounts as an activator of insulin,[1] but it exists predominantly in nature in cationic forms that are typically only sparingly-soluble in near-neutral pH soils, plants, cells, and natural waters.[2] In contrast, Cr(VI) is anionic and much more soluble than Cr(III) over the pH range of natural systems. It is toxic to many cells, is classified by USEPA as a Class A carcinogen by inhalation, and is a regulated contaminant of drinking water supplies.[3] When soluble Cr is detected in natural waters, especially at high concentrations, it is usually Cr(VI) derived from industrial wastes containing Cr(VI) or possibly resulting from the oxidation of certain forms of Cr(III) in soils or sediments.[4,5] The balance of the different forms and the solubilities of Cr(III) and Cr(VI) in natural waters is governed by pH, aeration status (Eh or oxidation–reduction potential), and other environmental conditions

(Table 1). Understanding and predicting the oxidation state, solubility, mobility, and bioavailability of Cr in water are further complicated by the fact that Cr(III) can be oxidized (lose three electrons) to form Cr(VI); whereas Cr(VI) can gain three electrons and be reduced to Cr(III).[6,7] Natural variation and human-induced changes in pH and the oxidation– reduction status of soil and water can control the solubility of Cr. As a result, purification of drinking water supplies and treatment of waste waters contaminated with Cr are possible through chemical and microbiological processes that modify the acidity and the relative abundance of oxidizing and reducing agents for Cr.[8,9] Chromium is the seventh most abundant metal on earth with an average content of 100 mg/kg in the earth’s crust and 3700 mg/kg for the earth as a whole,[10] principally as Cr(III) in unreactive, insoluble minerals, such as chromite (FeO
Cr2O3). Roasting chromite ore under alkaline, high temperature conditions oxidizes Cr2O3 to soluble Cr(VI), a widely-used starting material for production of stainless steel, pressure-treated lumber, chrome-tanned leather, pigments, chrome-plated metals, and other common products used in modern societies.[11] As a result, Cr(VI) remaining in chromite ore processing residue, chrome plating bath waste, paint aerosols, and other industrial wastes may enrich soils and contaminate surface waters and groundwater that are supplies for domestic uses, irrigation, and industrial processes. In contrast to these concentrated, anthropogenic sources of Cr(VI); naturally-occurring sources of Cr are predominantly Cr(III) and occur at low concentrations. Ultramafic and basaltic rocks (and soils developed from these parent materials), however, may contain up to 2400 mg Cr/kg, and can release small fractions of the Cr contained in them as Cr(VI), either through dissolution of Cr(VI) minerals or possibly via oxidation of Cr(III). As a result, Cr(VI) has been detected in groundwater ( 3.5 upon dilution of or addition of base to solutions of Cr(III); green

Cr(H2O)4(OH)þ 2

Dihydroxychromium(III)

Second hydrolysis product of Cr(III); may dimerize and polymerize to form large molecular weight cations in planes of octahedra; green

Cr(H2O)3(OH)03

Chromium hydroxide

Metastable, uncharged hydrolysis product that precipitates as the sparingly-soluble Cr(OH)3

Cr(H2O)2(OH) 4

Hydroxochromate

Fourth hydrolysis product of Cr(III) that may form at pH > 11; may oxidize to Cr(VI) by O2

Cr(III)–organic acid complexes and chelates

For example: chromium citrate, chromium picolinate, chromium fulvate

Soluble complexes and chelates in which water molecules of hydration surrounding Cr(H2O)3þ 6 are displaced by carboxylic acid and N-containing ligands; formation is pH- and concentration-dependent; blue–green–purple colors, depending on ligand binding Cr(III)

H2CrO4

Chromic acid

Fully-protonated form of Cr(VI) formed at pH < 1; see Fig. 2 for key Eh values for redox

HCrO 4

Bichromate

Form of Cr(VI) that predominates at 1 < pH < 6.4; yellow; see Fig. 2 for key Eh values for redox

CrO2 4

Chromate

Form of Cr(VI) that predominates at pH > 6.4; yellow; see Fig. 2 for key Eh values for redox

Cr2O2 7

Dichromate

Form of Cr(VI) that predominates at pH < 3 and in concentrated solutions (>1.0 mM); rapidly reverts to 2 HCrO 4 or CrO4 upon dilution or pH change; orange

Chromium (VI) (hexavalent chromium)

Name

Chemical conditions of water under which it is found and pertinent reactions in natural waters

principally as Cr(VI) at concentrations in the range of 3–7.3 nM (0.16–0.38 mg/L).[12] Based on the known carcinogenicity of Cr(VI) to humans by inhalation, and due to uncertainty about its long-term effects on human health via ingestion in drinking water, the USEPA has set a maximum contaminant level for total Cr [Cr(III)-plus-Cr(VI)] in drinking water in the United States of 100 mg/L.[3] This valence-independent standard is based on research results that showed no observed adverse effects of Cr(III) or Cr(VI) at 25,000 mg/L in drinking water given to rats, and after factoring in ‘‘uncertainty’’ and ‘‘safety’’ factors. The standard is based on total, soluble Cr (rather than Cr(VI) alone) because USEPA assumed that (a) Cr(III) is in dynamic equilibrium with Cr(VI) and could be oxidized, (b) the reduction of Cr(VI) to Cr(III) in the stomach and digestive tract is incomplete, and (c) despite the low toxicity of Cr(III), it may react with DNA in cells. The State of California has proposed the first valence-specific drinking water standard (public health goal) for Cr(VI) at 2.5 mg/L,

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an action based on a desire to be highly-protective of human health and drinking water quality.[13]

SOLUBILITY CONTROLS OF CHROMIUM CONCENTRATIONS IN WATER Most inorganic compounds of Cr(III) are less soluble in water than are those of Cr(VI) because Cr(III) cations have high ionic potentials (charge-to-size ratio) and hydrolyze to form covalent bonds with OH ions (Table 1). When three OH anions surround the Cr3þ cation, it is particularly stable in water as the sparinglysoluble compound, Cr(OH)3 (Table 2). Upon aging and dehydration, Cr(OH)3 slowly converts to the more crystalline, less soluble Cr2O3.[12] Incorporation of Fe(III) or Fe(II) into solid phases and precipitates containing Cr(III) renders the Cr(III) less soluble, often by a factor of 1000 in the solubility product (Ksp).[14,15] In the pH range of 5.5–8, Cr(III) reaches minimum solubility in water due to this hydrolysis and precipitation

Chromium

107

Table 2 Solubility in water at pH 7 of selected chromium compounds Chromium (III)

Chromium (VI)

Compound name

Formula

Chromium(III)hydroxide

Cr(OH)3

Chromium(III) oxide

Cr2O3

Chromite

FeO
Cr2O3

Approximate solubility (moles Cr/L) 1012

(am)

1017

(cr)

1020

(cr)

Chromium chloride

CrCl3

Highly soluble

Chromium sulfate

Cr2(SO4)3

Highly soluble

Chromium phosphate

CrPO4

Chromium fluoride

CrF3

Chromium arsenate

CrAsO4

1010

Potassium chromate

K2CrO4

3.2

Sodium chromate

Na2CrO4

5.4

Calcium chromate

CaCrO4

0.14

Barium chromate

BaCrO4

1.7  103

‘‘Zinc yellow’’ pigment

3ZnCrO4
K2CrO4
Zn(OH)2
2H2O

8.2  103

Strontium chromate

SrCrO4

5.9  103

Lead chromate

PbCrO4

1.8  106

Chromium jarosite

KFe3(CrO4)2(OH)6

reaction, an important process that controls the movement of Cr(III) in soils enriched with industrial waste waters and solid materials. Under strongly acidic concations ditions (pH < 4), unhydrolyzed Cr(H2O)3þ 6 exist in solution; while Cr(OH) 4 forms under strongly alkaline conditions (pH > 11), particularly in response to adding base to solutions of soluble salts of Cr(III), e.g., CrCl3, Cr(NO3)3, or Cr2(SO4)3. Other anions besides OH coordinate with Cr(H2O)3þ 6 and displace water molecules of hydration to form sparingly-soluble compounds and soluble chelates (Table 2). In water treatment facilities and in 2 3 natural waters; phosphate (H2PO 4 HPO4 , PO4 ),  2 3 arsenate (H2AsO4 , HAsO4 , AsO4 ) and fluoride (F) may form low solubility compounds with Cr(III). Organic complexes of Cr(III) with carboxylic acids (RCOOH, e.g., citric, oxalic, tartaric, fulvic) remain soluble at pH values above which Cr(OH)3 forms. By increasing the solubility of Cr(III) in neutral and alkaline waters, such organic complexes enhance the potential for absorption of Cr(III) by cells. Stable, insoluble complexes of Cr(III) also form with humic acids and other high molecular aggregate weight organic moieties in soils, sediments, wastes, and natural waters.[16] With the exception of chromium jarosite (Table 2), Cr(VI) compounds are more soluble over the pH range of natural waters than are those of Cr(III); thereby leading to the greater concern about the potential mobility and bioavailability of Cr(VI) than Cr(III) in natural waters. The alkali salts of Cr(VI) are highly soluble, CaCrO4 is moderately soluble, and PbCrO4 and BaCrO4 are only sparingly-soluble. In colloidal environments containing aluminosilicate clays and

© 2008 by Taylor & Francis Group, LLC

1010 1.2  103

(cr)

1030

(hydr)oxides of Al(III), Fe(II,III), and Mn(III,IV) (e.g., in soils and sediments), Cr(VI) anions may be adsorbed similarly to SO2 4 . Low pH and high ionic 2 strength promote retention of HCrO 4 and CrO4 on positively-charged sites, especially those associated with colloidal surfaces dominated by pH-dependent charge. Such electrostatic adsorption may be reversible, or the sorbed Cr(VI) species may gradually become incorporated into the structure of the mineral

Fig. 1 Seesaw model depicting a balance of the oxidation of Cr(III) by Mn(III,IV)(hydr)oxides and the reduction of Cr(VI) by organic compounds, with the pH acting as a sliding control (master variable) on the seesaw to set the redox balance for given quantities and reactivities of oxidants and reductants. The equilibrium quantity of Cr(VI) in the water is indicated by the pointing arrow from the fulcrum.

Chemical– Desalination

Oxidation state of Cr

108

Chromium

Chemical– Desalination

Fig. 2 Eh–pH diagram illustrating the stability fields defined by Eh (redox potential relative to the standard hydrogen electrode, SHE) and pH for Cr(VI) and Cr(III) at 104 M total Cr. The vertical dashed lines indicate semi-quantitatively the pH range in which Cr(OH)3 is expected to control Cr(III) cation activities in the absence of other ligands besides OH.

surface (chemisorption). Recently-precipitated Cr(OH)3 can adsorb Cr(VI) or incorporate Cr(VI) within its structure as it forms, thereby forming a Cr(III)–Cr(VI) compound.[17]

OXIDATION–REDUCTION CHEMISTRY OF CHROMIUM IN NATURAL WATERS The paradox of the contrasting solubilities and toxicities of Cr(III) and Cr(VI) in natural waters and living systems is complicated by two electron transfer reactions: Cr(III) can oxidize to Cr(VI) in soils and natural waters; and Cr(VI) can reduce to Cr(III) in the same systems, and at the same time. Understanding the key electron transfer processes (redox) and predicting environmental conditions governing them are central

to treatment of drinking water, waste waters, contaminated soils, and to predicting the hazard of Cr in natural systems.[18] The metaphor of a seesaw (Fig. 1) is useful in picturing the undulating nature of the changes in Cr speciation in water due to oxidation of Cr(III) and reduction of Cr(VI). A balance for the two redox reactions is achieved in accordance with the quantities and reactivities of reductants and oxidants in the system (e.g., organic matter and Mn(III,IV) (hydr)oxides), as modulated by pH as a master variable.[8] The thermodynamics (energetics predicting the relative stability of reactants and products of a chemical reaction) of the interconversions of Cr(III) and Cr(VI) compared to other redox couples can be used to predict the predominance of Cr(III) or Cr(VI) in water supplies (Fig. 2). The Eh variable defines the predicted

Fig. 3 Eh–pH diagram showing potential oxidants for Cr(III) in natural waters as dashed lines above the bold Cr(VI)–Cr(III) line; and potential reductants for Cr(VI) below the line. Each line for an oxidant (first species of the pair) and reductant (second species) combination represents the reduction potential (in mV) at a given pH established by that oxidant–reductant pair (e.g., O3–O2). The oxidant member of a pair for a higher line is expected to oxidize the reductant member of the lower line, thereby establishing the area and species between the lines as thermodynamically favored to exist at chemical equilibrium.

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voltage (electron pressure) that must be applied at a given pH to reduce Cr(VI) to Cr(III), and this pressure increases (lower Eh values) as pH increases, as shown in an Eh–pH diagram (Fig. 2). Certain electron-poor species may act as oxidants (electron acceptors) for Cr(III), especially soluble forms of Cr(III), in the treatment of water supplies or in soils enriched with Cr(III) (Ref.[19], Fig. 3). Examples are those above the bold line for Cr(VI)–Cr(III) on the Eh–pH diagram: Cl2, OCl, H2O2, O3, and MnOOH. In contrast, electronrich species may donate electrons to electron-poor Cr(VI) and reduce it to Cr(III): Fe2þ [or Fe(0)], H2S, H2, ascorbic acid (and organic compounds, generally), and SO2. Sunlight may affect the kinetics of both oxidation and reduction reactions for Cr, a relevant fact for natural processes in lakes and streams and for treatment technologies for drinking water purification. Depending on pH, temperature, and the concentrations of oxidants and reductants, Cr(VI)-to-Cr(III) ratios in natural waters may be predicted. Predictions of the likelihood of Cr(III) oxidation and Cr(VI) reduction occurring are important for water treatment and for establishing health-based regulations and allowable limits for Cr(VI) and Cr(III) in water supplies. In agricultural soil–plant–water systems, Cr(VI) added in irrigation water or formed via oxidation of Cr(III) will reduce to Cr(VI) if electron donors (e.g., Fe2þ, H2S, and organic matter) and Eh– pH conditions are sufficiently reducing (Refs.[4,17] Fig. 2). If not reduced, Cr(VI) may leach from surface soils to subsoils and groundwater. Therefore, predictions of Cr bioavailability and mobility in natural waters must consider redox reactions of this heavy metal.

REFERENCES 1. Anderson, R.A. Essentiality of chromium in humans. Sci. Tot. Environ. 1989, 86, 75–81. 2. Kimbrough, D.E.; Cohen, Y.; Winer, A.M.; Creelman, L.; Mabuni, C. A critical assessment of chromium in the environment. Crit. Rev. Environ. Sci. Technol. 1999, 29 (1), 1–46. 3. Goldhaber, S.; Vogt, C. Development of the revised drinking water standard for chromium. Sci. Tot. Environ. 1989, 86, 43–51. 4. Bartlett, R.; James, B. Behavior of chromium in soils: III. oxidation. J. Environ. Qual. 1979, 8, 31–35.

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5. James, B.R.; Petura, J.C.; Vitale, R.J.; Mussoline, G.R. Oxidation–reduction chemistry of chromium: relevance to the regulation and remediation of chromatecontaminated soils. J. Soil Contamin. 1997, 6 (6), 569–580. 6. Bartlett, R.J. Chromium redox mechanisms in soils: should we worry about Cr(VI)? Chromium Environmental Issues; FrancoAngeli: Milan, 1997; 1–20. 7. James, B.R. Remediation-by-reduction strategies for chromate-contaminated soils. Environ. Geochem. Health 2001, 23 (3), 175–179. 8. James, B.R. The challenge of remediating chromiumcontaminated soil. Environ. Sci. Technol. 1996, 30, 248A–251A. 9. Fendorf, S.; Wielenga, B.W.; Hansel, C.M. Chromium transformations in natural environments: the role of biological and abiological processes in chromium(VI) reduction. Intern. Geol. Rev. 2000, 42, 691–701. 10. Nriagu, J.O. Production and uses of chromium. In Chromium in the Natural and Human Environments; Nriagu, J.O., Nieboer, E., Eds.; Wiley-Interscience: New York, 1988; 81–103. 11. Barnhart, J. Chromium chemistry and implications for environmental fate and toxicity. J. Soil Contamin. 1997, 6 (6), 561–568. 12. Ball, J.W.; Nordstrom, D.K. Critical evaluation and selection of standard state thermodynamic properties for chromium metal and its aqueous ions, hydrolysis species, oxides, and hydroxides. J. Chem. Eng. Data 1998, 43, 895–918. 13. Morry, D. Public health goal for chromium in drinking water. Office of Environmental Health Hazard Assessment, California Environmental Protection Agency, 1999; 1–20. 14. Rai, D.; Sass, B.M.; Moore, D.A. Chromium(III) hydrolysis constants and solubility of chromium(III) hydroxide. Inorg. Chem. 1987, 26, 345–349. 15. Sass, B.M.; Rai, D. Solubility of amorphous chromium(III)–Iron(III) hydroxide solid solutions. Inorg. Chem. 1987, 26, 2228–2232. 16. Nieboer, E.; Jusys, A.A. Biologic chemistry of chromium. In Chromium in the Natural and Human Environments; Nriagu, J.O., Nieboer, E., Eds.; WileyInterscience: New York, 1988; 21–80. 17. James, B.R.; Bartlett, R.J. Behavior of chromium in soils: VII. adsorption and reduction of hexavalent forms. J. Environ. Qual. 1983, 12, 177–181. 18. James, B.R. Redox phenomena. In Encyclopedia of Soil Science, 1st Ed.; Lal, R., Ed.; Marcel Dekker, Inc.: New York, 2002; 1098–1100. 19. James, B.R.; Bartlett, R.J. Redox phenomena. In Handbook of Soil Science; Sumner, M.E., Ed.; CRC Press: Boca Raton, 2000; B169–B194.

Chemical– Desalination

Chromium

Conservation: Tillage and No-Tillage Paul W. Unger Conservation and Production Research Laboratory, Agricultural Research Service (USDA-ARS), U.S. Department of Agriculture, Bushland, Texas, U.S.A.

Chemical– Desalination

INTRODUCTION Conservation tillage is any tillage or tillage and planting system that results in at least a 30% cover of crop residues on the soil surface after planting the next crop.[1] It is used mainly to control soil erosion, but it also helps conserve water. In comparison, conventional tillage refers to tillage operations normally used for crop production that bury most residues and result in 0.

Plant Water Relations Crop water uptake, soil evaporation, rainfall infiltration and runoff, and soil water redistribution and drainage are simulated within a crop model’s soil water balance. Daily crop water uptake from the soil is a consequence of the balance between crop water demand and soil water supply. Crop water use is strongly correlated with biomass production[7]—as the leaf stomata open in order to take up CO2 for photosynthesis, water is also lost in transpiration. This relationship between the potential amount of daily biomass produced relative to the

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123

amount of water transpired represents an apparent transpiration efficiency (TE, g m2 mm1). However, any direct measure of TE will change from day to day depending upon the humidity of the atmosphere, quantified as a vapor pressure deficit (VPD, kPa). On low humidity (high VPD) days, more water is required to be transpired to produce the same amount of biomass as on high humidity (low VPD) days. In many crop models, a T coefficient (TEC, kPa) is set for each crop species and it represents the inherent efficiency of biomass production per water use—its units are kPa/ g m2/mm1, which collapses to simply kPa if one considers that 1 kg ¼ 1 m3 water. Values of TEC are generally higher in C4 crops (0.009 kPa) than in C3 crops (0.005 kPa). Daily crop transpiration demand (DEp, mm) can thus be estimated by DEp ¼ DW  n=ðt  1000Þ where t is TEC (kPa), n is VPD (kPa), DW (g m2) is the potential daily increase in crop biomass and 1000 is the factor for converting weight:volume of water. An alternative approach to estimating crop transpiration demand is to assume that atmospheric demand, set by daily potential evapotranspiration (EO), drives crop transpiration such that DEp ¼ EO I=I o where I/Io is proportional daily light interception as calculated from Beer’s Law. Potential evapotranspiration can be calculated from climatic parameters by either the Penman–Monteith[8] or the Priestley– Taylor[9] or simply derived from measurements of pan evaporation. Soil water supply to a crop is defined as the maximum amount of soil water that can be extracted on a daily basis from the root zone. Crop water supply is therefore a function of rooting depth, the amount of plant available water in each soil layer, and the ability of the crop roots to extract soil water. Depth of rooting is generally assumed to increase from soon after emergence at a constant rate (10–30 mm day1) until either the maximum depth of the soil profile is reached or until root extension ceases at a nominated phenological stage (usually around flowering). The calculation of available water for each soil layer is the difference between the soil water content (SW, mm mm1) on a day and the crop lower limit (CLL, mm mm1) of soil water content. The potential root water uptake (o, mm) by plants from each soil layer can be calculated as a function of available soil water, root length density, and the diffusivity of water per unit of root length

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Crop Development Models

124

(mm3 mm1).[5] However, neither root length density nor water uptake per unit of root length is easily determined experimentally. Alternatively, soil water supply from a layer (Si, mm) can be simulated as Si ¼ fi kli Chemical– Desalination

where fi (mm) is the available soil water and kli is the rate constant for layer i. The kl constant for each layer is empirically derived from experimental data on crop water extraction and it amalgamates the effects of both root length density and soil water diffusivity, which limit the rate of water uptake.[10] Values of kl typically vary between 0.01 for deep layers with low root length densities to 0.10 for surface layers with high root length densities. Daily assimilate accumulation, transpiration, and leaf development are decreased below potential values when water deficits occur by using the ratio of potential root water uptake to actual plant evaporative demand. Similar methods of simulating water deficit have been used to predict delays in plant phenology and seedling mortality.

APPLICATION OF CROP DEVELOPMENT MODELS Modeling is not new to research on cropping systems. In fact, modeling goes back to at least the 1950– 60s.[11,12] Since then, investment in simulation modeling has grown in line with the rapid advances in computers themselves. Well-known examples of significant and sustained modeling efforts would include the models developed at the University of Wageningen,[13] the CERES,[5] and CROPGRO[14] suite of crop models contained within the DSSAT software[15] and the APSIM systems simulation model.[16] Crop development models can be used to simulate the effects of agronomic management on crop growth and development—the comparison of alternative production scenarios using simulated crop performance is a key component of agricultural operations research. The effects of site selection, crop genotype, sowing time, sowing depth, plant population, irrigation regime, nitrogen fertilizer rate, previous cropping history, and fallowing may all be dealt with by many crop models. However, not all determinants of system performance (e.g., the incidence of pest and disease) may be addressed in any one crop model. Nevertheless, there are numerous examples of the use of simulation models in the assessment of agricultural production strategies (e.g., Ref.[17]).

© 2008 by Taylor & Francis Group, LLC

Crop Development Models

REFERENCES 1. Whisler, F.D.; Acock, B.; Baker, D.N.; Fye, R.E.; Hodges, H.F.; Lambert, J.R.; Lemmon, H.E.; McKinion, J.M.; Reddy, V.R. Crop simulation models in agronomic systems. Adv. Agron. 1986, 40, 141–208. ¨ ber den lichtfaktor in den pflan2. Monsi, M.; Saeki, T. U zengesellschaften und sein bedeutung fu¨r die stoffproduction. Jpn. J. Bot. 1953, 14, 22–52. 3. Sinclair, T.R.; Muchow, R.C. Radiation use efficiency. Adv. Agron. 1999, 65, 215–265. 4. Duncan, W.G.; Loomis, R.S.; Williams, W.A.; Hanau, R. A model for simulating photosynthesis in plant communities. Hilgardia 1967, 38, 181–205. 5. Jones, C.A.; Kiniry, J.R.; Eds. CERES-Maize: A Simulation Model of Maize Growth and Development; Texas A&M University Press: College Station, Texas, 1986. 6. Sinclair, T.R. Water and nitrogen limitations in soybeans crop production. I. Model development. Field Crops Res. 1986, 15, 125–141. 7. Tanner, C.B.; Sinclair, T.R. Efficient water use in crop production: research or re-search? In Limitations to Efficient Water Use in Crop Production; Taylor, H.M., Jordon, W.R., Sinclair, T.R., Eds.; American Society of Agronomy: Madison, WI, 1983; 1–27. 8. Monteith, J.L. Evaporation and environment. Symp. Soc. Exp. Biol. 1965, 19, 205–234. 9. Priestley, C.H.B.; Taylor, R.J. On the assessment of surface heat flux and evaporation using large-scale parameters. Mon. Weather Rev. 1972, 100, 81–92. 10. Monteith, J.L. How do crops manipulate supply and demand? Phil. Trans. R. Soc. Lond. A 1986, 316, 245–259. 11. van Bavel, C.H.M. A drought criterion and its application in evaluating drought incidence and hazard. Agron. J. 1953, 45, 167–172. 12. De Wit, C.T. Dynamic Concepts in Biology; IBP/PP Technical Meeting, Productivity of Photosynthetic Systems Models and Methods, Trebon: Czechoslovakia, 1969. 13. Penning de Vries, F.W.T.; van Laar, H.H.; Eds. Simulation of Plant Growth and Crop Production, Purdoc. Wageningen, 1982. 14. Boote, K.J.; Jones, J.W.; Hoogenboom, G. Simulation of crop growth: CROPGRO model. In Agricultural Systems Modeling and Simulation; Peart, R.M., Curry, R.B., Eds.; Marcel Dekker: New York, 1998; 651–692. 15. Uehara, G. Technology transfer in the tropics. Outlook Agric. 1989, 18, 38–42. 16. McCown, R.L.; Hammer, G.L.; Hargreaves, J.N.G.; Holzworth, D.P.; Freebairn, D.M. APSIM: a novel software system for model development, model testing, and simulation in agricultural systems research. Agric. Syst. 1996, 50, 255–271. 17. Muchow, R.C.; Bellamy, J.A.; Eds. Climatic Risk in Crop Production: Models and Management in the Semiarid Tropics and Subtropics; CAB International: Wallingford, 1991.

Crop Plants: Critical Developmental Stages of Water Stress Zvi Plaut Chemical– Desalination

Volcani Center, Institute of Soil, Water and Environmental Sciences, Agricultural Research Organization (ARO), Bet-Dagan, Israel

INTRODUCTION

Corn

Developmental stages at which crop plants are more sensitive to water deficit as compared to others are known as critical stages. Restricting water supply during these stages may affect productivity more severely than during other periods. Early irrigation timing studies[1] demonstrated that stress sensitivity was greatest from floral development through pollination. The possibility to increase water-use efficiency with minimal damage to crops by determining sensitive growth stages will be outlined based on recent studies. Most studies are concerned with two major objectives: 1) to determine sensitive growth stages in order to avoid any stress during this period; and 2) to determine insensitive growth stages in order to save water and cause minimal damage to the crop. Differences in sensitivity at different developmental stages of a given crop may depend on growing conditions, environmental factors, and crop cultivars and may thus result in disagreement among investigators.

Full irrigation of corn throughout its entire growing season was claimed to be more profitable than any irrigation regime applying deficit irrigation.[10] It was, thus, advised to reduce the area of grown corn rather than the application of water, if water is rate limiting. Other investigators claimed that corn is able to tolerate short periods of water deficit during its vegetative stage, and is more sensitive between late vegetative growth and grain filling. Optimal irrigation scheduling, at this critical stage, decreased the consumption of water with minimal yield losses.[11] Insufficient water application during this period resulted mainly in a decrease in kernel number. It seems that the discrepancies shown for corn response to irrigation timing may be explained on the basis of different plant biomass available to support grain yield. It was found that grain yield of corn was closely linked to the accumulated biomass.[12] Fig. 1 presents the range of grain yield and crop biomass of two different field experiments, which differ markedly in initial biomass production, but show similar relationship between biomass production and grain yield. In order to produce maximal biomass, sufficient water supply is needed throughout the season as shown earlier. When the produced biomass is limiting, optimal water supply is especially needed at the critical time of flowering and grain filling, when the utilization of constituents stored in the vegetative organs takes place.

FIELD CROPS Wheat Wheat was shown to be most sensitive to water stress during booting through early grain filling.[2] Water application prior to boot stage and during advanced grain filling was found to have limited effect on grain yield. The decrease in grain yield was most marked when the optimal water availability, which was 70% of total available soil water within the root zone, was reduced by 33% during the sensitive growth stage.[3] Similar results were obtained with various cultivars as well as at very different locations.[4–7] The effect of water stress at the vegetative stage was not only relatively low, but plants could easily recover from this stress.[6] However, the stage of tillering, which is prior to or at the beginning of the vegetative stage, was found to be quite sensitive to water stress.[8,9] Water stress at this stage may reduce the number of tillers, which can result in severe yield losses.

Sorghum Sorghum is sensitive to water stress at equivalent growth stages as corn, but its sensitivity is lower. The sensitive stage is from heading through grain filling.[13] A stress sensitivity index was introduced by Meyer, Hubbard, and Wilhite [14] which can be calculated from the following equation: X X Y =Yp ¼ Pð ETi = ETpi Þli ð1Þ in which Y and Yp are actual and potential yields, when moisture is not limiting. ETi and ETpi are actual and potential evapotranspiration rates for the i growth stages. The symbol P is a multiplication 125

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126

Crop Plants: Critical Developmental Stages of Water Stress

The effect of water stress on yield was most severe when it occurred during panicle development. Stress at this stage delayed anthesis and reduced the number of spikiests per panicle to 60% of the fully irrigated control while drought during grain filling decreased yield by 40% of the control only. Chemical– Desalination

Soybean No distinct sensitivity of specific developmental stages was outlined for soybean. Internode length and plant height were affected when plants were stressed during both vegetative and flowering stages.[18] This may repress the number of flowers and pods, but could also be a result for enhanced flower and pod abortion.[19] Stress at the stage of seed filling reduced the size of the seeds. Beans

factor and li is the sensitivity index of the crop to water stress at the i stage. The calculated values of li for sorghum at different growth stages were 0.04, 0.20, and 0.18 for vegetative, from panicle initiation through anthesis, and grain filling, respectively.[15] These values were much lower than those calculated for corn: 0.06–0.18, 1.54, and 0.03 for vegetative, reproductive, and ripening growth stages, respectively.[14] In sweet sorghum, the critical stage of water stress was found to be at a much earlier stage, namely during leaf growth.[16] Water stress at this stage reduced biomass by 30% when compared with unstressed plants.

The analysis of crop sensitivity to water stress at different growth stages is more appropriate if stress intensity is comparable and the quantities of water, which are applied, are similar. Typical results of such an experiment are presented in Table 1 for Black beans.[20] Seed yield was mostly decreased in both years, when water was eliminated during the reproductive stage and all yield components (pods per plant, seeds per pod, and seed weight) were affected. Moreover, plants did not make use of later applied water. Application of water to field beans (Vicia faba), during or shortly after flowering, increased bean yield even under high rates of rainfall.[21] There are other indications,[22] showing that a period of mild water stress during flowering of field beans followed by large quantities of water after flowering was optimal for achieving high grain yield. Conditions of extremely low water stress throughout the growing period may result in decreased dry-matter partitioning to reproductive organs. The sensitivity of mung bean to water stress at different growth stages was not well defined. It was outlined that irrigation at vegetative and pod development stages was needed in order to assure and improve yield.[23] However, when irrigation is applied during one developing stage only, flowering seems to be the most critical stage.[24] Nonetheless, it was recommended to avoid water stress during two out of three growth stages—vegetative, flowering, and pod filling and maturation.

Rice

Peas

Water stress occurring during the vegetative stage had a relatively small effect on grain yield in rice.[17]

Field peas responded positively to irrigation at the vegetative growth stage.[21] This was found, in particular,

Fig. 1 Maize grain yield vs. crop biomass for individual plots in Florida Exp. Solid line was obtained from linear regression analysis.

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Table 1 Seed yield, yield components, water use, and water use efficiency for black bean for 1995 and 1996 at Akron, Colorado

Treatment

Water withheld duringa

Population (plants ha
1)

Pods plant
1

Seeds pod
1

Seed wt (mg)

Seed yieldb (kg ha
1)

Water use (cm)

Water use efficiency (kg ha
1 cm
1)

— GF R V

176,700 160,300 158,500 154,800 0.5757 39,500

20.3 15.4 12.5 19.4 0.2190 8.9

3.1 3.1 2.4 3.4 0.0030 0.4

182.6 156.1 219.2 188.3 0.000 13.9

1,975 1,280 1,035 2,511 0.009 719

45.5 45.0 44.7 41.7 0.011 2.00

43.4 28.4 23.2 60.2 0.005 16.2

— GF R V

196,700 202,200 202,200 176,700 0.0494 19,200

14.6 14.5 12.1 15.1 0.0154 1.6

4.2 4.4 3.8 3.5 0.0763 0.7

203.9 186.7 180.2 215.2 0.0030 13.9

2,758 2,672 1,881 2,197 0.0012 303

31.7 35.2 31.9 26.5 0.053 5.74

87.2 76.5 59.2 83.3 0.0101 13.7

1 2 3 4 Pc LSD(0.05) 1996 1 2 3 4 P LSD(0.05) a

V ¼ vegetative state, R ¼ reproductive stage, GF ¼ grain-filling stage. Yield reported at moisture content of 0.14 kg H2O/kg dry matter. c P is the probability level of significant differences due to water stress treatments. b

for more drought-sensitive cultivars. Irrigation of field peas throughout the growing season gave, however, the highest yield but a lower water-use efficiency. Groundnuts Groundnuts seem to be most sensitive to water stress during pod development rather than during flowering or even pegging.[25] Water stress during this stage reduced pod yield by 56% as compared to 27% and 45% at the other two stages, respectively. Moreover, moderate water stress during the preflowering phase was outlined by other investigators to enhance subsequent pod growth.[26] Greater synchrony of pod set in moderately stressed plants resulted in a greater proportion of mature pods at the final harvest.

growth. The timing of the first irrigation, which terminates the early stress period, can be set according to several parameters, out of which plant water potential is mostly recommended.[28] The optimal leaf water potential for the initiation of the irrigation season was about
1.8 MPa.[28] Sunflowers Sunflowers are known as being fairly tolerant to water stress and may produce an acceptable yield under dryland farming.[29] A relief of water stress by irrigation will, however, enhance production of achenes and of achene oil considerably.[30] The most sensitive growth stage was found to be between stem elongation and the end of flowering. All yield components were found to be affected by irrigation.

Cotton Cotton is a crop of different water requirements throughout its growing season. The requirement is quite low during vegetative growth but increases during the reproductive period, which occurs in most cotton growing regions when transpiration demand is also maximal. The peak in water requirement remains throughout flowering until maturation (opening) of the first boll, when the requirement decreases again (Fig. 2). In order to avoid stress during flowering, it was recommended to apply 3–4 irrigations according to ET.[27] Exposure of the crop to water stress during vegetative growth will avoid excessive vegetative growth, and minimize the consumption of stored assimilates that may compete with reproductive

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Fig. 2 Estimated typical water use for cotton based on 30yr-mean maximum temperatures recorded at the University of Arkansas Northeast Research and Extension Center.

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VEGETABLE CROPS AND POTATOES

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Flowering and fruit set are known to be the most sensitive stage for all crops in which the fruit or grains are the edible organs like garden peas, fresh corn, tomatoes, oilseed rape, watermelon, and others. Water stress at this stage reduced yield by 43–60%, but only by 32–50% at the second most sensitive stage, which was fruit enlargement or grain filling. The vegetative and ripening stages were much less sensitive.[31,32] In vegetable crops, which are grown for their vegetative organs (as cabbage and onion), no distinct sensitivity was found at any particular developmental stage. Katerji, Mastrorolli, and Hamdy[33] who maintained similar stress intensities (according to predawn Cp) at different growth stages, came to a similar conclusion for pepper. In potatoes, which are essentially parts of the group in which vegetative organs are consumed, withholding water during tuberization caused a sharp decrease in yield and hindered physiological processes.[34] Continuous drought between tuber initiation and final tuber growth reduced yield as well. As far as quality is concerned, even slight water stress during tuber bulking was found to have deleterious effects on specific tuber gravity, dry matter and starch content, and chip yield. This implies that frequent irrigations are essential at this stage.[35] Cassava, which is mainly grown for animal feed and alcohol production from its starchy tubers, is also part of this group. Water stress during bulking of the underground tubers was found to be very detrimental.[36] This was in fact related to the leaf area and assimilates production rather than to tuber bulking.

FRUIT TREE CROPS Apples Reducing the amount of applied water during the growing season was shown to have only slight affects on fruit yield and quality.[37,38] Since restriction of shoot growth is desirable, mainly due to practical reasons, this may even be considered as a positive effect of stress. Water stress may also improve quality of apple fruits depending on timing. It was shown that withholding irrigation during the last 90 day prior to harvest resulted in advanced fruit maturity, more yellow skin color, higher total soluble solids (TSS), and increased flesh firmness during storage, and had hardly any effect on yield. Application of a similar stress during the initial 100 day after full bloom hardly improved the fruit quality. The response of irrigation withdrawal throughout the season gave similar results to late withdrawal, while the control (fully irrigated) trees

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Crop Plants: Critical Developmental Stages of Water Stress

responded similarly to the early withdrawal of irrigation.[39]

Prunes It was shown many years ago that prunes are tolerant to water stress, as it took 4 yr of no irrigation to decrease trunk growth and 5 yr to decrease fruit yield.[40] These results were obtained with widely spaced trees on a deep soil. It was found much later that prunes responded by decreased fruit load and fruit hydration, smaller fruits, and increased flowering when subjected to water stress for 75 day of the lag phase during fruit growth. This effect on fruit load was even more critical under conditions of shallow soil, as water storage was limiting.[41]

Apricots Two critical periods of water stress were outlined: the second exponential fruit growth period and immediately after harvest.[42] Water stress during the first period resulted in decreased yield and quality, as the harvested fruits were smaller. The response to the second period of stress was a reduction in yield, in the following year, due to increased drop of young fruits.

Citrus Fruit yield of clementine citrus was especially sensitive to water stress during flowering and fruit set. This was in addition to the effect of water stress on fruit yield throughout the year.[43] Fruit quality and vegetative growth were affected by water stress during the ripening period. Clementines seem to be more sensitive to water stress as compared to other citrus species, which were studied earlier.[44] Fruit size seems to be the most sensitive parameter responding to stress. Valencia oranges were also found to be stress sensitive and their critical sensitivity periods were flowering and fruit cell enlargement. In grapefruit and naval oranges, flowering and fruit cell division were most sensitive.[45] Juice content and the ratio of TSS to acid (TSS/acid) of the juice were also reduced if conditions of water stress prevailed during cell enlargement.

REFERENCES 1. Salter, P.J.; Goode, J.E. Crop Responses to Water at Different Stages of Growth; Ommonw. Agric. Bur.: Farnham Royal, Buck, England, 1967.

2. Dusek, D.A.; Musick, J.T. Deficit Irrigation of Winter Wheat: Southern Plains; ASAE Paper No. 92-2608, ASAE: St. Joseph, MI, 1992. 3. Schneider, A.D.; Howell, T.A. Methods, amounts and timing of sprinkling irrigation for winter wheat. Trans. ASAE 1997, 40 (1), 137–142. 4. Hamdy, A.; Lacirignola, C.; Akl, A. Competing Drought Conditions Through Supplementary Irrigation with Saline Water. Proceedings of 6th Drainage Workshop on Drainage and the Environment, Ljubljana, Slovenia, Apr 21–29, 1996; 718–726. 5. Ravichandran, V.; Mungse, H.B. Response of wheat to moisture stress at critical growth stages. Ann. Plant Physiol. 1997, 11 (2), 208–211. 6. Abayomi, Y.A.; Wright, D. Effects of water stress on growth and yield of spring wheat (Tricticum Aestivum L.) Cultivars. Trop. Agric. 1999, 76 (2), 120–125. 7. Zhang, H.; Wang, X.; You, M.; Liu, C. Water-yield relations and water-use efficiency of winter wheat in North China. Irrig. Sci. 1999, 19 (1), 37–45. 8. Christen, O.; Sieling, K.; Richterharder, H.; Hanus, H. Effects of temporary water-stress before anthesis on growth, development and grain-yield of spring wheat. Eur. J. Agron. 1995, 4 (1), 27–36. 9. Pal, S.K.; Verma, U.N.; Thakur, R.; Singh, M.K.; Upasani, R.R. Dry-matter partitioning of late sown wheat under different irrigation schedules. Indian J. Agric. Sci. 2000, 70 (12), 831–834. 10. Lamm, F.R.; Nelson, M.E.; Rogers, D.H. Resource allocation in corn production with water resource constraints. Trans. ASAE 1993, 9 (4), 379–385. 11. D’Andria, R.; Chiaranda, F.Q.; Lavini, A.; Mori, M. Grain yield and water consumption of ethephon-treated corn under different irrigation regimes. Agron. J. 1997, 89, 104–112. 12. Sinclair, T.R.; Bennett, J.M.; Muchow, R.C. Relative sensitivity of grain yield and biomass accumulation to drought in field-grown maize. Crop Sci. 1990, 30, 690–693. 13. Krieg, D.R.; Lascano, R.J. Sorghum. In Irrigation of Agricultural Crops; Agr. Monogr. 30, Stewart, E.A., Nielsen, D.R., Eds.; ASA, CSSA, SSSA: Madison, WI, 1990. 14. Meyer, S.J.; Hubbard, K.G.; Wilhite, D.A. A crop special drought index for Corn. I. Model development and validation. Agron. J. 1993, 85, 388–395. 15. Paes de Camargo, M.B.; Hubbard, K.G. Drought sensitivity indices for a sorghum crop. J. Prod. Agric. 1999, 12 (2), 312–316. 16. Mastrorilli, M.; Katerji, N.; Rana, G. Productivity and water use efficiency of sweet sorghum as affected by soil water deficits occurring at different vegetative growth stages. Eur. J. Agron. 1999, 11 (3/4), 207–215. 17. Boonjung, H.; Fukai, S. Effects of soil water deficit at different growth stages on rice growth and yield under upland conditions. 2. Phenology, biomass production and yield. Field Crops Res. 1996, 48 (1), 47–55. 18. Desclaux, D.; Huynh, T.T.; Roumet, P. Identification of soybean characteristics that indicate the timing of drought stress. Crop Sci. 2000, 40 (3), 716–722.

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19. Saitoh, K.; Mahmood, T.; Kuroda, T. Effect of stress at different growth stages on flowering and pod set in determinate and indeterminate soybean cultivars. Jpn. J. Crop Sci. 1999, 68 (4), 537–544. 20. Nielsen, D.C.; Nelson, N.A. Black bean sensitivity to water stress at various growth stages. Crop Sci. 1998, 38, 422–427. 21. Knott, C.M. Irrigation of spring field beans (Vicia Faba): to timing at different crop growth stages. J. Agric. Sci. 1999, 132, 407–415. 22. Grashoff, C. Effect of pattern of water supply on Vicia Faba L. 2. Pod retention and filling, and dry matter partitioning, production and water use. Neth. J. Agric. Sci. 1990, 38, 131–143. 23. Haqqani, A.M.; Pandey, R.K. Response of mung bean to water stress and irrigation at various growth stages and plant densities: II. Yield and yield components. Trop. Agric. 1994, 71 (4), 289–294. 24. De Costa, W.A.J.M.; Shanmugathsan, K.N. Effects of irrigation at different growth stages on vegetative growth of mung bean, Vigna Radiata (L.) Wilczek, in dry and intermediate zones of Sri Lanka. J. Agron. Crop Sci. 1999, 183, 137–143. 25. Golakiya, B.A. Drought response of groundnut: VII. Identification of critical growth stages most acceptable to water stress. Adv. Plant Sci. 1993, 6 (1), 20–27. 26. Nageswara Rao, R.C.; Williams, J.H.; Sivakumar, M.V.K.; Wadia, K.D.R. Effect of water deficit at different growth phases of peanut. II. Response to drought during preflowering phase. Agron. J. 1988, 80, 431–438. 27. Vories, E.D.; Glover, R.E. Effect of irrigation timing on cotton yield and earliness. Proc. Beltwide Cotton Conf. 2000, 1439–1440. 28. Wrona, A.F.; Kerbt, T.; Shouse, P. Effect of irrigation timing on yield and earliness of five cotton varieties. Proc. Beltwide Cotton Conf. 1995, 1108–1109. 29. Cox, W.J.; Jolliff, G.D. Growth and yield of sunflower and soybean under soil water deficit. Agron. J. 1986, 78, 226–230. 30. Hedge, M.R.; Havanagi, G.V. Effect of moisture stress at different growth phases on seed setting and yield of sunflower. Karnataka Jour. Agric. Sci. 1989, 2 (3), 147–150. 31. Morse, R. Efficient water use—the role of irrigation at critical growth stages. Vegetable Growers News 1983, 38 (3), 1–3. 32. Champolivier, L.; Merrien, A. Effects of water stress applied at different growth stages to Brassica Napus L. Var Oleifera on yield components and seed quality. Eur. J. Agron. 1996, 5 (3–4), 153–160. 33. Katerji, N.; Mastrorolli, M.; Hamdy, A. Effect of water stress at different growth stages on pepper yield. Acta Hort. 1993, 335, 165–171. 34. DallaCosta, L.; DelleVedove, G.; Gianquinto, G.; Giovanardi, R.; Peressoti, A. Yield, water use efficiency and nitrogen uptake in potato: influence of drought stress. Potato Res. 1997, 40 (1), 19–34. 35. Gunel, E.; Karadogan, T. Effect of irrigation applied at different growth stages and length of irrigation period on quality characters of potato tubers. Potato Res. 1998, 41, 9–19.

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36. Baker, G.R.; Fukai, A.C.; Wilson, G.L. The response of cassava to water deficits at various stages of growth in the subtropics. Aust. J. Agric. Res. 1989, 40, 517–528. 37. Ebel, R.C.; Proebsting, E.L.; Patterson, M.E. Regulated deficit irrigation may alter apple maturity, quality and storage life. Hortscience 1993, 28, 141–143, E. L. 38. Ebel, R.C.; Proebsting, E.L.; Evans, R.G. Deficit irrigation to control vegetative growth in apples and monitoring fruit growth to schedule irrigation. Hortscience 1995, 30, 1229–1232. 39. Kilili, A.W.; Behboudian, M.H.; Mills, T.M. Postharvest performance of ‘‘Braeburn’’ apples in relation to withholding of irrigation at different stages of the growing season. J. Hort. Sci. 1996, 71, 693–701. 40. Hendrickson, A.H.; Veihmeyer, F.J. Irrigation experiments with prunes. Univ. Calif. Agric. Exp. Sta. Bull. 1934, 573, 1–44.

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41. Lampinen, B.D.; Shackel, K.A.; Southwick, S.M.; Olson, B.; Goldhamer, D. Sensitivity of yield and fruit quality of french prune to water deprivation at different fruit growth stages. J. Am. Soc. Hort. Sci. 1995, 120 (2), 139–147. 42. Torrecillas, A.; Domingo, R.; Galego, R.; Ruiz-Sanchez, M.C. Apricot tree response to withholding irrigation at different phenological periods. Sci. Hort. 2000, 85 (3), 201–215. 43. Ginestar, C.; Castel, J.R. Responses of young clementine citrus trees to water stress during different phenological periods. J. Hort. Sci. 1996, 71, 551–559. 44. Shalhevet, J.; Levy, Y. Citrus trees. In Irrigation of Agricultural Crops; Agr. Monogr. 30, Stewart, B.A., Nielsen, D.R., Eds.; ASA, CSSA, SSSA: Madison, WI, 1990. 45. Mostert, P.G.; Zyl, J.L.; Proft, M.P.; Verhoyn, M.N.J. Gains in citrus fruit quality through regulated irrigation. Acta Hort. 2000, 516, 123–130.

Crop Residues: Snow Capture by Donald K. McCool Chemical– Desalination

Pacific West Area, Agricultural Research Service (USDA-ARS), U.S. Department of Agriculture, Pullman, Washington, U.S.A.

Brenton S. Sharratt Agricultural Research Service (USDA-ARS), U.S. Department of Agriculture, Washington State University, Pullman, Washington, U.S.A.

John R. Morse Land Management and Water Conservation Research Unit, Agricultural Research Service (USDA-ARS), U.S. Department of Agriculture, Washington State University, Pullman, Washington, U.S.A.

INTRODUCTION The primary purpose of retaining or capturing snow by residue management in semiarid regions is to increase the supply of soil moisture for subsequent crops. Snow retention and capture are of major benefit in regions, such as the North American Great Plains, where blowing snow can result in considerable loss of moisture from the landscape. This moisture is often needed to supplement seasonal rainfall and to bolster spring crop production. In the Great Plains, rainfall during the growing season is typically low and is quickly lost from the surface or shallow depth in the soil due to high evaporative demand.[1] Managing snow on the landscape is also beneficial for moderating soil temperatures and protecting dormant plants from freezing and desiccating. This is particularly important in cold regions where lethal soil temperatures can occur in the absence of snow cover during winter. Snow cover aids in reducing the depth of soil freezing which, in combination with duration of soil freezing, can affect hydrological processes such as infiltration and runoff. Snow retention or trapping also reduces the loss of water associated with sublimation of blowing snow. Sublimation can result in a substantial loss of precipitation in cold regions. Indeed, sublimation of blowing snow has been found to comprise 15–40% of the annual snowfall in the Canadian Prairies and up to 45% of annual snowfall in the arctic region of North America.[2]

POTENTIAL FOR SOIL WATER INCREASE A relatively large portion of the annual precipitation occurs in the form of snow in the cold, semiarid regions of the United States and Canada. In the Northern

Great Plains of the United States, about 20% of the annual precipitation of 250 mm to over 500 mm occurs in the form of snow.[3,4] Steppuhn[1] indicated that in the Canadian Prairies, 20–30% of the annual precipitation of 300 mm yr 1 to over 500 mm yr 1 occurs as snowfall. Staple and Lehane,[5] Zentner et al.,[6] and Pomeroy and Gray[7] report somewhat different values for precipitation falling as snow on the Canadian Prairies. These studies suggest that from 30% to nearly 40% of annual precipitation occurs in the form of snow. The shortage of precipitation to sustain maximum crop production in cold, semiarid regions requires the use of management techniques that conserve winter precipitation. Conservation of winter precipitation is vital in the Great Plains where economical production of spring wheat requires the annual withdrawal of about 250–400 mm of water from the soil.[8] Indeed, Greb[9] suggested that sustainable crop production can only be attained in the Great Plains as a result of soil water recharge occurring from snowmelt. Crop residues are only effective in retaining or trapping blowing snow to a depth equivalent to the height of the residue.[10] Surface winds are moderated by exposed residue elements, thus deposition will occur within the residue canopy as long as the elements can effectively retard wind velocity. Once the residue canopy is filled with snow, there is little or no obstruction of the horizontal wind by the residue elements; thus, no further deposition will occur. Implicit in the capture of snow by crop residues is the recharge of moisture within the soil profile. Staple, Lehane, and Wenhardt[11] found in Saskatchewan that recharge of the soil profile from snowmelt was greater in fields covered with stubble than in low-residue fallow. Stubble height also influences snow depth and therefore soil water recharge during winter. Soil water recharge is generally accentuated by taller stubble as a result of greater 131

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retention or trapping of snow in taller stubble during winter.[12,13] The extent of recharge, however, is dependent on the rapidity of snowmelt as well as on other soil physical properties such as soil water content and frost depth. Soil frost may prevent infiltration of snowmelt and enhance runoff. Thus, snow retention does not always impact soil water. In fact, Sharratt[10] found, in the northern U.S. Corn Belt, that stubble height had little effect on over-winter changes in soil water content despite large differences in snow depth. Soil water recharge was greater for stubble cut near the soil surface than at either a 30-cm or 60-cm height despite a thicker snow pack in the 30-cm and 60-cm stubble. Although Greb, Smika, and Black[14] found that loose stubble, as a result of autumn tillage, was less efficient at retaining blowing snow than undisturbed, well-anchored stubble at several locations across the Great Plains, soil water storage in early spring was the same for the loose and well-anchored stubble.

EFFECT OF ADDITIONAL WATER ON CROP YIELD In the semiarid region of Saskatchewan, yield of wheat is dependent on precipitation received during the growing season as well as on soil water reserves at the time of sowing. de Jong and Rennie[15] found that spring wheat yield was influenced to a greater extent by precipitation than by soil water reserves. They reported a yield increase of 70–200 kg ha 1 for every 25-mm increase in growing season precipitation, but did not find a positive yield response to an increase in soil water storage at the time of sowing. In contrast, a 10-yr study at Swift Current, Saskatchewan indicated an average spring wheat yield increase of 80 kg ha 1 for a 13-mm increase in soil water storage due to enhanced snow cover from a trap strip maintained in a wheat stubble field. However, the yield response to soil water storage doubled during years with a dry growing season.[6] Staple and Lehane[16] summarized 12 yr of data on water use by spring wheat at seven Experimental Substations in southern Saskatchewan and found an average yield increase of 235 kg ha 1 for each additional 25 mm of water used by the crop. In the Northern Great Plains of the United States, wheat yield is also dependent on precipitation and soil water storage. Cole[17] summarized results from studies at a number of sites of the relationship of spring wheat yield to annual precipitation and found yield increases of 145–215 kg ha 1 from each 25-mm increase in precipitation above a base of 205–255 mm. A positive yield response of wheat to stored soil moisture was found by Johnson[18] who reported an increase in yield

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Crop Residues: Snow Capture by

of 50–290 kg ha 1 for every 25 mm of water stored over winter. Crop residues may not always bolster wheat yield as a result of enhancing soil water recharge over winter. Indeed, Cutworth and McConkey[19] found that yield was greater when wheat was grown in taller stubble due to a more favorable microclimate in taller rather than shorter stubble during the growing season. In the absence of differences in soil water content in the early growing season, greater yield in taller stubble was attributed to lower evaporative demands in taller than in shorter stubble.

EFFECT ON SOIL TEMPERATURE, FROST, AND RUNOFF A series of studies on the effect of corn stubble height and residue cover on soil temperature, frost depth, and spring thaw have been conducted by the Agricultural Research Service in West-Central Minnesota. In a 3-yr study by Benoit et al.,[20] tillage and residue practices, which retained more residue on the soil surface, resulted in greater snow cover and therefore reduced frost depth and hastened thaw and warming of the soil in early spring. They found practices that retained or trapped an additional 0.1 m of snow on the surface reduced seasonal frost penetration by 0.21 m. In a subsequent study, Sharratt, Benoit, and Vorhees[21] found that standing corn stubble trapped more snow during winter and hastened warming and thawing (by as much as 20 day) of the soil as compared with prostrate corn residue or a bare soil. They later found that soil under 60-cm height corn stubble thawed as much as 15 day earlier than under 30-cm height stubble, and at least 25 day earlier than under 0-cm height stubble. However, net radiation and maximum soil temperature in the early spring were generally higher for soils without residue cover vs. soils with residue cover.[10] Taller stubble not only has a larger capacity to trap more snow during winter, but also prolongs the period of snow cover during winter.[10] The potential for runoff, therefore, is likely greater for taller stubble. Indeed, Willis, Haas, and Carlson[3] found that taller stubble not only hastened snowmelt runoff, but also partitioned more of the snowmelt to runoff than did shorter stubble.

TECHNIQUES TO ENHANCE SNOW CAPTURE Uniform Height Stubble Leaving a uniform cover of standing stubble is the simplest technique of retaining snow on the soil surface

Crop Residues: Snow Capture by

Alternate Height Stubble Stubble can be cut at alternate heights, high (30–60 cm) and low (15–30 cm), in subsequent passes of a swather. Snow might fill all of the short stubble and part of the tall stubble. The taller stubble might not fill completely. Pomeroy and Gray[7] indicated that using the technique added 31 mm of water to the soil via snowmelt as compared to a uniform canopy of short stubble. Leave Strips Narrow (0.30 m) barriers of a standing crop can be left without harvesting. Under proper conditions, the loss of income from the grain would be offset by the cost of additional water gained from snow trapped between barriers. The barriers should be oriented perpendicular to the prevailing wind direction and spacing would be about 20 times the height, based on results generated from several locations.[1]

vs. 32 mm). Increase ranged from 0 mm in years with minimal snowfall to 48 mm in years when snow accumulations were favorable.[6] Permanent Vegetation Barriers Permanent vegetation barriers consist of rows of trees, shrubs, or plants. When used for snow management, they are often referred to as living snow fences. The amount of snow trapped by this type of barrier is influenced by the porosity and height of the vegetation. Porosity varies with species and spacing between individual trees, shrubs, or plants. For uniform spreading of snow across a field, vegetation density should be no more than 40%.[22] Snow capture is optimized when the porosity of the vegetation approaches 50%; at this porosity, the snowdrift can be expected to extend as much as 25 times the barrier height.[22] Greater barrier densities result in shorter, deeper deposits and less benefited area. Greater non-uniformity of depth of snow results in greater differences in soil drying and problems in seeding; part of the area may be too dry while other areas may be too wet to be seeded in a timely manner.[22] Snow Ridges Snow ridges formed mechanically and perpendicular to the prevailing wind can act as wind barriers and collect blowing snow. The practice has potential value but it has not been widely used. The snow trapping effect is much the same as from a solid fence.[7] If the ridge does not consolidate after plowing it can be removed by high winds,[1] and success will be diminished if melting occurs before the snow fall period is concluded.

Trap Strips

CONCLUSION

Trap strips are similar to leave strips in that narrow (0.40–0.60 m) strips of tall stubble are left in the field with stubble rows oriented perpendicular to the prevailing wind direction. Trap strips are formed by the use of a deflector attachment on the swather or combine, which bends the stems sideways, and only the heads and a small part of the stem are removed at harvest. The strips are 250–350 mm taller than adjacent stubble. The strips are more effective at trapping snow when strips and spacing are relatively narrow (width 0.75 m and spacing 5 m) than when strips are wider and more widely spaced (width 1.5 m and spacing 10 m).[7] Results of a 10-yr study at Swift Current, Saskatchewan showed trap strips conserved 13 mm more soil water than short standing stubble (45 mm

Snow capture by standing stubble has potential to increase soil moisture to enhance spring cropping in cold semiarid regions of the United States and Canada. Snow capture techniques such as permanent vegetation barriers or snow ridges are useful but require maintenance or construction each winter and are not widely used.

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REFERENCES 1. Steppuhn, H. Snow Management for Crop Production on the Canadian Prairies, Proceedings of the 48th Annual Meeting, Western Snow Conference, Laramie, WY, Apr 15–17, 1980; 50–61.

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in windy regions. If sufficient snow is available and the residue is dense, snow will fill to the top of the stubble.[10] The water density of freshly fallen snow is typically about 10% of its depth (e.g., 300 mm of snow will contain 30 mm of liquid water), but when blown and settled into stubble or drifts, the water density of snow may range from 18% to 35%.[7] Thus, stubble of 40-cm height might capture as much as 140 mm of water. The quality of standing stubble will also influence snow capture. Stubble weakened by subsurface tillage in autumn will likely bend as a result of the weight of the snow load or shear stresses exerted by the wind and thus reduce the efficiency of snow capture compared to undisturbed, well-anchored stubble.[14]

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2. Pomeroy, J.W.; Essery, R.L.H. Turbulent fluxes during blowing snow: field tests of model sublimation predictions. Hydrol. Proc. 1999, 13, 2963–2975. 3. Willis, W.O.; Haas, H.J.; Carlson, C.W. Snowpack runoff as affected by stubble height. Soil Sci. 1969, 107 (4), 256–259. 4. Chow, V.T. Handbook of Applied Hydrology; McGraw-Hill Book Company: New York, NY, 1964; 1–1418. 5. Staple, W.J.; Lehane, J.J. The conservation of soil moisture in southern Saskatchewan. Sci. Agric. 1952, 32, 36–47. 6. Zentner, B.; Campbell, C.; Seles, F.; McConkey, B. Benefits of Enhanced Snow Trapping, Agriculture Canada Research Newsletter No. 8; Swift Current, SK, Oct 15, 1993. 7. Pomeroy, J.W.; Gray, D.M. Snowcover Accumulation, Relocation and Management, National Hydrology Research Institute Science Report No. 7; 1995. 8. Gray, D.M.; Male, D.H. Handbook of Snow: Principles, Processes, Management and Use; Pergamon Press: Willowdale, Ontario, Canada, 1981; 1–776. 9. Greb, B.W. Snowfall characteristics and snowmelt storage at Akron, Colorado. Proc. Symp. Snow Manag. Great Plains, Univ. Nebraska Agric. Exp. Stn Publ. 1975, 73, 45–64. 10. Sharratt, B.S. Corn stubble height and residue placement in the northern U.S. Corn Belt. Part I. Soil physical environment during winter. Soil Tillage Res. 2002, 64 (3–4), 243–252. 11. Staple, W.J.; Lehane, J.J.; Wenhardt, A. Conservation of soil moisture from fall and winter precipitation. Can. J. Soil Sci. 1960, 40, 80–88. 12. Aase, J.K.; Siddoway, F.H. Stubble height effects on seasonal microclimate, water balance, and plant

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13.

14.

15.

16.

17.

18.

19.

20.

21.

22.

development of no-till winter wheat. Agric. Meteorol. 1980, 21, 1–20. Smika, D.E.; Whitfield, C.J. Effect of standing wheat stubble on storage of winter precipitation. J. Soil Water Conserv. 1966, 21, 138–141. Greb, B.W.; Smika, D.E.; Black, A.L. Effect of straw mulch rates on soil water storage during summer fallow in the great plains. Proc. Soil Sci. Soc. Am. 1967, 31, 556–559. de Jong, E.; Rennie, D.A. Effect of soil profile type and fertilizer on moisture use by wheat grown on fallow or stubble. Can. J. Soil Sci. 1969, 49, 189–197. Staple, W.J.; Lehane, J.J. Wheat yield and use of moisture on substations in southern Saskatchewan. Can. J. Agric. Sci. 1954, 34, 460–468. Cole, J.S. Correlations Between Annual Precipitation and Yield of Spring Wheat in the Great Plains; USDA Tech. Bull. No. 636, 1938. Johnson, W.C. Some observations on the contribution of an inch of seeding-time soil moisture to wheat yield in the great plains. Agron. J. 1964, 56, 29–35. Cutworth, H.W.; McConkey, B.G. Stubble height effects on microclimate, yield and water use efficiency of spring wheat grown in a semiarid climate on the Canadian Prairies. Can. J. Plant Sci. 1997, 77, 359–366. Benoit, G.R.; Mostaghimi, S.; Young, R.A.; Lindstrom, M.J. Tillage-residue effects on snow cover, soil water, temperature and frost. Trans. ASAE 1986, 29 (2), 473–479. Sharratt, B.S.; Benoit, G.R.; Vorhees, V.B. Winter soil microclimate altered by corn residue management in the northern Corn Belt of the U.S.A. Soil Tillage Res. 1998, 49 (3), 243–248. Brandle, J.R.; Nickerson, H.D. Windbreaks for Snow Management; EC 96-1770-X, University of Nebraska: Lincoln, 1996; 1–6.

Cyanobacteria in Eutrophic Freshwater Systems Anja Gassner Chemical– Desalination

Institute of Science and Technology, Universiti Malaysia Sabah, Kota Kinabalu, Sabah, Malaysia

Martin V. Fey Department of Soil Science, University of Stellenbosch, Matieland, Stellenbosch, South Africa

INTRODUCTION Aquatic systems in an urban environment are subjected to massive anthropogenic nutrient input in the form of either non-point (stormwater) sources or point sources (industry, sewage). Most of these water bodies progress from low-productivity or oligotrophic settings to productive mesotrophic conditions to overenriched eutrophic or hypertrophic conditions. The response to the so-called ‘‘cultural’’ eutrophication is excessive production of undesirable algae and aquatic weeds and oxygen shortages caused by their senescence and decomposition. Cyanobacterial (blue-green) algal blooms have become a serious water-quality problem around the world. From a human perspective it is desirable to prevent or minimize such processes for both aesthetic and health reasons. Algal blooms lower drinking water quality by production of often odorous and toxic compounds; the toxins produced in many blue-green algae have caused health problems for wildlife, livestock, pets, and humans in contact with contaminated water. Given the variety of uses of urban water bodies for recreation, housing development, fish farming, and nature reserves, management guidelines and increasing awareness are urgently needed. The objectives of this article are to give a short introduction to freshwater blue-green algae, the key environmental factors that lead to their proliferation, and the subsequent environmental problems and to present management strategies.

FACTORS LEADING TO CYANOBACTERIAL DOMINANCE The taxonomic composition of phytoplankton communities, the abundance and the relative dominance of the different species and groups present, undergo seasonal changes. This process of continuous community change is termed succession. Under undisturbed conditions, most phytoplankton populations are of relatively short duration. Typically, the growth

and decline cycle of one specific population lasts, on average, 4 to 8 weeks. The ‘‘seasonal paradigm’’ of phytoplankton succession[1] describes the typical pattern of phytoplankton succession corresponding to the prevailing nutrient cycle in temperate, undisturbed lakes: a spring maximum of diatoms, sometimes followed by a second maximum in the autumn, an early summer maximum of Chlorophyceae (green algae) and a late summer maximum of Cyanophyta (blue-green algae). It is generally accepted that with excess nutrients in the water column, in particular phosphorus, the phytoplankton flora deviates from the traditional seasonal community pattern with a shift toward cyanobacterial dominance. However, it must be stressed that nutrient limitation does not, in itself, provide cyanobacteria with the ability to become dominant; it is the combination of a multitude of abiotic and biotic factors. Enrichment experiments demonstrated that the maximum biomass of temperate lakes is ultimately limited by the phosphorus supply.[2] Increasing supplies of phosphorus lead to an increase of phytoplankton growth until other essential nutrients become limited. The first nutrient to become limited after phosphorus is usually nitrogen. Cyanobacteria are the only species that are able to fix atmospheric nitrogen. Whereas other algae become nitrogen limited, the ascendancy of nitrogen-fixing cyanobacteria is favored. Apart from their ability of fixing atmospheric nitrogen, cyanobacteria feature some adaptations that enable them to outcompete other species. Eutrophic conditions result in large suspended stocks of phytoplankton, which reduce light penetration. Cyanobacteria possess gas vacuoles to control buoyancy. When subjected to suboptimal light conditions, they respond by increasing their buoyancy (regulated by the rate of photosynthesis) and move nearer to the surface and hence to the light.[2] Additionally, the possession of chlorophyll a together with phycobiliproteins allows them to harvest light efficiently and to grow in the shade of other species. Cyanobacteria are supposed to be more tolerant of high pH conditions and have an additional selective advantage at times of high 135

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photosynthesis because of their ability to use CO2 as carbon source.[3] Some genera are able to offset the effects of photoinhibiting UV radiation encountered by near-surface populations. The resistance to photoinhibition is achieved by producing increased amounts of carotenoid pigments, which act as ‘‘sunscreens.’’[4] Once established, cyanobacteria are able to inhibit the growth of other algae by producing secondary metabolites that are toxic to species of other genera.[5]

CONSEQUENCES OF CYANOBACTERIAL BLOOMS Like any phytoplankton, bloom proliferation of bluegreen algae reduces water quality in terms of human water use but also results in a reduction in diversity of the aquatic species assemblage at all trophic levels. The presence of ‘‘pea soup green’’ water, the accumulation of malodorous decaying algal cells, and the buildup of sediments rich in organic matter lead to user avoidance with the associated problems and implications for water quality management. The most obvious sign of an advanced blue-green algae bloom is the formation of green ‘‘scum,’’ which leads to deoxygenation of underlying waters, subsequent fish kills, foul odors, and lowered aesthetic values of affected waters.[6] In addition, certain genera and species produce taste and odor compounds, typically geosmin and 2-methyl isoborneol, which cause non-hazardous but unpleasant problems for suppliers and users of potable water.[4] The most serious public health concerns associated with cyanobacteria arise from their ability to produce toxins. Since the first published reported incidence of mammal deaths related to a toxic cyanobacterial bloom in 1978, more then 12 species belonging to nine genera of blue-green algae have been implicated in animal poisoning.[7] For human exposure, routes are the oral route via drinking water, the dermal route during recreational use of lakes and rivers, or consumption of algal health food tablets. Toxins produced in a random and unpredictable fashion by cyanobacteria are called cyanotoxins and classified functionally into hepatotoxins, neurotoxins, and cytotoxins. Additionally, some cyanobacteria produce the lesser toxic lipopolysaccharides (LPS) and other secondary metabolites that may be of potential pharamacological use.[8] One of the most tragic encounters of humans with cyanobacterial toxins led to the deaths of 60 dialysis patients due to contaminated water supply used in a hemodialysis unit.[9] Presently, a drinking water guideline of 1 mg L 1 of toxin has been developed and implemented only for microcystin-LR.[4] Haider et al.[8] stress that the biggest challenge for water treatment procedures for the removal of cyanobacterial toxins is that one is faced with soluble and suspended substances.

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Cyanobacteria in Eutrophic Freshwater Systems

Thus, the most common treatment, chlorination, in general has been found not to be an effective process in destroying cyanotoxins.

MONITORING AND MANAGEMENT OF ALGAL BLOOMS Drinking water treatment strategies are not always successful in removing algal toxins. Thus, detection of early-stage (emergent) blooms of cyanobacteria, especially if the bloom has not started to produce toxins, is important to allow municipalities and recreation facilities to implement a response plan. It has been shown that remote sensing technology can be used to estimate the concentration and distribution of cyanobacteria through measurement of the concentration of the pigment phycocyanin.[10] Once detected, the growth of nuisance algae is prevented by the use of chemicals; the commonest is copper sulfate. Other algicides include phenolic compounds, amide derivatives, quaternary ammonium compounds, and quinone derivatives. Dichloronaphthoquinone is selectively toxic to blue-greens. The inherent problem of algicides is that on cell lysis, toxins contained in the algae cell are released into the surrounding water. In 1979, almost 150 people had to be hospitalized for treatment of liver damage after a reservoir contaminated with Cylindrospermopsis was treated with copper sulfate.[4] Biological control by zooplankton is, in principle, possible, although not always practical or effective because of the low nutrient adequacy, toxicity, and inconvenient size and shape of most blue-green algae. The only zooplankton reported to successfully graze on blue greens is Daphnia sp., but it tends to decrease with increasing nutrient content of the water[11] More effective is the use of microorganisms, as certain chytrids (fungal pathogens) and cyanophages (viral pathogens) specifically infest akinetes and other heterocysts, whereas Myxobacteriales (bacterial pathogens) can affect rapid lysis of a wide range of unicellular and filamentous blue-greens, although heterocysts and akinetes remain generally unaffected.[12] The consensus regarding the management of bluegreen algal blooms is the management of excess nutrient loads into receiving water bodies.[13,14] Management options can be divided into two broad categories: catchment management (decrease of nutrient export) or lake management (decrease of internal nutrient supply). Catchment options are, e.g., management of urban and agricultural runoff, biological and chemical treatment of wastewater, nutrient diversion, and implementation of legislation. Lake management options are dredging, chemical sediment treatment, and biomanipulation.[13]

CONCLUSIONS Cyanobacteria pose a serious threat to ecosystem health and human livelihood. From a human perspective, the most serious threat associated with bluegreens are their toxins. Routes for human exposure are the oral route via drinking water, the dermal route during recreational use of lakes and rivers, or consumption of algal health food tablets. Removal of these algae and their toxins from water bodies poses a great logistical problem. However, it is important to understand that the proliferation of blue-greens and thus the presence of their toxins is a response to human-induced ‘‘cultural’’ eutrophication. Increasing awareness of the need of proper watershed management is urgently needed among municipalities and stakeholders, especially because chlorination has been shown not to be very effective in removing toxins from the water.

REFERENCES 1. Reynolds, C.S. The Ecology of Freshwater Plankton; Cambridge University Press: Cambridge, 1983. 2. Havens, K.E.; James, R.T.; East, T.L.; Smith, V.H. N:P ratios, light limitation, and cyanobacterial dominance in a subtropical lake impacted by non-point source nutrient pollution. Environ. Pollut. 2003, 122 (3), 379–390. 3. Shapiro, J. Current beliefs regarding dominance by blue-greens: the case for the importance of CO2 and pH. Proceedings of the International Association of Theoretical and Applied Limnology, Verh. Int. Verein. Theor. Angew. Limnol. 1990, 24, 38–54.

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4. Chorus, I., Bartram, J., Eds.; WHO (World Health Organization) Toxic Cyanobacteria in Water: A Guide to Their Public Health Consequences, Monitoring and Management, 1st Ed. E and F Spon: London, 1999. 5. Carr, N.G.; Whitton, B.A. The biology of blue-green algae. Bot. Monogr. Vol. 9. University of California Press, Berkeley. 6. Codd, G.A. Cyanobacterial toxins, the perception of water quality, and the prioritisation of eutrophication control. Ecol. Eng. 2000, 16 (1), 51–60. 7. Carmichael, W.W. Health effects of toxin-producing cyanobacteria: ‘‘The cyanoHABs.’’ Hum. Ecol. Risk Assess. 2001, 7, 1393–1407. 8. Haider, S.; Naithani, V.; Viswanathan, P.N.; Kakkar, P. Cyanobacterial toxins: a growing environmental concern. Chemosphere 2003, 52 (1), 1–21. 9. Pouria, S.; de Andrade, A.; Barbosa, J.; Cavalcanti, R.; Barreto, V.; Ward, C. Fatal microcystin intoxication in haemodialysis unit in Caruaru, Brazil. Lancet 1998, 352, 21–26. 10. Richardson, L.L. Remote sensing of algal bloom dynamics. BioScience 1996, 44, 492–501. 11. Epp, G.T. Grazing on filamentous cyanobacteria by Daphnia pulicaria. Limnol. Oceanogr. 1996, 41 (3), 560–567. 12. Philips, E.J.; Monegue, R.L.; Aldridge, F.J. Cyanophages which impact bloom-forming cyanobacteria. J. Aquat. Plant Manage. 1990, 28, 92–97. 13. Harding, W.R.; Quick, A.J.R. Management options for shallow hypertrophic lakes, with particular reference to Zeekoevlei, Cape Town. S. Afr. J. Aquat. Sci. 1992, 18 (1/2), 3–19. 14. Herath, G. Freshwater algal blooms and their control: comparison of the European and Australian experience. J. Environ. Manage. 1997, 51, 217–227.

Chemical– Desalination

Cyanobacteria in Eutrophic Freshwater Systems

Dam Removal Desiree Tullos Biological and Ecological Engineering, Oregon State University, Corvallis, Oregon, U.S.A.

Chemical– Desalination

Gregory Jennings Biological and Agricultural Engineering, North Carolina State University, Raleigh, North Carolina, U.S.A.

INTRODUCTION Why Dam Removal? There are more than 75,000 dams over 5 ft in height in the United States and 40,000 dams over 50 ft tall worldwide.[1–3] The deleterious effects of these large and small dams are well documented in the literature. Dams fragment the continuity of rivers both longitudinally and laterally, restricting and delaying the exchange of materials and migration and dispersal of organisms from upstream and from the floodplain,[4–7] fragmenting key elements of the interactions and life history of riverine ecosystems.[8–10] Dams generally modify natural flow regimes by storing larger flood events and increasing base flows, consequently altering the biological and physical features of rivers.[7,11–16] Reservoirs also store sediment and modify habitats,[17,18] with geomorphic responses of the downstream reaches varying,[19,20] but generally characterized by coarsening of the surface bed material, incision of the riverbed, homogenization of bed features, bank erosion and failure, and loss of riparian vegetation and complexity[17,21–25] in response to the reduced supply of sediment. The shift from lotic to lentic environment upstream of dams results in a shift in species composition,[13] frequently displacing native taxa with exotics[26,27] and increasing opportunities for predators of endangered species.[28] Modification of temperature regimes may also occur,[11,13,15] obscuring emergence or growth cues.[11] The spatial extent of effects from larger dams is greater than small dams, with trapped sediment resulting in habitat loss and change in biological community compositions at the coast.[29] A growing concern over these adverse ecological, social, and economic impacts[30] is reflected in the increasing frequency of dam removal (Fig. 1). As Federal Energy Regulatory Commission (FERC) licenses on non-federal structures expire, dam owners may be required to meet new fish passage criteria, with the expense of fishway design and construction making removal more economically favorable. Further, with 85% of dams in the United States at the end of their 138

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working life by 2020,[31] the age of structures with their associated safety declines, as well as new policies and funding sources to support removal projects[32] also explain the increased frequency of dam removal.

LOGISTICS OF DAM REMOVAL The strategy for removing a dam has been reported as the single most critical factor for managers influencing the outcome of removal[33] and the removal of dams may occur in a number of ways. Dams may be completely removed from the river or breached and partially removed, leaving bulkheads and sills in the river as structural artifacts. Dams may be removed instantaneously or dewatered and removed in stages, allowing the channel to erode reservoir sediments more slowly over time. The previous option (instantaneous removal) tends to be more common at small dam sites where rapid headcut erosion is less likely to initiate lateral erosion through gully walls. In contrast, larger dams tend to be slated for removal by staged breaching, allowing terraces to form in the stored sediment, stabilizing the material and preventing lateral erosion. Dams tend to be removed during periods of low flow when transport capacity limits the suspension of fine sediment.[17] To further reduce the rate and extent of erosion during removal, reservoirs may be drawn down gradually, sediment screens and traps may be installed, and stability measures may be taken.[34] Whether in response to concern for downstream habitats or owing to a hazard posed by contaminants attached to reservoir sediments,[18,35] managers may choose to: (1) remove the sediment stored behind the dam, typically by dredging or by conventional excavation after reservoir drawdown; or (2) leave the sediment in place, with or without some structural stabilization of sediment stored behind the dam. Stabilization practices may include regrading, revegetating, or armoring exposed sediment to reduce erosion,[36,37] or construction of grade control structures to fix the elevation of the riverbed. Owing to the cost of removal and structural stabilization, sediment is often left in place at small dam removals, allowing the river to

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Dam Removal

Fig. 1 Frequency of dam removal in the United States in the 20th Century. (Courtesy of the H. John Heinz III Center for Science, Economics, and the Environment. Hart, D.D; Johnson, T.E; Bushaw-Newton, K.L; Horwitz, R.J; Bednarek, A.T; Charles, D.F; Kreeger, D.A; Velinsky, D.J. Dam removal: challenges and opportunities for Ecological Research and River Restoration Bioscience 2002, 52, 669–681. Copyright, American Institute of Biological Sciences.)

erode through the reservoir sediments at a rate that depends on the hydrologic regime, sorting and volume of stored sediments, removal strategy, and width of the reservoir relative to the width of the channel.[32]

UNCERTAINTY IN THIS EMERGING SCIENCE A great deal of uncertainty about the consequences of dam removal exists,[38,39] particularly related to the extent, magnitude, and timing of physical and ecological outcomes.[32,39] Confident predictions of ecosystem responses to dam removal are unrealistic,[40] and experience with and documentation of dam removal is limited.[32] Although approximately 450 dams have been removed in the last 100 years,[41] less than 5% of these removals resulted in published ecological research.[39] This work is both difficult to obtain and of limited value because of unsystematic data collection and lack of comprehensive reporting.[32,39] As a consequence of a lack of examples from which to derive expectations and insufficient documentation and analysis, the practice and science of dam removal are essentially new. An integrated scientific approach including both assessments and experiments is considered by some[38] to be our greatest need in the current development of conceptual models for the dominant processes influencing ecosystem responses to dam removal.[25] The information gained by more rigorous studies and reporting of current dam removals will help to more realistically assess and predict impacts of future dam removals. With improved quality and consistency of published monitoring data and methods, scientists, managers, and the public will be better prepared for holding informed discussions on the benefits and risks of dam removal.[32]

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OUTCOMES OF DAM REMOVAL While there is substantial uncertainty about the outcomes of dam removal, some general conclusions may be drawn about ecological responses, especially in light of some baseline information regarding the spatial and temporal influence of the dam on geomorphology of the river. Removal of dams may not reverse all of the adverse impacts previously described over all spatial scales, and recovery of river ecosystems following dam removal may not occur over immediate temporal scales. Biological recovery most likely follows geomorphic processes returning to equilibrium (Fig. 2), the timing of which will be driven by various features of the river and removal, including time of sediment accumulation, river velocity, channel gradient, and removal strategy.[42] Some features of ecosystem recovery are expected immediately following removal while others will take much longer to respond (Fig. 2), with the timing of these responses varying with dam size and influence on the river. Immediately following removal, sediment stored behind the dam is expected to mobilize both episodically and chronically.[43] An equilibrium channel will incise through the stored sediment, forming a new floodplain and increasing the supply of sediment downstream.[25,44] The short-term outcomes of the disturbance associated with removal and the subsequent reservoir erosion may initially inhibit downstream recovery. Downstream increases in suspended sediment occur for some period following removal,[45] resulting in homogenization and modification of bed features (e.g. pools and riffles) and burial of coarsegrained material,[25] spawning grounds downstream may be smothered with fine sediments, abrasive sediment may damage macrophytes,[18] algae and invertebrates may be scoured from mobile substrates and unable to attach to fine material deposited

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Chemical– Desalination Fig. 2 Timescale of geomorphic processes associated with dam removal. (Pizzuto, JE. Effects of dam removal on river form and process. Bioscience 2002, 52, 683–691. Copyright, American Institute of Biological Sciences.)

downstream[18,46] while other habitats may be smothered and inaccessible.[45] However, it may be argued that these short-term risks associated with dam removal are largely outweighed by the long-term benefits (Fig. 3) of reconnecting fragmented rivers. For example, the removal of two

dams and subsequent restoration of natural sediment transport regime along the Elwha River in Washington is expected to promote tremendous recovery of nearshore habitats for native biota, including fish, shrimp, and hardshell clams, the latter of which were replaced by exotic biota when shoreline erosion owing to

Fig. 3 Spatial and temporal framework of key responses to dam removal, including the direction of change associated with removal. (Pizzuto, JE. Effects of dam removal on river form and process. Bioscience 2002, 52, 683–691. Copyright, American Institute of Biological Sciences.)

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decreased sediment supply from the dams resulted in armoring of the substrates.[29] In many rivers regulated by large dams, it is hypothesized that temperatures in the river will decrease following dam removal. As is the case again in the Elwha River, this restoration of more natural temperature regimes is critical to recovery of salmon as current maximum daily water temperature during low water years are attributed to increased infection of Dermocystidium bacteria, which attack salmon as they migrate inland from the ocean.[40] Another benefit of dam removal is the reconnection of the channel to an active floodplain, including the backwater areas that are used for spawning and habitat by various aquatic organisms.[47,48] In addition to providing access to instream and riparian habitats, dam removal promotes restoration of food chains and habitat building processes, supporting piscivorous predators (i.e., common mergansers, great blue heron, and belted kingfishers) and juvenile fishes and other aquatic predators that will benefit from the nutrients provided by salmon carcasses.[40]

REFERENCES 1. National Research Council. Restoration of Aquatic Ecosystems: Science, Technology and Public Policy; National Academy Press: Washington, DC, 1992; 552 pp. 2. Dynesius, M.; Nilsson, C. Fragmentation and flow regulation of river systems in the Northern third of the world. Science 1994, 266, 753–762. 3. McCully, P. Silenced Rivers: The Ecology and Politics of Large Dams; Zed Books: London, 1997; 350 pp. 4. Naiman, R.J.; Decamps, H.; Pollock, M. The role of riparian corridors in maintaining regional biodiversity. Ecol. Appl. 1993, 3 (2), 209–212. 5. Drinkwater, K.F.; Frank, K.T. Effects of river regulation and diversion on marine fish and invertebrates. Aquatic Conserv.: Freshwater Marine Ecosystems 1994, 4, 135–151. 6. Staggs, M.; Lyons, J.; Visser, K. Habitat restoration following dam removal on the Milwaukee River at West Bend. Wisconsin’s Biodiversity As a Management Issue: A Report to Department of Natural Resources Managers; Wisconsin Department of Natural Resources, 1995; 202–203. 7. Stanford, J.A.; Ward, J.V.; Liss, W.J.; Frissell, C.A.; Williams, R.N.; Lichatowich, J.A.; Coutant, C.C. A general protocol for restoration of regulated rivers. Regulated Rivers: Res. Management 1996, 12, 391–413. 8. Andersson, E.; Nilsson, C.; Johansson, M.E. Effects of river fragmentation on plant dispersal and riparian flora. Regulated Rivers: Res. Management 2000, 16, 83–89. 9. Jansson, R.; Nilson, C.; Dynesius, M.; Andersson, E. Effects of river regulation on river-margin vegetation: a comparison of eight boreal rivers. Ecol. Appl. 2000, 10, 203–224.

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10. Morita, K.; Yamamoto, S.; Hoshino, N. Extreme life history change of white-spotted char (Salvelinus leucomaenis) after damming. Can. J. Fish. Aquatic Sci. 2000, 57, 1300–1306. 11. Petts, G.E. Impounded Rivers: Perspectives for Ecological Management; Wiley: Chichester, UK, 1984; 326 pp. 12. Chisholm, I.; Aadland, L. Environmental Impacts of River Regulation; Minnesota Department of Natural Resources: St. Paul, Minnesota, 1994; 31 pp. 13. Yeager, B.L. Impacts of Reservoirs on the Aquatic Environment of Regulated Rivers. TVA/WR/AB-93/1 Tennessee Valley Authority, Water Resources, Aquatic Biology Department: Norris, Tennessee, 1994. 14. Ligon, F.K.; Dietrich, W.E.; Trush, W.J. Downstream ecological effects of dams. BioScience 1995, 45 (3), 183–192. 15. Ward, J.V.; Stanford, A. The Ecology of Regulated Streams; Plenum Publishing: New York and London, 1979; 398 pp. 16. Poff, N.L.; Allan, J.D.; Bain, M.B.; Karr, J.R.; Prestegaard, K.L.; Richter, B.D.; Sparks, R.E.; Stromberg, J.C. The natural flow regime: a paradigm for conservation and restoration of river ecosytems. BioScience 1997, 47, 769–784. 17. Kondolf, G.M. Hungry water: effects of dams and gravel mining on river channels. Environ. Management 1997, 21, 533–551. 18. Wood, P.J.; Armitage, P.D. Biological effects of fine sediment in the lotic environment. Environ. Management 1997, 21, 203–217. 19. Williams, G.P.; Wolman, M.G. Downstream Effects of Dams on Alluvial Rivers; U.S. Geological Survey Professional Paper 1286: Washington, DC, 1984. 20. Collier, M.; Webb, R.H.; Schmidt, J.C. Dams and Rivers: A Primer on the Downstream Effects of Dams; US Geological Survey Circular No. 1126: Menlo Park, CA, 1996. 21. Dietrich, W.E.; Kirchner, J.W.; Ikeda, H.; Iseya, F. Sediment supply and development of coarse surface layer in gravel bedded rivers. Nature 1989, 340, 215–217. 22. Quinn, J.M.; Hickey, C.W. Magnitude of effects of substrate particle size, recent flooding, and catchment development on benthic invertebrates. New Zealand J. Mar. Freshwater Res. 1990, 24, 411–427. 23. Sear, D.A. Morphological and sedimentological changes in a gravel-bed river following 12 years of flow regulation for hydropower. Regulated Rivers: Res. Management 1995, 10, 247–264. 24. Gillilan, D.M.; Brown, T.C. Instream Flow Protection: Seeking a Balance in Western Water Use; Island Press: Washington, DC, 1997; 417 pp. 25. Pizzuto, J.E. Effects of dam removal on river form and process. BioScience 2002, 52, 683–691. 26. Moyle, P.G. Fish introduction in California: history and impact on native fishes. Biol. Conserv. 1976, 9, 101–118. 27. Meffe, G.K.; Minkley, W.I. Differential selection by flooding in stream fish communities of the arid American Southwest. In Community and Evolutionary

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28.

29.

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30.

31.

32.

33.

34.

35.

36.

37.

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Ecology of North American Stream Fishes; Matthews, W.J., Heins, D.C., Eds.; University of Oklahoma Press: Norman, Oklahoma, 1987; 93–104. Wik, S.J. Reservoir drawdown: case study in flow changes to potentially improve fisheries. J. Energy Eng. 1995, 121 (2), 89–96. DOI (Department of the Interior). Final Environmental Impact Statement: Elwha River Ecosystem Restoration; Olympic National Park: Washington, 1995; 674 pp. Pejchar, L.; Warner, K. A river might run through it again: criteria for consideration of dam removal and interim lessons from California. Environ. Management 2001, 28, 561–575. Federal Emergency Management Agency (FEMA). National dam safety program. FEMA: Washington, DC, 1999. Graf, W.L., Ed. Heinz center dam removal research: status and prospects. Proceedings of the H. John Heinz III Center for Science, Economics, and the Environment Workshop on Dam Removal, Oct 23–24, 2002. Thompson, J. Out, out, out, dam spot! the geomorphic response of river to dam removal. Science Findings 71; U.S. Department of Agriculture, Pacific Northwest Research Station: Portland, OR, 2006; 5 pp. American Society of Civil Engineers (ASCE). Task committee on guidelines for retirement of dams and hydroelectric facilities of the hydropower committee of the energy division. Guidelines for the Retirement of Dams and Hydroelectric Facilities; ACSE: New York, 1997. Stone, M.; Droppo, I.G. In-channel surficial fine grained sediment laminae. Part II: Chemical characteristics and implications for contaminant transport in fluvial systems. Hydrol. Processes 1994, 8 (2), 113–124. Harbor, J.M. Proposed measures to alleviate the environmental impacts of hydroelectric dams on the Elwha River, Washington, U.S.A. In Industrial and Agricultural Impacts on the Hydrologic Environment, Proceedings of the Second USA/CIS Joint Conference on Environmental Hydrology and Hydrogeology; Eckstein, Y., Zaporozec, A., Eds.; American Institute of Hydrology, 1993. Kanehl, P.D.; Lyons, J.; Nelson, J.E. Changes in the habitat and fish community of the Milwaukee River, Wisconsin, following removal of the Woolen Mills

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48.

Dam. North Am. J. Fish. Management 1997, 17, 387–400. Aspen Institute. Dam removal: A new option for a new century. The Aspen Institute Program on Energy, Environment and the Economy: Washington, DC, 2002. Hart, D.D.; Johnson, T.E.; Bushaw-Newton, K.L.; Horwitz, R.J.; Bednarek, A.T.; Charles, D.F.; Kreeger, D.A.; Velinsky, D.J. Dam removal: Challenges and opportunities for ecological research and river restoration. BioScience 2002, 52, 669–681. Gregory, S.; Li, J.; Li, H. The conceptual basis for ecological responses to dam removal. BioScience 2002, 52, 713–723. (AR/FE/TU) American Rivers, Friends of the Earth, Trout Unlimited. Dam Removal Success Stories: Restoring Rivers Through Selective Removal of Dams That Don’t Make Sense. Washington, DC, 1999. Bednarek, A.T. Undamming rivers: a review of the ecological impacts of dam removal. Environ. Management 2001, 27, 803–814. Stewart, G.B. Patterns and processes of sediment transport following sediment-filled dam removal in gravel bed rivers, PhD dissertation; Oregon State University: Corvallis, Oregon, 2006. Stanley, E.H.; Doyle, M.W. A geomorphic perspective on nutrient retention following dam removal. BioScience 2002, 52, 693–701. Doeg, T.J.; Koehn, J.D. Effects of draining and desilting a small weir on downstream fish and macroinvertebrates. Regulated Rivers: Res. Management 1994, 9, 263–277. Newcombe, C.P.; MacDonald, D.D. Effects of suspended sediments on aquatic ecosystems. North Am. J. Fish. Management 1991, 11, 72–82. Hill, M.J.; Long, E.A.; Hardin, S. Effects of dam removal on Dead Lake, Chipola River, Florida. Apalachicola River Watershed Investigations, Florida Game and Fresh Water Fish Commission. A WallopBreaux Project F-39-R, 1993; 12 pp. Kaufman, J.H. Effects on wildlife of restoring the riverine forest in the Rodman pool area. The Case for Restoring the Free-Flowing Ocklawaha River; Florida Defenders of the Environment: Gainesville, FL, 1992; 38–41.

Darcy’s Law

Centre for Soil and Environmental Quality, Lincoln University, Canterbury, New Zealand

INTRODUCTION In simple flow systems in nature, the fundamental law of flow is linear, i.e., the flow rate increases in direct proportion to the ‘‘driving force’’ for the flow, where the driving ‘‘force’’ is the gradient (rate of change with distance) of some so-called ‘‘potential.’’ For example, the flow of heat obeys Fourier’s law, with the heat flux proportional to the gradient of temperature, T. (T is sometimes called the thermodynamic potential). For the flow of liquid water in permeable materials, it fell to the French hydraulics engineer, Henry Philibert Gaspard Darcy (1800–1858) to show that the flow obeys a similar law (‘‘Darcy’s law’’), with the flow rate proportional to the gradient of the hydraulic potential. More generally, we now know that Darcy’s law applies to flow of most simple liquids in any porous (and permeable) medium. Historical In the early 19th century, the city of Dijon in France had a water supply among the worst in Europe.[1] Darcy was a civil engineer, born in Dijon, and set himself the task of improving the city’s water supply. He decided to investigate the filtration of water by sands and gravels, reported in 1856 in his report Les Fontaines Publiques de la Ville de Dijon.[2] Using simple but ingenious equipment (see Fig. 1), he arrived at his universal law for the mass flow of liquids in permeable materials.[3] Thus, Darcy joined the ‘‘famous four’’ who revealed the simplicity (yet paradoxically the complexity) of Nature’s most basic laws of flow: Fourier’s law for heat flow; Ohm’s law for electric current; Fick’s law for gas diffusion; and Darcy’s law for liquid flow in materials.

THEORY OF LIQUID FLOW IN PERMEABLE MEDIA Saturated Flow: Darcy’s Law Darcy[2] first established his flow equation for water flow in saturated sand (Fig. 1). He found that the flow

rate per unit cross-sectional area, q, through the pipe in Fig. 1 is proportional to the difference in head h between the ends, and inversely proportional to length L: q ¼ KðhB
hC Þ=L

ð1Þ

where K is a proportionality constant. In more general form, for flow along the x-direction: q ¼
K dh=dx

ð2Þ

Here q (in units of m sec
1) is the ‘‘Darcy velocity.’’ This is the average apparent velocity of the water, as if it were flowing across the entire area, solids as well as pores. K ¼ Ksat (also in m sec
1) is the saturated hydraulic conductivity, and dh/dx is the driving force for flow, i.e., the gradient of the hydraulic head h (meters head of water) in the direction of flow, x (m). Ksat is strongly controlled by the pore space of the permeable medium, especially the pore sizes.[4] For soils, Ksat varies enormously with texture. See Table 1. Ksat is approximately a measure of the maximum drainage rate of a soil. For example, a sandy soil may have Ksat  5  10
5 m sec
1 ¼ 180 mm hr
1 while a clay soil may have Ksat  10
8 m sec
1 ¼ 0.036 mm hr
1. Thus, a rainfall of only 1 mm hr
1 would drain freely into the sand, but would cause surface ponding on the clay. Soil structure also controls pore sizes and hence Ksat, e.g., a soil with a tightly packed platy structure will have lower Ksat than one with open, porous ‘‘granular’’ structure.

Unsaturated Flow: Buckingham–Darcy Equation Soils are mostly unsaturated, and the generalization of Darcy’s law to unsaturated flow was developed by Buckingham.[5] He reasoned that hydraulic conductivity K(y) is a function of the soil volume occupied by the conducting water, i.e., the volumetric water content y. Also, the pressure potential h becomes negative, since water is now under suction. Thus we can again assume Darcy’s law, Eq. (2), but now K ¼ K(y) decreases very rapidly as soil loses water. See Fig. 2. 143

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Chemical– Desalination Fig. 1 Horizontal pipe filled with sand to demonstrate the experiment of Darcy (1856). His original equipment was actually vertically oriented, which would add an extra term to the head difference in Eq. 1, equal to the height difference zB
zC. Source: From Ref.[6].

The actual water velocity between the grains is greater than the Darcy velocity, with an average value v ¼ q/y. Note that n is an average value, and masks a microscopically complex flow pattern in the tortuous, multisized pore space, with a range of speeds and directions. An extreme example of the microscopic variability of flow velocity occurs in water flow through soils with strong structure development, e.g., with large cracks or wormholes. Here, water can be fast-tracked along the large macropores by so-called ‘‘bypass flow.’’ (This phenomenon can fast-track contaminants to groundwater, and also contributes to the dispersion of solutes during their transport.) Note that the hydraulic conductivity depends not only on the architecture of the soil pore space, but also on the properties of water itself, especially its viscosity. For example, water’s viscosity almost doubles from 30 to 0 C. So soil water flows more slowly in winter than in summer. In order to extend Darcy’s law to other liquids, it is desirable to transform the hydraulic

Table 1 Typical values of saturated hydraulic conductivity for soils, ranging from sand to clay

Drainage class of soil Class 1: very slow

Saturated hydraulic conductivity, Ksat (mm hr
1) 250

conductivity (which is specific to water only) to a more absolute measure of the ‘‘conductivity’’ of the permeable material, independent of the fluid. This leads to the ‘‘intrinsic permeability:’’ k ¼ Kðm=rgÞ

ð3Þ

Here m is the viscosity of water, r is the density of water, and g is the acceleration due to gravity. In Eq. (3), the dependence of K on the properties specific to water has been factored out. (rg represents the ‘‘heaviness’’ of water, or the amount of pressure produced by a water column of unit height). k is now controlled only by the properties of the permeable medium, and can be used for any other liquid (e.g., oil) that might flow in the medium. Incidentally, like many famous scientists, Darcy has a unit named after him. The ‘‘darcy’’ is a unit of the intrinsic permeability k, but is in such antiquated (pre-SI) units, that it is now little used, except by petroleum engineers.[6]

Clay

Class 2: slow

Class 7: very rapid

Fig. 2 Hydraulic conductivity K(y) of two soils of contrasting texture. Ks1 and Ks2 are the conductivities at saturation. K decreases dramatically as water content y decreases. Source: From Hillel, D. Environmental Soil Physics; Academic Press: San Diego, CA, 1998; 208.

Sand

APPLICATIONS Soil Water Flow Darcy’s law is used extensively in soil science, in drainage theory, and in most models of water and solute transport in soil. Fig. 2 shows how the hydraulic conductivity function K(y) is strongly influenced by soil

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non-miscible liquids share the pore space. It occurs in oil reservoirs if they contain both oil and water. Darcy’s law was generalized in 1949 by Muskat (see Ref.[8]) to describe the more complex flow of two phases together. The generalized form of Darcy’s law is used by oil exploration scientists. This form also applies in groundwater that has been contaminated by so-called NAPLs: ‘‘non-aqueous phase liquids,’’ or organic liquids immiscible in water. Thus, Darcy’s law can be applied to oil extraction, and to the remediation of polluted groundwater.

Further Complexities Fig. 3 Schematic of an aquifer layer supplying water to a groundwater well. The aquifer’s supply capacity depends on its ‘‘transmissivity,’’ i.e., its saturated hydraulic conductivity Ksat multiplied by its thickness d. The aquifer is ‘‘confined’’ by an impermeable layer above.

texture. Paradoxically, coarse-texture soils (e.g., sands), while more conductive than fine-texture soils (e.g., silt loams) at saturation (Table 1), lose their conductive capacity at low suctions, due to rapid desaturation of their large pores. This has interesting practical applications, e.g., in sports turf soils. A gravel layer beneath a sandy root zone provides excellent drainage at saturation, but then desaturates, becomes almost nonconductive, arrests drainage from the root zone, and hence enhances root zone moisture retention.

Anisotropic materials Some properties of materials may be anisotropic, i.e., for directional phenomena (such as water flow) the controlling property depends on the direction. For example, in sedimentary deposits K perpendicular to the stratification is usually less than K parallel to it. In clay subsoils, the clay particles (typically plate-shaped) may be preferentially orientated in the horizontal plane, and vertical conductivity will then be much less than horizontal conductivity, improving the layer’s ability to act as a barrier layer to vertical contaminant transport. A result of anisotropy is that in 3-D flow, the velocity flow lines are not parallel to the head gradient. Vapor flow

Groundwater Flow Groundwater is a major source of irrigation and municipal water in many parts of the world, and is commonly pumped either from shallow groundwater layers, or from deeper aquifers, which are either permeable gravels or rock layers. Darcy’s law again applies to the saturated flow. However, groundwater hydrologists are interested in the ability of an aquifer, with thickness d, to deliver water to a well which cuts across the aquifer. See Fig. 3. The aquifer’s supply capacity is thus controlled by the product K  d of aquifer conductivity K and its thickness d, a quantity called ‘‘transmissivity.’’[7]

Two-Phase Flow: Flow in Oil Reservoirs Above, we described Darcy’s law for a single fluid. Multiphase flow occurs where several (usually two)

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Fluids can be transported in the vapor as well as the liquid phase. However, Darcy’s law applies strictly only to the mass flow of liquids, not to the flow of vapor, which moves by gas diffusion. Vapor diffusion also obeys a simple proportional law (Fick’s Law), but is driven most strongly by gradients of temperature T. This leads to ‘‘coupled flows.’’ For example, in a moist soil near the ground surface, the vertical (z-) gradient dT/dz can be very strong. Then heat and moisture flows occur simultaneously and in linkage: water is distilled from warmer to cooler regions, and carries latent heat energy, as well as the water itself.[9]

REFERENCES 1. Brown, G. Darcy’s Law: Basics and More. 2000. http:// biosystems.okstat.edu/darcy/LaLoi/index.htm (accessed May 2001).

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2. Darcy, H. Les Fontaines Publiques de la Ville de Dijon; V. Dalmont: Paris, 1856. 3. Lage, J.L. The fundamental theory of flow through permeable media, from darcy to turbulence. In Transport Phenomena in Porous Media, 1st Ed.; Ingham, D.B., Pop, I., Eds.; Pergamon: Oxford, UK, 1998; 1–4. 4. Cameron, K.C.; Buchan, G.D. Porosity and pore-size distribution. In Encyclopedia of Soil Science; Lal, R., Ed.; Marcel Dekker: NY, 2001. 5. Buckingham, E. Studies on the Movement of Soil Moisture; USDA Bur. Soils Bull.; USDA, 1907; 38 pp. 6. Fetter, C.W. Applied Hydrogeology, 4th Ed.; Prentice Hall: NJ, 2001.

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Darcy’s Law

7. De Wiest, R.J.M. Fundamental principles of groundwater flow. In Flow Through Porous Media; De Weist, R.J.M., Ed.; Academic Press: NY, 1969; Chap. 1. 8. Morel-Seytoux, H.J. Flow of immiscible liquids in porous media. In Flow Through Porous Media; De Wiest, R.J.M., Ed.; Academic Press: NY, 1969; Chap. 11. 9. Buchan, G.D. Soil temperature regime. In Soil and Environmental Analysis: Physical Methods; Smith, K.A., Mullins, C., Eds.; Marcel Dekker, Inc.: New York, 2001; 539–594. 10. McLaren, R.G.; Cameron, K.C. Soil Science. Sustainable Production and Environmental Protection; Oxford University Press: New Zealand, 1996; 122 pp.

Databases Rosemary Streatfeild

INTRODUCTION A database is an electronic bibliographic or full-text product providing access to publications in either a general or subject specific area. Information on water science can be accessed through a number of different databases, some with a focus on water resources, while others provide relational coverage. Most researchers, looking for as much information on their topics as available, will use more than one product to locate materials. Databases are available through vendors on different platforms and through variable subscription options. They differ in coverage, content, search and retrieval methods, source materials, and access modes. Most provide more than one search level, varying in the number of searchable fields and level of expertise, and employ search features such as Boolean operators, truncation and proximity methods, phrasing and limiting that differ by vendor. Help pages, indexes, and controlled-language thesauri accompany most packages to assist with searching. Other features commonly engaged by database vendors include display options, selection of citations (‘‘marking’’) for printing, downloading and emailing, saving and retrieving searches, combining searches, linking to online catalogs and full-text electronic journals, and alerting services. The addition of new materials (‘‘updating’’) occurs regularly, but each vendor follows its own schedule.

DATABASES Agnic[1] offers agricultural information provided by the National Agricultural Library (NAL), Land-Grant Universities, and other institutions, each providing information on a discrete area of emphasis. Subject coverage includes Water Quality; Government, Law and Regulations; Plant Sciences; Aquaculture and Fisheries; Earth and Environmental Sciences, and Forestry. Descriptions of each record are included, along with accessibility links. A thesaurus and search tips are provided. Update schedules and years of coverage are established by the respective institutions that provide the information.

Agricola,[2] a database provided free by the NAL, includes citations from materials relating to agriculture, forestry, life sciences, and other related disciplines dating from the 15th century. It is available also on CSA (2), Community of Science (4), Dialog (5), Ebsco (6), OCLC (9), Ovid (10), Silverplatter (11), and STN (12) platforms. The NAL version is divided into the Online Public Access Catalog (Books, etc.) and Journal Article Citation Index (Articles, etc.). It is updated daily from over 5000 sources. AGRIS,[3] sponsored by the Food and Agricultural Organization of the United Nations and available in English, Spanish, and French, offers an international perspective on agriculture and related subjects. Available on CD-ROM (updated quarterly) and on the Internet (updated monthly) from 1975, there are over 144,000 records. Sources include journal articles, books, technical reports, and gray literature from all countries that participate in the UN program. A thesaurus (AGROVOC) is provided. Coverage is in many subject areas including water resources and management, aquatic sciences and fisheries, agriculture, natural resources, and pollution. The database is bibliographic only, but information is provided about document availability. There is also a personalized search profiling service provided. Dialog (5) and Silverplatter (11) supply AGRIS on their platforms also. AgroBase, produced by the Government Research Center[4] at the National Technical Information Service, combines AGRIS and AGRICOLA into one database, with a total of over 5.5 million records, many including abstracts, with coverage from 1970 to present.[5] Topics include water quality, aquatic sciences and fisheries, hydrology, and hydroponics. It is available on the NISC (6) BiblioLine platform and is updated monthly. Applied Science and Technology Abstracts, from H. W. Wilson Co. (13), provides over 1,000,000 records from more than 600 sources in the fields of chemistry, engineering, physics, and others. The dates covered are from 1983, with abstracts provided from 1994. The database is updated weekly (WilsonWeb) or monthly (WilsonDisc). There is also a full-text version dating from 1997, which is updated four times weekly. Other platforms include Dialog (5), Ebsco (6), OCLC (9), Ovid (10), and Silverplatter (11). 147

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Aquatic Biology, Aquaculture and Fisheries Resources is an anthology of thirteen files available through NISC (6) on their BiblioLine platform, and on CD-ROM. The database dates from 1971 and is updated quarterly. It contains over 888,600 records with abstracts on the science and management of aquatic organisms and environments. Subject coverage includes the ecology, biology, nutritional and environmental aspects of fish and aquatic environments. There are 13 file sources, which include ASFA: Aquatic Sciences and Fisheries Abstracts Part1: Biological Sciences and Living Resources, and several abstracting services relating to aquaculture. ASFA: Aquatic Sciences and Fisheries Abstracts, produced by Cambridge Scientific Abstracts (2), is available through the Internet and on CD-ROM. It provides international information in the science, technology, and management of marine, freshwater, and brackish water environments and organisms. The database dates from 1978 and is updated monthly. Internet platforms include Dialog (5), Ovid (10), SilverPlatter (11), and STN (12). There are over 717,000 bibliographic records, many with abstracts, in aquaculture, biology, ecology, the environment, marine sciences, oceanography, pollution, and water. A thesaurus is available. BIOSIS Previews, from the BIOSIS organization,[6] combines Biological Abstracts with Biological Abstracts/Reports, Reviews, Meetings (RRM) to provide references from journals, books, meetings, reviews, and other publications. Platforms include Dialog (5), Ebsco (Biological Abstracts and BasicBIOSIS only) (6), ISI (7), Ovid (10), Silverplatter (11), and STN (12). It has approximately 13 million records dating from 1969 to the present from over 5500 international sources, and is updated weekly. Subject coverage is life science topics including sources relating to water. The records are bibliographic with abstracts. Biological and Agricultural Index, from H. W. Wilson Co. (13), covers the literature in the fields of biology and agriculture since 1983, including subject areas relating to water. It is available on CD-ROM (updated monthly) and through the Internet (updated weekly) on OCLC (9), Ovid (10), Silverplatter (11), and WilsonWeb and WilsonDisc platforms and is bibliographic only. Biological and Agricultural Index Plus is also available, providing full-text and abstracts of 45 journals from 1994, and updated weekly. CAB ABSTRACTS, available on CD-ROM and the Internet, is a bibliographic database compiled by CAB International (1) of its print abstracts. It covers the international literature in areas such as agriculture, forestry, horticulture, and the management and conservation of natural resources dating back at least 10 yr. Sources include scientific journals, monographs, books, technical reports, theses, reviews, conference

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Databases

proceedings, patents, annual reports, bibliographies and guides, and translated journals. There are over 3.5 million records in the database, which is updated weekly, and a thesaurus is provided. Other platforms include Dialog (5), Ovid (10), and Silverplatter (11). Chemical Abstracts is produced by Chemical Abstracts Service (CAS) (2) and made available electronically on their SciFinder Scholar platform. It is also available through Dialog (5). The database contains over 16 million citations and abstracts from journal articles, patents, reviews, technical reports, monographs, conference and symposium proceedings, dissertations, and books dating back to 1967 from over 8000 sources worldwide. Subject coverage is of all facets of pure and applied chemistry, including water and aquatic sciences. The database is updated weekly. Current Contents is available in CD-ROM and Internet formats from ISI (7) in seven discipline areas including Agriculture, Biology and Environmental Sciences, Life Sciences, and Physical, Chemical and Earth Sciences. It provides access to complete bibliographic information for a 2-yr backfile, which is updated daily, from articles, editorials, meeting abstracts, commentaries, and other significant items of over 8000 scholarly journals and more than 2000 books. Dialog (5), Ovid (10), and Silverplatter (11) make Current Contents available on their platforms. EiCompendex, available on the Internet through Engineering Village, is produced by Engineering Information, Inc.[7] and provides comprehensive interdisciplinary bibliographic information relating to engineering and technology dating back to 1970. It is updated monthly. There are over three million summaries from over 5000 sources including journal articles, technical reports, conference papers and proceedings, and Web sites. This database is also available on Dialog (5), Ebsco (6), Ovid (10), and Silverplatter (11) platforms. Environmental Sciences and Pollution Management, produced by CSA (2) and available on OCLC (9) and Ovid (10) platforms, contains over one million records about aquatic pollution, water resource issues, and other subject coverage from over 4000 sources since 1981. It is updated monthly. Abstracts and bibliographic citations are provided. General Science Abstracts, from the H.W. Wilson Co. (13), provides indexing since 1984, and abstracts since 1993 of 191 periodicals. The database focuses on student and non-specialist coverage of several fields, many relating to water resources. There are 615,000 records which are updated weekly (WilsonWeb) or monthly (WilsonDisc). A full-text version is also available for 57 periodicals dating from 1996, which is updated four times weekly. General Science Abstracts is available as well on the OCLC (9), Ovid (10), and Silverplatter (11) platforms with abstracts only.

GeoArchive, produced by Geosystems (United Kingdom) and available on circular CD-Rom and the Internet from Oxmill Publishing,[8] contains international information covering geological, hydrological, and environmental sciences dating from 1974. It is also available through Dialog (5) with over 628,000 bibliographic records and is updated monthly. It is also included in NISC’s Marine, Oceanographic and Freshwater Resources database. Sources include journals, magazines, conference proceedings, doctoral dissertations, technical reports, maps, and books. Indexing is by the thesaurus, Geosaurus. GeoRef, provided by the American Geological Institute,[9] covers the geology of North America since 1785 and international geology since 1933. It is available on CD-ROM (with monthly updates) and through the Internet (updated twice monthly) from many vendors including CSA (2), Community of Science, Inc. (4), Dialog (5), Ebsco (6), OCLC (9), SilverPlatter (11), and STN (12). There are over 2.2 million bibliographic records with abstracts from journals, books, maps, conference papers, reports, and theses, and a thesaurus is provided. The print equivalent is Bibliography and Index of Geology. Groundwater and Soil Contamination Database, provided by the American Geological Institute,[9] provides complete bibliographic information for over 60,000 references from 2500 serial titles published since 1975. It is updated quarterly with worldwide coverage of the literature in geology, hydrology, and the environment, with emphasis on reports of the U.S. Geological Survey and other US government departments. Hydrology InfoBase, produced by Geosystems (United Kingdom) and available on CD-Rom and the Internet from Oxmill Publishing,[8] is a database subset of GeoArchive (above). It covers information from international sources in all fields relating to hydrology, including geomorphology, soil science, water–rock interactions, water resources, energy, pollution, agriculture, forestry, engineering, and environment dating from 1970. It includes bibliographical references with abstracts. Marine, Oceanographic and Freshwater Resources contains materials dating back to 1964 from 14 different sources, and is available from NISC (8) on BiblioLine or CD-ROM. It is updated quarterly, with over 1,000,000 records, and provides coverage on international marine and oceanic information, and estuarine, brackish water, and freshwater environments. Subject areas include environmental quality; limnology and freshwater environments; physical oceanography; pollution, acid rain, and global warming; sea-level fluctuations; biological oceanography and ecology. Sources include Part 1: Aquatic Sciences and Fisheries Abstracts (above), Part 2: Ocean Technology, Policy

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and Non-Living Resources and Part 3: Aquatic Pollution and Environmental Quality, Oceanic Abstracts, National Oceanic and Atmospheric Administration, and others available from United States and international institutions. National Oceanic Atmospheric Administration’s Library and Information Network[10] is a 23-institution consortium providing nine collections dating back to 1820. Subject areas include hydrographic surveying, oceanography, meteorology, hydrology, living marine resources, and meteorological satellite applications. The database contains more than 127,000 bibliographic records from over 9000 serial titles, 1500 active journal subscriptions, 35,000 reports, and meteorological data publications from 100 countries, as well as 1000 rare books. Materials can be requested for loan. NTIS (National Technical Information Service) database[11] is available from the Government Research Center (URL: http://grc.ntis.gov/) and is a central source for government information. It provides access to over 2,000,000 titles produced by government agencies since 1964. Subjects include agriculture, energy, the environment, and other science and technology areas. Descriptive summaries and bibliographic records are provided. Ebsco (6), NISC (8), Ovid (10), Silverplatter (11), and USGovSearch[12] also offer access to this database through their platforms. Science Citation Index Expanded, from ISI (7) and available on CD-ROM and through the Internet as part of the Web of Science, is a citation index dating back to 1945, updated weekly. It provides bibliographic information, author abstracts, and cited references found in 3500 of the world’s leading scholarly science and technical journals covering more than 150 disciplines. The Web of Science covers more than 8000 international journals in the sciences, social sciences, arts and humanities, and offers access to electronic full-text journal articles. Selected Water Resources Abstracts (SWRA)[13] provides over 10,000 abstracts dating from 1977 (and earlier) to 1997 from worldwide technical literature covering a wide variety of topics relating to water resources. Sources include journals, monographs, conference proceedings, reports, and U.S. Government documents. Subject coverage includes groundwater, water quality, water planning, and water law and rights. The complete SWRA print records dating back to 1967 are available through Water Resources Abstracts. See also the USGS WRSIC Research Abstracts database[14] and the Universities Water Information Network.[15] Waternet, a bibliographic database provided by the American Water Works Association (AWWA),[16] provides over 50,000 citations and abstracts from journals, books, proceedings, government reports, and technical papers from publishers around the world. It is

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available in CD-ROM format, and updated twice a year. Document delivery services are offered. Water Resources Abstracts is a database maintained by CSA (2). It was formerly produced by the Water Resources Scientific Information Center[14] of the U.S. Geological Survey.[17] It is available in CDROM and Internet formats on Dialog (5), NISC (8), and Silverplatter (11) platforms. With over 360,000 records, the database provides citations and abstracts for water resources from 1967 and is updated monthly. Print sources include Water Resources Abstracts (1994–present), Selected Water Resources Abstracts (1967–1994), Water Quality Instructional Resources Information System (1979–1989), and the WRSIC Thesaurus. Water Resources Worldwide, available from NISC (8) on its BiblioLine platform and CD-ROM, provides coverage of industrial and environmental aspects of water, wastewater, and sanitation from international sources including South Africa’s WATERLIT, Canada’s AQUAREF, CAB Abstract’s Aquatic Subset and the Netherlands’ DELF T HYDRO databases. Emphases include water in arid lands, aquatic information relevant to agricultural practice, and engineering and related technological disciplines. Updated quarterly, there are over 531,300 citations and abstracts dating back to 1970.

PLATFORMS 1. CAB International:[18] CAB Abstracts. 2. CAS: Chemical Abstracts Service:[19] SciFinder Scholar. 3. CSA: Cambridge Scientific Abstracts:[20] Agricola; ASFA: Aquatic Sciences and Fisheries Abstracts; Biotechnology and Bioengineering Abstracts; Environmental Sciences and Pollution Management; GeoRef; Water Resources Abstracts. 4. Community of Science, Inc.:[21] Agricola; GeoRef. 5. Dialog:[22] Agricola; Agris; ASFA: Aquatic Sciences and Fisheries Abstracts; Applied Science and Technology Abstracts; BIOSIS Previews; CAB Abstracts; Chemical Abstracts; Current Contents; EiCompendex; GeoArchive; GeoRef; Water Resources Abstracts. 6. Ebsco Information Services[23] Ebscohost: Agricola; Applied Science and Technology Abstracts; BasicBIOSIS and Biological Abstracts; GeoRef; NTIS. 7. ISI: Institute of Science Information:[24] BIOSIS; Current Contents; Science Citation Index Expanded (Web of Science).

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8. NISC: National Information Services Corporation[25] BiblioLine: AgroBase; Aquatic Biology, Aquaculture and Fisheries Resources; Marine, Oceanography and Freshwater Resources; NTIS; Water Resources Abstracts; Water Resources Worldwide. 9. OCLC[26] FirstSearch: Agricola; Applied Science and Technology Abstracts; Biological and Agricultural Index; Environmental Sciences and Pollution Management; General Science Abstracts; GeoRef. 10. Ovid Technologies:[27] Agricola; ASFA: Aquatic Sciences and Fisheries Abstracts; Applied Science and Technology Abstracts; Biological and Agricultural Index; BIOSIS Previews; CAB Abstracts; Current Contents; EiCompendex; Environmental Sciences and Pollution Management; General Science Abstracts; NTIS. 11. Silverplatter Information, Inc.[28] SPIRS: Agricola; Agris; ASFA: Aquatic Sciences and Fisheries Abstracts; Applied Science and Technology Abstracts; Biological and Agricultural Index; BIOSIS Previews; CAB Abstracts; Current Contents; EiCompendex; GeoRef; NTIS; Water Resources Abstracts. 12. STN:[29] Agricola; ASFA: Aquatic Sciences and Fisheries Abstracts; BIOSIS Previews; GeoRef. 13. H. W. Wilson Co.[30] WilsonDisc and WilsonWeb: Applied Science and Technology Abstracts; Biological and Agricultural Index; General Science Abstracts.

REFERENCES 1. URL: http://www.agnic.org/ (accessed 3/02/2001). 2. URL: http://www.nal.usda.gov/ag98/ (accessed 9/11/ 2001). A list of journals indexed by Agricola is available at URL: http://www.nal.usda.gov/indexing/ljiarch. htm. 3. URL: http://www.fao.org/agris/ (accessed 9/17/2001). 4. URL: http://grc.ntis.gov/ (accessed 9/04/2001). 5. Email confirmation of coverage dates 9/05/2001 from GRC Help Desk: [email protected]. 6. URL: http://www.biosis.org/ (accessed 9/07/2001). 7. URL: http://www.ei.org/ (accessed 9/10/2001). 8. URL: http://www.oxmill.com/ (accessed 9/10/2001). 9. URL: http://www.agiweb.org/ (accessed 9/17/2001). 10. URL: http://www.lib.noaa.gov/ (accessed 3/03/2001). 11. URL: http://grc.ntis.gov/ntisdb.htm (accessed 9/10/ 2001). 12. URL: http://govsearch.northernlight.com/ (accessed 9/10/2001). 13. URL: http://water.usgs.gov/swra/ (accessed 3/03/2001). 14. URL: http://www.uwin.siu.edu/databases/wrsic/ 15. URL: http://www.uwin.siu.edu/dir_database/index. html

16. URL: http://www.awwa.org/waternet/index.html (accessed 9/12/2001). 17. URL: http://www.usgs.gov/ 18. URL: http://www.cabi.org/ (accessed 9/17/2001). 19. URL: http://www.cas.org/ (accessed 9/04/2001). 20. URL: http://www.csa.com/ (accessed 9/17/2001). 21. URL: http://www.cos.com/ (accessed 9/17/2001). 22. URL: http://www.dialog.com (accessed 2/24/2001). 23. URL: http://www.ebsco.com/ (accessed 9/17/2001). 24. URL: http://www.isinet.com/ (accessed 9/17/2001). 25. URL: http://www.nisc.com/Frame/NISC_products-f. htm (accessed 3/02/2001 and 9/04/2001). 26. URL: http://www.oclc.org/firstsearch/ (accessed 3/02/ 2001). 27. URL: http://www.ovid.com (accessed 9/07/2001). 28. URL: http://www.silverplatter.com (accessed 9/07/ 2001). 29. URL: http://www.cas.org/stn.html (accessed 9/18/ 2001). 30. URL: http://www.hwwilson.com/ (accessed 9/12/ 2001).

ADDITIONAL RESOURCES 1. The National Agricultural Library maintains a list with short descriptions of additional water resources data-

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2.

3. 4.

5.

6.

bases at its Water Quality Information Center web site (URL: http://www.nal.usda.gov/wqic/dbases.html). Ahn, Myeonghee Lee; Walker, R.D. Retrieval effectiveness of water resources literature. Proceedings of the 5th International Conference on Geoscience Information, Prague, Czech Republic, 1994; Geoscience Information Soc., 1996, 1–23. Clark; Katharine, E. Water resources abstracts: a review. CD-ROM Professional May 1990, 3, 102–107. Haas, Stephanie; Mae, Clark. Research journals and databases covering the field of agrochemicals and water pollution. Sci. Technol Libr. Winter 1992, 13, 57–63. Keeley; Kurt, M. AWWA’s (American Water Works Association) private files. In Private File Creation/Database Construction: A Proceeding with Five Case Studies; Hlava, M.M.K., Ed.; Special Libraries Association: New York, NY, 1984; 59–69. Pretorius, Martha. Water research, electronic journals and databases: the South African way. In Electronic Information and Publications: Looking to the Electronic Future, Let’s Not Forget the Archival Past, Proceedings of the 24th Annual Conference of the International Association of Aquatic and Marine Science Libraries and Information Centers (IAMSLIC) and the 17th Polar Libraries Colloquy (PLC), Reykjavik, Iceland, Fort Pierce, Sept 20–25, 1998; Markham, J.W., Duda, A.L., Martha, Andrews, Eds.; IAMSLIC: FL, 1999; 171–174.

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Deltas Paul F. Hudson Department of Geography and the Environment, University of Texas at Austin, Austin, Texas, U.S.A.

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INTRODUCTION The world’s major deltas are relatively young geomorphic features, having formed since the middle Holocene when sea level reached its current height. The term ‘‘delta’’ was coined by the Greek historian Herodotus about 2500 B.C. because of the similarity in shape between the Nile delta and the Greek letter D. Although marine deltas are the largest and most studied, the first conceptual model for understanding deltaic processes and sedimentary deposits was established by the famous geomorphologist G. K. Gilbert in the late 1800s in a study of paleo Lake Bonneville, U.S.A. The Mississippi delta is the most studied deltaic system in the world, although the geometry of this fluvial dominated delta is quite distinct from most other large deltas. Nevertheless, the conceptual framework developed from numerous early studies of the Mississippi delta system has been employed as a form-process model for understanding deltas around the world. Deltas are formed where a river discharges into a marine or lacustrine receiving basin and represent unique coastal environments. Debouching of sediment at a river mouth results in immediate sorting and deposition, the process of delta formation. An individual delta typically has multiple channels that branch from the main-stem river channel. These distributary channels deliver sediment to a broad area, resulting in the formation of a delta lobe. Such distributaries are typically in multiple stages of development and carry different proportions of streamflow. While deltas form at the river mouth, the delta plain includes inactive delta lobes adjacent to the active delta lobe. The major controls on delta plain evolution include those associated with the watershed (i.e., fluvial) and receiving basin. Because deltas are transitional boundaries between land and water, they are associated with a variety of freshwater, brackish, and marine environments, and thus represent diverse ecological settings.

because lake levels fluctuate more rapidly than global sea levels, and thus represent an ideal opportunity to examine the influence of base-level changes.[2] Indeed, global proliferation of dam construction during the 20th century provides a unique opportunity to examine deltaic sedimentation because of a rapid increase in base level imposed by infilling of large reservoirs, while controlling for scale (drainage area), geology, and climate. Sediment sorting and deltaic sedimentation begin as sediment is discharged into a standing body of water. This initiates a regressive sequence of deltaic deposits that prograde over marine deposits, as illustrated in Fig. 1. The basic model of sedimentation includes three major deltaic deposits: a distributary mouth bar comprises sands and coarse silts, low-angle delta front deposits comprise fine silts and clay, and thinly laminated clay deposited beyond the river mouth as a prodelta base.[3] These deposits coincide with topset, foreset, and bottomset beds, respectively, originally described by G. K. Gilbert in 1890.[4] A delta lobe is associated with the position of the main-stem channel, whereas delta plains comprise multiple delta lobes in various stages of growth and erosion. Thus, it is important to note that a delta plain is dominated by fluvial controls at the river mouth, while other processes dominate distal regions of a delta plain. A combination of these processes produces a suite of landforms associated with the stage of delta lobe development or the ‘‘delta cycle.’’ The occurrence of multiple delta lobes is evidence for delta switching, or avulsion of the main-stem channel. Subsidence begins as coarser sediments prograde over the clayey prodelta base deposits. As surface accretion rates become less relative to rates of subsidence, marine processes begin to rework older deltaic deposits.[5,6] The ‘‘delta cycle’’ (Fig. 2) is associated with distinct sequences of change, including: 1) initial progradation; 2) enlargement of delta lobe; 3) abandonment and transgression; and 4) reoccupation and growth of a new delta.

FORM AND PROCESS Initial Progradation While our understanding of deltaic processes stems largely from research on marine deltas,[1] the study of lacustrine deltas should be of increased interest 152

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Deltaic sedimentation begins because of an abrupt reduction in stream competence at the transition from

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Fig. 1 Planform: sediment mixing where river suspended sediment loads are discharged into a marine setting, a hypopycnal condition occurs because the streamflow is less dense than the saline marine waters of the receiving basin. Longitudinal: model of sediment sorting and typical deltaic deposits of a delta lobe.

channelized flow to open water, resulting in deposition of the river’s sandy bed load and formation of a river mouth bar. Channel bifurcation occurs as flow is diverted around the bar, initiating a distributary network. The distance to which the suspended sediment is transported into open water depends on discharge magnitude and wave energy, in addition to density differences between streamflow and water in the receiving basin (fresh or saline). Streamflow density is less than saline ocean water, a hypopycnal condition, resulting in a buoyant sediment plume transporting fine sediments far beyond the river mouth (Fig. 1). Under low energy conditions, silt is deposited as a sloping delta-front, but clay is transported over a much broader prodelta base. Together, these three deltaic deposits form a large subaqueous delta lobe extending far beyond subaerial distributary channels. Denser sandy distributary mouth bar deposits prograde over fine-grained delta-front and pro-delta base deposits, resulting in a coarsening-up grain size sequence, the classic sedimentological signature of a delta facies.

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Annual flooding and deposition of coarse sediments along natural levees increase their height and weight and result in channel extension.

Enlargement of the Delta Lobe After bifurcation of the main-stem river mouth, distributary channels prograde and eventually bifurcate, resulting in a distributary channel.[5] Progradation of sandy distributary channels results in a subaerial delta finger. A continuum of environments is located between adjacent distributary channels, including freshwater swamps near the channel, brackish marsh, and saline interdistributary bays toward the coast.[7] Landward of the coastal interface flooding delivers inorganic silts and clays, resulting in accretion of the delta plain. Marsh and swamp deposits result in organic delta plain peats, which can be radiocarbon dated to establish the chronology of delta evolution.[8] Large flood events occasionally breach natural levees

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Chemical– Desalination Fig. 2 The delta cycle, showing the time-dependent nature of deltaic environments: 1) initial progradation; 2) enlargement of delta lobe; 3) abandonment and transgression; and 4) reoccupation and growth of a new delta.

and deliver coarse silts and sands to low energy finegrained marsh environments. Continued diversion of suspended sediments through the crevasse outlet produces a subdelta, replicating the form and functioning of main-stem deltas within compressed spatial (approximately 10 2 km2) and temporal scales (approximately 10 2 yr).

lobe, after fluvial processes have ceased, marine processes become dominant and rework deltaic deposits into a transgressive suite of coastal landforms. The delta finger is initially reworked into flanking recurved spits, which eventually are separated from the transgressive (landward retreating) mud flats and are converted to narrow barrier islands.[9]

Abandonment and Transgression Reoccupation and Growth of a New Delta Distributary network extension and progradation has important consequences to the life-span of a delta lobe and diversity of ecological environments. Channel extension reduces stream gradient and, with diversion of streamflow into multiple channels, reduces hydraulic efficiency. This increases the probability of main-stem channel avulsion and formation of a new delta lobe. Progradation also establishes a vertical coarsening-up particle size trend. Fine-grained pro-delta base deposits are compacted by heavier sandy deposits, which initiate delta subsidence; a critical control on delta evolution. A major reason for delta subsidence is clay flocculation (clumping), which occurs when negatively charged colloidal clay enters a marine (saline) environment (Fig. 1). As denser delta finger sands prograde onto the prodelta base, porous flocculated clays are compacted, ultimately causing delta lobe subsidence. The amount of active delta lobe subsidence is generally countered by accretion of fresh flood deposits, preventing major erosion. However, in an abandoned delta

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The final stage of the delta cycle is initiation of a new delta lobe complex, or reoccupation of an old distributary course, following avulsion of the main-stem channel. This results in a vertical ‘‘stacking’’ of delta lobes, representing predictable changes in depositional and ecological environments.[7] Until the 1950s, it was believed that delta lobe switching (main-stem avulsion) was an abrupt process. However, sedimentologists now understand that the process requires hundreds of years, with the discharge of a drainage basin being shared between old and new delta lobes.[6,8] The bathymetry of the receiving basin influences the new delta lobe geometry, which is in part a function of the Holocene depositional history. A shallow and wide receiving basin results in a broad lobate delta lobe with a dendritic pattern of distributaries, such as the older Lafourche or newer Atchafalaya delta lobes within the Mississippi delta plain. In contrast, a narrow and elongate delta lobe, which extends further out to sea,

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Fig. 3 Dominant controls on delta morphology with examples of end-members. Fluvial: the delta finger (sandy distributary mouth bar and levee deposits) is seen prograding over the silty delta front and the large fine-grained prodelta base. Wave: course river deposits are reworked by strong littoral drift processes into parallel beach ridges. Tidal: the large tidal flux reworks coarser river deposits into elongated sand bars oriented with (normal) to the river channel.

is formed where the receiving basin is confined by older subaqueous delta lobes. Indeed, the modern Mississippi delta lobe extends to the edge of the continental shelf. Thus, in comparison to other major delta lobes the geometry of the classic ‘‘bird’s foot’’ Mississippi delta may be quite unique.

DELTA PLAIN STYLE Although the above model of delta formation remains widely accepted, it is important to note that the style and pattern of deltaic sedimentation varies tremendously depending on river discharge and sediment load (fluvial controls) as well as wave energy and tidal regime (marine controls) of the receiving basin.[1] Delta plain geometry is strongly related to the dominant processes that influence dispersal and reworking of fluvial sediments (Fig. 3). Early researchers[1,3,5] placed deltas into a tripartite classification scheme that considered planform geometry and fluvial, wave, or tidal

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processes. Fluvial dominated deltas extend far beyond the primary coastline and form when discharge and sediment load are deposited without being immediately reworked by waves or tides. Such deltas are ‘‘constructional,’’ with the Mississippi being the prime example (Fig. 3). Small rivers discharging to open marine environments are unlikely to form fluvial dominated deltas because of low discharge and sediment loads. However, smaller rivers flowing into coastal bays or lakes may develop elongated deltas because the energy of the receiving basin is comparatively lower. Fluvial dominated deltas, however, are probably less common than marine dominated deltas. Wave dominated deltas, such as the Sao Francisco delta in Brazil, have coarser sediments rapidly reworked into linear sand bodies, becoming spits and sand ridges oriented parallel to the coastline because of the significance of littoral drift (Fig. 3). In contrast to wave dominated deltas, tidal dominated deltas such as the Ord of Australia have large embayed river mouths and estuaries. The large tidal flux reworks sands into linear sand bodies

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oriented with the river channel or perpendicular to the coastline (Fig. 3).

CONCLUSIONS Chemical– Desalination

River deltas are young but complex geomorphic environments that exhibit spatial and temporal variability in form and process in response to fluvial and marine controls. Delta formation begins when a river debouches sediment into a standing body of water. This initiates the formation of three distinctive deltaic deposits: sandy distributary mouth bar, silty delta front, and a prodelta base of flocculated clay. River mouth progradation results in a coarsening-up vertical sequence, and, with time, denser sands compact soft underlying clay. Subsidence occurs as the distributary network shuts down because of a reduction in hydraulic efficiency. This is followed by main-stem avulsion and delta lobe switching, which may involve reoccupation of an older delta lobe. Deltas have been important to humans for millennia and continue to be of great interest because of dependence upon deltaic resources. In an era of global sea level rise and rapidly growing populations along the world’s coastlines, it is critical to understand the fundamental processes, which influence deltaic environments. The model of delta formation discussed here provides a general framework for understanding deltas. Because each river delta and the anthropogenic factors that influence it are unique, study of deltaic processes will continue to be a rich area of research for a variety of disciplines.

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Deltas

REFERENCES 1. Wright, L.D.; Coleman, J.M. Variations in Morphology of Major River Deltas as Functions of Ocean Wave and River Discharge Regimes; The American Association of Petroleum Geologists, 1973; Vol. 57, 370–398. 2. Butzer, K. Recent history of an Ethiopian delta: the Omo river and the level of lake Rudolf. In Department of Geography Research Papers; University of Chicago: Chicago, IL, 1971; 136 pp. 3. Frazier, D.E. Recent deltaic deposits of the Mississippi river. Their development and chronology. Trans. Gulf Coast Assoc. Geol. Soc. 1967, 17, 287–315. 4. Gilbert, G.K. Lake Bonneville. U.S. In Monograph 1980; Geological Survey; 1. 5. Fisk, H.N.; McFarlan, E.; Kolb, C.R.; Wilbert, L.J. Sedimentary framework of the modern Mississippi delta. J. Sediment. Petrol 1954, 24, 76–99. 6. Saucier, R.T. Geomorphology and Quaternary Geologic History of the Lower Mississippi Valley; Waterways Experiment Station, U.S. Army Corps of Engineers: Vicksburg, MS, 1994; 364 pp. 7. Coleman, J.M. Ecological changes in a massive freshwater clay sequence. Trans. Gulf Coast Assoc. Geol. Soc. 1966, 165, 159–174. 8. To¨rnqvist, T.E.; Kidder, T.R.; Autin, W.J.; Van der Borg, K.; De Jong, A.F.M.; Klerks, C.J.W.; Snijders, E.M.A.; Storms, J.E.A.; Van Dam, R.L.; Wiemann, M.C. A revised chronology for Mississippi river subdeltas. Science 1996, 273, 1693–1696. 9. Penland, S.; Boyd, R.; Suter, J.R. Transgressive depositional systems of the Mississippi delta plain: a model for barrier shoreline shelf sand development. J. Sediment. Petrol 1988, 58, 932–949.

Desalination Fawzi Karajeh Fethi BenJemaa

INTRODUCTION Desalination, also known as desalinization or desalting, is the process of removing dissolved salts and minerals from saline water to produce fresh potable water. Desalination was explored very early in history and its use goes back to ancient times, where simple distillation techniques were employed. Nowadays, brackish water and seawater desalination is becoming a common practice to meet fresh water needs in many parts of the world, especially in the arid regions of the Middle East and North Africa, and in many island communities such as the Canary Islands and Malta. Desalination technology is also used to treat wastewater effluents for reuse and to improve the quality of fresh water for potable and industrial uses. Most desalination processes are based either on thermal distillation or membrane separation technologies. Among the technologically proven and commercially utilized desalination processes are: reverse osmosis (RO), electrodialysis (ED), electrodialysis reversal (EDR), multieffect distillation (MED), multistage flash distillation (MSF), and vapor compression [mechanical (MVC) or thermal (TVC)]. Key issues associated with the use of desalination include cost, energy use, and environmental impacts of brine disposal and feedwater intake.

DESALINATION PROCESSES Thermal Processes Thermal desalination processes are based on distillation, which consists of heating a saline solution to induce water to evaporate leaving the salt behind, the vapor is then condensed to produce pure water. Simple distillation consumes a considerable amount of energy, making it impractical as a water supply option. However, modern thermal desalination processes employ more efficient distillation techniques through the regulation of the evaporation pressure and the use of more efficient heat exchangers and heat recovery methods in a multistage setting. These processes take advantage of the relationship between pressure and the boiling point, making it possible to evaporate water

Chemical– Desalination

Office of Water Use Efficiency, California Department of Water Resources, Sacramento, California, U.S.A.

at a number of successive stages, each operating at lower temperature and pressure than the preceding stage. Among such processes are MED, MSF, and vapor compression (VC). Multieffect distillation is based on the concept of a multitude of successive effects where water is repeatedly evaporated at increasingly lower pressures and temperatures without the need for additional energy. Vapor produced at a given effect is used for heating and evaporating additional quantities of water in the subsequent effect, while operating at a slightly lower pressure (Fig. 1). The vapor then passes through a heat exchanger where it condenses while preheating the incoming saline feedwater. MSF is similar to MED and consists of a multitude of successive stages operating at progressively lower pressures. Feedwater enters the system backward from the last stage toward the first; as it travels, it gains energy and serves for condensing vapor flashing at each stage. The feedwater gains sufficient heat at a high pressure until it reaches the first stage where the pressure drops and sudden evaporation (flashing) occurs. Flashing continues in each stage at lower temperatures and pressures (Fig. 2). Unlike MED, the evaporation and condensation phases in the MSF process both occur within the stage. MSF plants are generally of large scale, with capacities reaching tens of MGD, and a typical plant can be composed of 15–25 stages. VC is a distillation-based process where the evaporating heat is obtained via compressing vapor rather than direct heat exchange. Depending on the procedure used for compression, we identify two vapor compression processes: MVC, where a mechanical compressor is used (Fig. 3), and TVC, where a steam ejector is used. Generally, VC plants have built-in capacities ranging from few gallons to few MGD.

Membrane Processes Membranes are used as barriers to filter out particles, salts, and chemicals from water in a selective manner. Various types of membrane separation methods have been developed. The most widely used processes for water desalination are RO, ED, and EDR. 157

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Fig. 1 Multieffect distillation process.

RO, a pressure-driven process, is a relatively new technology gaining ground over other processes by the end of the twentieth century. Feed saline water is pressurized against a semipermeable membrane that allows the passage of water whereas salts are retained; as a result, high-quality product water is obtained and the remaining portion of feedwater, which becomes more concentrated in salt, will be rejected as brine. Osmosis is a natural phenomenon that occurs when two solutions are separated with a semipermeable membrane, causing water to flow from the side of low solute concentration to the side of high solute concentration. RO employs pressure to counteract and exceed the natural osmotic pressure, reversing the normal osmotic flow, in order to squeeze water out of a concentrated solution (Fig. 4). A typical RO plant (Fig. 5) operates in three main stages: 1) the pretreatment stage—fine filtration and addition of acid and other chemicals to inhibit bacteria growth and precipitation of sparingly soluble salts; 2) the RO module— where high-pressure pumps are used to enable water passage through the membrane; 3) the posttreatment stage—stabilization of water before its use (this includes pH adjustment and the removal of residual chemicals and gases). RO membranes are generally encapsulated in cylindrical modules in a spiral wound configuration or as hollow fibers or capillary tubes. ED and EDR are electrically driven processes where an electric potential causes the ions of the saline solution to migrate toward the electrodes passing

through selective membranes. Successions of two types of selective membranes (cation-permeable and anionpermeable) are alternatively arranged between the electrically charged electrodes. In EDR, polarity of the electrodes is periodically reversed to prevent scaling. The spaces between successive membranes (anionic and cationic) are called cells. Anions (
) pass through the anion-permeable membrane toward the positive pole. Likewise, cations (þ) pass through the cationpermeable membrane toward the negative pole. As a result of this electric separation process, the concentration of ions increases in some cells and decreases in others. Demineralized water is then collected from the cells in which the solution is diluted and a concentrate is discharged from cells where concentration increases (Fig. 6). Other membrane processes that are commonly used for water softening and treatment include nanofiltration (NF), ultrafiltration (UF), and microfiltration (MF). NF is similar to RO, except that the membranes remove multivalent salts more efficiently than monovalent salts. UF and MF have larger pore sizes that allow the excellent removal of suspended solids, but not the removal of metallic ions and salts. Other Processes A number of other desalination processes that are not widely used include freezing, air humidification and dehumidification, and membrane distillation.

Fig. 2 Multistage flash distillation process.

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water. Vapor is then condensed on the other side of the membrane to produce fresh water.

Prior to any desalination process, feedwater requires adequate pretreatment consisting primarily of the removal of suspended solids and particulates to prevent fouling and the addition of antiscalants to prevent mineral scale build-up. An optimized pretreatment process allows desalination plants to run more efficiently and at higher water recovery rates. After passing through the desalination process, desalinated water is stabilized by adding back some mineral components so as to ensure balance and optimum taste for public consumption and for compatibility with the existing distribution system. The water is then chlorinated to ensure conformity with health and drinking water quality standards. Fig. 3 Mechanical vapor compression process.

KEY ISSUES FACING DESALINATION Freezing is based on the fact that ice crystals, even when formed within a saline solution, are constituted of pure water only. This process consists of separating ice crystals from the saline solution to produce fresh water. Air humidification is a process that mimics the natural hydrologic cycle, which perpetually purifies water through an evaporation and condensation sequence. Air is heated, then placed in contact with water to produce vapor (humidification). The moisture-saturated air is then cooled, causing moisture to condense. The historical glass-covered solar stills can be classified under this category. However, more complex humidification and dehumidification systems were developed in recent years. Membrane distillation is a process combining distillation and membranes. Saline water is heated to produce vapor that will pass through a hydrophobic membrane, which is permeable to vapor but not to

The traditional and main constraint preventing the widespread use of desalination is the associated cost. The high cost is often attributed to the large quantity of energy needed by the desalination process. Besides energy use, others factors may also substantially drive up the cost such as chemicals needed for treatment and cleaning, fouling and corrosion of various components, source water quality, operation and maintenance costs, and requirements for concentrate disposal. Other issues and concerns that may need special attention include the environmental and ecological impacts associated with brine disposal (effects of high salinity, residual chemicals as well as high discharge temperatures to receiving water ecosystems including aquatic life, plants and wildlife), the effects of surface feedwater intakes on aquatic life (entrainment and impingement of fish, larvae, and marine organisms), and the effects of well intakes on groundwater

Fig. 4 Osmosis and reverse osmosis concepts.

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Chemical– Desalination Fig. 5 Basic components of a reverse osmosis desalination plant.

(e.g., overdraft and seawater intrusion). Some of the concerns and impacts associated with feedwater intake and brine discharge may be offset by colocating desalination plants with power plants so as to take advantage of existing intake and outfall structures and to dilute the brine discharge. Discharge to a wastewater treatment plant or mixing the brine with the treated wastewater effluent will also contribute to its dilution.

CONCLUSION As more and more stress is exerted on conventional water sources worldwide and given the rising cost of

developing new water supply sources, desalination of seawater, brackish, and impaired waters is becoming a viable alternative for meeting the ever-growing demand for fresh water. Various processes have been proven to successfully produce potable water from saline sources; and recent technological improvements, especially in the domain of membrane technology, have made desalination more affordable and, in some instances, competitive with other water supply options. A desalination process that is well disposed toward the environment can play an important role as part of a diversified water supply portfolio to meet current and future water needs for many arid and water-stressed regions. Although many of the desalination processes

Fig. 6 Electrodialysis process.

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currently in use have reached maturity, there is still room for improvement and perfection. Research needs deemed crucial in advancing desalination use include the following factors: more efficient pretreatment and posttreatment methods, improved process designs and the development of cheaper corrosion-resistant materials, improved membranes with higher salt rejection rates, improved scaling and fouling prevention techniques, environmentally acceptable strategies for brine disposal and concentrate management, advanced technologies that reduce entrainment and impingement of aquatic organisms at the feedwater intake, and opportunities for energy efficiencies, energy recovery methods, cogeneration, and the use of renewable energies. BIBLIOGRAPHY 1. American Water Works Association. Water Desalting Planning Guide for Water Utilities; John Wiley & Sons: Hoboken, NJ, 2004.

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2. Bureau of Reclamation. Desalting Handbook for Planners; US Department of Interior, Desalination and Water Purification Research and Development Program, Report No. 72, July 2003. 3. Buros, O.K. The ABCs of Desalting, 2nd Ed.; International Desalination Association: Topsfield, MA, 2000. 4. California Coastal Commission. Seawater Desalination and the California Coastal Act; March, 2004. http:// www.coastal.ca.gov/pubs.html (accessed August 2004). 5. California Department of Water Resources. Water Desalination—Findings and Recommendations; State of California, October 2003. 6. National Water Research Institute. Seawater Desalination: Opportunities and Challenges; March 2003. 7. Pitzer, G. Tapping the world’s largest reservoir: desalination. In Western Water; January/February 4–13, 2003. 8. Speigler, K.S.; El-Sayed, Y.M. A Desalination Primer; Balaban Desalination Publications, 1994. 9. Wangnick, K. IDA Worldwide Desalting Plants Inventory; International Desalination Association, Report No.17, 2002.

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Drainage and Water Quality James L. Baker Department of Agricultural and Biosystems Engineering, Iowa State University, Ames, Iowa, U.S.A.

INTRODUCTION Drainage–Dust

Artificial subsurface drainage is required on many agricultural lands to remove excess precipitation and/or irrigation water in order to provide a suitable soil environment for plant growth and a soil surface capable of physically supporting necessary traffic, e.g., for tillage, planting, and harvesting. Although this drainage makes otherwise wet soils very productive, subsurface drainage alters the time and route by which excess water reaches surface waters and can carry nutrients, pesticides, bacteria, and suspended solids to surface waters and cause nonpoint source pollution problems. The net water quality impact of subsurface drainage considered here is determined by comparison to the same cropping system not having subsurface drainage (not on the fact that the existence of adequate drainage will affect land use). The degree of pollutant transport with surface runoff and subsurface drainage is determined by the product of the volumes of water and the pollutant concentrations in the water. These are both influenced by environmental conditions, pollutant properties, and management factors, and their interactions with subsurface drainage are discussed.

ENVIRONMENTAL CONDITIONS Soils/Hydrology Whether water added to the soil surface as precipitation or irrigation infiltrates or becomes surface runoff is critical to water quality. Topography/slope, soil moisture content, texture, and soil structure, including the existence of preferential flow paths or ‘‘macropores,’’ can affect both the rate and route of water infiltration (as shown in Fig. 1). While different drain spacings are used to provide a desired ‘‘drainage coefficient’’ (e.g., 0.5 in./day) based on the internal drainage characteristics of the subsoils, the conditions of the surface soil will determine what percentage of applied water (rain or irrigation) will infiltrate. In general, the existence of subsurface drainage increases the volume of infiltration and thus decreases the volume of surface runoff (and pollutant loss with that runoff), 162

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increases shallow percolation, and lowers water tables. This effect will be more pronounced for ‘‘lighter’’ soils that have higher infiltration rates and lower water holding capacities. These changes affect the final quality of cropland drainage because of differences in time and type of soil-water–chemical interactions. Surface runoff allows water to come in contact only with the surface soil and materials such as crop residue and surface-applied fertilizers, manures, and pesticides present on it for short periods of time. On the other hand, water that infiltrates and percolates through the soil comes in intimate contact with all the soils in the profile to at least the depth of the tile drain, and generally, for much longer periods of time. The soil residence time is shorter for that portion of infiltrated water intercepted by the tile drains than for water that must flow through the underground strata to appear as base flow.

Climate/Precipitation The timing, amount, and intensity of water inputs, including precipitation and irrigation, relative to evapotranspiration (ET) determines the timing and amount of excess water that will leave agricultural land as surface runoff and subsurface drainage. Inputs at low intensities generally will totally infiltrate, but as intensities increase past the rate of infiltration, surface runoff begins. As just discussed, because decreased antecedent soil moisture contents, which result from the existence of artificial subsurface drainage, increase infiltration rates, the volume of surface runoff generally is decreased when subsurface drainage exists. This effect will be more pronounced for areas where input intensities often exceed the rates of infiltration. For a given crop and climatic region, the amount of ET is relatively constant, so the volume of subsurface drainage is very much dependent on the amount of input water above that value. As an example, in a 4-yr Iowa tile drainage experiment,[1] during a low rainfall year, only a trace of subsurface flow occurred; in a wet year (116 cm of precipitation vs the average of 80 cm), there was 29 cm of flow; and the overall average flow for the four yr was 15 cm.

Fig. 1 Schematic of transport processes.

POLLUTANT PROPERTIES Persistence Pollutants that have a limited existence in agricultural lands because of plant uptake/chemical transformation (i.e., nutrients), degradation (i.e., pesticides), and die-off (i.e., micro–organisms) have different potentials for off-site transport with water based on their persistence. Because surface runoff is much more of an immediate process than subsurface drainage, concentrations of nonpersistent pollutants in subsurface drainage are usually lower than in surface runoff. This effect will be more pronounced, the lesser the persistence is. However, the ‘‘route’’ of infiltration, where in some cases the drainage water moves quickly through the soil profile because of macropores (see Fig. 1), plays a role and can somewhat negate the expected dissipation effect.

Adsorption/Filtration Based on their chemical properties and/or their physical size, some pollutants transported down into and through the soil with subsurface drainage are removed from the flow stream by the soil. In particular, pollutants that are positively charged (e.g., ammoniumnitrogen (NH4-N)) and larger, less soluble, organic compounds (e.g., some pesticides) can be attenuated by adsorption to soil clay and organic matter. Inorganic P ions can be removed by complexation or precipitation with soil cations. Microorganisms and sediment can be filtered out by small soil pores. Thus, in general, with the exception of soluble salts of nitratenitrogen (NO3-N), sulfate (SO4), and chloride (Cl) anions, pollutant concentrations in subsurface drainage are lower than in surface runoff. This effect will be more pronounced, the greater the interaction between the pollutant and soil is.[2]

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For sediment itself, because soil erosion is dependent on the erosive ability and transport capacity of surface runoff, reducing the runoff flow rate and volume with subsurface drainage reduces sediment loss. For example, in a 6-yr study in the lower Mississippi Valley, surface runoff volumes from plots on a clay loam soil with subsurface drainage were 34% less than for plots without subsurface drainage, and the corresponding soil loss of 3500 kg/ha/yr represented a decrease of 30%. For the plots with subsurface drainage, both sediment concentrations and losses in subsurface drainage were about one-tenth those in surface runoff.[3] For nitrogen (N), loss from poorly drained soils is usually much less than that from soils with improved drainage systems.[4] While significant N can be transported with sediment (often sediment has at least 1000 ppm N), land needing subsurface drainage is usually not highly susceptible to erosion, and N loss is dominated by soluble inorganic-N loss. In a 3-yr tile-drained watershed study in northeast Iowa,[5] NO3-N losses in solution represented over 85% of the total N losses, including NH4-N, organic-N in solution, and N associated with sediment. In a 5-yr study in east-central Iowa,[6,7] where nutrient concentrations in both surface runoff and subsurface drainage from cropland were monitored, NO3-N concentrations in subsurface drainage averaged about 12 mg/L and 2–3 times those in surface runoff. While NH4-N concentrations were usually 2–10 times higher in surface runoff, on an absolute scale, the concentrations were overall much lower than those for NO3-N and constituted only a fraction of the total N loss. For phosphorus (P), transport is primarily in surface runoff with sediment (often sediment has at least 500 ppm P) and dissolved in surface runoff. For conventionally tilled cropland, about 75–90% of P transported in surface runoff is with sediment. In areas where soil erosion is minimal, soluble P in surface runoff water can dominate transport. The soluble P in surface runoff (and subsurface drainage) is regulated by adsorption/desorption characteristics of soil. Therefore, with P in surface soils generally much higher than in subsoils, subsurface drainage usually has much lower soluble P concentrations than in surface runoff. For pesticides, because of their adsorption characteristics, like with P, concentrations in surface runoff water are usually much greater than in subsurface drainage. However, this effect is even more dramatic for pesticides because unlike P, pesticides have a limited persistence, which decreases their potential for movement with subsurface flows with long travel times. In a series of studies on herbicides in surface and subsurface drainage,[8–10] atrazine concentrations in May (the period of application) were 75 mg/L in

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surface runoff for plots with surface drainage only, and were 51 mg/L and 1 mg/L in surface runoff and subsurface drainage, respectively, for plots that also had subsurface drainage. Not only were there much lower atrazine concentrations in subsurface drainage water, but the presence of the subsurface drains delayed and reduced surface runoff such that concentrations in surface runoff from the plots with subsurface drainage were reduced about one-third. For bacteria, a review by Crane et al.[11] showed that fecal coliform counts in surface runoff from manured lands often were greater than 10,000/100 mL. In a comparison of surface runoff and subsurface drainage, Culley and Phillips[12] found similar counts (>10,000/ 100 mL) for fecal coliform in surface runoff from both manured and fertilized plots, but with much, much lower counts ( Taiwan.

GROUNDWATER ARSENIC CONTAMINATION IN WEST BENGAL—INDIA West Bengal—India’s groundwater arsenic contamination in villages was first reported in 1982, and arsenical skin lesions were first detected in 1983. Twenty-two patients with arsenical skin lesions were known from five villages in four districts. About 50% of the districts in West Bengal—India reported groundwater arsenic concentration above 50 mg/L. Six million people are drinking arseniccontaminated water above 50 mg/L from 74 police stations/blocks in 9 arsenic affected districts including a part of Calcutta city in West Bengal (Fig. 2). In 2600 villages/wards, arsenic in groundwater has been found above 50 mg/L. In a preliminary study from 255 villages, 86,000 people were examined and 8500 people have been registered with arsenical skin lesions. Fig. 3 shows an arsenic patient with severe keratosis. In affected villages, the following skin manifestations and other symptoms of arsenic toxicity were detected—diffuse melanosis; mucous membrane pigmentation on tongue,

gum, and lips; spotted melanosis; leuco-melanosis; spotted and diffuse keratosis; and dorsal and limb keratosis. The following non-dermatological complications were also observed in victims suffering from arsenic toxicity—weakness and anemia, muscle pain, nonpetting oedema, conjunctival congestion, laryngitis, myopathy, neurological problem, chronic bronchitis, asthmatic bronchitis, hepatomegaly, splenomegaly, ascitis, and various types of external and internal cancer. Arsenical skin lesions from nine affected districts of West Bengal affect an estimated 300,000 people.[1] From arsenic-affected areas of West Bengal, over 99,000 water samples from hand tubewells have been analyzed by flow injection hydride generation atomic absorption spectrometry. Fifty-five percent had arsenic concentrations above 10 mg/L and 25% above 50 mg/L. The highest concentration of arsenic found in a hand tubewell was 3880 mg/L. About 25,000 biological samples (hair, nail, urine, skin scale) have been analyzed from villagers living in arsenic-affected villages (about 40% samples of total 25,000 are from arsenic patients) and on average 80% of the biological samples had arsenic above normal arsenic level in human body. This indicates many more are subclinically affected.

GROUNDWATER ARSENIC CONTAMINATION IN BANGLADESH Groundwater arsenic contamination and sufferings of people in Bangladesh surfaced in 1995. At that time, there was information about three affected villages in two police stations of two districts (Narayanganj and Faridpur). During the last 7 yr, a tremendous amount of survey work was done to determine the magnitude of the arsenic calamity in Bangladesh. Present survey reports indicate that 2000 villages in 178 police stations of 50 districts out of total 64 districts in Bangladesh, groundwater contains arsenic 421

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Dhaka Community Hospital, Dhaka, Bangladesh

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Fig. 1 Shows groundwater arsenic incidents round the world.

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above 50 mg/L. Bangladesh comprises four existing geo-morphological regions: 1) Deltaic region (including coastal region); 2) Flood Plain; 3) Tableland; and 4) Hill Tract. Of these four regions, Hill Tract is free of arsenic contamination. Most of the Tableland region is also contamination-free (except Flood Plain deposition on the eroded surface of Tableland). The highly arsenic-contaminated areas of Bangladesh are Deltaic region followed by Flood Plain (Fig. 4). Huge arsenic-free groundwater aquifers remain in selected areas of Bangladesh.[2] Arsenic-contaminated areas of Bangladesh belong to arsenic-bearing holocene sediments. Bangladesh’s arsenic calamity is considered the worst in the world. The World Bank and World Health Organization (WHO) described the magnitude of arsenic contamination in Bangladesh.[3] The World Bank’s local chief stated that tens of millions of people are at risk from health effects, and that 43,000 of the 68,000 villages are presently at risk or could be at risk in future. According to the prediction of WHO, within a few years, death across much of southern Bangladesh (1 in 10 adults) could be from cancers triggered by arsenic.[3] The area and population of Bangladesh are 148,393 km2 and 120 million, respectively. Thirty-four thousand hand tubewell water samples from 64 districts in Bangladesh have been analyzed and 56% contained arsenic above 10 mg/L, that the WHO recommended level of arsenic in drinking water with 37% contained more than 50 mg/L, the WHO maximum permissible limit. Maximum concentration of arsenic

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found in groundwater of Bangladesh was 4730 mg/L. Overall result shows only 25% and 37% of hand tubewells contain arsenic above 50 mg/L in arsenic-affected areas of West Bengal and Bangladesh, respectively, but there are many villages in West Bengal and Bangladesh where 80–90% of hand tubewells contain arsenic above 50 mg/L. It has been estimated that at the present time, more than 25 million people in Bangladesh are drinking arsenic-contaminated water above 50 mg/L, and 51 million people are drinking water above 10 mg/L. Analyses of more than 9900 biological samples from arsenic-affected villages of Bangladesh indicate that 95% of samples contain arsenic above normal level. So far in a preliminary survey, over 10,000 people have been identified with arsenical skin lesions from 222 out of 253 villages surveyed for patients. Fig. 5 shows an arsenic patient with squamous cell carcinoma.

SOCIAL PROBLEM AND IGNORANCE Arsenic poisoning in villages of West Bengal and Bangladesh are causing social problems that are the biggest curse. The prevailing social problems in the villages are as follows. 1. Due to ignorance, the villagers assume the arsenical skin lesions are a case of leprosy and force arsenic patients to maintain an isolated

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Fig. 2 Map shows the present arsenic affected areas and blocks of West Bengal—India.

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Fig. 3 Shows an arsenic patient with severe keratosis.

2. 3. 4. 5.

life or avoid them socially. It is a social curse and human tragedy. Affected wives are sent back to their parents and often with their children. Marriages in the affected villages have become a serious problem because of skin lesions. Jobs/services have been denied/ignored to the arsenic-affected people. When a husband or a wife has been singled out as an arsenic patient, the social problem has increased and destroyed the social fabric.

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Most of the people in the affected villages are not aware of the serious consequence of arsenic toxicity. People think arsenical skin lesions are just a single skin disease and will be cured with ointment. Some of them also think the skin lesions are the ‘‘Wrath of God or Curse of God.’’ A group also think the skin manifestation is due to the sin committed in their last birth.

SOURCE OF ARSENIC A single Rural Water Supply Scheme (RWSS) from Malda, one of the arsenic-affected districts of West Bengal—India, is withdrawing 147 kg of arsenic with groundwater in a year and 6.4 t of arsenic is being withdrawn in a year from 3000 shallow, large-diameter tubewells in use for agricultural irrigation in Deganga police station of North 24-Parganas district, West Bengal.[4] It indicates that the source of arsenic is not antropogenic and is geologic. Although the source of arsenic is believed to be aquifer sediments, the chemistry and mineralogy of the sediments of Ganges– Brahmaputra–Meghna (GBM) delta and arsenic

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leaching from the aquifer are not well understood. Reports[5,6] show existence of arsenic-rich pyrite in sediments of the delta region of Gangetic West Bengal. A probable explanation of arsenic contamination to the aquifer was predicted due to breakdown of arsenic-rich pyrite that occurred due to heavy groundwater withdrawal (i.e., underground aquifer is aerated and oxygen causes degradation of pyrite, the arsenic rich source). The cause of groundwater arsenic contamination in West Bengal and Bangladesh was also predicted due to reduction of arsenic-rich iron oxy-hydroxide in anoxic groundwater.[7,8,9] Whatever may be the mechanism of arsenic leaching to the aquifer, in West Bengal—India 38,865 km2 and in Bangladesh 118,849 km2 are arsenic affected areas, and population in West Bengal—India living in arsenic affected areas is 42.7 million and 104.9 million in Bangladesh. This does not mean that the total population (147.6 million) is drinking arsenic-contaminated water in West Bengal and Bangladesh and will suffer from arsenic toxicity, but it does indicate the risk levels. Our knowledge about long-term effects on those who have stopped drinking arsenic-contaminated water, those drinking contaminated water, and those suffering from arsenical skin lesions is not complete. A limited follow-up study for the last 10 yr indicates that a percentage of those suffering from severe skin lesions are getting internal/external cancers. A future danger to those living in West Bengal—India and Bangladesh is that arsenic is entering the food chain. Of great concern is the huge amount of arsenic applied to agricultural land from contaminated water from hand tube-wells used for irrigation.

HOW TO COMBAT THE PRESENT ARSENIC CRISIS The mistakes made in the past and that are persisting even today are due to the exploitation of groundwater for irrigation without even trying to adopt effective watershed management to harness the huge surface water resources and rain water. In West Bengal—India and Bangladesh, huge amounts of arsenic-free surface water is in ponds, canals, rivers, wetlands, flooded river basins, and ox-bow lakes. Per capita available surface water in Bangladesh is about 11,000 m3. West Bengal—India and Bangladesh are known as the land of rivers and have approximately 2000 mm annual rainfall. Instead of using those resources, groundwater is being pumped without proper management. Proper watershed management and villager participation are needed to combat the present arsenic crisis.

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Fig. 4 Map shows the status of arsenic in groundwater in all 64 districts of Bangladesh and in four geo-morphological regions.

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3. 4.

5.

Fig. 5 Shows an arsenic patient with Squamous cell carcinoma on head.

6.

7.

REFERENCES 1. Chowdhury, U.K.; Biswas, B.K.; Roy Chowdhury, T.; Samanta, G.; Mandal, B.K.; Basu, G.K.; Chanda, C.R.; Lodh, D.; Saha, K.C.; Mukherjee, S.C.; Roy, S.; Kabir, S.; Quamruzzaman, Q.; Chakraborti, D. Groundwater arsenic contamination in Bangladesh and West Bengal, India. Environ. Health Perspect. 2000, 108 (5), 393–397. 2. Chakraborti, D.; Biswas, B.K.; Basu, G.K.; Chowdhury, U.K.; Roy Chowdhury, T.; Lodh, D.; Chanda, C.R.; Mandal, B.K.; Samanta, G.; Chakraborti, A.K.; Rahman, M.M.; Paul, K.; Roy, S.; Kabir, S.; Ahmed, B.; Das, R.; Salim, M.; Quamruzzaman, Q. Possible

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8.

9.

arsenic contamination free groundwater source in Bangladesh. J. Surf. Sci. Technol. 1999, 15 (3–4), 180–188. Pearce, F. Arsenic in the Water. The Guardian (UK) 1999, 19/25 Feb, 2–3. Mandal, B.K.; Roy Chowdury, T.; Samanta, G.; Basu, G.K.; Chowdhury, P.P.; Chanda, C.R.; Lodh, D.; Karan, N.K.; Tamili, D.K.; Das, D.; Saha, K.C.; Chakraborti, D. Arsenic in groundwater in seven districts of West Bengal, India—The biggest arsenic calamity in the world. Curr. Sci. 1996, 70 (11), 976–986. Das, D.; Samanta, G.; Mandal, B.K.; Roy Chowdhury, T.; Chanda, C.R.; Chowdhury, P.P.; Basu, G.K.; Chakraborti, D. Arsenic in groundwater in six districts of West Bengal, India. Environ. Geochem. Health 1996, 18 (1), 5–15. Roy Chowdhury, T.; Basu, G.K.; Mandal, B.K.; Biswas, B.K.; Samanta, G.; Chowdhury, U.K.; Chanda, C.R.; Lodh, D.; Roy, S.L.; Saha, K.C.; Roy, S.; Kabir, S.; Quamruzzaman, Q.; Chakraborti, D. Arsenic poisoning in the Ganges Delta. Nature 1999, 401, 545–546. Nickson, R.; McArthur, J.M.; Burgess, W.G.; Ahmed, K.M.; Ravenscroft, P.; Rahman, M. Arsenic poisoning in Bangladesh Groundwater. Nature (Lond.) 1998, 395, 338. Bhattacharya, P.; Larson, M.; Leiss, A.; Jacks, G.; Sracek, A.; Chatterjee, D. Genesis of arseniferous groundwater in the alluvial aquifers of Bengal Delta plains and strategies for low cost remediation. International Conference on Arsenic Pollution of Groundwater in Bangladesh: Cause, Effects and Remedies, Dhaka, Bangladesh, Feb 8–12, 1998; 120 pp. Nickson, R.T.; McArthur, J.M.; Ravenscroft, P.; Burgess, W.G.; Ahmed, K.M. Mechanism of arsenic release to groundwater, Bangladesh and West Bengal. Appl. Geochem. 2000, 15, 403–413.

Groundwater: Mapping Levels Marios A. Sophocleous Kansas Geological Survey, University of Kansas, Lawrence, Kansas, U.S.A.

Maps of groundwater levels are used to estimate groundwater flow direction and velocity, to assess groundwater vulnerability, to locate landfills and wastewater disposal sites, and as input to hydrologic and pollutant transport models. Because groundwater is hidden from view beneath the land surface, groundwater can only be directly observed through monitoring wells. However, because these observations are limited to specific points, mapping groundwater levels requires hydrogeologically appropriate techniques to generalize the point measurements. Rules or models for spatially and temporally generalizing monitoring (sample) data across the groundwater system are inherent and essential to hydrogeologic science. Our understanding of ground water is the product of a long history of hypothesis and model development, testing, and refinement.[1] The position of the water table is the product of a wide range of static and dynamic environmental conditions and processes affecting the rate at which water enters and leaves the saturated zone of the aquifer. The water table rises if the rate of water added (recharge) exceeds the rate of water leaving (discharge); conversely, the water table falls if discharge exceeds recharge. The water-table surface is therefore not static, nor flat (as the name implies), but responsive to climatic, vegetative, geomorphic, and geologic conditions. As Matson and Fels[1] also pointed out, traditional water-table mapping uses graphical methods to interpolate between water-table measurements and hydrogeologic boundaries, with professional judgment and experience filling the gaps in sampling. Computer assisted approaches may incorporate surface mapping methods such as trend surface interpolation and kriging;[2] many of these tools are currently provided in Geographic Information Systems (GIS) software. Other methods employ mathematical modeling to predict water-table elevation from hydrogeologic conditions and processes.

DESIGNING A MONITORING SYSTEM Setting up a monitoring system requires careful consideration of both the hydrogeologic setting and the

data needed. It is premature and wasteful to locate monitoring wells without first synthesizing what is known about the setting—in other words, without formulating a sound conceptual model of the system under study. For example, water-supply wells drilled without understanding area hydrogeology may be placed where 1) the aquifer is thin or missing altogether, 2) the aquifer is present but not very productive, or 3) the aquifer contains water of poor quality.[3] Areas in which the geology is highly variable require more extensive (and costly) water-level monitoring systems than comparatively more homogeneous areas. The degree of geologic complexity is often not known or appreciated during the early phases of a testing program, and it may require several stages of drilling, well installation, water-level measurement, and analysis of hydrogeologic data before the required understanding is achieved. Due to space limitations, the design for an optimal spacing of groundwater-level monitoring wells cannot be covered in this article; however, the reader is referred to Refs.[4,5] for examples of such an observation well network design.

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INTRODUCTION

NATURAL PROCESSES CAUSING GROUNDWATER-LEVEL FLUCTUATIONS To interpret the monitored water levels, one needs to understand the various processes causing fluctuations in groundwater level. These are the effects of hydrologic processes active in the atmosphere, land surface, and subsurface, the groundwater movement in hydrodynamic flow systems, groundwater recharge and discharge processes, atmospheric pressure changes, plant transpiration, aquifer compression and dilation, and others. In addition to natural processes, human activities also cause groundwater-level fluctuations. Major among them are: 1) groundwater withdrawals from wells; 2) artificial recharge; 3) irrigation; 4) land clearing; 5) pumping of hydrocarbons and brine from reservoirs; 6) construction of water reservoirs; 7) mining; and 8) loading and unloading by heavy equipment, such as freight trains. 427

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Primary uses of groundwater-level data are to understand and predict water-level changes and to assess the direction of flow beneath an area. The usual procedure is to plot the location of wells on a base map, convert the depth-to-water measurements to elevations, plot the water-level elevations on the base map, and then construct a groundwater elevation contour map. Constructing a water-level change map, as will be explained later on (see section on ‘‘Examples of Groundwater-level Data Interpretation’’), will indicate the extent and severity of water-level declines resulting from a variety of factors, including human development and droughts. The direction of groundwater flow is estimated by drawing ground-water flow lines perpendicular to the ground-water elevation contours (Figs. 1 and 2) if the aquifer can be considered homogeneous and isotropic. The relatively simple approach to estimating ground-water flow directions described above is suitable where wells are screened in the same zone and the flow of groundwater is predominantly horizontal. However, as attention has focused on detecting the subsurface position of contaminant plumes or predicting possible contaminant migration pathways, this simple approach has been shown to be not always valid.[6] Increasingly, flow lines shown on vertical sections are required to complement the planar maps showing horizontal flow directions to illustrate how groundwater is flowing either upward or downward beneath a site. Groundwater flows in three dimensions, and as such can have both horizontal and vertical (either upward

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or downward) flow components. The magnitude of either the horizontal or the vertical flow component and the direction of groundwater flow are dependent on several factors: recharge and discharge conditions, aquifer heterogeneity, and aquifer anisotropy. Dalton, Huntsman, and Bradbury[6] summarized these factors, and the following draws on their summary. In recharge areas, groundwater flows downward (or away from the water table), whereas in discharge areas groundwater flows upward (or toward the water table). Groundwater migrates nearly horizontally in areas where neither recharge nor discharge conditions prevail. For example, in Fig. 1 well cluster A is located in a recharge area, well cluster B is located in an area where flow is predominantly lateral, and well cluster C is located in a discharge area.[7] Note in Fig. 1 that wells located adjacent to one another, but finished at different depths, may display different water-level elevations. In a heterogeneous aquifer, hydrogeologic properties are dependent on position within a geologic formation,[8] and thus the geology needs to be considered in evaluating water-level data. While recharge or discharge may cause vertical gradients to be present within a discrete geologic zone, vertical gradients may also be caused by the contrast in hydraulic conductivity between aquifer zones. This is especially evident where a deposit of low hydraulic conductivity overlies a deposit of relatively higher hydraulic conductivity. Aquifer anisotropy refers to an aquifer condition in which aquifer properties vary with direction at a point within a geologic formation.[8] For example, many aquifer zones were deposited in more or less horizontal layers, causing the horizontal hydraulic conductivity

Fig. 1 Ideal flow system showing recharge and discharge relationships. Source: Adapted from Saines, 1981.[7].

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to be greater than the vertical hydraulic conductivity. In anisotropic zones, where the horizontal component of hydraulic conductivity is higher than the vertical one, flow will be restricted to higher elevations compared to an equivalent flow system in isotropic zones showing the same water-level conditions. The practical significance of the three factors discussed earlier is that groundwater levels can be a function of either well-screen depth or of well position along a groundwater flow line or, more commonly, a combination of the two.[6] For these reasons, considerable care needs to be taken in evaluating water-level data. Interpreting Water-Level Data Dalton, Huntsman, and Bradbury[6] also summarized the various steps in groundwater-level data interpretation. The first step in interpreting groundwater-level data is to make a thorough assessment of the site geology. The vertical and horizontal extent and relative positions of aquifer zones and the hydrologic properties of each zone should be determined to the fullest extent possible. It is extremely important to have as detailed an understanding of the site geology as possible. Detailed surficial geologic maps and geologic sections should be constructed to provide the framework to interpret data on groundwater levels. The next step in interpreting these data is to review monitoring wells with respect to screen elevations and the various zones in which the screens are situated. The objective of this review is to identify whether vertical hydraulic gradients are present beneath the site and to determine the probable cause of the gradients. Once the presence and magnitude of vertical gradients and the distribution of data with respect to each zone are established, the direction of groundwater flow can be assessed. If the geologic system is relatively simple and substantial vertical gradients are not

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present, a planar groundwater elevation contour map can be prepared which shows the direction of groundwater flow. However, if multiple zones of differing hydraulic conductivity are present beneath the site, several planar maps may be required to show the horizontal component of flow within each zone (typically the zones of relatively higher hydraulic conductivity) and vertical sections are required to illustrate how groundwater flows between each zone.[6] The presence of vertical gradients can be anticipated in areas where sites are underlain by a layered (heterogeneous) geologic sequence, especially where deposits of lower hydraulic conductivity overlie deposits of substantially higher hydraulic conductivity; or are located within recharge or discharge areas. Site activities can modify local conditions to such an extent that groundwater flows in directions contrary to what would be expected for ‘‘natural’’ conditions. For example, drainage ditches can modify flow within near-surface deposits, and facility-induced recharge can create local downward gradients in regional discharge areas.[6] As mentioned previously, groundwater flow directions and water levels are not static and can change in response to a variety of factors, such as seasonal precipitation, irrigation, well pumping, changing river stage, and fluctuations caused by tides. Fluctuations caused by these factors can modify, or even reverse, horizontal and vertical flow gradients and thus alter groundwater flow directions. Contouring of Water-Level Elevation Data Typically, as Dalton, Huntsman, and Bradbury[6] also outlined, groundwater flow directions are assessed by preparing groundwater elevation contour maps. Water-level elevations are plotted on base maps and linear interpolations of data between measuring points

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Fig. 2 Contour map of the water table in a small hypothetical groundwater basin. If the aquifer is homogeneous and isotropic and if the slope of the water table is not large, the map can be used to construct a flow net. A small number of flowlines (shown as dash lines) have been drawn on the map. Excessive convergence of the flow lines suggests a changing transmissivity of the aquifer. Source: From Ref.[9].

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are made to construct contours of equal elevation (Fig. 2). These maps should be prepared using data from wells screened in the same zone, where the horizontal component of the groundwater flow gradient is greater than the vertical gradient. The greatest amount of interpretation is typically required at the periphery of the data set. A reliable interpretation requires that at least a conceptual analysis of the hydrogeologic system be made. The probable effects of aquifer boundaries, such as valley walls or drainage features, need to be considered. Computer contouring and statistical analysis (such as kriging) of water-level elevation data are becoming more popular. These tools offer several advantages, especially for large data sets. However, the approach and assumptions that underlie these methods should be thoroughly understood before they are applied, and the computer output should be critically reviewed. The most desirable approach would be to interpret the water-level data using both manual and computer techniques.[6] If different interpretations result, then the discrepancy between the interpretations should be resolved by further analysis of the geologic and water-level data.

Examples of Groundwater-Level Data Interpretation

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Several common errors in interpreting and contouring groundwater-level data are summarized by Davis and DeWiest.[9] Fig. 2 presents a number of water-table configurations related to common geologic or hydrologic causes. Area A is an area of recharge within an alluvial fan where the surface is 24 m above the water table. Here the stream continually loses water to the

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permeable substrata. Streams with this relationship to the water table are called influent or losing streams. In such cases, ground-water contours form a V, pointing downstream when they cross a losing stream. At point B, the water in the stream is at the same elevation as the water table. The water-table contour is normal to the stream at this point because there is no flow from the stream and groundwater flowlines are therefore tangent to the direction of the stream. At C the surface of the stream is below the water table, and the stream receives groundwater discharge. At C the stream is called an effluent or gaining stream. Groundwater contours bend upstream when they cross a gaining stream. At F the stream is still an effluent stream, but most of the groundwater has already been discharged into the stream so the contours no longer bend sharply upstream.[9] Point D is an area of heavy pumping in which the water has been lowered to 6 m below the stream level at B. After a short period, the pumping at D should make the contours shift so the river will be influent at B. Area E is an area of recharge in which surplus irrigation water has produced a ground-water mound 3 m above the stream surface at B. The stream at K and I is flowing in an impervious channel. The difference between the discharges at K and I is equal to the water lost or gained within the ground-water basin. Common mistakes in mapping groundwater levels are a failure to distinguish between the water levels of different aquifers and to identify wells that have contact with more than one aquifer (Fig. 3). If the area is one of complex stratigraphy or structure, the data should be interpreted with maximum use of geologic information. Similar problems occur if observation wells completed at different depths in recharge and/ or discharge areas are all combined to produce a

Fig. 3 Observation wells in a region having two confined aquifers under separate pressures. Correct interpretation of water levels is almost impossible unless details of well construction are known. Source: From Ref.[9].

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is flat and the water table beneath it is also flat.[10] Hence, groundwater contours must go around it (Fig. 4A). The only exception to this rule is when the lake is perched on low-permeability sediments and has a surface elevation above the main water table.[10] Mistakes in constructing water-table maps are often associated with purely mechanical extrapolation of contours between measured water levels. The water table thus can be placed mistakenly above the land surface (Fig. 4A), or obvious geologic structures are ignored (Fig. 4B; Ref.[9]). In areas where the groundwater levels exhibit a gentle gradient, the groundwater contours will be spaced well apart. If the gradient is steep, the groundwater contours will be closer together. Groundwater will flow in the general direction that the water-level surface is sloping. Water-level change maps are constructed by plotting the change of water levels in wells during a given span of time. If the study is of a short span of time, data from the same wells can be used. If, however, the time span is long (of the order of 50 yr or more), it is impossible in some areas to measure the same wells, owing to their rather rapid destruction or failure.

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groundwater elevation contour map. In such areas, vertical flow components are significant (Fig. 1) and water levels in wells completed at different depths will be at different elevations. In such cases, only shallow wells screened at or near the water table should be used for constructing water-table maps. Surface-water features such as springs, ponds, lakes, streams, and rivers can interact with the water table. In addition, the water table is often a subdued reflection of the surface topography. All this must be taken into account when preparing a water-table map.[10] A base map showing the surface topography and the locations of surface-water features should be prepared. The elevations of lakes and ponds can be helpful information. The locations of the wells are then plotted on the base map, and the water-level elevations are noted. The datum for the water level in wells should be the same as the datum for the surface topography. Interpolation of contours between data points is strongly influenced by the surface topography and surface-water features. For example, groundwater contours cannot be higher than the surface topography. The depth to groundwater will typically be greater beneath hills than beneath valleys. If a lake is present, the lake surface

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Fig. 4 Common errors encountered in contouring water-table maps in areas of (A) topographic depressions occupied by lakes, and (B) fault zones. Source: From Ref.[9].

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Fig. 5 Construction of a water-level change map by superimposing water-level contour maps. Source: From Ref.[9].

The best procedure in this case is to draw two watertable maps of the years of interest.[9] The maps are then superimposed and the water-level changes at contour intersections are recorded. The values can then be transferred to a separate map and lines of equal water-level change can be drawn (Fig. 5). Modern technology, especially the use of GIS, has made such procedures much easier and faster.

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REFERENCES 1. Matson, K.C.; Fels, J.E. 1996. Approaches to automated water table mapping. In: Proceedings, Third International Conference/Workshop on Integrating GIS and Environmental Modeling, Santa Fe, New Mexico, January, 21–26, 1996; Santa Barbara, CA: National Center for Geographic Information and Analysis, http://www.ncgia. ucsb.edu/conf/SANTA_ FE_CD-ROM/sf_papers/matson_kris/santa-fe.2.html (accessed May 11, 2001).

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2. Olea, R.A. Geostatistics for Engineers and Earth Scientists; Kluwer Academic Publishers: Boston, MA, 1999. 3. Stone, W.J. Hydrogeology in Practice: A Guide to Characterizing Groundwater Systems; Prentice Hall: Upper Saddle River, NJ, 1999. 4. Olea, R.A. Sampling design optimization for spatial functions. Math. Geol. 1984, 16 (4), 369–392. 5. Sophocleous, M.A. Groundwater observation network design for the Kansas groundwater management districts, USA. J. Hydrol. 1983, 61 (4), 371–389. 6. Dalton, M.G.; Huntsman, B.E.; Bradbury, K. Acquisition and interpretation of water-level data. In Practical Handbook of Ground-Water Monitoring; Nielsen, D.M., Ed.; Lewis Publishers: Chelsea, MI, 1991; 367–395. 7. Saines, M. Errors in interpretation of ground-water level data. Ground Water Monit. Rev. 1981, 1 (1), 56–61. 8. Freeze, R.A.; Cherry, J.A. Groundwater; Prentice Hall: Englewood Cliffs, NJ, 1979. 9. Davis, S.N.; DeWiest, R.J.M. Hydrogeology; John Wiley and Sons, Inc.: New York, 1966. 10. Fetter, C.W. Applied Hydrogeology, 4th Ed.; PrenticeHall, Inc.: Upper Saddle River, NJ, 2001.

Groundwater: Measuring Levels Paul F. Hudak Department of Geography, University of North Texas, Denton, Texas, U.S.A.

A ‘‘groundwater level’’ is the elevation of water in a well tapping an aquifer. Well construction in addition to hydraulic conditions in the aquifer influence measured groundwater levels. Hydraulic head, the mechanical energy per unit weight of water,[1] is equal to the elevation to which water rises in a cased well open to a ‘‘point’’ in an aquifer. The hydraulic head measurement pertains only to that point and is normally expressed in units of length above mean sea level. Groundwater levels from several cased wells, at several points in time, illustrate spatial and temporal patterns in hydraulic head within an aquifer.

WELL CASING AND SCREENED INTERVAL For the purpose of monitoring hydraulic head, wells should have as short a screened interval (intake) as possible, generally less than 3 m long.[2] Short intakes are especially important if there are strong vertical flow components. Under these conditions, piezometers (wells with intakes less than 0.3 m long) provide more accurate hydraulic head data.[3] A hydraulic head measurement can be obtained by subtracting the depth to water in a well from the elevation of a reference point at the top of the well casing. The well casing should be permanently marked at the reference point—depth to water measurements should always be made from that point. The reference point must be accurately surveyed, to within 0.01 ft (3 mm). It should be resurveyed every 5 years to account for settling. Unstable terrain, such as expansive clay soils or bogs, requires more frequent surveying. The initial survey should also establish x–y coordinates of each well. Each well at a field site should be permanently marked with a unique identifier (ID). The water table represents the surface of an unconfined aquifer. Wells used to measure water table elevations should be screened across or just beneath the water table. In a well tapping a confined aquifer, groundwater levels will rise higher than the top of the aquifer (where it contacts an overlying confining layer). A flowing artesian condition exists if the water level rises above the land surface.

Measuring hydraulic head at flowing wells requires an extension pipe or pressure gage.[3] An extension pipe, tightly fitted to the top of a well casing, must be tall enough to contain the rising water. Alternatively, a pressure gage can be attached to the top of the well casing. The gage measures pressure head (height of water level above gage) or water pressure (pressure head times specific weight of water). The pressure head measurement should be added to the height of the gage above the reference point.

MEASURING DEVICES Groundwater levels can be measured with several devices, including measuring tapes and poppers, chalk-coated tapes, acoustic probes, electrical sensors, pressure transducers, air lines, time domain reflectometry (TDR), floats and pulleys, and vibrating wire (VW) piezometers. Poppers (Fig. 1) make an audible sound when dropped onto a water column.[4] The tape should be read at the reference point when the popper just reaches the water column. Length of the popper should be accounted for in the water depth measurement. Weighted, chalk-coated tapes are similarly lowered down a well, but should penetrate the water column. The tape should be marked where it touches the reference point and then withdrawn from the well. Depth to water equals the distance from the marked point to the top of the wetted portion of the tape. Chalk-coated tapes are one of the most common and accurate methods for measuring groundwater levels.[5] Acoustic probes transmit sound waves from the top of a well casing to the water level in the well. They measure sound-wave travel times and convert them to distance (depth to water). Electrical sensors (Fig. 1) transmit sound or light signals when a probe enters the water column. The measurement should be made as the probe enters the water column. Submerging and raising the probe, and taking a measurement as the signal stops, is less accurate because dripping water may prolong the signal. False signals from water condensed on the sides of a well should also be considered when using electrical sensors. Pressure transducers (Fig. 1), air lines, and TDR measure the height of a water column above a 433

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Fig. 1 Pressure transducer (left), electrical sensor (middle), and vinyl tape and popper (right).

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submerged probe or tube. They can be connected to data loggers that store water level measurements and corresponding times. Pressure transducers are often used to obtain frequent water level measurements in observation wells during pumping tests. Air lines are less accurate and used mainly in wells being pumped.[5] The TDR devices transmit pulses down a coaxial cable and analyze the reflected voltage signature.[6] A strong voltage drop at the air–water interface is produced by the difference in dielectric constant between air and water. Time domain reflectometry cables can be used in riser pipes as small as 12 mm in diameter. Floats sit on the water column in a well. One end of a cable is attached to the float, and the other to a counter weight. The cable is draped over a pulley at ground level, and the pulley rotates as water levels in the well rise or fall. Water levels can be recorded with a penand-chart or digital system. Vibrating wire piezometers can be lowered down wells, buried in boreholes, or pushed into unconsolidated sediment. One end of a stretched magnetic wire is anchored and the other attached to a diaphragm, which deflects in proportion to pore-water pressure. Any deflection of the diaphragm changes the tension in the wire, thus affecting the resonant frequency of the VW. Measured pore-water pressures can be converted to pressure head by dividing by the specific weight of water. Adding the pressure head measurement to the elevation of the sensor gives the hydraulic head at the sensor.

FIELD CONSIDERATIONS Prior to measuring groundwater levels, they should be allowed to recover a minimum of 24 hr following any

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well construction, development, purging and sampling, or aquifer testing.[5] Recovery may take longer in aquifers with a low hydraulic conductivity. Measuring devices should be inert and regularly calibrated, taking into account stretch of tapes, wires, or cables. Water level measurements should be made to the nearest 0.01 ft (3 mm) and repeated for accuracy. Well depths should also be measured during each field visit. These do not require as much accuracy as water level measurements and can be accomplished with a weighted tape measure. Water levels should be measured before collecting water samples or performing aquifer tests, which disturb static water elevations. Ideally, the same device should be used to measure all wells (except in pumping tests requiring frequent or simultaneous measurements at different wells). In a contaminated aquifer, the first water level measurement should be made at the cleanest well, and subsequent measurements should be made at progressively more contaminated wells. The measuring device should be thoroughly cleaned between wells. At wells with floating immiscible contaminants, both depth to the immiscible layer and depth to water should be measured. This can be done with interface probes or tapes coated with reactive paste, which transmit different signals or colors when contacting different fluids. An immiscible layer depresses the water column in a well—measured depth to water should be corrected by subtracting the product of immiscible layer thickness and specific gravity. Unless a data logger is being used, each water level measurement should be recorded in a field book, along with the well ID, time of measurement, and device used. Weather conditions and the name of the person making the measurements should also be recorded in the field book.

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MAPS AND GRADIENTS

water levels in the piezometer and water body, divided by the distance between the bottom of the water body and bottom of the piezometer. A higher water level in the piezometer indicates an upward gradient and gaining condition, whereas a lower level in the piezometer indicates a downward gradient and that surface water is seeping into the ground.

REFERENCES 1. Fetter, C.W. Applied Hydrogeology, 4th Ed.; Prentice Hall: Upper Saddle River, NJ, 2000. 2. EPA (U.S. Environmental Protection Agency). RCRA Ground Water Monitoring: Draft Technical Guidance; Government Institutes: Rockville, MD, 1994. 3. Sanders, L.L. A Manual of Field Hydrogeology; Prentice Hall: Upper Saddle River, NJ, 1998. 4. Hudak, P.F. Principles of Hydrogeology, 2nd Ed.; Lewis Publishers: Boca Raton, FL, 2000. 5. Driscoll, F.G. Groundwater and Wells; Johnson Division: St. Paul, MN, 1986. 6. Dowding, C.H.; Nicholson, G.A.; Taylor, P.A.; Agoston, A.; Pierce, C.E. Recent advancements in TDR monitoring of ground water levels and piezometric pressures. In Proceedings of the 2nd North American Rock Mechanics Symposium, Montreal, Quebec, June 19–21, 1996; Aubertin, M., Hassani, F., Mitri, H., Eds.; Balkema: Rotterdam, The Netherlands, 1996.

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When using water level measurements to construct a contour map of the water table (unconfined aquifer) or potentiometric surface (confined aquifer), the wells should be measured during the same time interval (typically less than 24 hr) and open to the same hydrostratigraphic interval.[2] As many wells as possible should be measured, without sacrificing the above considerations. Moreover, the wells should be spread throughout the study area to avoid inaccurate hydraulic head extrapolations. Hydraulic gradient, change in hydraulic head with distance, should be calculated along a flow line in a water table or potentiometric surface map. Flow lines should be constructed perpendicular to hydraulic head contours (equipotential lines), unless the aquifer is anisotropic. A minimum of three wells defines a sloping plane and local flow direction. However, three wells allow for only a local, linear approximation of the groundwater flow direction. Vertical gradients in groundwater can be computed from hydraulic head measurements at adjacent wells open at different depths. A vertical gradient can also indicate the gaining or losing status of a surface water body such as a lake or stream. This can be accomplished by driving a narrow steel pipe with a slotted conical tip about 0.5 m into the bottom of the water body. The vertical gradient is the difference between

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Groundwater: Mining Hugo A. Loa´iciga Department of Geography, University of California–Santa Barbara, Santa Barbara, California, U.S.A.

INTRODUCTION

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Groundwater mining is defined as the extraction of ground water from aquifers by humans. This definition is analogous to that concerning the mining of mineral resources. There is, however, a fundamental difference between groundwater mining and the mining of minerals. Groundwater is, in most cases, a renewable resource. On the other hand, mineral resources, such as silver and gold ores, are non-renewable. Groundwater is a renewable resource because it is replenished naturally by fluxes that arise in the hydrologic cycle. The sum of the fluxes that replenish ground water is called recharge. During periods of plentiful precipitation, and in the absence of human intervention, aquifers are replenished by recharge. During droughts, groundwater storage and groundwater levels decline due to low levels of recharge. There are also groundwater deposits of ‘‘fossil’’ ground waters that have become isolated from the hydrologic cycle. These deposits resemble in many respects oil reservoirs. One important shared characteristic is that extraction of the resource, be it ground water or oil, produces an irreversible reduction in its stock. Continued mining of such fossil deposits leads to their eventual depletion. This article is devoted to an analysis of the effects of groundwater mining on renewable ground water. The latter constitutes most of the ground water used by humans. Principles of sustainable groundwater mining are presented and illustrated with data from one of the most productive aquifers in the world.

GROUNDWATER MINING AND THE WATER BALANCE Let us consider an aquifer that is subject to groundwater mining. Assume that the amount of groundwater storage is denoted by S, and that recharge (R), groundwater pumping (W ), and outflow (G) affect the status of storage as shown in Fig. 1. Groundwater pumping is the means by which ground water is mined. The recharge is the net water flux into groundwater storage from surface water sources. It includes percolation, seepage (from rivers and lakes), and artificial recharge (by wells and spreading basins). Groundwater 436

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uptake by plants, baseflow, and spring flow abstractions from groundwater are also included in the calculation of aquifer recharge. The groundwater outflow (G) term is the net of subsurface fluxes in and out of groundwater storage across the (subsurface) aquifer boundaries. From water-balance considerations for a period of duration T, it is evident that the change in groundwater storage is given by the following equation: SðTÞ
Sð0Þ ¼

Z

t¼T

½RðtÞ
WðtÞ
GðtÞ T  0

t¼0

ð1Þ in which S(0) and S(T) are the storages at time zero (initial storage) and time T, respectively. Pumping may be measured accurately with well meters. It commonly exhibits a strong seasonal pattern, rising during periods of low precipitation (i.e., during dry seasons) and subsiding during wet seasons. This is true for urban and agricultural groundwater uses. In addition, as a result of population growth, urban groundwater mining typically exhibits an increasing trend over time.[1] The recharge flux in Eq. (1) is, in general, difficult to estimate. Recharge depends strongly on the amount of precipitation, and, thus, it tends to replicate the seasonality and inter-annual variability observed in the climate specific to the region where the aquifer is found.[2] The groundwater outflow term (G) is not amenable to direct measurement. Instead, it must be estimated by indirect methods.[3] When the aquifer boundaries coincide with groundwater divides, the outflow term (G) is negligible. Fig. 2 shows the evolution of annual groundwater recharge, pumping, and spring flow from 1934 to 1995 in the Edwards Aquifer of Texas, one of the most productive groundwater systems in the world.[1] The subsurface outflow term (G) in the Edwards aquifer is negligible.[4] In this instance, it is advantageous to treat separately the flux of water into the aquifer (i.e., recharge) from the discharge of ground water at several large springs (i.e., spring flow). Recharge takes place primarily by means of stream seepage along aquifer outcrops. It is seen in Fig. 2 that recharge shows large inter-annual fluctuations, and that those fluctuations appear to become larger and larger over time. Groundwater pumping displays a

Groundwater: Mining

long-term increasing trend until year 1985, even during the drought of 1936–1959. The intermittent lows in groundwater pumping after 1985 were caused by Court orders imposed on the mining of the Edwards Aquifer to protect aquatic habitats in the discharge zone near springs.[1] Spring flow is concentrated along large fault springs that define the discharge zone of the Edwards aquifer.[1] As shown in Fig. 2, it is a smoothed-out and dampened replica of annual recharge. It lags recharge by a short period of time, typically less than 2 yr. The time series of spring flow shown in Fig. 2 does not represent natural groundwater discharge because of the effect that groundwater pumping had on spring flow. If the Edwards aquifer had not been mined in the period 1934–1995, the amount of spring flow would have been roughly equal to the amount of recharge. The latter is demonstrated in Fig. 3, where the cumulative recharge and the cumulative pumping plus spring flow time series are plotted. By adding pumping to spring flow, the latter is reconstructed to what would have been its natural value during the period

of analysis. The differences between the two time series plotted in Fig. 3 arise from unequal beginning and ending aquifer storages. Fig. 4 shows the change in storage, S(T)
S(0), calculated from Eq. (1) for the Edwards Aquifer data shown in Fig. 2. It is seen there that during the drought period between 1936 (point 1) and 1956 (point 2) aquifer storage dropped by 3500  106 m3 as a result of groundwater mining. Between 1956 and 1992 (point 3) ground water continued to be mined, yet, there was a recovery of aquifer storage equal to 5100  106 m3. Since the Edwards Aquifer was severely de-watered in 1956—demonstrated by the drying of major springs—and in 1992 water levels rose to historically high levels after heavy El Nin˜o rainfall, it can be concluded that the Edwards Aquifer extractable storage must be on the order of 5100  106 m3. The evolution of storage S(T ) captures one important aspect of groundwater mining in any aquifer. For a full grasp of groundwater mining, however, one must broaden the scope of its analysis.

GROUNDWATER MINING AND SUSTAINABLE AQUIFER USE The key question regarding groundwater mining is how to pump ground water from an aquifer without compromising the availability of ground water in storage, while maintaining its natural water quality and protecting water bodies that may depend on the status of groundwater storage (e.g., influent streams and lakes, springs, and wetlands). Groundwater mining concerns must go beyond the amount of ground water extracted or left in aquifer storage. Other environmental considerations must be taken into account in determining the best way to mine an aquifer. Groundwater mining that ensures a long-term supply of good-quality water while protecting the environment is what we call sustainable groundwater mining. Simplistic rules such

Fig. 2 Groundwater pumping (W), recharge (R), and spring flow (Sp) in the Edwards Aquifer.

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Fig. 1 Schematic of a mined aquifer.

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Fig. 3 Cumulative recharge and cumulative pumping plus spring flow from 1934 to 1995.

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as ‘‘groundwater pumping shall not exceed the longterm average recharge’’ are inadequate to cope with the spectrum of impacts associated with groundwater mining. This is so because even if pumping does not exceed the long-term recharge (which, by the way, may be difficult to estimate accurately), groundwater storage may still reach levels that are detrimental from the perspective of water-quality protection and environmental conservation. The cyclic and variable nature of recharge, and the instinctive drive to intensify groundwater mining during periods of low precipitation (to irrigate crops for example, or to water lawns and gardens[5]) pose serious challenges to sustainable groundwater mining during periods of low precipitation, be they seasonal or associated with protracted drought.[6] Fig. 5 shows a graph of the cumulative recharge in the Edwards Aquifer. The cumulative or mass

recharge is expressed by:

Mass recharge ¼

T X

rechargeðtÞ

T  0

ð2Þ

t¼0

The curve shown in Fig. 5 is called a ‘‘mass curve’’ for the Edwards Aquifer. Mass curves are widely used in the analysis of stream flow time series for the purpose of sizing surface reservoirs or determining reservoir releases.[7] The mass curve is used herein to provide a first estimate of long-term groundwater pumping. Assume an extractable groundwater storage of 5100  106 m3, which was estimated from Fig. 4. One finds that the minimum-slope tangent to the mass curve (in this case drawn through point A in Fig. 5) that encompasses the estimated groundwater storage of 5100  106 m3 has a slope of 690  106 m3 yr
1

Fig. 4 Changes in aquifer storage as a result of groundwater mining and climate.

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Fig. 5 Mass curve and the estimation of an average groundwater mining rate.

CONCLUSIONS The former example illustrates important factors that must be considered in the planning of sustainable groundwater mining. The first is the long-term behavior of aquifer recharge and aquifer discharge (besides artificial pumping). Secondly, one must have an in-depth understanding of the hydraulic and ecological linkages of aquifer storage and discharge to dependent ecosystems. The rate of pumping must be adjusted to the natural fluctuations of recharge. This requires detailed numerical simulations of aquifer response to pumping under specific recharge conditions. Although water-quality deterioration effected by groundwater

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mining was not specifically addressed in this work, it is another consideration that must be taken into account in planning sustainable groundwater mining strategies. The excessive lowering of aquifer storage may induce the upwelling of poor-quality groundwater and/or the intrusion of saltwater.

ACKNOWLEDGMENTS This article was supported in part by U.S. Geological Survey grant HQ-96-GR-02657 and by California Water Resources Center grant WR-952.

REFERENCES 1. Loa´iciga, H.A.; Maidment, D.; Valdes, J.B. Climatechange impacts in a regional karst aquifer. J. Hydrol. 2000, 227, 173–194. 2. Loa´iciga, H.A. Climate-change impacts in regional-scale aquifer: principles and field application. In Ground Water Updates; Sato, K., Iwasa, Y., Eds.; Springer-Verlag: Tokyo, 2000; 247–252. 3. Zektser, I.S.; Loa´iciga, H.A. Ground-water fluxes in the global hydrologic cycle: past, present, and future. J. Hydrol. 1993, 144, 405–427. 4. Edwards Underground Water District. Edwards/Glen Rose Hydrologic Communication, San Antonio Region, Texas, Report 95-03; Edwards Underground Water District: San Antonio, Texas, USA, 1995. 5. Loa´iciga, H.A.; Renehan, S. Municipal water use and water rates driven by severe drought: a case study. J. Am. Water Res. Assoc. 1997, 33, 1313–1326. 6. Loa´iciga, H.A.; Michaelsen, J.; Garver, S.; Haston, L.; Leipnik, R.B. Droughts in river basins of the Western United States. Geophys. Res. Lett. 1992, 19 (20), 2051–2054. 7. Linsley, R.K.; Franzini, J.B. Water Resources Engineering, 3rd Ed.; McGraw-Hill: New York, 1979.

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(see Fig. 5). Ignoring spring flow and related impacts associated with groundwater mining, the magnitude of that slope equals the average long-term groundwater pumping that would be consistent with an usable aquifer storage of 5100  106 m3. It turns out, however, that a pumping rate of 690  106  m3  yr
1 would cause exceedingly low spring flow values during low-recharge years and adverse and irreversible impacts on aquatic ecosystems supported by the Edwards Aquifer springs.[1] More detailed simulations by the author, which were carried out with a specially calibrated numerical groundwater model for the Edwards Aquifer,[1] indicated that during low-recharge periods (1947–1956, for example) the aquifer may not be mined at a rate greater than 123  106 m3 yr
1 in order to protect minimum spring flow levels and aquatic habitats. For comparison, during the period 1934– 1995, the average pumping in the Edwards Aquifer was on the order of 360  106 m3 yr
1, while during the high-growth period 1970–1995 pumping averaged 514  106 m3 yr
1, a mining strategy that has left a legacy of adverse ecological impacts.

Groundwater: Modeling Jesu´s Carrera Technical University of Catalonia (UPC), Barcelona, Spain

INTRODUCTION

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A model is an entity built to reproduce some aspect of the behavior of a natural system. In the context of groundwater, aspects to be reproduced may include: groundwater flow (heads, water velocities, etc.); solute transport (concentrations, solute fluxes, etc.); reactive transport (concentrations of chemical species reacting among themselves and with the solid matrix, minerals dissolving or precipitating, etc.); multiphase flow (fractions of water, air, non-aqueous phase liquids, etc.); energy (soil temperature, surface radiation, etc.); and so forth. Depending on the type of description of reality that one is seeking (qualitative or quantitative), models can be classified as conceptual or mathematical. A conceptual model is a qualitative description of ‘‘some aspect of the behavior of a natural system.’’ This description is usually verbal, but may also be accompanied by figures and graphs. In the groundwater flow context, a conceptual model involves defining the origin of water (areas and processes of recharge) and the way it flows through and exits the aquifer. In contrast, a mathematical model is an abstract description (abstract in the sense that it is based on variables, equations, and the like) of ‘‘some aspect of the behavior of a natural system.’’ However, the motivation of mathematical models is not abstraction, but rather quantification. For example, a groundwater flow mathematical model should yield the time evolution of heads and fluxes (water movements) at every point in the aquifer. Both conceptual and mathematical models seek understanding. Some would argue that understanding is not possible without quantification. Reversely, one cannot even think of writing equations without some sort of qualitative understanding. The methods of conceptual modeling are those of conventional hydrogeology (study geology, measure heads and hydraulic parameters, hydrochemistry, etc). On the other hand, the methods of mathematical modeling (discretization, calibration, etc.) are more specific. Yet, it should be clear from the outset that conceptualization is the first step in modeling and that mathematical modeling helps in building firm conceptual models. Depending on the manner in which equations are solved, models can be classified as: analog, analytical, and numerical. Analog models are based on a physical 440

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simulation of a phenomenon governed by the same equation(s) as that of our natural system. For example, because of the equivalence between electrostatics and steady state flow, one may use conductive paper subject to an electrical current to solve the flow equation (a parallelism can be established between electric potential and hydraulic head). This kind of application, however, is restricted mainly to teaching. Boxes of resistances and condensators were used in the 1950s and 1960s as analog aquifer models, but they have become inefficient compared to computers. As a result, analog models are no longer used in practice. Analytical models are based on closed-form solutions to the groundwater flow and transport equations. They are convenient in the sense that they are easy to evaluate and intuitive (visual inspection of the equation may yield an idea of the phenomenon). As a result, they are used very frequently. Examples include solutions of problems in well hydraulics, tracer movement, etc. Numerical models are based on discretizing the partial differential equations governing flow and transport. This leads to linear systems of equations that can only be solved with the aid of computers. The advantage of numerical models lies in their generality. Analytical models are constrained to homogeneous domains and very simple geometry and boundary conditions. Numerical models, on the other hand, can handle spatially and temporally variable properties, arbitrary geometry and boundary conditions, and complex processes. The price to pay is methodological singularity. Analytical models are easy to use. Numerical models can be complex and, often, difficult. Because of the methodological singularity mentioned above, this chapter concentrates on mathematical numerical models. Analytical solutions are not discussed. In addition, conceptual modeling will be discussed as the first step in modeling, but not by itself.

WHAT CAN BE MODELED AND WHAT FOR? Modeled Phenomena The most basic phenomenon is groundwater flow (Fig. 1) because of its intrinsic importance and because it is needed for subsequent processes. In essence, the flow equation expresses two things. First, groundwater

Fig. 1 A groundwater flow model involves using a flow simulator to take aquifer parameters, boundary conditions, and internal sink and sources as inputs and obtain heads and water fluxes as output. Water fluxes are used in conservative transport models, together with porosity, diffusivity, and solute mass inflows, to yield time evolution and spatial distribution of inert tracers.

moves according to Darcy’s law. Second, a mass balance must be satisfied in the whole aquifer and in each of its parts. Therefore, the main output from flow models is a mass balance: classified inflows, outflows, and storage variations. The output also includes where water flows through the aquifer (water fluxes) and heads (water levels in the aquifer). In essence, input data are a thorough description of hydraulic conductivity (and/or transmissivity), storativity, recharge/ discharge throughout the model domain, as well as conditions at the model boundaries. Obviously, these data are never available, and the modeler has to use a good deal of ingenuity to generate them. This is where the conceptual model becomes important. Specific cases of flow phenomena are unsaturated and multiphase flow. In the first case, one models water flow in the vadose zone or, in general, in areas where water does not fill all the pores.[1] Therefore, besides heads and fluxes, one must work with water contents (volume of water per unit volume of aquifer), capillary pressures and suctions (difference between water and air pressure). From the input viewpoint, the main singularity of unsaturated flow is the need to specify the retention curve (water content vs. suction) and relative permeability (permeability vs. water content). The multiphase flow case is similar, but includes several fluids (phases). It is used to represent the flow of air or mixtures of liquids, singularly non-aqueous phase liquids (NAPLs), which have been the subject of much research in recent years.[2]

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Conservative transport refers to the movement of inert substances dissolved in water. Solutes are affected by advection (displacement of the solute as linked to flowing water) and dispersion (dilution of contaminated water with clean water, which causes the size of the contaminated area to grow while reducing peak concentrations). The main input to a solute transport model is the output of a flow model (water fluxes). Additionally, porosity and dispersivity need to be specified (Fig. 1). The output is the time evolution and spatial distribution of concentrations. While the amount of data needed for solute transport modeling is relatively small, it must be stressed that solute transport is extremely sensitive to variability and errors in water fluxes. A flow model may be good enough for flow results (heads and water balances) but insufficiently detailed to yield water fluxes good enough for solute transport. Therefore, modeling solute transport ends up being rather difficult. Reactive transport refers to the movement of solutes that react among themselves and with the soil phase. Reactions can be of many kinds, ranging from sorption of a contaminant onto a solid surface to redox phenomena controlling the degradation of an organic pollutant. Input for reactive transport modeling includes not only the output of flow and conservative transport models but also the equilibrium constants of the reactions (usually available from chemistry databases) and the parameters controlling reaction kinetics. However, the most difficult input is the proper identification of relevant chemical processes. Model output includes the concentrations of all chemical species, the reaction rates, etc. Coupled models refer to models in which different phenomena are affected reciprocally. Density dependent flow is a typical example. Variations in density affect groundwater flow (e.g., dense sea water sinks under light fresh water), which in turn affects solute transport and, hence, density distribution. Other coupled phenomena are the non-isothermal flow of water (coupling flow and energy transport) and the mechanically driven flow of water (coupling flow and mechanical deformation equations).

What Are Models Built For? While discussing the usage of models, it is convenient to distinguish between site-specific models and generic models. The former are aimed at describing a specific aquifer while the latter emphasize processes, regardless of where they take place. Groundwater management is the ideal use of sitespecific models. Management involves deciding where to extract and/or inject water to satisfy water needs while ensuring water quality and other constraints.

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In this context, it is important to point out that a model is essentially a system for accounting water fluxes and stores (Fig. 2) in the same way that the accounting system of a company keeps track of money fluxes and reserves. No one would imagine a wellmanaged company without a proper accounting system. Aquifers will not be managed accurately until they have a model running on real time. Unfortunately, at present, this is still a dream. Because of the difficulties in building and maintaining models and because of legal and practical difficulties to manage aquifers in real time, models are rarely, if ever, used in this fashion. Instead, models are often used as decision support tools. Building an accurate model is very difficult and time consuming. As a result, one can rarely expect models to yield exact predictions. However, approximate models are much easier to build. These do not result in precise forecasts but normally allow reasonable assessments of the outcome of different management alternatives, i.e., the relative advantages and disadvantages of each alternative can be evaluated and the options ranked. This is usually all one needs for decision making. This type of use is very frequent in aquifer rehabilitation, where one has to choose among several alternatives, including the option of doing nothing.[3] Models are also used for supporting aquifer exploration policies, i.e., for answering questions such as ‘‘how much water can be extracted?,’’ ‘‘where should one pump to minimize environmental impact?,’’ etc. In fact, a large body of literature is devoted to this kind of questions in an optimal fashion.[4] Site-specific models are most frequently used, however, as a tool to support aquifer characterization efforts. This is somewhat ironic because a model is

Fig. 2 A groundwater model is the accounting system of an aquifer. It keeps track of the balance of each section (cells or compartments in the groundwater language) by evaluating exchanges with the outside (pumping Qi, recharge, ri, etc.) and with the adjacent sections (fij). The difference between inflow and outflow is equal to the variation in reserves (storage variation, DSi). A well-managed company needs an accounting system, and so does an aquifer.

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Groundwater: Modeling

an essentially quantitative tool while site characterization is rather qualitative. Yet, experience dictates that modeling is the only way to consistently integrate the kind of data available in site characterization. These data are very diverse and range from geologic maps to isotope concentrations. One can use vastly different models to verbally explain all observations. Quantitative consistency is not so easy to check and requires the use of a model. Because of the difficulties in fully describing all data, this kind of model use is rarely described in the scientific literature. Models can also be used in generic fashion as teaching or research tools to gain understanding on physicochemical phenomena. In these cases, they do not aim at representing a specific aquifer, but at evaluating the role of some processes under idealized conditions. A classical example of this type of use is the analysis of flow on regional basins.[5] Models are used in this fashion to explain geological processes.[6,7] Much emphasis has been placed in recent years on the evaluation of the effects of spatial variability. This involves issues such as upscaling, i.e., finding the relationship between large-scale effective parameters and small-scale measurements;[8] or analysis of hydraulic tests.[9]

HOW ARE MODELS BUILT: THE MODELING PROCESS The procedure to build a model is outlined in Fig. 3. First, one defines a conceptual model (i.e., zones of recharge, boundaries of aquifers, etc). Second, one discretizes the model domain into a finite element or finite difference grid. This can be entered as input data for a simulation code. Unfortunately, output data will rarely fit the observed aquifer heads and concentrations. This is what motivates calibration, i.e., the modification of model parameters to ensure that model output is indeed similar to what has been observed in reality. The model thus calibrated can be considered a ‘‘representation of the natural system’’ and can be used for management or simulation purposes. The above procedure is formally described in Fig. 4. This section is devoted to discussing in detail the modeling steps as previously described.[10–12] In practice, the effort behind each of these tasks may be very sensitive to the objectives of the studies and model. For the time being, we will assume that one is building a model aimed at describing reality in detail for the purpose making predictions. Conceptualization Modeling starts by defining which processes are important and how they are represented in the model.

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Fig. 3 Building a model involves three basic steps: conceptualization, discretization, and calibration. Example from the Almonte-Marismas aquifer.

Definition of the relevant processes is termed ‘‘process identification’’ and it is needed for several reasons. First, the number of processes that may affect flow and transport is very large. For practical reasons, the modeler is forced to select those that affect the phenomenon under study, most significantly. Second, not all processes are well understood and they have to be treated in a simplified manner. In short, process identification involves simplifications, both in the

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choice of the processes and in the way they are implemented in the model. Model structure identification refers to the definition of parameter variability, boundary conditions, etc. In a somewhat narrower but more systematic sense, model structure identification implies expressing the model in terms of a finite number of unknowns called model parameters. Parameters controlling the above processes are variable in space. In some cases,

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In spite of the large amount of data usually available, their qualitative nature prevents a detailed definition of the conceptual model. Thus, more than one description of the system may result from the conceptualization step. Selecting one conceptual model among several alternatives is sometimes performed during calibration, as discussed later. Discretization

Fig. 4 A formal description of the modeling process. Modeling starts with an understanding of the natural system (conceptual model), which is based on experience about such kind of systems (science) and on data from the site. Writing the conceptual model in a manner adequate for computer solution requires discretization. The resulting model is still dependent on many parameters that are uncertain. During calibration, these parameters are adjusted so that model outputs are close to measurements (recall Fig. 3). Model predictions may be uncertain because so are the fitted parameters or because different models are consistent with observations. If uncertainty is unacceptably high, one should perform additional measurements or experiments and redo the whole process.

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they also vary in time or depend on heads and/or concentrations. As discussed earlier, data are scarce so that such variability cannot be expressed accurately. Therefore, the modeler is also forced to make numerous simplifications to express the patterns of parameter variations, boundary conditions, etc. These assumptions are reflected on what is denoted as model structure. The conceptualization step of any modeling effort is somewhat subjective and dependent on the modeler’s ingenuity, experience, scientific background, and way of looking at the data. Selection of the physicochemical processes to be included in the model is only rarely the most difficult issue. The most important processes affecting the movement of water and solutes underground (advection, dispersion, sorption, etc.) are relatively well known. Ignoring a relevant process will only be caused by misjudgments and should be pointed out by reviewers, which illustrates why reviewing by others is important. Difficulties arise when trying to characterize those processes and, more specifically, the spatial variability of controlling parameters.

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Strictly speaking, discretization consists of substituting a continuum by a discrete system. However, we are extending this term here to describe the whole process of going from mathematical equations, derived from the conceptual model, to numerical expressions that can be solved by a computer. Closely related is the issue of verification, which refers to ensuring that a code accurately solves the equations that it is claimed to solve. As such, verification is a code-dependent concept. However, using a verified code is not sufficient for mathematical correctness. One should also make sure that time and space discretization is adequate for the problem being addressed. Moreover, numerical implementation of a conceptual model is not always straightforward international code comparison projects; INTRANCOIN and HYDROCOIN have shown the need for sound conceptual models and independent checks of calculation results. Even well-posed mathematical problems lead to widely different solutions when solved by different people, because of slight variations in the solution methodology or misinterpretations in the formulation.[13] The reasons behind these differences and ways to solve them only become apparent after discussions among them. The main concern during discretization is accuracy. In this sense, it is not conceptually difficult, although it can be complex. Accuracy is not only restricted to numerical errors (differences between numerical and exact solutions of the involved equations) but also refers to the precision with which the structure of spatial variability reproduces the natural system. Calibration and Error Analysis The choice of numerical values for model parameters is made during calibration, which consists of finding those values that grant a good reproduction of head and concentration data (Fig. 3) and are consistent with prior independent information. Calibration is rarely straightforward. Data come from various sources, with varying degrees of accuracy and levels of representativeness. Some parameters can be measured directly in the field, but such measurements are usually scarce and prone to error. Furthermore, since measurements are most often performed

on scales and under conditions different from those required for modeling purposes, they tend to be both numerically and conceptually different from model parameters. The most dramatic example of this is dispersivity, whose representative value increases with the scale of measurement so that dispersivities derived from tracer tests cannot be used directly in a largescale model. As a result, model parameters are calibrated by ensuring that simulated heads and concentrations are close to the corresponding field measurements. Calibration can be tedious and time consuming because many combinations of parameters have to be evaluated, which also makes it prone to be incomplete. This, coupled to difficulties in taking into account the reliability of different pieces of information, makes it very hard to evaluate the quality of results. Therefore, it is not surprising that significant efforts have been devoted to the development of automatic calibration methods.[14–16]

Model Selection The first step in any modeling effort involves constructing a conceptual model, describing it by means of appropriate governing equations, and translating the latter into a computer code. Model selection involves the process of choosing between alternative model forms. Methods for model selection can be classified into three broad categories. The first category is based on a comparative analysis of residuals (differences between measured and computed system responses) using objective as well as subjective criteria. The second category is denoted parameter assessment and involves evaluating whether or not computed parameters can be considered as ‘‘reasonable.’’ The third category relies on theoretical measures of model validity known as ‘‘identification criteria.’’ In practice, all three categories will be needed: residual analysis and parameter assessment suggest ways to modify an existing model and the resulting improvement in model performance is evaluated on the basis of identification criteria. If the modified model is judged an improvement over the previous model, the former is accepted and the latter discarded. The most widely used tool of model identification is residual analysis. In the groundwater context, the spatial and frequency distributions of head and concentration residuals are very useful in pointing towards aspects of the model that need to be modified. For example, a long tail in the breakthrough curve not properly simulated by a single porosity model may point to a need for incorporating matrix diffusion or a similar mechanism. These modifications should, whenever possible, be guided by independent

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information. Qualitative data such as lithology, geological structure, geomorphology, and hydrochemistry are often useful for this purpose. A particular behavioral pattern of the residuals may be the result of varied causes that are often difficult to isolate. Spatial and/or temporal correlation among residuals may be a consequence of not only improper conceptualization, but also measurement or numerical errors. Simplifications in simulating the stresses exerted over the system are always made and they lead to correlation among residuals. Distinguishing between correlations caused by improper conceptualization and measurement errors is not an easy matter. This makes analysis of residuals a limited tool for model selection. An expedite way of evaluating a model concept is based on assessing whether or not the parameters representing physico-chemical properties can be considered ‘‘reasonable;’’ i.e., whether or not their values make sense and/or are consistent with those obtained elsewhere. Meaningless parameters can be a consequence of either poor conceptualization or instability. If a relevant process is ignored during conceptualization, the effect of such process may be reproduced by some other parameter. For example, the effect of sorption is to keep part of the solute attached to the solid phase, hence retarding the movement of the solute mass; in linear instantaneous sorption, this effect cannot be distinguished from standard storage in the pores. Therefore, if one needs an absent porosity (e.g., larger than one) to fit observation, one should consider the possibility of including sorption in the model. However, despite this example, parameter assessment tends to be more useful for ruling out some model concepts than for giving a hint on how to modify an inadequate model. Residual analysis is usually more helpful for this purpose. Instability may also lead to unreasonable parameter estimates during automatic calibration, despite the validity of the conceptual model. When the number of data or their information content is low, small perturbations in the measurement or deviations in the model may lead to drastically different parameter estimates. When this happens, the model may obtain equally good fits with widely different parameter sets. Thus, one may converge to a senseless parameter set while missing other perfectly meaningful sets. This type of behavior can be easily identified by means of a thorough error analysis and corrected by fixing the values of one or several parameters.[14]

Predictions and Uncertainty Formulation of predictions involves a conceptualization of its own. Quite often, the stresses, whose response is to be predicted, lead to significant changes

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in the natural system, so that the structure used for calibration is no longer valid for prediction. Changes in the hydrochemical conditions or in the flow geometry may have to be incorporated into the model. While numerical models can be used for network design or as investigation tools, most models are built in order to study the response of the medium to various scenario alternatives. Therefore, uncertainties on future natural and man-induced stresses also cause model predictions to be uncertain. Finally, even if future conditions and conceptual model are exactly known, errors in model parameters will still cause errors in the predictions. In summary, three types of prediction uncertainties can be identified: conceptual model uncertainties; stresses uncertainties; and parameters uncertainties. The first group includes two types of problems. One is related to model selection during calibration. That is, more than one conceptual model may have been properly calibrated and data may not suffice to distinguish which one is the closest to reality. It is clear that such indetermination should be carried into the prediction stage because both models may lead to widely different results under future conditions. The second type of problems arise from improper extension of calibration to prediction conditions, i.e., from not taking into account changes in the natural system or in the scale of the problem. The only way we think about dealing with this problem consists of evaluating carefully whether or not the assumptions in which the calibration was based are still valid under future conditions. Indeed, model uncertainties can be very large. We do not think that, strictly speaking, the second type of uncertainties, those associated with future stresses, falls in the realm of modeling. While future stresses may affect the validity of the model, they are external to it. In any case, this type of uncertainty is evaluated by carrying out simulations under a number of alternative scenarios, whose definition is an important subject in itself. The last set of prediction uncertainties is the one associated with parameter uncertainties, which can be quantified quite well.

CONCLUSION Groundwater modeling involves so many subjective decisions that it can be considered as an art. This is somewhat contrary to the widely accepted perception of models as something objective. The fact is that numerous assumptions need to be made both about the selection of relevant processes and about the manner of representing them in the computer. All these assumptions are specified in the conceptual model.

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The result relies so heavily on conceptualization that models ought to be viewed as theories about the behavior of natural systems. Model predictions should rarely be viewed as firm statements about the future evolution of aquifers. Rather, they should be considered references against which actual data has to be compared. Codes do exist for modeling most processes affecting groundwater (flow, transport, reactions, thermomechanics, etc). It is lack of understanding and lack of data what limits the actual application of those codes. Having specified a conceptual model, the remaining steps (discretization, calibration, uncertainty analysis, prediction) are relatively objective, in the sense that systematic procedures can be followed. This explains why conceptualization is so important. It also explains why modeling is the best way of integrating widely different data. Uncertain as it is, it may represent unambiguously the overall knowledge of the aquifer. Models represent the water balance (or solute balance, or energy balance) at the overall aquifer and at each of its parts. Therefore, they can also be viewed as accounting systems. It is argued that well managed aquifers need real time models to help decision making, the same way that well managed companies need financial accounting systems. This is the challenge modelers must meet in the near future.

REFERENCES 1. Neuman, S.P. Saturated–unsaturated seepage by finite elements. ASCE, J. Hydraulics Div. 1973, 99 (12), 2233–2250. 2. Abriola, L.M.; Pinder, G.F. A multiphase approach to the modeling of porous media contamination by organic compounds. Water Resour. Res. 1985, 21 (1), 11–26. 3. Konikow, L.F.; Bredehoeft, J.D. Groundwater models cannot be validated. Adv. Water Resour. 1992, 15 (1), 75–83. 4. Gorelick, S. A review of distributed parameter groundwater management modeling methods. Water Resour. Res. 1983, 19, 305–319. 5. Freeze, R.A.; Witherspoon, P.A. Theoretical analysis of regional groundwater flow. II, effect of water table configuration and subsurface permeability variations. Water Resour. Res. 1966, 3, 623–634. 6. Garven, G.; Freeze, R.A. Theoretical analysis of the role of groundwater flow in the genesis of stratabound ore deposits. II, quantitative results. Am. J. Sci. 1984, 284, 1125–1174. 7. Ayora, C.; Tabener, C.; Saaltink, M.W.; Carrera, J. The genesis of dedolomites: a discussion based on reactive transport modeling. J. Hydrol. 1998, 209, 346–365. 8. Sa´nchez-Vila, X.; Girardi, J.; Carrera, J. A synthesis of approaches to upscaling of hydraulic conductivities. Water Resour. Res. 1995, 31 (4), 867–882.

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13. NEA-SKI. The International HYDROCOIN Project, Level 2: Model Validation; OECD: Paris, France, 1990; 194 pp. 14. Carrera, J.; Neuman, S.P. Estimation of aquifer parameters under steady state and transient conditions: I through III. Water Resour. Res. 1986, 22 (2), 199–242. 15. Loaiciga, H.A.; Marino, M.A. The inverse problem for confined aquifer flow: identification and estimation with extensions. Water Resour. Res. 1973, 23 (1), 92–104. 16. Hill, M.C. A Computer Program (MODFLOWP) for Estimating Parameters of a Transient, Three Dimensional, Groundwater Flow Model Using Nonlinear Regression, USGS Open-File Report, 1992; 91–484.

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9. Meier, P.; Carrera, J.; Sa´nchez-Vila, X. An evaluation of Jacob’s method work for the interpretation of pumping tests in heterogeneous formations. Water Resour. Res. 1998, 34 (5), 1011–1025. 10. Carrera, J.; Mousavi, S.F.; Usunoff, E.; Sa´nchez-Vila, X.; Galarza, G. A discussion on validation of hydrogeological models. Reliab. Eng. Syst. Saf. 1993, 42, 201–216. 11. Anderson, M.P.; Woessner, W.W. Applied Groundwater Modeling; Academic Press: San Diego, 1992. 12. Konikow, L.F.; Bredehoeft, J.D. Computer Model of Two-Dimensional Solute Transport and Dispersion in Groundwater: Reston, VA; U.S. Geo. Surv. TWRI, Book 7, 1, 1978; 40 pp, Chap. C2.

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Groundwater: Modeling Using Numerical Methods Jesu´s Carrera Technical University of Catalonia (UPC), Barcelona, Spain

INTRODUCTION Numerical methods are tools used by people who develop codes for solving equations governing groundwater problems. All problems that we are interested in are governed by partial differential equations. The computer cannot directly solve these and one needs numerical methods to transform them into a solvable form. In essence, all numerical methods are based on, first, discretizing (i.e., substituting the continuum by a discrete medium) and, second, approximating the differential equation by a system of equations. Numerical methods differ in the way discretization and approximations are performed. To illustrate these two steps, we will first develop them in detail for a generic numerical method. We will, then, introduce the classical numerical methods (finite element, finite differences, etc.). This section ends with a discussion on specific methods for solute transport.

A GENERIC NUMERICAL METHOD FOR SOLVING GROUNDWATER FLOW Global– Groundwater

ð1Þ

where DSi is the rate of change in storage during one time step (say, between time tk and time tkþ1); fij is the inflow into cell i from cell j (and the same for fil, fim, and fin); and gi are external inflows into cell i (for example, recharge, minus pumping, minus evaporation, 448

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DSi ¼ SAi

ðhkþ1
hki Þ i Dt

ð2Þ

where Ai is the surface area of cell i, hki is the head in node i at time k and Dt ¼ tkþ1
tk is the time step. Darcy’s law gives lateral inflows f ij ¼
Twij

hi
hj ¼ aij ðhi
hj Þ Lji

ð3Þ

where T is transmissivity; wij is the width of the connection between nodes i and j; Lji is the length of such connection; and aij is implicitly defined as
Twij/Lji. The remaining inflow terms, fil, fim, and fin are defined likewise. Changing these terms to the left-hand side of Eq. (1) and rearranging terms yields: ðhkþ1
hki Þ i þ aii hi þ aij hj þ ail hl Dt þ aim hm þ ain hn ¼ gi

SAi

As mentioned earlier, all methods require, first, discretizing and, second, approximating the physical phenomenon. For the generic method we are going to present here, discretization will be performed as shown in Fig. 1. That is, the continuum aquifer domain will be substituted by a discrete number of cells. Furthermore, the continuum aquifer heads, h(x,y), are substituted by a discrete number of model heads, hi. The second step, approximation, can be made in different manners. For the purpose of this section, it is sufficient to bear in mind that the flow equation is nothing but a mass balance. Therefore, we will express the mass balance in cell i as change in storage equals inflows minus outflows. DSi ¼ f ij þ f il þ f im þ f in þ gi

minus river outflow, etc.). Each of the terms in Eq. (1) is relatively easy to approximate. Storage variation can be derived from the definition of storage coefficient, (S is the change in volume of water stored per unit surface area of aquifer and per unit change in head):

ð4Þ

where aii ¼
aij
ail
aim
ain. If an equation like Eq. (4) is written for all cells from i ¼ 1 through N, N being the number of cells (nodes), the resulting system of equations can be rewritten in matrix form as: D

ðhkþ1
hk Þ þ Ah ¼ g Dt

ð5Þ

where D is a diagonal matrix whose i-th diagonal term is precisely SAi. This matrix is often called storage matrix. A is the conductance matrix, a square symmetric matrix whose components are aij. Finally, g is the source vector. All the numerical methods to be outlined in subsequent sections lead to equations analogous to Eq. (5). Moreover, the meaning of the terms in such equations is always similar to that in Eq. (5). Namely, the system represents the mass balance at each of the N nodes (cells); specifically the i-th equation represents the mass balance at the i-th node. The first

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term, D(hkþ1
hk)/Dt, always represents storage variations. The second term, Ah, represents outflows from minus inflows into the i-th cell from the adjacent cells. Finally, term g represents external inflows minus outflows (recharge, pumping, etc.) at all i. Eq. (5) needs to be integrated in time. For this purpose, let us assume that Ah is evaluated at time k þ 1 (Ahkþ1). Then, Eq. (5) can be rewritten as:
A þ

 D kþ1 D k h h ¼ g þ Dt Dt

ð6Þ

This is simply a linear system, which can be solved using conventional methods. Bhkþ1 ¼ b

ð7Þ

where B ¼ A þ D/Dt and b ¼ g þ Dhk/Dt. This system is solved sequentially in time. That is, most codes solve Eq. (7) using the following steps (Fig. 2): 1. Input all data. Set k ¼ 0. 2. Compute g, A [Eq. (3)]; D [Eq. (5)] and B [Eq. (7)]. 3. Set k ¼ k þ 1. 4. Build b [Eq. (7)]. 5. Solve Bhkþ1 ¼ b. 6. If k ¼ kmax (maximum number of time steps), end. Otherwise, return to step 3.

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Fig. 2 Basic steps involved in simulating groundwater flow. Heads hkþ1 are computed by solving equation Bhkþ1 ¼ b. They may be written for later drawings. They are used as initial head for the next time increment. These steps (time loop) are repeated sequentially until the last time is reached.

Most codes follow a structure such as this, although each method displays specific features. Some of these are outlined below.

FINITE DIFFERENCES (FD) As mentioned at the beginning, numerical methods differ in the way in which the domain is discretized and in the way in which the partial differential equation is transformed into a linear system of equations. In finite differences, the problem domain is discretized in a regular grid (Fig. 3A), usually rectangular (equilateral triangles or hexagons are possible, but very rare). The grid may be centered at the corners (nodes are located at the vertices of the squares) or at the cells (nodes are located at the center of the squares, such as in Fig. 3A.)

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Fig. 1 The system of equations for a generic groundwater model is obtained by establishing a water balance (inputs minus outputs equal storage variations) at each cell. Inputs and outputs include water exchanges with the outside (pumping Qi, recharge, ri, etc.) and with adjacent cells (fij). The latter are expressed, using Darcy’s law, as fij ¼ Tijwij (hj
hi)/Lij.

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nodes. Frequently, the cells are the Thiessen polygons of the set of nodes. This allows adapting the node density to the problem (e.g., increasing nodes density where accuracy is needed most). Model equations can be derived using a mass balance approach, such as in the generic method section. Integrating the flow equation over each cell and applying Green’s identity to transform volume integrals in boundary fluxes can also yield model equations. This type of approach is the basis of the finite volume method, which is widely used nowadays.

FINITE ELEMENT METHOD

Fig. 3 The most widely spread methods of discretization are Finite Differences, which consists of subdividing the model domain into regular rectangles, and Finite Elements, which is based on dividing the aquifer region into elements of arbitrary shape (often triangles). Finite Volumes, also called Integrated Finite Differences, divides the region into polygons. The Boundary Element Method is very convenient, when applicable, because it only requires discretizing boundaries (both internal and external).

Regarding the approximation of the partial differential equations, several alternatives are possible. The most intuitive consists of substituting all derivates by an incremental ratio. That is, the derivative between adjacent nodes i and j is approximated as: Global– Groundwater

@h hi
hj ¼ Dx @x

ð8Þ

where hi and hj are heads at nodes i and j, respectively, and Dx is the distance between them. Approximating all derivatives by means of equations analogous to Eq. (8) leads to a system identical to Eq. (5). In fact, the finite differences method is often introduced using a mass balance approach such as the one in the section ‘‘A Generic Numerical Method for Solving Groundwater Flow,’’ only using a regular instead of a generic grid. This is the method used in MODFLOW,[1] HST3D,[2] and their children.

INTEGRATED FINITE DIFFERENCES (IFD) The basic philosophy of this method is very similar to that of the generic method introduced in the generic numerical section. Basically, the domain is discretized in a number of cells centered around arbitrarily located

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Finite element method (FEM) discretization consists of elements and nodes. Elements are generalized polygons (normally triangles or curvilinear quadrilaterals). Nodes are points located at the vertices and, sometimes, at the sides or the middle of the element. Unlike FD or IFD, cells around the nodes are not defined. Still, in many cases, one may write the equations in such a way that the mass balance formulation of the generic method section is still valid. However, the most singular feature of the FEM is the way the solution is interpolated, so that it becomes defined at every point. That is, head (or concentrations) is approximated as: ^ðxÞ ¼ hðxÞ ffi h

X

hi N i ðxÞ

ð9Þ

i

where hi are nodal heads and Ni are interpolation func^ is not the exact solution, it would yield a tions. Since h residual if substituted in the flow equation. Minimizing this residual, which requires somewhat sophisticated maths, leads to a system similar to Eq. (5).

BOUNDARY ELEMENT METHOD The idea behind the boundary element method (BEM) is similar to that of the FEM. The main difference stems from the choice of interpolation functions, which are taken as the fundamental solutions of the flow equation (or whatever equation is to be solved). As a ^ is substituted in the result, when the corresponding h flow equation, the residuals are zero. Since the equation is satisfied exactly in the model domain, one is only left with boundary conditions. In fact, as shown in Fig. 3D, discretization is only required at the boundaries, where boundary heads are defined so as to satisfy approximately the boundary conditions. This method is extremely accurate, but its applicability is limited by the need of finding the fundamental solutions. This constrains the BEM to flow problems in relatively homogeneous domains.

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so, it introduces numerical dispersion. As a result, the wiggles at the solute front in Fig. 4 are substituted by an artificially smeared front. However, the vast majority of alternative formulations for solute transport are Lagrangian in the sense that time variations are written in terms of the material derivative, which expresses the rate of change in concentration of a particle that moves with the water. In this way, the advective term, which is the cause of problems in eulerian formulations, disappears. The number of Lagrangian methods is very large and many researchers have devoted much effort to find one, which is universal. The fact that so many methods have survived to date suggests that the effort has not been fully successful. Still, in practical problems, one can usually find a suitable method. Following is an outline of some of the most popular methods, with a discussion of their advantages and disadvantages and early references. The interested reader should seek further. The most natural Lagrangian method is to write the equations on a moving grid, that is on a grid whose nodes move with water. This method is highly accurate, but expensive because the grid has to be updated every time step. Moreover, the grid can become highly deformed over time. To avoid problems with moving grids it is frequent to work with particles. Displacing the particles with the moving water represents advection, while dispersion can be represented with a variety of methods. One such possibility is to add a random component to each particle basis displacement. This is statistically equivalent to each particle basis dispersion and is the basis of the ‘‘random walk’’ method.[4] The method requires careful implementation, but its main drawback is the fact that the solution is given in terms of number of particles per cell. If one is interested in spatial distributions of the solute, a huge number of particles may be needed. The method of characteristics (MOC) overcomes the above difficulty by assigning concentrations to

All the methods discussed above can be used for simulating solute transport. They are called Eulerian methods because they are based on a fixed (as opposed to moving) grid and all derivatives are based on a fixed coordinate system. They work fine when dispersion is dominant. Otherwise, they may lead to numerical problems (Fig. 4). Two dimensionless numbers are used to anticipate numerical difficulties. Specifically, discretization must satisfy the following conditions. Peclet number Pe ¼

vDx Dx 1 ffi < D a 2

ð10Þ

Courant number Co ¼

vDt < 1 Dx

ð11Þ

where v is the solute velocity, a is dispersivity, Dx is the distance between nodes and we have assumed that D ffi av. The condition on the Courant number implies that the solute at one node will not move beyond the following node downstream during one time step. This condition is easy to meet because usually groundwater moves slowly and also reducing Dt to satisfy Eq. (11) is not difficult. The condition on the Peclet number, Eq. (10), implies that Dx is smaller than a/2, which may require very small elements, leading to a huge computational burden. Because of this, conventional Eulerian methods are not applicable to many groundwater problems, which have motivated the search of alternative methods. Alternative methods can be Eulerian or Lagrangian. Among the former, the most popular is upstream weighting, introduced by Heinrich et al.[3] in the FEM, but with a huge number of papers thereafter. It consists of slightly modifying Eulerian equations so as to ensure stability. The problem is that, in doing

Fig. 4 Difficulties typically associated to numerical simulation of solute transport: A) front smearing: the concentration front is more dispersed than it should; B) instability oscillations: too high and/or too low (even negative) concentrations.

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particles and interpolating them onto a fixed grid, where dispersion and, possibly, other transport processes are modeled. Concentrations are then interpolated back onto the particles. The method has become very popular in groundwater because of the USGS MOC Code.[5] The method is very practical, but the interpolation back-and-forth between particles and grid may introduce numerical dispersion and mass balance errors. The modified method of characteristics[6] tries to overcome the problems associated to interpolating particle concentrations by redefining them in each time interval so that at the end of the time step they coincide with a node location. The method is very accurate, although some interpolation errors still occur when the front is abrupt. Some of these problems are overcome by the Eulerian–Lagrangian Localized Adjoint Method,[7] which looks as the most promising method.

CONCLUSION Computer codes are available for simulating all phenomena affecting groundwater. In essence, they represent the balance of water (or salt, contaminants, or energy) in a manner that can be solved by the computer. This is achieved by, first, discretizing the problem and, second, rewriting it as a system of equations. This type of approach has been successful for flow problems. Solute transport, on the other hand, remains ellusive. No single method is universally

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successful. Instead, one must seek the appropriate code in each case.

REFERENCES 1. McDonald, M.G.; Harbaugh, A.W. A modular threedimensional finite difference groundwater flow model. Techniques of Water Resources Inv., of the USGS, Book 6, 1988, Ch. A1. 2. Kipp, K.L. HST3D, A Computer Code for Simulation of Heat and Solute Transport in Three Dimensional Groundwater Flow Systems. USGS Water Recourses Inv. Rep. 86-4095; 1987. 3. Heinrich, J.C.; Huyakorn, P.S.; Mitchell, A.R.; Zienkiewicz, O.C. An upwind finite element scheme for two-dimensional transport equation. Int. J. Numer. Methods Eng. 1977, 11, 131–143. 4. Prickett, T.A.; Naymik, T.G.; Lonnquist, C.G. A ‘‘Random Walk’’ solute transport model for selected groundwater quality evaluations. Illinois State Water Survey Bull. 1981, 55, 62. 5. Konikow, L.F.; Bredehoeft, J.D. Computer Model of Two-Dimensional Solute Transport and Dispersion in Ground Water; U.S. Geo. Surv. TWRI: Reston, Va., 1978; Book 7, 40 pp. 1 Chap. C2. 6. Neuman, S.P. A Eulerian–lagrangian numerical scheme for the dispersion–convection equation using conjugate space–time grids. J. Comput. Phys. 1981, 41, 270–294. 7. Celia, M.A.; Russell, T.F.; Herrera, I.; Ewing, R.E. An Eulerian–lagrangian localized adjoin method for the advection–diffusion equation. Adv. Water Resour. 1990, 13, 187–206.

Groundwater: Pollution from Mining Jeffrey G. Skousen West Virginia University, Morgantown, West Virginia, U.S.A.

George F. Vance Agricultural Experiment Station, University of Wyoming, Laramie, Wyoming, U.S.A.

Surface and underground mining activities can have direct and indirect impacts on the quantity, quality, and usability of groundwater supplies. The nature of the mining activity, geological substrata, and redistribution of surface and subsurface materials will determine to a large degree how groundwater supplies will be impacted. As waters interact and alter the disturbed geologic materials, constituents such as salts, metals, trace elements, and/or organic compounds become mobilized.[1,2] Once mobilized, the dissolved substances can leach into deep aquifers, resulting in groundwater quality impacts. In addition to concerns due to naturally occurring contaminants, mining activities may also contribute to groundwater pollution from leaking underground storage tanks, improper disposal of lubricants and solvents, contaminant spills as well as others. In the United States, the Clean Water Act (CWA), which was enacted in 1948 as the Water Pollution Control Act (WPCA), and the CWA amendments in 1977, establishes the authority for all water pollution control actions at the federal level.[3] The Safe Drinking Water Act (SDWA), which was enacted in 1974 and amended in 1996, was promulgated to protect drinking water supplies by legislating maximum contaminant levels (MCLs) above which waters are considered unsafe for human consumption, and defined enforcement standards that states are required to use for determining minimum treatments needed to improve water quality.[4] Examples of some MCLs that may be associated with water quality issues relating to mining activities are listed in Table 1. Because mining activities can result in poor quality groundwaters, enforcement of regulations is needed to minimize and/or eliminate potential problems. The Surface Mining Control and Reclamation Act (SMCRA) of 1977 specifies policies and practices for mining and reclamation to minimize water quality impacts.[6] The SMCRA requires that specific actions be taken to protect the quantity and quality of both

on- and off-site groundwaters. All mines are required to meet either state or federal groundwater guidelines, which are generally related to priority pollutant standards described in the CWA.

GROUNDWATER RESOURCES Our groundwater resources are the world’s third largest source of water and represent 0.6% of the earth’s water content. Approximately 53% of the U.S. population uses groundwater as a drinking water source, but this percentage increases to almost 97% for rural households. In areas of low rainfall, weathering and translocation of dissolved constituents is relatively slow compared to high rainfall areas. In addition, physical disruption of rocks into small particles can enhance mineral weathering that results in mineral dissolution and migration of dissolved substances. Transport of contaminants from surface and subsurface environments to groundwaters is generally accelerated as the amount of percolating water increases. Infiltrating water moves through the vadose zone (unsaturated region) into groundwater zones (saturated region). The upper boundary of the groundwater system (e.g., water table) fluctuates depending on the amount of water received by, or depleted from, the groundwater zone. Groundwater movement is a function of hydraulic gradients and hydraulic conductivities, which represent the ease with which water moves as a function of gravitational forces and the permeability of substrata materials. Groundwater moves faster in coarse textured substrata and as the slope of the water table increases. Aquifers are groundwater systems that have sufficient porosity and permeability to supply enough water for a specific purpose. In order for an aquifer to be useful, it must be able to store, transmit, and yield sufficient amounts of good quality water. Important hydrogeological characteristics of a site that determine groundwater quantity and quality are listed in Table 2. 453

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Table 1 Select contaminants in drinking waters that may be influenced by mining activities Contaminant

MCLa (mg/L)

MCLGb (mg/L)

0.006 0.005 0.1 LVc 0.2 4 0.015 0.002 0.1 10 0.05 500 0.002

0.006 0.005 0.1 1.3 0.2 4 0 0.002 0.1 10 0.05 500 0.0005

300 q/L 0.02 q/L

0 0

Groundwater flow directions (hydraulic gradients, both horizontal and vertical), volumes (specific discharge), rate (average linear velocity)

0.005 0.005 0.001 1 10

0 0 0 1 10

Recharge and discharge areas

Inorganics Arsenic Cadmium Chromium Copper Cyanide Fluoride Lead Mercury Nickel Nitrate (NO3-N) Selenium Sulfate Thallium Radionuclides Radon Uranium

Microbiological Total coliforms Viruses

Geological Type of water-bearing unit or aquifer (overburden, bedrock) Thickness, areal extent of water-bearing units and aquifers Type of porosity (primary, such as intergranular pore space, or secondary, such as bedrock discontinuities, e.g., fracture or solution cavities) Presence or absence of impermeable units or confining layers. Depths to water tables; thickness of vadose zone. Hydraulic Hydraulic properties of water-bearing unit or aquifer (hydraulic conductivity, transmissivity, storability, porosity, dispersivity) Pressure conditions (confined, unconfined, leaky confined)

Organics Benzene Carbon tetrachloride Pentachlorophenol Toluene Xylenes

Table 2 Important hydrogeological characteristics of a site that determine groundwater quantity and quality

Groundwater or surface water interactions; areas of ground water discharge to surface water Seasonal variations of groundwater conditions Groundwater use

LV LV

0 0

a

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MCL ¼ maximum contaminant levels permissible for a contaminant in water that is delivered to any user of a public water system. b MCLG ¼ maximum contaminant level goals of a drinking water contaminant that is protective of adverse human health effects and which allows for an adequate margin of safety. c LV ¼ lowest value that can be achieved using best available technology. Source: From Ref.[5].

Existing or potential underground sources of drinking water Existing or near-site use of groundwater

be immediately excavated and disposed of by approved methods. The chemistry of groundwaters and potential levels of naturally occurring contaminants are related to:

GROUNDWATER CONTAMINANTS

Table 3 Different classes of groundwater contaminants and their origins

There are several types of substances that can affect groundwater quality.[1,7] Water contaminants include inorganic, organic, and biological materials, of which some have a direct impact on water quality, whereas others indirectly cause physical, chemical, or biological changes. Substances that can impact groundwaters include nutrients, salts, heavy metals, trace elements, and organic chemicals, as well as contaminants such as radionuclides, carcinogens, pathogens, and petroleum wastes (Table 3). Some groundwaters are derived from mining activities contain natural (e.g., methane gas) and synthetic organic chemicals. Organic contamination may result from leaking gas tanks, oil spills, or run-off from equipment-servicing areas. In these cases, the source of the contamination must be identified and removed. Gasoline, diesel, or oil-soaked areas should

Water Contaminant Class

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Contributions

Inorganic chemicals

Toxic metals and acidic substances from mining operations and various industrial wastes

Organic chemicals

Petroleum products, pesticides, and materials from organic wastes industrial operations

Infectious agents

Bacteria and viruses from sewage and other organic wastes

Radioactive substances

Waste materials from mining and processing of radioactive substances or from improper disposal of radioactive isotopes

Source: From Ref.[8].

1) groundwater hydrologic conditions; 2) mineralogy of the mined and locally impacted geological material; 3) mining operation (e.g., extent of disturbed materials and its exposure to atmospheric conditions); and 4) time. Movement of metal contaminants in groundwater varies depending on the chemical of concern, and include considerations such as with cobalt (Co), copper (Cu), nickel (Ni), and zinc (Zn) mobility being greater than silver (Ag) and lead (Pb), which tend to be more mobile than gold (Au) and tin (Sn).[1] As conditions such as pH, redox, and ionic strength change over time, dissolved constituents in groundwaters may decrease due to adsorption, precipitation, and chemical speciation reactions and transformations. Acid mine drainage (AMD) is most prevalent at inactive and abandoned surfaces and underground mine sites. If geological substrata containing reduced S minerals [e.g., pyrite (FeS2)] is exposed to oxygen (O2), such as when pyritic overburden materials are brought to the earth surface during mining activities and then re-buried, high concentrations of sulfuric acid (H2SO4) can develop and form acid waters with pH levels below 2. Neutralization of some of the acidity produced during the oxidation of reduced S-compounds occurs when silicate minerals dissolve; however, during this process, high levels of potentially toxic metals such as aluminum (Al), copper (Cu), cadmium (Cd), iron (Fe), manganese (Mn), Ni, Pb, Zn, may be released. For example, mining of coal in the Toms Run area of northwestern Pennsylvania resulted in groundwater contamination by AMD containing high concentrations of Fe and sulfate (SO4) that leached into the underlying aquifer through joints, fractures, and abandoned oil and gas wells. The Gwennap Mining District in the United Kingdom contained numerous mines that operated over several centuries to extract various mineral resources. One of these mines, the Wheal Jane metalliferous mine in Cornwall, extracted ores that included cassiterite (Sn-containing mineral), chalcopyrite (Cu), pyrite (Fe), wolframite [tungsten (W)], arsenopyrite [arsenic (As)], in addition to smaller deposits of Ag, galena (Pb), and other minerals.[9] After closure in the early 1990s, extensive voids remaining in the Wheal Jane mine that contained oxidized and weathered minerals were flooded. Initial groundwater quality was poor with a pH of 2.8 and a total metal concentrations close to 5000 mg/L, which contained high levels of Fe, Zn, Cu, and Cd. Water quality worsened with depth, and at 180 m the groundwater had a pH of 2.5 and metal concentrations of 2200, 1500, 44, and 5 mg/L for Fe, Zn, Cu, and Cd, respectively. Current treatment of discharge waters originating from the

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mine involves an expensive process and will continue to be long-term if environment quality in the region is to be preserved. A similar situation occurred when a Zn mine in southwestern France was closed. However, after flooding, discharge mine waters contained high concentrations of Zn, Cd, Mn, Fe, and SO4 even though the solution pH was near neutral. Within the Coeur D’Alene District of Idaho, location of the Bunker Hill Superfund site, groundwater samples have been found to contain high concentrations of Zn, Pb, and Cd.[10] The contamination was believed to originate from the leaching of old mine tailings that were deposited on a sand and gravel aquifer. When settling ponds were developed nearby the old tailings, re-charge of the local groundwaters resulted in a rise in the water table that saturated the tailings causing considerable metal leaching to occur. Gold mining operations have used cyanide as a leaching agent to solubilize Au from ores, which often contain arsenopyrite [As, Fe, sulfur (S)], and in some cases pyrite.[1] During the leaching process, Ag is also recovered as a by-product if present. Unfortunately, cyanide is a powerful non-selective solvent that will solubilize numerous substances that can be environmental contaminants. These ore waste materials are often stored in tailing ponds, and depending on the local geology and climate, cyanide present in the tailings can exist as free cyanide (CN
, HCN), inorganic compounds (NaCN, HgCN2), metal–cyanide complexes with Cu, Fe, Ni, and Zn, and/or the compound CNS. Because cyanide species are mobile and persistent under certain conditions, there is the potential for trace element and cyanide migration into groundwaters. For example, a tailings dam failure resulted in cyanide contamination of groundwater at a gold mining operation in British Columbia, Canada.[1] Arsenic and uranium (U) contamination has resulted from extensive mining and smelting of ores containing various metals (Ag, Au, Co, Ni, Pb, and Zn) and/or non-metals (As, phosphorus (P), and U). Contaminated As groundwaters have been a source of surface re-charge and drinking water supplies; As in a contaminated river of Canada were 7 and 13 times greater than the recommended national and local drinking water standards, respectively.[1] Arsenic is known as a carcinogen and has been the contributing cause of death to humans in several parts of the world that rely on As-contaminated drinking waters.[7] Waters from dewatering a U mine in New Mexico had elevated levels of U and radium (Ra) activities as well as high concentrations of dissolved molybdenum (Mo) and selenium (Se), which were detected in stream waters 140 km downstream from the mine.

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GROUNDWATER ANALYSIS

STRATEGIES FOR REMEDIATING CONTAMINATED GROUNDWATERS

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Both remediation and prevention of groundwater contamination by nutrients, salts, heavy metals, trace elements, organic chemicals (natural and synthetic), pathogens, and other contaminants requires the evaluation of the composition and concentration of these constituents either in-situ or in groundwater samples.[2] Monitoring may require the analysis of physical properties, inorganic and organic chemical compositions, and/or microorganisms according to well-established protocols for sampling, storage, and analysis.[11] For example, if groundwaters will be used for human or animal consumption, the most appropriate tests would be nitrate-nitrogen (NO3-N), trace metals, pathogens, and organic chemicals. Several common constituents measured in groundwaters are listed in Table 4; however, other tests can be conducted on waters including tests for hardness, chlorine, radioactivity, or water toxicity.[12] Recommendations based on interpretation of the groundwater test results should be related to the ultimate use of the water.[2] The interpretation and recommendation processes may be as simple as determining that a drinking water well exceeds the established MCLs for NO3-N and recommending the well should not be used as a drinking water source or that a purification system be installed. However, interpretations of most groundwater analyses can be quite complicated and require additional information for proper interpretation. If a contaminant exceeds an acceptable concentration, all potential sources contributing to the pollution and pathways by which the contaminant moves must be determined. In many cases, multiple groundwater contaminants are present at different concentrations. Because the interpretation of water analyses is a complex process, recommendations should be based on a complete evaluation of the water’s physical, chemical, and biological properties. Integrating water analyses into predictive models that can assess the effects of mining activities on water quality is needed in the long term to determine the most effective means to preserve and restore water quality.

Mine sites that have been contaminated generally contain mixtures of inorganic and/or organic constituents, so it is important to understand these multicomponent systems in order to develop remediation strategies. Therefore, a proper remediation program must consider identification, assessment, and correction of the problem.[13] Identification of a potential problem site requires that either the past history of the area and activities that took place are known, or when a water analysis indicates a site has been contaminated. Assessment addresses questions such as is there a problem, where is the problem, and what is the extent of the problem? Afterwards a remediation action plan must be developed that will address the specific problems identified. A remediation action program may require that substrata materials (e.g., backfill) and groundwater be treated. If remedial action is considered necessary, three general options are available—containment, in-situ treatment, or pump-and-treat (Fig. 1). The method(s) used for the containment of contaminants are beneficial for restricting contaminant movement. Of the remediation techniques, in-situ treatment measures are the most appealing because they generally do less surface damage, require a minimal amount of facilities, reduce the potential for human exposure to contaminants, and when effective, reduce or remove the contaminant. In-situ remediation can be achieved by physical, chemical, and/or biological techniques. Biological in-situ techniques used for groundwater bioremediation can either rely on the indigenous (native) microorganisms to degrade organic contaminants or on amending the groundwater environment with microorganisms (bioaugmentation). The pump-and-treat method, however, is one of the more commonly used processes for remediating contaminated groundwaters. With the pumpand-treat method, the contaminated waters are pumped to the surface where one of many treatment processes can be utilized. A major consideration in the pump-and-treat technology is the placement of

Table 4 Groundwater quality parameters and constituents measured in some testing programs Physical parameters Conductivity, salinity, sodicity, dissolved solids, temperature, odors

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Metals and trace elements

Non-metallic constituents

Organic chemicals

Al, Ag, As, Ca, Cd, Cr, Cu, Fe, Mg, Mn, Na, Ni, Pb, Se, Sr, Zn

pH, acidity, alkalinity, dissolved O2, B, CO2, HCO3, Cl, CN, F, I, NH4, NO2, NO3, P, Si, SO4

Methane, oil and grease, organic acids, volatile acids, organic C, pesticides, phenols, surfactants

Microbiological parameters Coliforms, bacteria, viruses

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Fig. 1 Remediation options to consider if cleanup of contaminated groundwater is required.

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non-ionic exchange resins can adsorb contaminants, reducing their leaching potential. Biological methods for contaminant remediation are less extensive than physical and chemical techniques. Land treatment is an effective method for treating groundwaters by applying the contaminated waters to lands using surface, overland flow, or subsurface irrigation. Activated sludge and aerated surface impoundments are used to precipitate or degrade contaminants present in water and include both aerobic and anaerobic processes. Biodegradation is one of several biological-mediated processes that transform contaminants and utilizes vegetation and microorganism.

REFERENCES 1. Ripley, E.A.; Redmann, R.E.; Crowder, A.A. Environmental Effects of Mining; St. Lucie Press: Delray Beach, FL, 1996; 356 pp. 2. Pierzynski, G.M.; Sims, J.T.; Vance, G.F. Soils and Environmental Quality, 2nd Ed.; CRC Press, Inc.: Boca Raton, FL, 2000; 459 pp. 3. U.S. Congress. In Clean Water Act; Public Law 95-217; 1977. 4. U.S. Congress. Safe Drinking Water Act and Amendments; Public Law 104-182; 1996. 5. U.S. Environmental Protection Agency Office of Water. Drinking Water Regulations and Health Advisories; USEPA Report 822-B-96; USEPA: Washington, DC, 1996. 6. U.S. Congress. Surface Mining Control and Reclamation Act; Public Law 95-87; 1977. 7. Manahan, S.E. Environmental Chemistry, 5th Ed.; Lewis Publishers: Chelsea, MI, 1991.

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wells, which is dependent on site characteristics (see Table 2. Extraction wells are used to pump the contaminated water to the surface where it can be treated and re-injected or discharged. Injection wells can be used to re-inject the treated water, water containing nutrients and other substances that increase the chances for chemical alteration or microbial degradation of the contaminants, or materials for enhanced oil recovery. Treatment techniques can be grouped into three categories including physical, chemical, and biological methods.[2,13] Physical methods include several techniques. Adsorption methods physically sorb or trap contaminants on various types of resins. Separation treatments include physically separating contaminants by forcing water through semipermeable membranes (e.g., reverse-osmosis). Flotation, or density separation, is commonly used to separate low-density organic chemicals from groundwaters. Air and steam stripping can remove volatile organic chemicals. Isolation utilizes barriers placed above, below, or around sites to restrict movement of the contaminant; containment systems should have permeabilities of 10
7 cm/sec (approximately 0.1 ft/yr) or less. Chemical methods are also numerous. Chemical treatment involves addition of chemical agent(s) in an injection system to neutralize, immobilize and/or chemically modify contaminants. Extraction (leaching) of contaminants uses one of several different aqueous extracting agents such as an acid, base, detergent, or organic solvent miscible in water. Oxidation and reduction of groundwater contaminants is commonly done using air, oxygen, ozone, chlorine, hypochlorite, and hydrogen peroxide. Ionic and

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8. Nielsen, D.M. A Practical Handbook of Groundwater Monitoring; Lewis Publishers: Boca Raton, FL, 1991. 9. Bowen, G.G.; Dussek, C.; Hamilton, R.M. Pollution resulting from the abandonment and subsequent flooding of wheal jane mine in Cornwall, UK. In Groundwater Contaminants and Their Migration, Mather, J., Banks, D., Dumpleton, S., Fermor, M., Eds.; Special publication 128; 1998; 93–99. 10. Todd, D.K.; McNulty, D.E.O. Polluted Groundwater; Water Information Center, Inc.: Huntington, NY, 1975; 32–33.

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11. Clesceri, L.S.; Greenberg, A.E.; Eaton, A.D.; Eds. Standard Methods for the Examination of Water and Wastewater, 20th Ed.; American Public Health Association: Washington, DC, 1998; 1220. 12. U.S. Environmental Protection Agency. Quality Criteria for Water, Document 440/5-86-001; EPA: Washington, DC, 1986. 13. Hyman, M.H. Groundwater and Soil Remediation. In Encyclopedia of Environmental Pollution and Cleanup; Meyers, R.A., Editor-in-Chief; John Wiley & Sons, Inc.: New York, NY, 1999; Vol. 1, 684–712.

Groundwater: Pollution from Nitrogen Fertilizers Lloyd B. Owens Agricultural Research Service (USDA-ARS), U.S. Department of Agriculture, Coshocton, Ohio, U.S.A.

INTRODUCTION Why is nitrogen in groundwater a problem? High nitrate levels in water consumed by humans can cause adverse health problems, and groundwater is a major source of water for human consumption. A part of groundwater resurfaces to feed surface water from streams to oceans. High levels of nitrogen can cause excess plant and bacterial growth, which upon death and decay can deplete much of the oxygen in water. This causes fish kills and ‘‘dead zones,’’ such as the area of hypoxia in the Gulf of Mexico. This encyclopedia article discusses agricultural practices, e.g., row crops, grasslands/turf, container horticultural crops, that contribute to nitrogen in groundwater. Some agricultural practices to reduce the contributing factors are also presented.

receives much of the attention in regards to eutrophication in fresh waters because it often is the limiting nutrient. But as water systems become more brackish, there is a shift to N limitation.[2] A major example of this situation is the area of hypoxia in the Gulf of Mexico.[3] Hypoxia occurs when the concentration of dissolved oxygen is less than 2 mg L 1. Nitrogen contributions to the Gulf of Mexico, and other large bodies of water, come via surface runoff and resurfacing of groundwater. There are also several agricultural sources of nitrogen, e.g., nitrogen fertilizer, surface application of manure, manure from grazing systems, and mineralization of organic matter. However, the focus of this chapter will be only on groundwater, and how it is impacted by the leaching of nitrogen fertilizers. Although several aspects of nitrate leaching will be addressed and accompanied by supporting references, space does not permit this chapter to be a comprehensive literature review.

WHY NITROGEN IN GROUNDWATER IS A PROBLEM

Groundwater is a major source of water for human consumption. Nitrogen present in groundwater is usually in the form of nitrate (NO3), and at high levels can pose major human health concerns, especially for infants. The link between high NO3 in polluted water and serious blood changes in infants was first reported in 1945. From 1947 to 1950, 139 cases of methemoglobinemia were reported, including 14 deaths in Minnesota alone. Thus, a standard has been set that NO3 in excess of 45 mg L 1 (10 mg L 1 NO3-N) is considered hazardous to human health.[1] Environmental Impacts A part of groundwater, especially shallow groundwater, resurfaces to feed streams, rivers, and reservoirs and eventually estuaries and oceans. Nutrients, pollutants, in the groundwater are carried via these routes as well and can cause excess plant and bacterial growth in aquatic systems. The decay of this organic matter can deplete much of the oxygen in the water causing fish kills and ‘‘dead zones’’ to occur. Phosphorus

AGRICULTURAL PRACTICES CONTRIBUTING TO THE NITROGEN IN GROUNDWATER PROBLEM Row Crops High levels of NO3-N in subsurface drainage from row crops, especially corn (Zea mays L.), are well documented. Nitrate-N concentrations in tile lines draining silt loam soils in Iowa with fertilized, continuous corn, or corn in rotation already exceeded 10 mg L 1 two decades ago.[4,5] Other high NO3-N levels have been reported in tile lines with clay loams in Minnesota;[6–8] with silty clay loams in Illinois;[9] with silt loams in Indiana;[10] with silt loams/silty clay loams in Ohio;[11] and with clay over silty clay loam and fine sand over clay in Ontario, Canada.[12] Analyses of subsurface water collected with monolith lysimeters[13,14] and ceramic porous-cup samplers[15,16] are in agreement with these findings. Nitrate-N concentrations have been studied in tile drains frequently because of their wide spread use and the relative ease of collecting a sample. The majority of NO3-N moves in the subsurface water during the winter recharge period.[9,10,13] There are several factors 459

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that impact the amount of N export from tiles, including timing and area of N fertilization.[9] Increasing the drain spacing decreases the NO3-N losses in tiles[6,10] although the NO3-N concentration in the tiles may change very little.[10] Even though the increased drain spacing should reduce the NO3-N losses in the tile, it probably increases the NO3-N losses in seepage below the drains. Model simulation studies show that reducing N fertilization rates will have much greater impact for reducing NO3-N losses than changing tile drain spacing or depth.[6] Too often, inexpensive N fertilizer has been applied in excess to crops to ensure that inadequate N will not limit crop yields. The difficulty in synchronizing N applications with crop needs contributes to such practices. It has been shown that there is a direct relationship between NO3-N loss by leaching and application rates of N that exceed crop needs.[15,17] Excess N in soil can result from overapplication of N fertilizers or manure or from residual N from the previous year (as well as from mineralization of organic N). This can be a particular problem following a dry year because reduced crop growth will not utilize as much N fertilizer as during a year when a ‘‘normal’’ amount of water was available.[8] Therefore, there is an increased amount of residual N to begin the next cropping season. Even at economic optimum N (EON) levels, considering all sources of N, concentrations of NO3-N in subsurface water have been found to exceed the 10 mg L 1 maximum contaminant level (MCL).[15,17] The conclusion can be drawn that optimum corn production will likely produce elevated NO3-N concentrations in groundwater.[17] In irrigated agriculture, a similar impairment to groundwater quality exists from N fertilizer management. High concentrations of NO3-N were found in subsurface water under a sprinkler irrigated crop rotation in Spain,[18] a sprinkler irrigated corn–soybean rotation in Nebraska,[19] and flood irrigated wheat in Arizona.[20] Even with irrigation BMPs, NO3-N concentrations in groundwater above the MCL can be expected.

Grasslands/Turf Because of the animal component, NO3-N leaching in grazed grassland is quite complex. Leaching of NO3-N from grasslands is greatly increased with the presence of grazing livestock.[21,22] Even on highly fertilized pastures, much of the leached NO3-N has been attributed to excreta.[23,24] Studies in England,[25,26] the Netherlands,[27] and the eastern USA[28,29] have shown that NO3-N concentrations in subsurface water are often greater than 10 mg L 1 when >100 kg N ha 1 is applied annually to grazed grasslands. Other processes,

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such as the accumulation of fertilizer-N during drought or the release of N from decaying plant material, e.g., resulting from tilling or killing the sod in preparation for reseeding, may influence N leaching from the pasture as a whole, rather than acting specifically on areas affected by urine.[23] In some non-grazed systems, NO3 leaching from highly fertilized systems is low, e.g., 29 kg N ha 1 lost from ryegrass (Lolium perenne L.) receiving 420 kg N ha 1.[30] Fertilized turf, whether it be home lawns or golf courses, raises environmental issues. Annual applications up to 244 kg N ha 1 to turfgrass on sandy loam soils in Rhode Island do not appear to pose a threat to drinking water aquifers,[31] although overwatering can cause increased N loadings to bays and estuaries in coastal areas. The excess N movement would be more prevalent with late summer N applications. Nitrate-N concentrations in subsurface water were the highest on an Ohio silt loam in the late summer and early autumn but did not exceed the MCL when 220 kg N ha 1 per year was applied to turfgrass.[32] The exception was the occurrence of high NO3-N concentrations with the soil disturbance during establishment of the turf. Grass sod has the capacity to use large amounts of N; 85–90% of fertilizer N can be retained in the turf–soil ecosystem.[33] Roots and thatch can represent a large N pool because it becomes available for mineralization and subsequent leaching if disturbed.[34] Reseeding and sod establishment within 2 mo of ‘‘turf death’’ can stabilize this N pool.[33] High rates of NO3 leaching can occur at very high N fertilizer rates, e.g., 450 kg N ha 1 per year. Even though most of the NO3 leaching occurred in the autumn and winter, it was an accumulation of all N fertilizer application and not just the autumn application.[35] Excess NO3 in the fall is the driving force that causes NO3 leaching, regardless of the N source or time of application. Therefore, high rates of N application to turf should be avoided in the fall, because it can result in high NO3 leaching rates. A survey of several golf courses across the USA indicated that NO3-N concentrations above the MCL occurred in only 4% of the samples;[36] most of these were apparently due to prior agricultural land use. Pollution of groundwater by NO3 leaching from N fertilized turf should be minimal with good management, which includes consideration of soil texture, N source, rate and timing, and irrigation/rainfall.[37]

Container Horticultural Crops Although the acreage for container horticultural crops is small compared to row crops or grasslands, the production intensity is great and ‘‘hot spots’’ of potential NO3 leaching could develop. Assuming 80,000 pots ha 1

Groundwater: Pollution from Nitrogen Fertilizers

AGRICULTURAL PRACTICES TO MITIGATE THE NITROGEN IN GROUNDWATER PROBLEM Use of Winter Cover Crops Winter cover crops have been shown to be an effective strategy in reducing NO3 leaching during the winter period.[41–44] A variety of crops, e.g., annual grasses, cereals, legumes, have been used with varying degrees of success depending on soils, climate, cropping sequences, etc. Care needs to be exercised with longterm cover crops, because if they are disturbed, some of the accumulated N may become mineralized and actually increase NO3 leaching.[45] Sometimes cover crops cannot be counted on as a best management practice (BMP) to reduce NO3 leaching.[46] On the Delmarva Peninsula in the Mid-Atlantic U.S.A, a rye winter cover crop following corn did not reduce NO3 leaching. One factor was that the existing crop did not permit a sufficiently early seeding of the cover crop.

Use of Soil Nitrate Tests Preplant N tests (PPNT) or presidedress N tests (PSNT) can assess the N stored in soil from cover crops and help to give adequate N credits for legume N carry-over in a crop rotation, such as a soybean N credit in a corn–soybean rotation. Nitrification inhibitors used with N fertilizer in the ammoniacal form can slow the rate of oxidation of reduced forms of N to NO3-N, and subsequently decrease the amount of NO3-N leaching,[47] especially with fall N applications. Even with these improved practices, it may be necessary to reduce the N fertilization rate below

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the EON level to achieve NO3-N concentrations in groundwater below the MCL. Use of Alternate Grassland/Turf Management Several management options to reduce nitrate leaching from grasslands include the use of grass–legume mixtures instead of highly fertilized grass;[24,48] coordinating the timing and N fertilizer application rate with other N sources, e.g., manure applications, to avoid excessive N application;[49] use of irrigation, especially during dry periods, to encourage N uptake;[24] and an integration of cutting forage and grazing, especially cutting in late summer areas that have been intensively grazed earlier in the year. In areas where NO3 contamination from turf is a concern, late summer N fertilizer applications should be reduced and watering should be limited.[31]

REFERENCES 1. U.S. Public Health Service. Public Health Drinking Water Standards, U.S. Public Health Publ. 956; U.S. Government Printing Office: Washington, DC, 1996. 2. Correll, D.L. The role of phosphorus in the eutrophication of receiving waters: a review. J. Environ. Qual. 1998, 27 (2), 261–266. 3. Burkart, M.R.; James, D.E. Agricultural-nitrogen contributions to hypoxia in the Gulf of Mexico. J. Environ. Qual. 1999, 28 (3), 850–859. 4. Baker, J.L.; Campbell, H.P.; Johnson, H.P.; Hanway, J.J. Nitrate, phosphorus, and sulfate in subsurface drainage water. J. Environ. Qual. 1975, 4 (3), 406–412. 5. Baker, J.L.; Johnson, H.P. Nitrate-nitrogen in tile drainage as affected by fertilization. J. Environ. Qual. 1981, 10 (4), 519–522. 6. Davis, D.M.; Gowda, P.H.; Mulla, D.J.; Randall, G.W. Modeling nitrate nitrogen leaching in response to nitrogen fertilizer rate and tile drain depth or spacing for Southern Minnesota, USA. J. Environ. Qual. 2000, 29 (5), 1568–1581. 7. Randall, G.W.; Huggins, D.R.; Russelle, M.P.; Fuchs, D.J.; Nelson, W.W.; Anderson, J.L. Nitrate losses through subsurface tile drainage in conservation reserve program, alfalfa, and row crop systems. J. Environ. Qual. 1997, 26 (5), 1240–1247. 8. Randall, G.W.; Iragavarapu, T.K. Impact of long-term tillage systems for continuous corn on nitrate leaching to tile drainage. J. Environ. Qual. 1995, 24 (2), 360–366. 9. Gentry, L.E.; David, M.B.; Smith, K.M.; Kovacic, D.A. Nitrogen cycling and tile drainage nitrate loss in corn/soybean watershed. Agric. Ecosyst. Environ. 1998, 68, 85–97. 10. Kladivko, E.J.; Grochulska, J.; Turco, R.F.; Van Scoyoc, G.E.; Eigel, J.D. Pesticide and nitrate transport into subsurface tile drains of different spacings. J. Environ. Qual. 1999, 28 (3), 997–1004.

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for a typical foliage plant nursery and using a soluble granular fertilizer, over 650 kg N ha 1 could be lost through leaching annually.[38] During a 10-week greenhouse study of potted flowers, average NO3-N in the leachate ranged from 250 mg N L 1 to 450 mg N L 1.[39] As long as the amount of water applied to the plants did not exceed plant usage (and the greenhouse canopy remained intact to prevent precipitation inputs), there would be little NO3 movement from the soil beneath the pots, unless there was a high water table. Nevertheless, this area of N accumulation would eventually need to be addressed. The use of controlled release fertilizers is one practice that can significantly reduce leaching losses.[38] Also, vegetable crops that have high N demand but low apparent N recovery, e.g., sweet peppers, can leave large amounts of N in the soil and residues at harvest.[40]

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11. Logan, T.J.; Schwab, G.O. Nutrient and sediment characteristics of tile effluent in Ohio. J. Soil Water Conserv. 1976, 31 (1), 24–27. 12. Miller, M.H. Contribution of nitrogen and phosphorus to subsurface drainage water from intensively cropped mineral and organic soils in Ontario. J. Environ. Qual. 1979, 8 (1), 42–48. 13. Cookson, W.R.; Rowarth, J.S.; Cameron, K.C. The effect of autumn applied 15N-labelled fertilizer on nitrate leaching in a cultivated soil during winter. Nutr. Cycl. Agroecosyst. 2000, 56, 99–107. 14. Owens, L.B.; Malone, R.W.; Shipitalo, M.J.; Edwards, W.M.; Bonta, J.V. Lysimeter study on nitrate leaching from a corn–soybean rotation. J. Environ. Qual. 2000, 29 (2), 467–474. 15. Andraski, T.W.; Bundy, L.G.; Brye, K.R. Crop management and corn nitrogen rate effects on nitrate leaching. J. Environ. Qual. 2000, 29 (4), 1095–1103. 16. Steinheimer, T.R.; Scoggin, K.D.; Kramer, L.A. Agricultural chemical movement through a field-size watershed in Iowa: subsurface hydrology and distribution of nitrate in groundwater. Environ. Sci. Technol. 1998, 32, 1039–1047. 17. Jemison, J.M., Jr.; Fox, R.H. Nitrate leaching from nitrogen-fertilized and manured corn measured with zero-tension pan lysimeters. J. Environ. Qual. 1994, 23 (2), 337–343. 18. Diez, J.A.; Caballero, R.; Roman, R.; Tarquis, A.; Cartegena, M.C.; Vallejo, A. Integrated fertilizer and irrigation management to reduce nitrate leaching in Central Spain. J. Environ. Qual. 2000, 29 (5), 1539–1547. 19. Klocke, N.L.; Watts, D.G.; Schneekloth, J.P.; Davison, D.R.; Todd, R.W.; Parkhurst, A.M. Nitrate leaching in irrigated corn and soybean in a semi-arid climate. Trans. ASAE 1999, 42 (2), 1621–1630. 20. Ottman, M.J.; Tickes, B.R.; Husman, S.H. Nitrogen-15 and bromide tracers of nitrogen fertilizer movement in irrigated wheat production. J. Environ. Qual. 2000, 29 (5), 1500–1508. 21. Ball, P.R.; Ryden, J.C. Nitrogen relationships in intensively managed temperate grasslands. Plant Soil 1984, 76, 23–33. 22. Ryden, J.C.; Ball, P.R.; Garwood, E.A. Nitrate leaching from Grassland. Nature 1984, 311, 50–53. 23. Cuttle, S.P.; Scurlock, R.V.; Davies, B.M.S. A 6-year comparison of nitrate leaching from grass/clover and N-fertilized grass pastures grazed by sheep. J. Agric. Sci., Cambridge 1998, 131, 39–50. 24. Whitehead, D.C. Leaching of nitrogen from soils. In Grassland Nitrogen; CAB International: Wallingford, UK, 1995; 129–151. 25. Haigh, R.A.; White, R.E. Nitrate leaching from a small, underdrained, grassland, clay catchment. Soil Use Manag. 1986, 2, 65–70. 26. Roberts, G. Nitrogen inputs and outputs in a small agricultural catchment in the eastern part of the United Kingdom. Soil Use Manag. 1987, 3, 148–154. 27. Steenvoorden, J.H.A.M.; Fonck, H.; Oosterom, H.P. Losses of nitrogen from intensive grassland systems by leaching and surface runoff. In Nitrogen Fluxes in Intensive Grassland Systems; van der Meer, H.G., Ryden,

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28.

29.

30.

31.

32.

33.

34.

35.

36.

37. 38.

39.

40.

41.

42.

43.

J.C., Ennik, G.C., Eds.; Martinus Nijhoff Publ.: Dordrecht, The Netherlands, 1986; 85–97. Owens, L.B.; Van Keuren, R.W.; Edwards, W.M. Nitrogen loss from a high-fertility, rotational pasture program. J. Environ. Qual. 1983, 12 (3), 346–350. Owens, L.B.; Edwards, W.M.; Van Keuren, R.W. Nitrate levels in shallow groundwater under pastures receiving ammonium nitrate or slow-release nitrogen fertilizer. J. Environ. Qual. 1992, 21 (4), 607–613. Garwood, E.A.; Ryden, J.C. Nitrate loss through leaching and surface runoff from Grassland: effects of water supply, soil type and management. In Nitrogen Fluxes in Intensive Grassland Systems; van der Meer, H.G., Ryden, J.C., Ennik, G.C., Eds.; Martinus Nijhoff Publ.: Dordrecht, The Netherlands, 1986; 99–113. Morton, T.G.; Gold, A.J.; Sullivan, W.M. Influence of overwatering and fertilization on nitrogen losses from home lawns. J. Environ. Qual. 1988, 17 (1), 124–130. Geron, C.A.; Danneberger, T.K.; Traina, S.J.; Logan, T.J.; Street, J.R. The effects of establishment methods and fertilization practices on nitrate leaching from turfgrass. J. Environ. Qual. 1993, 22 (1), 119–125. Bushoven, J.T.; Jiang, Z.; Ford, H.J.; Sawyer, C.D.; Hull, R.J.; Amador, J.A. Stabilization of soil nitrate by reseeding with ryegrass following sudden turf death. J. Environ. Qual. 2000, 29 (5), 1657–1661. Jiang, Z.; Bushoven, J.T.; Ford, H.J.; Sawyer, C.D.; Amador, J.A.; Hull, R.J. Mobility of soil nitrogen and microbial responses following the sudden death of established turf. J. Environ. Qual. 2000, 29 (5), 1625–1631. Roy, J.W.; Parkin, G.W.; Wagner-Riddle, C. Timing of nitrate leaching from turfgrass after multiple fertilizer applications. Water Qual. Res. J. Canada 2000, 35 (4), 735–752. Cohen, S.; Svrjcek, A.; Durborow, T.; Barnes, N.L. Water quality impacts by golf courses. J. Environ. Qual. 1999, 28 (3), 798–809. Petrovic, A.M. The fate of nitrogenous fertilizers applied to turfgrass. J. Environ. Qual. 1990, 19 (1), 1–14. Broschat, T.K. Nitrate, phosphorus, and potassium leaching from container-grown plants fertilized by several methods. Hort. Sci. 1995, 30 (1), 74–77. McAvoy, R.J. Nitrate nitrogen movement through the soil profile beneath a containerized greenhouse crop irrigated with two leaching fractions and two wetting agent levels. J. Am. Soc. Hort. Sci. 1994, 119 (3), 446–451. Tei, F.; Benincasa, P.; Guiducci, M. Nitrogen fertilisation of lettuce, processing tomato and sweet pepper: yield, nitrogen uptake and the risk of nitrate leaching. Acta Hort. 1999, 506, 61–67. McCracken, D.V.; Smith, M.S.; Grove, J.H.; MacKown, C.T.; Blevins, R.L. Nitrate leaching as influenced by cover cropping and nitrogen source. Soil Sci. Soc. Am. J. 1994, 58 (5), 1476–1483. Rasse, D.P.; Ritchie, J.T.; Peterson, W.R.; Wei, J.; Smucker, A.J. Rye cover crop and nitrogen fertilization effects on nitrate leaching in inbred maize fields. J. Environ. Qual. 2000, 29 (1), 298–304. Zhou, X.; MacKenzie, A.F.; Madramootoo, C.A.; Kaluli, J.W.; Smith, D.L. Management practices to

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reducing nitrogen leaching. J. Contam. Hydrol. 1998, 34 (1), 1–15. 47. Owens, L.B. Nitrate leaching losses from monolith lysimeters as influenced by nitrapyrin. J. Environ. Qual. 1987, 16 (1), 34–38. 48. Owens, L.B.; Edwards, W.M.; Van Keuren, R.W. Groundwater nitrate levels under fertilized grass and grass–legume pastures. J. Environ. Qual. 1994, 23 (4), 752–758. 49. Jarvis, S.C. Progress in studies of nitrate leaching from grassland soils. Soil Use Manag. 2000, 16, 152–156.

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conserve soil nitrate in maize production systems. J. Environ. Qual. 1997, 26 (5), 1369–1374. 44. Ball-Coelho, B.R.; Roy, R.C. Overseeding rye into corn reduces NO3 leaching and increases yield. Can. J. Soil Sci. 1997, 77, 443–451. 45. Hansen, E.M.; Djurhuus, J.; Kristensen, K. Nitrate leaching as affected by introduction or discontinuation of cover crop use. J. Environ. Qual. 2000, 29 (4), 1110–1116. 46. Ritter, W.F.; Scarborough, R.W.; Christie, A.E.M. Winter cover crops as a best management practice for

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Groundwater: Pollution from Phosphorous Fertilizers Bahman Eghball Soil and Water Cons Research Unit, Agricultural Research Service (USDA-ARS), U.S. Department of Agriculture, Lincoln, Nebraska, U.S.A.

INTRODUCTION Phosphorus (P) is a primary nutrient necessary for plant growth. When the P level in soil is below what is essential for plant needs, P is supplied to the soil by the addition of P fertilizer or organic residuals (i.e., manure). Because of the P fertilizer use in the past few decades or application of manure or other organic residuals, a greater portion of the soils in each state in the United States have soil test P levels that exceed the critical level for plant growth. The excess P in soil is then subjected to leaching loss or transport in surface runoff either in soluble or in particulate (sediment-bound) forms. Phosphorus that is moving downward in the soil profile can eventually reach the ground water, especially in areas with shallow or perched groundwater. Phosphorus moving downward in the soil may also be intercepted by artificial drainage systems (i.e., tile drains) that are located within 1–2 m from the soil surface.

PHOSPHOROUS LEACHING AND FIXATION

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Phosphorus leaching can occur as a slow process in the soil or rapidly with preferential flow, which is the movement of water and solute through cracks and earthworm holes in soil. The point at which P might come in contact with groundwater depends on soil properties and the proximity of the groundwater to the soil surface. The U.S. Environmental Protection Agency has no safe drinking water concentration limit for P. The major concern about P enrichment of groundwater is that groundwater frequently emerges as surface water and if it contains sufficient P, can cause eutrophication (nutrient enrichment that causes algae bloom and oxygen depletion in water). Phosphorus movement in soil is primarily through the diffusion process with the rate influenced by the amount of P applied, soil water content, bulk density (i.e., porosity), and chemical reaction of P with the soil constituents. The average rate of diffusion in three Nebraska medium and fine-textured soils was 0.00011 cm2 hr 1 (0.000017 in.2 hr 1) following the application of P fertilizer (15 kg P ha 1) to replace what is needed by a corn crop with the expected yield of 5600 kg ha 1 (90 bu acre 1).[1] The P in soil is not 464

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usually an environmental concern until the soil test P is in the very high (excessive) category for plant needs (Fig. 1). High levels of P in soil can be a source of groundwater pollution when P is leached in soil. This is especially of concern when the groundwater is near the soil surface; groundwater has an upward movement toward the soil surface in certain times of the year, and in coarse-textured soils. In areas where groundwater is deep, the pollution of groundwater with P is of little concern even if the P level in soil is excessive. For example, P fertilizer applied at a rate of 100 kg ha 1 yr 1 to a sandy loam soil with a P adsorption capacity of 150 mg kg 1 and a bulk density of 1.4 kg m 3 would take 21 yr to reach a 1 m (3.3 ft) soil depth, assuming no preferential flow. If the water table was located several meters deep, it would take many years for the applied P to reach the groundwater. Factors that can influence P leaching in soil are given in Table 1. Applied fertilizer P interacts with various constituents in soil and can be readily immobilized. Phosphorus in all chemical fertilizers (except rock phosphate) is about 100% plant-available and therefore, their reaction in soil should be similar. The mechanism involved in reducing P movement in acid soils includes the reaction of orthophosphate ions (H2PO4 and HPO24 ) with iron and aluminum to form insoluble compounds. In alkaline soils, the P retention mechanisms include precipitation of calcium phosphate compounds, surface precipitation of P on solid phase calcium carbonate, and retention of P by clay particles that are saturated with calcium. Therefore, P leaching in soil is very limited since P interacts with the soil constituents. Eghball, Sander, and Skopp[1] found that maximum fertilizer P movement from a band applied at 60 kg P ha 1 was about 4 cm in 3 mo in three different soils. The size of the band was not expected to expand much after 3 mo. The most P movement occurred in the first few weeks after application. However, P can leach deep into the soil with preferential flow (a small number of pores is used to move water) where P moves with water through cracks and earthworm holes. Preferential flow is the primary mechanism for deep movement of P in fine-textured soils (clayey types) either in particulate form or in soluble form. There are several factors that influence fixation of P in the soil. These include amount and type of clay, time

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of reaction, soil pH, temperature, and organic matter. The greater the clay content of a soil, the more P is adsorbed. More P is retained by the 1 : 1 clays (i.e., kaolonitic type) that are found in the humid and tropic areas (high rainfall high temperature) than other clay types. Phosphorus fixation in soil increases with time after P addition indicating that P gradually becomes more insoluble with time unless plants remove P from soil. Phosphorus fixation also increases with increasing temperature. Increasing soil organic matter usually results in increased P solubility and thus reduces P fixation in soil. Soils that are primarily made up of muck and peat are subject to increased P leaching from added P fertilizer.

PHOSPHOROUS IN TILE DRAINS Phosphorus can leach into the drainage water in fields with tile drains. Tiles are placed 1–2 m (3.3–6.6 ft) deep in poorly drained soils, so there is a potential for the Table 1 Factors that can influence phosphorus leaching in the soil Factor

Phosphorus leaching can also occur in areas with a naturally high P level in the soil. In these areas, P can leach deep into the soil, especially coarse-textured soils, and reach the groundwater. The closer the ground water is to the soil surface, the greater the potential of P reaching the water body. The capacity of a soil to retain P is limited. Repeated and/or heavy application of P fertilizer can saturate the upper soil layers with P. Usually the fraction of P saturated soil decreases with depth. However, when the upper layers of soil have been saturated, P movement to the subsoil can occur. Long-term (>50 yr) application of beef cattle feedlot manure and chemical fertilizer resulted in P leaching to a maximum of 1.8 m (6 ft) depth in a sandy loam soil.[3] Phosphorus from manure source moved deeper in the soil than P from chemical fertilizer indicating that some of the manure P components were not subject to P fixation by the calcium carbonate layer located about 0.75 m (2.5 ft) deep in this soil.

REFERENCES

P leaching risk

Excessive soil P level

High

Shallow or perched water table, waterlogged conditions

High

Fine-textured soils

Low

Fine-textured soils with preferential flow

High

Coarse-textured soils (sandy)

High

Tile drain

High

Soils with high organic matter

Low

Organic soils (soil with organic matter >20%)

High

High soil Ca, Al, or Fe contents

Low

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PHOSPHOROUS SATURATION

1. Eghball, B.; Sander, D.H.; Skopp, J. Diffusion, adsorption and predicted longevity of banded phosphorus fertilizer in three soils. Soil Sci. Soc. Am. J. 1990, 54, 1161–1165. 2. Beauchemin, S.; Simard, R.R.; Cluis, D. Forms and concentration of phosphorus in drainage water of twentyseven tile-drained soils. J. Environ. Qual. 1998, 27, 721–728. 3. Eghball, B.; Binford, G.D.; Baltensperger, D.D. Phosphorus movement and adsorption in a soil receiving long-term manure and fertilizer application. J. Environ. Qual. 1996, 25, 1339–1343. 4. Sharpsburg, A.N.; Daniel, T.C.; Edwards, D.R. Phosphorus movement in the landscape. J. Prod. Agric. 1999, 6, 492–500.

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Fig. 1 Relative crop yield as influenced by soil test P level. Source: From Ref.[4].

applied fertilizer P to leach into the soil and reach the tile drain. The tile water usually empties into the surface water and if it contains sufficient P, can cause eutrophication. Since most of the tile drains are located in humid and semi-humid regions with large rainfall potential, leaching of P through thin soil layers above the tile-drains can especially be high. In a study conducted in Canada,[2] tile-drain water samples were collected from 27 fields that mainly received P fertilizer. Drain water from 14 out of 27 fields exceeded the Canadian standard of 0.03 mg total PL 1 in the water. More than 80% of the P in tile drain was particulate (sediment-bound) and dissolved organic P. Of those exceeding the standard, 10 out of 14 were clayey soils with medium to high soil P levels indicating loss of P through preferential flow.

Groundwater: Pumping and Land Subsidence Steven P. Phillips Devin Galloway U.S. Geological Survey (USGS), Sacramento, California, U.S.A.

INTRODUCTION Land subsidence is a gradual settling or sudden sinking of the Earth’s surface owing to subsurface movement of earth materials. Subsidence is a global problem and, in the United States, more than 17,000 mi2 have been directly affected by subsidence. The associated costs from flooding and structural damage exceeded an estimated $125 million by 1991.[1] About 60% of the subsidence is attributed to permanent compaction of subsurface sediments caused by the withdrawal of underground fluids—groundwater, oil and gas.[1] This article will discuss the single largest cause of subsidence—the compaction of susceptible aquifer systems resulting from groundwater pumping for water supply. Thus, the development of groundwater resources has had a major impact on the landscape and the increasing development of land and water resources threatens to exacerbate subsidence problems.[2,3]

Reversible Deformation Occurs in all Aquifer Systems The relation between changes in groundwater levels and deformation of the aquifer system is based on the principle of effective stress.[4] By this principle, when the support provided by pore-fluid pressure is reduced, such as when groundwater levels are lowered, this support is transferred to the aquifer-system skeleton, which compresses. Conversely, when the porefluid pressure is increased, support previously provided by the skeleton is transferred to the fluid and the skeleton expands. As the pore-fluid pressure fluctuates within the aquifer system the skeleton alternately undergoes compression and expansion. When the load (stress) on the skeleton remains less than any previous maximum load, the groundwater-level fluctuations create small elastic (reversible) deformations of the aquifer system. This recoverable deformation occurs to some degree in all aquifer systems in response to seasonal changes in groundwater levels.

MINING GROUNDWATER Irreversible Compaction of Aquifer Systems Global– Groundwater

The overdraft of susceptible aquifer systems has resulted in regional, permanent subsidence and related ground failures. In the affected alluvial aquifer systems, especially those that include semi- and unconsolidated silt and clay layers (aquitards), long-term groundwater-level declines created a vast one-time release of ‘‘water of compaction’’ from compacting aquitards, which manifests as land subsidence. A largely non-recoverable reduction in the pore volume of the compacted aquitards and of the associated storage capacity accompanied this release of water. The reduction of water levels and pore-fluid pressure in unconsolidated aquifer systems is inevitably accompanied by deformation of the aquifer system. Because the granular structure, or ‘‘skeleton,’’ of the aquifer system is not rigid, a shift in the balance of support for the overlying material causes the skeleton to deform. Both the aquifers and aquitards that constitute the aquifer system undergo deformation, but to different degrees. Almost all permanent subsidence occurs as a result of the irreversible compression or compaction of aquitards during the typically slow process of aquitard drainage. 466

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The maximum previous stress on the skeleton is termed the preconsolidation stress. When the load on the aquitard skeleton exceeds the preconsolidation stress, the aquitard may undergo permanent rearrangement, resulting in irreversible compaction and a permanent reduction of pore volume. In confined aquifer systems, the volume of water derived from irreversible aquitard compaction is equal to the volume of subsidence, which can represent a substantial portion of the total volume of water pumped. This represents a onetime mining of stored groundwater and a small permanent reduction in the storage capacity of the aquifer system.[5] Aquitards—Important Role in Compaction In many confined alluvial aquifer systems, aquitards in the form of discontinuous interbedded layers of silts and clays constitute the bulk of the groundwater storage capacity. This is by virtue of their greater porosity and compressibility and, often, their greater

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aggregate thickness compared to coarser-grained sand and gravel layers. Because aquitards are much less permeable than aquifers, the vertical drainage of aquitards into adjacent pumped aquifers may lag far behind the seasonal fluctuations in water levels in adjacent aquifers. The lagged response in the middle of a thick aquitard may be less influenced by seasonal fluctuations and more influenced by longer-term trends in groundwater levels. When the internal stresses in an aquitard exceed the preconsolidation stress, the compressibility increases, typically by a factor of 20 to 100 times, and the resulting compaction is largely irreversible. The delay in aquitard drainage increases by comparable factors owing to decreased aquitard permeability, and compaction may occur over decades or centuries.

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Subsidence measurement Relative changes in the position of the land surface are measured using geodetic methods such as leveling and satellite-based global positioning system (GPS) surveys. Leveling is accurate and suitable for small

ROLE OF SCIENCE A combination of scientific understanding and careful management can minimize subsidence that results from developing our land and water resources. A key role of science is in the recognition and assessment of subsidence.[1]

Recognition

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Where groundwater mining is involved, subsidence is typically gradual and widespread, and its discovery becomes an exercise in detection. Gazing out over the San Joaquin Valley, California, one would be hard pressed to recognize that nearly 30 ft of subsidence has occurred in some locations (Fig. 1).[6] Possible indicators of land subsidence include protruding wells (Fig. 2) and failed well casings; the formation of earth fissures; changes in flood-inundation frequency and distribution; stagnation or reversals of streams, aqueducts, storm drainages, or sewer lines; failure, overtopping, or reduction in freeboard along reaches of levees, canals, and flood-conveyance structures; and cracks and (or) changes in the gradient of linear-engineered structures such as pipelines and roadways. In the absence of these indicators, measurements of landsurface elevation changes are needed to reveal the subsidence.

Assessment The principal methods for assessing land subsidence are measurement of the magnitude and extent of subsidence, and characterization of aquifer systems undergoing compaction.

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Fig. 1 Approximate location of maximum subsidence in United States identified by Joseph Poland, U.S. Geological Survey (pictured). Signs on pole show approximate altitude of land surface in 1925, 1955, and 1977 in the San Joaquin Valley southwest of Mendota, California. Source: From Ref.[2].

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Fig. 2 Two photographs of a well near Las Vegas, Nevada, show the well progressively protruding from the ground as the surrounding land-surface subsides.

areas; large regional networks warrant use of more efficient GPS surveying. In GPS surveys, the relative positions of two points can be determined when receivers at each point receive signals simultaneously from the same set of GPS satellites. When the same points are later reoccupied, relative motion between the points can be measured. Subsidence can be monitored over hundreds of square miles with a geodetic network. Borehole extensometers (Fig. 3) can generate precise, continuous measurements of vertical displacement between the land surface and a reference point at the bottom of a borehole.[7] Used in conjunction with

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water-level data, extensometer measurements can provide the basis to estimate the average compressibility and vertical hydraulic conductivity of the aquitards, and constrain subsidence predictions.[8,9] Interferometric synthetic aperture radar (InSAR) is a tool that uses radar signals to measure land-surface deformation at a high spatial resolution. For landscapes with stable reflectors, high-precision measurements of the change in the position of these features are made by subtracting or ‘‘interfering’’ two radar scans. Regional-scale land subsidence has been mapped using InSAR, and hydrogeologic understanding has

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non-unique and limited by simplifying assumptions of groundwater flow and aquifer-system compaction. Models remain non-unique because the available hydrogeologic data are always limited. Nevertheless, simulation models may be used cautiously to evaluate management strategies.

CONCLUSIONS

Fig. 3 Conceptual diagram of a borehole extensometer used to measure compaction occurring in a specific vertical interval of an aquifer system.

improved as geohydrologic features, such as buried faults, are revealed.[10] Aquifer-system characterization The stress history of subsidence-prone materials governs the potential for aquifer-system compaction and land subsidence. Preconsolidation stress is estimated from paired measurements of groundwater levels and land subsidence, groundwater flow model calibration, or consolidation tests on sediment cores. All estimates are highly uncertain because initial preconsolidation stresses are unknown. Determining the new preconsolidation stresses in a system that has subsided is equally difficult. Groundwater flow models are important tools for analyzing, visualizing, and managing subsidence.[11,12] Simulation models are powerful tools, but are

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REFERENCES 1. National Research Council Mitigation Losses from Land Subsidence in the United States; National Academy Press: Washington, DC, 1991; 1–58. 2. Galloway, D.; Jones, D.R.; Ingebritsen, S.E. Land subsidence in the United States; Circular 1182; U.S. Geological Survey: Reston, VA, 1999; 1–175. 3. Holzer, T.L.; Galloway, D.L. Impacts of land subsidence caused by withdrawal of underground fluids in the United States; Ehlen, J., Haneberg, W.C., Larson, R.A., Eds.; Humans as Geologic Agents, Reviews in Engineering Geology, V. XVI, Geological Society of America, 2005; 87–99. 4. Terzaghi, K. Erdbaumechanik auf Bodenphysikalisher Grundlage; Wien: Deuticke, 1925; 1–399. 5. Riley, F.S. Mechanics of aquifer systems—The scientific legacy of Dr. Joseph F. Poland. In Land Subsidence Case Studies and Current Research, Proceedings of the Dr. Joseph F. Poland Symposium on Land Subsidence, Association of Engineering Geologists Special Publication No 8, Sacramento, CA, Oct 5–6, 1995; Borchers, J.W., Ed.; Star Publ. Co.: Belmont, CA, 1998, 1–576.

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With adequate monitoring programs and institutional mechanisms in place, optimal benefits may be achieved for both subsidence mitigation and resource development. Effective management of our land and water resources requires definition of the relevant interacting processes. In the case of land subsidence and groundwater resources, this means understanding the hydrogeologic framework of the resource as well as the demands placed on it, and identifying a set of objectives and policies to guide usage of the resources. For alluvial groundwater basins subject to aquifersystem compaction, the preconsolidation stress defines the threshold between recoverable compaction, and non-recoverable compaction and associated subsidence. Consequently, management of land subsidence is inextricably linked to other facets of water-resource management. One such facet is the conjunctive use of surface water and groundwater, which serves to augment and stabilize water supplies, and often plays a critical role in maintaining minimum groundwater levels and arresting or reducing land subsidence.

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6. Poland, J.F.; Lofgren, B.E.; Ireland, R.L.; Pugh, R.G. Land Subsidence in the San Joaquin Valley, California, as of 1972; Professional Paper 437-H; U.S. Geological Survey, 1975; 1–78. 7. Riley, F.S. Developments in borehole extensometry. In Land Subsidence; International Association of Hydrological Sciences,Publication No. 151; Johnson, A.I., Carborgnin, L., Ubertini, L., Eds.; 1986; 169–186. 8. Riley, F.S. Analysis of borehole extensometer data from central California. International Association of Scientific Hydrology Publication 89, 1969, 423–431. 9. Helm, D.C. One-dimensional simulation of aquifer system compaction near Pixley, California. Water Resources Res. 1975, 11 (3), 465–478.

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Groundwater: Pumping and Land Subsidence

10. Galloway, D.L.; Hoffmann, J. The application of satellite differential SAR interferometry-derived ground displacements in hydrogeology. Hydrogeology J. 2007, 15 (1), 133–154. 11. Hoffmann, J.; Leake, S.A.; Galloway, D.L.; Wilson, A.M. MODFLOW-2000 Ground-Water Model—User Guide to the Subsidence and Aquifer-System Compaction (SUB) Package; Open-File Report 03-233; U.S. Geological Survey: Sacramento, CA, 2003; 1–44. 12. Leake, S.A.; Galloway, D.L. MODFLOW Ground-Water Model—User Guide to the Subsidence and AquiferSystem Compaction Package (SUB-WT) for Water-Table Aquifers, Techniques and Methods, Book 6, Chap. A23; U.S. Geological Survey, 2007, 42 p.

Groundwater: Pumping Methods Dennis E. Williams

INTRODUCTION Groundwater has been used for municipal, industrial, irrigation, and other purposes since prehistoric times. In today’s world, groundwater is becoming increasingly more important as a reserve against drought, especially in the arid and semiarid lands. Extraction of groundwater from aquifers beneath the earth is the subject of this section. Various methods of pumping groundwater will be discussed ranging from simple hand-powered systems to high-capacity deep-well turbine pumps.

REVIEW OF BASIC GEOHYDROLOGIC PRINCIPLES Prior to any discussion of groundwater pumping methods, the reader should be familiar with some fundamental principles of groundwater—how it occurs, how it moves, and what governs its movement from areas of recharge to areas of discharge—irrespective of whether the discharge is natural or withdrawn through man-made devices (e.g., wells and pumps). Aquifers are geologic formations or groups of formations capable of yielding water in usable quantities. Groundwater is the subsurface runoff component of the hydrologic cycle, which moves through and is stored in the interstitial spaces found between the solid particles of geologic formations. In unconsolidated materials such as sand and gravel, groundwater moves through the pore space, which occurs, between individual grains of solid material. This pore space is called primary porosity. In consolidated rocks (e.g., granite, volcanics, and limestone), groundwater moves through secondary porosity created as the result of fracturing, fissuring, or weathering. Groundwater flows from areas of recharge to areas of discharge with the rate and direction of flow governed by both the magnitude of the decreasing hydraulic head and the nature of the aquifer materials. Under the same hydraulic gradient, groundwater moves faster through more permeable materials (e.g., coarse sand and gravel) and slower through less permeable materials such as silty sands. Aquifers may be grouped into three main types: confined, unconfined, and semiconfined, depending

upon their subsurface layering and permeability. Confined aquifers, also known as artesian aquifers, are saturated formations found between low permeability materials. The low permeability materials prevent movement of water into or out of the saturated zone. Unconfined or water table aquifers have no upper confining layers. Semiconfined or ‘‘leaky’’ aquifers have semipervious layering either above or below and as a result, may allow water to flow vertically into or out of the aquifer depending on the difference in vertical hydraulic gradients. As groundwater moves, it may discharge naturally to the earth’s surface resulting in a spring, or contributing to the inflow of a lake or stream. Groundwater may also be artificially extracted from the subsurface through pumping or flowing wells. Flowing wells occur in confined (artesian) aquifers where the hydraulic head rises above the top of the well casing. Groundwater discharge to springs occurs when the groundwater surface intersects the land surface—usually in the sides of steep canyons (Fig. 1). Similarly, groundwater may flow into a subsurface drain or trench when groundwater levels in the aquifer are higher than that in the drain or trench. The Ghanats of Iran are an example of groundwater flowing into man-made subterranean tunnels. A Ghanat consists of a series of vertical (hand-dug) shafts typically spaced approximately 100 m (
300 ft) apart roughly paralleling the slope of alluvial fans located near the base of mountain ranges. Starting in the lowermost vertical shaft, a horizontal tunnel is dug which laterally connects the vertical shafts, working upslope until the water table is encountered. At this point, groundwater flows by gravity into the tunnel and is conveyed downslope for irrigation or domestic use. Ghanats are still used extensively throughout Iran as a method for tapping deep groundwater without the use of any type of pumping equipment. In ancient times, the Romans also used the technique in conjunction with aqueducts to serve urban water supply systems.[1] Shallow, hand-dug wells have been used for centuries for irrigation and domestic use where surface water is not a reliable source. In modern times, deep vertical wells are used extensively in arid and semiarid environments to supply water for all purposes including domestic, industrial, agricultural, and municipal applications. 471

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Geoscience Support Services, Inc., Claremont, California, U.S.A.

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Fig. 1 Hydrologic cycle.

WELLS AND GROUND WATER PUMPING SYSTEMS

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Water wells may be constructed in a variety of different aquifer materials in order to supply water for different uses. Most wells are vertical (Fig. 2); however, in specialized cases, wells may be horizontal (to enhance the flow from springs or seeps) or may include a central caisson with lateral ‘‘spokes’’ to induce greater infiltration from the aquifer (i.e., Ranney collector well—Fig. 3A and B). To withdraw water from non-flowing water wells, a variety of pumping methods may be employed. The simplest of these do not require electrical energy or fuel-powered motors. These methods include positive displacement-type pumps such as windmills or hand pumps, or more simply a bucket attached to a rope used to raise water to the surface. Most pumps, however, require some sort of mechanical energy—supplied by a drive motor or engine—to lift water to the land surface. When a well pump is turned on, groundwater first flows into the pump intake from the volume stored within the well casing and borehole area itself. As this volume is typically small compared to the capability of the pump to produce water, a hydraulic gradient forms between the pumping level inside the well and the groundwater level in the near-well zone. A ‘‘cone of depression’’ thus develops around the well, which assumes a general logarithmic shape (Figs. 4 and 5). As pumping continues, the cone of depression expands outward from the well until the recharge captured by the cone of depression equals the discharge requirements of the well. When the cone of depression reaches a steady or non-changing condition the well discharge rate is said to be in equilibrium with the recharge rate

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to the well. The type of aquifer materials and amount of water being pumped from the well determine the size and shape of the cone of depression. For example, domestic wells generally pump for short periods of time [measured as gallons per minute (gpm)] at rates of 1–15 gpm. This results in small, poorly defined cones

Fig. 2 Example of municipal water well showing deep-well turbine pump (Roscoe Moss Company).

Groundwater: Pumping Methods

of depression. On the other hand, deep, large diameter, municipal water supply wells completed in coarsegrained alluvial aquifers can easily produce 2000– 4000 gpm for long periods of time with cones of depressions extending several thousand feet. Fig. 5 defines pumping well terminology as related to the cone of depression.

COMMON GROUND WATER PUMPING METHODS Types of Pumps

in the well casing with the pump intake submerged below the pumping level. Intake areas to deep-well pumps are always under a positive head and do not require suction to pump the water. Fig. 6 shows examples of centrifugal, jet, and rotary pump types. Positive displacement (e.g., piston) and variable displacement (e.g., centrifugal pumps) are the two types most commonly used in water wells. In positive displacement-pumps, water is moved mechanically (for a given pump) and directly related to the speed of the pump (e.g., hand strokes per minute) and independent of the total lift (i.e., head). Pump discharge rate is changed by varying pump speed and decreases only slightly with increasing head.[3] In variable displacement-pumps on the other hand (e.g., airlift or centrifugal), the discharge rate depends largely on the total dynamic head (TDH) and decreases as the head increases. Positive displacement-pumps are used for handpumped wells or windmill type of power with cylinders

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Pumps may be classified in accordance to use (e.g., shallow or deep wells), design (positive or variable displacement), and method of operation (rotary, reciprocating, centrifugal, jet, or airlift).[2] Shallow-well pumps (suction-lift pumps) are generally installed above ground. Deep-well pumps are always installed

473

Fig. 3 (A) Typical radial collector well (Ranney Water Systems, Inc.). (B) Horizontal well (Ground Water Publishing Co.).

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Global– Groundwater Fig. 4 Schematic of vertical deep water supply well, pump, and storage system.

mounted at the surface for shallow lifts (or down the well for deeper applications). Centrifugal pumps run at higher speeds than hand-operated pumps with electric, gasoline, or diesel motors typically providing

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the power source. Single-stage centrifugal pumps (Fig. 6(A)) can be used at the surface to pump water from shallow wells, but multiple stages are needed in wells where depths to pumping levels are deep.

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Fig. 5 Cone of depression around a pumping well.

Another type of pump is the jet pump. Jet pumps (Fig. 6(B)) may also be used at the surface for pumping shallow groundwater. A jet pump forces water down one pipe (through a high-pressure nozzle), and returns the water to the surface through a second pipe, where the discharge is used. Jet pumps are typically used for applications where the depth to water is less than approximately 22–25 ft. Power is required to lift groundwater to the surface, either indirectly by suction pumps or directly from hand or mechanically driven pumps. Windmills may be a good choice for lifting water from shallow or deepwater wells in rural communities, or where conventional power supplies or fuel costs are either unavailable or very expensive. Modern technology has also produced solar cells that convert sunlight directly into electricity. One of the most important applications for solar cells in rural areas all over the world is for pumping water.[4]

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Hand Pumps A variety of inexpensive positive displacement-pumps are available to pump water from wells. One such type is called a ‘‘pitcher’’ pump (Fig. 7) and is commonly used when the water table is less than approximately 22–25 ft below the surface.[2,5] (The theoretical maximum suction lift equal to atmospheric pressure
34 ft or 14.7 pounds per square inch (psi), cannot be achieved with these pumps). Pitcher pumps are surface-mounted, reciprocating or single-acting piston pumps utilizing a hand-operated plunger inside a cylinder set on top of the well casing. The pump suction pipe is attached to the bottom of the cylinder. The plunger has a simple ball valve that opens on the down stroke and closes on the upstroke. A check valve at the lower end of the cylinder opens on the upstroke of the pump and closes on the down stroke. Through

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Fig. 6 (A) Single-stage centrifugal pump (U.S. Government Printing Office). (B) Jet pump (U.S. Government Printing Office). (C) Rotary pump (U.S. Government Printing Office).

continuous upstroke and down stroke actions water flows out of the discharge pipe. For deeper depths (up to 250 ft), a surface pump stand and separate lift cylinder can be installed down the well, which effectively ‘‘pushes’’ the water to the surface with the help of interconnecting rods. The latter method is often employed in a windmill system. Fig. 7 illustrates a typical suction pump. Another manually operated water pump is the Treadle Pump. The Treadle Pump was developed to provide low cost, sustainable, environment friendly technology, which can be sold at a fair market price, in rural areas.[6] The Treadle Pump relies upon the basic suction lifting principle of the hand pump consisting of two

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barrels, plungers, and treadles. One person through use of foot pedals can operate it. The discharge rate of a Treadle Pump can achieve approximately 10–15 gpm depending on the size of the pump/suction depth etc.

Rotary Pumps These pumps use a system of rotating gears to create a suction at the inlet and force a water stream out of the discharge. The gears’ teeth move away from each other at the inlet port. This action causes a partial vacuum and the water in the suction pipe rises. In the pump, the water is carried between the gear teeth and around

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477

formed inside the eductor pipe results in the air/water fluid being lighter than the water outside the eductor pipe (i.e., inside the well casing). This results in the air/water fluid flowing upward and out the top of the eductor pipe. Airlift pumps produce the best results when the submergence ratio of the air and eductor pipe is approximately 60%, however, reasonable results can be obtained with submergence as low as 30%.[8] An example of a 60% submergence is when the length of the air pipe is 200 ft (B on Fig. 8(A)) and the pumping water level depth is 80 ft (C on Fig. 8(A)). This results in a submergence of 120/200 (60%).

Fig. 7 Example of suction pump (U.S. Government Printing Office).

both sides of the pump case. At the outlet, the teeth moving together and meshing causes a positive pressure that forces the water into the discharge line. In a rotary gear pump, water flows continuously and steadily with very small pulsations. The pump size and shaft rotation speed determine how much water is pumped per hour. Gear pumps are generally intended for low-speed operation. The flowing water lubricates all internal parts. Therefore, the pumps should be used for pumping water that is free of sand or grit. If sand or grit does flow through the gears, the close-fitting gear teeth will wear, thus reducing pump efficiency or lifting capacity. Airlift Pumps Water can also be pumped from a well using an airlift pump (Fig. 8(A) and (B)). The airlift pump assembly consists of a vertical discharge pipe (eductor pipe) and a smaller air pipe which are both submerged below the well’s pumping level for approximately two-third of their length.[7] Compressed air is forced through the air pipe to within a few feet of the bottom of the eductor pipe. The mixture of air bubbles and water

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Centrifugal pumps are variable displacement-pumps with the discharge rate being inversely related to the head supplied. That is, when the TDH increases, the discharge rate decreases.[9] A centrifugal pump contains a rotating impeller within a housing. The centrifugal forces generated by the spinning impeller impart kinetic energy to the water. This kinetic energy is converted into pressure at the discharge side of the pump. The general characteristic pump performance curves for centrifugal pumps are shown in Fig. 9 and relate TDH, pump efficiency, and horsepower. Centrifugal pumps may be used to pump water from shallow wells with high water tables and low drawdowns. Centrifugal pumps may also be used in deep wells (deep-well vertical centrifugal pumps or commonly known as deep-well turbine pumps), and are installed inside the well casing below the water level. These latter pumps consist of a number of pump bowls with impellers, each set above another, which are added so as to ‘‘build’’ the head required. The impellers may be driven by either a motor at the surface (typically, electric, gasoline, or diesel) and connected to the pump by a long shaft and tube assembly (surface drive). The impellers may also be powered by a submerged electric motor directly coupled to the pump (submersible drive). When a centrifugal suction pump is used to produce water from shallow wells, all pump components and suction lines must be completely filled with water or ‘‘primed’’ in order to operate. Hand-operated or motor-powered vacuum pumps are typically used for priming. Deep-Well Turbine Pumps Deep-well turbine pumps are the most common type of pump used in cased water wells where the groundwater surface is below the practical limits of centrifugal suction pumps. The turbine pump has three main parts: 1) the head assembly; 2) the column (tube) and shaft assembly; and

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Centrifugal Pumps

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Groundwater: Pumping Methods

Fig. 8 (A) Principle of airlift pump (U.S. Government Printing Office). (B) Example of airlift pump (U.S. Government Printing Office).

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3) the pump bowl assembly. The discharge head is typically cast iron or fabricated steel, and is designed to be installed on a foundation. The discharge head supports the column (tube), shaft, and bowl assemblies and directs the discharge of water. Additionally, it also provides a base to support an electric motor, a right angle gear drive or a belt drive (Fig. 2).

The column and shaft assembly connects the head and pump bowls. The line shaft transfers the power from the motor to the pump impeller(s). The impellers lift the water and the column conveys the lifted water to the surface. The line shaft on a deep-well turbine pump may be either water- or oil-lubricated. The oil-lubricated pump has an enclosed shaft (oil

Fig. 9 Typical pump performance curves for centrifugal pumps.

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Groundwater: Pumping Methods

Impellers used in turbine pumps may be either semiopen or enclosed. The vanes on semiopen impellers are open on the bottom and rotate with a very close tolerance to the bottom of the pump bowl (enclosure). The operating characteristics of deep-well turbine pumps are determined by laboratory testing and depend largely on bowl design, impeller type, and speed. Vertical turbine pumps are generally designed for specific speeds [measured as revolutions per minute (rpm)]; generally, either 1800 rpm or 3500 rpm for deep-well turbine pump applications. Other speeds are used for specialized applications.[11] Pump

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tube) into which oil drips at the surface, lubricating the bearings by gravity. The water-lubricated pump has an open shaft, where the pumped water itself lubricates the bearings. If a high content of sand in the discharge is anticipated, an oil-lubricated pump should be selected in order to keep the bearings clean. The pump bowl encloses the impeller. In most deepwell turbine installations, several bowls (stages) are stacked in series. A four-stage bowl assembly contains four impellers attached by a common shaft, and will operate at four times the discharge head of a single-stage pump.[10]

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Fig. 10 Vertical turbine multi-stage submersible pump (Roscoe Moss Company).

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480

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performance curves at these speeds can be obtained from the manufacturer of each pump.

Submersible Pumps A submersible pump is a turbine pump close-coupled to a submersible electric motor (Fig. 10). Both pump and motor are suspended below the water surface, eliminating the long drive shaft and bearing retainers required for a deep-well turbine pump. The pump bowl assembly is located above the motor. Water enters the pump through a screen located between the pump and motor. The pump curve for a submersible pump is very similar to a deep-well turbine pump.[10] Submersible motors are smaller in diameter and much longer than ordinary motors. Because of their smaller diameter, they are lower in efficiency than those used for centrifugal or deep-well turbine pumps. Most submersible pumps used for domestic purposes use either single or two-phase power, while larger pumps used for agricultural, industrial, or municipal purposes require three-phase power. Electrical wiring connecting the pump motor to the surface power supply must be watertight with all connections sealed. Submersible pumps can be selected to provide a wide range of flow rate and TDH combinations.

Pump Head and Power Requirements Before selecting a pump, a careful and complete inventory of the conditions under which the pump will operate must take place. The discharge rate and TDH will be determined by the specific use and distribution system (Figs. 4 and 11). The TDH of a pump is the sum of the elevation and pressure heads plus head losses due to friction and velocity[3] (Fig. 4). Friction head is the sum of the energy loss due to the flow of water through a pipe, and is a function of the velocity and pipeline diameter including losses through fittings and valves as well as changes in flow direction and pipeline diameter. Values for these losses can be calculated or obtained from friction loss tables. The discharge rate, which will be produced, is a function of both the system and pump characteristics. The intersection of the system head curve and the pump head curve determine the discharge rate at which the pump will operate (Fig. 11). Cavitation (i.e., implosion of air bubbles and water vapor on the impeller, causing pitting) occurs when the hydraulic head at the pump intake is too low. The head must be high enough so that as velocity increases (and pressure decreases), within the pump, the pressure cannot drop below the vapor pressure of the water. The minimum head needed at the pump intake is termed

Global– Groundwater Fig. 11 Typical system head and pump curves.

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Type of pump

Practical suction lifta

Usual well-pumping depths

Usual pressure heads

Advantages

Disadvantages

Remarks

Reciprocating 1. Shallow Well 2. Deep Well

22–26 ft 22–25 ft

22–26 ft Up to 600 ft

100–200 ft

1. Positive action 2. Discharge against variable heads 3. Pumps water containing sand and silt 4. Especially adapted to low capacity and high lifts

1. Pulsating discharge 2. Subject to vibration and noise 3. Maintenance cost may be high 4. May cause destructive pressure if operated against closed valve

1. Best suited for capacities of 5–25 gpm against moderate to high heads 2. Adaptable to hand operation 3. Can be installed in very small diameter walls (2-in. casing) 4. Pump must be set directly over well (deep well only)

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Table 1 Pump selection criteria

Centrifugal 1. Smooth, even flow 2. Pumps water containing sand and silt 3. Pressure on system is even and free from shock 4. Low-starting torque 5. Usually reliable and good service life

1. Shallow well

Straight centrifugal (single stage)

20 ft maximum

10–20 ft

100–150 ft

1. Loses prime easily 2. Efficiency depends on operating under design heads and speed

Very efficient pump for capacities above 50 gpm and heads up to about 150 ft

Regenerative vane turbine type (single impeller)

28 ft maximum

28 ft

100–200 ft

Same as straight centrifugal except maintains priming easily

Reduction in pressure with increased capacity not as severe as straight centrifugal

Impellers submerged

50–300 ft

100–800 ft

2. Deep well Vertical line shaft turbine (multi-stage)

Same as shallow-well turbine

1. Efficiency depends on operating under design head and speed 2. Requires straight well large enough for turbine bowl and housing 3. Lubrication and alignment of shaft critical 4. Abrasion from sand (Continued) 481

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Table 1 Pump selection criteria (Continued)

Type of pump Submersible turbine (multi-stage)

Practical suction lifta

Usual well-pumping depths

Usual pressure heads

Advantages

Disadvantages

Pump and motor submerged

50–400 ft

80–900 ft

1. Same as shallow-well turbine 2. Easy to frost proof installation 3. Short pump shaft to motor

1. Repair to motor or pump requires pulling from well 2. Sealing of electrical equipment from water vapor critical 3. Abrasion from sand

1. Shallow well

15–20 ft below ejector

Up to 15–20 ft below ejector

80–150 ft

1. High capacity at low heads 2. Simple in operation 3. Does not have to be installed over the well 4. No moving parts in the well

1. Capacity reduces as lift increases 2. Air in suction or return line will stop pumping

2. Deep well

15–20 ft below ejector

25–120 ft, 200 ft maximum

80–150 ft

Same as shallowwell jet

Same as shallow-well jet

1. Shallow well (gear type)

22 ft

22 ft

50–250 ft

1. Positive action 2. Discharge constant under variable heads 3. Efficient operation

1. Subject to rapid wear if water contains sand or silt 2. Wear of gears reduces efficiency

2. Deep well (Helical-rotary type)

Usually submerged

50–500 ft

100–500 ft

1. Same as shallow-well rotary 2. Only one moving pump device in well

Same as shallow well rotary except no gear wear

Remarks Difficulty with sealing has caused uncertainty as to service life in data

Jet

The amount of water returned to ejector increases with increased lift—50% of total water pumped at 50 ft lift and 75% at 100 ft lift

Rotary

Practical suction lift at sea level. Reduce lift 1 ft for each 1000 ft above sea level.

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Groundwater: Pumping Methods

a

A rubber stator increases life of pump; flexible drive coupling has been weak point in pump; best adapted for low capacity and high heads

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WHP ¼ ðQ  TDHÞ=ð3960Þ where, WHP ¼ water horsepower, Q ¼ discharge rate of the well pump, [gpm], TDH ¼ total dynamic head, [ft] ¼ s þ SWL þ hs þ hf, s ¼ drawdown in the pumping well, [ft], SWL ¼ depth to static water level below reference point, [ft], hs ¼ elevation of system head above reference point, [ft], hf ¼ friction (and velocity) head losses in piping system from pump to system storage, [ft]. However, the actual horsepower required to run a pump will be greater than the water horsepower as pumps and drivers are not 100% efficient. The horsepower required to pump a specified flow rate against a specified TDH therefore is the brake horsepower (BHP), and is calculated using the following formula:[10] BHP ¼ ðQ  TDHÞ=ð3960  eÞ where, BHP ¼ water horsepower (horsepower rating of the power unit), e ¼ pump efficiency  drive efficiency (expressed as a decimal). The pump efficiency (percentage) may be read directly from the pump curve provided by the manufacturer. The drive efficiency is the efficiency value (percentage) given the driver unit (source of power to the pump) by the manufacturer.

Pump Selection Criteria Proper selection of a pump must consider both anticipated pumping conditions and well type, which include the following main design parameters:[10]        

Diameter of the well. Required discharge rate. TDH. Depth to static ground water. Friction losses. Power requirements. Power source. Water quality and/or potential for sand production.

Centrifugal suction pumps are generally used for shallow groundwater levels (e.g., less than 22–25 ft) while most installations utilize deep-well turbine or submersible pumps. The discharge rate of the well is often overlooked when selecting a pump for small wells. If too large a pump is installed in a small

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capacity well, the result will be to either temporarily drain the well (i.e., ‘‘break suction’’), or exceed the maximum possible suction lift. It is very important, therefore, to match pumping requirements and well characteristics when selecting the optimum pump for each installation. Table 1 provides general guidelines for pump selection.[2] REFERENCES 1. Glick, T.F. Movement of ideas and techniques. In Islamic and Christian Spain in the Early Middle Ages; Princeton University Press, 1979. 2. Multiservice Procedures for Well-Drilling Operations. Army Manual No. 5-484; Navy Facilities Engineering Command Pamphlet No. 1065; Air Force Manual No. 32-1072; Washington DC, March 8, 1994. 3. Marin˜o, M.A.; Luthin, J.N. Seepage and Groundwater; Elsevier Scientific Publishing Company: New York, 1982. 4. Dankoff Solar Products. www.dankoffsolar.com (accessed January 2001). 5. Ashe, William. Understanding Water Wells, Technical Paper #68; Volunteers in Technical Assistance: Arlington, Virginia, 1990; 1–14. 6. Lifewater Canada. http://www.lifewater.ca/ndexpump. htm (accessed January 2001). 7. Wells, Technical Manual No. 5-297; Air Force Manual No. 85-23; Departments of the Army and the Air Force: Washington DC, Aug 1, 1957. 8. Driscoll, F.G. Ground Water and Wells; Johnson Division, Universal Products Co.: St. Paul, MN, 1972. 9. Helweg, O.J.; Scalmanini, J.C.; Scott, V.H. Improving Well and Pump Efficiency; American Water Works Association: U.S.A., 1983. 10. Roscoe Moss Company. Handbook of Ground Water Development; John Wiley & Sons: New York, NY, 1990. 11. Driscoll, F.G. Groundwater and Wells, 2nd Ed.; Johnson Division: St. Paul, MN, 1986.

BIBLIOGRAPHY 1. Campbell, M.D.; Lehr, J.H. Water Well Technology, McGraw-Hill Book Company: New York, NY, 1973. 2. Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. www. edis.ifas.ufl.edu/BODY_w1001 (accessed January 2001). 3. Hix, G.L. More on the development of horizontal wells. Water Well J. 1996, L (2), 47–49. 4. Honors Program, University of Nevada, Reno, Tony Zuliani. www.honors.unr.edu/
tzuliani/wtpaper.html# pump (accessed January 2001). 5. Microsoft Encarta Online Encyclopedia 2000. http:// encarta.msn.com (accessed January 2001). 6. Neptune Internet Services, Inc., John Yonge. www.neptune. on.ca/
jyonge/windmill.htm (accessed January 2001). 7. North Dakota State University. www.ext.nodak.edu/ extpubs/ageng/irrigate/ae1057w.htm#centrifugal (accessed January 2001).

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the net positive suction head (NPSH) and is specific to the operation and pump design. The power required to move water through a pump may be calculated using the following formula:

483

Groundwater: Quality Loret M. Ruppe Timothy R. Ginn Department of Civil and Environmental Engineering, University of California–Davis, Davis, California, U.S.A.

INTRODUCTION

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Groundwater quality refers to the type and concentration of constituents in a given source of groundwater. Constituents in groundwater may originate from the natural environment with which the groundwater comes in contact, or may be introduced as pollutants from external sources. Constituents can be dissolved solids and gases, suspended solids, hydrogen ions, and microorganisms. There is wide variation in the chemical and biological constituents in groundwater due to the differing qualities of water that recharge groundwater, and due to the different environments through which groundwater passes. The unique polar nature of the water molecule makes it a ready solvent, with the capacity to dissolve many solid-phase minerals into solution, and many of the elements comprising the subsurface environment dissolve into ground water. Groundwater is never free from all impurities. A principal and ubiquitous constituent class is that of ions in solution, that have dissolved into the groundwater from the earth materials in which it flows. Total ionic concentration includes various dissolved salts and associated mineral species as well as hydrogen, and is quantified as total dissolved solids (TDS). The concentration of TDS in units of mass of ions per volume of water is used to classify water, with fresh water (0–1000 mg/L TDS) differing from brackish (1000– 10,000 mg/L TDS), saline (10,000–100,000 mg/L TDS), and brine (greater than 100,000 mg/L TDS) waters in dissolved solids concentration. Potable water typically has less than 500 mg/L TDS, while the concentration in seawater is approximately 35,000 mg/L.[1]

TYPICAL CONSTITUENTS IN GROUNDWATER Inorganic solids comprising the geologic material of the subsurface constitute the greatest concentrations of constituents in groundwater, with bicarbonate, calcium, chloride, magnesium, sodium, and sulfate typically 90% of the TDS in groundwater.[1] As groundwater ages in an aquifer, dominant ions 484

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tend to shift from calcium (Ca2þ) and bicarbonate þ
(HCO
3 ) to sodium (Na ) and chloride (Cl ). Constituents generally found in uncontaminated groundwater are classified in Table 1 according to relative abundance.[2,3]

MECHANISMS INFLUENCING GROUND-WATER QUALITY The processes by which chemicals are dissolved into the groundwater are primarily: mineral dissolution and precipitation, microbially mediated oxidation and reduction reactions, ion exchange and adsorption, and hydrolysis.[4] Each of these processes is described later.

Mineral Dissolution and Precipitation Rainwater is slightly acidic (pH < 7), and more so in regions with acidic air pollutants that dissolve into water droplets. Once the rainwater reaches the ground and percolates through the root zone, the degree of acidity can increase further as the oxygen in the water is consumed by the decay of organic matter and by the respiration of plant roots. Oxygen removal creates an oxygen sink that is filled by the further dissociation of aqueous compounds containing oxygen such as bicarbonate, that also puts more hydrogen ions into solution, thus reducing pH. Water from rainfall, lakes, streams, and other sources travels through the unsaturated vadose zone, and accumulates in the saturated zone of an aquifer. As the water passes through the vadose zone and through the aquifer, the acidity of the water causes the dissolution of the geologic features into which it comes in contact. An example is the dissolution of calcite, CaCO3, the basic constituent of limestone, marble, and chalk that is commonly found in sedimentary rock. In the presence of acidity in the groundwater, in the form of carbonic acid, H2CO3, calcium and bicarbonate become dissolved ions in the groundwater solution. CaCO3 þ H2 CO3 ! Ca2þ þ 2HCO
3

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Major constituents (greater than 5 mg/L) Bicarbonate Silicon Calcium Sodium Chloride Sulfate Magnesium Carbonic acid Nitrogen Minor constituents (0.01–10.0 mg/L) Boron Nitrate Carbonate Potassium Fluoride Strontium Iron Bromide Oxygen Carbon dioxide Trace constituents (less than 0.1 mg/L) Aluminum Nickel Antimony Niobium Arsenic Phosphate Barium Platinum Beryllium Radium Bismuth Rubidium Cadmium Ruthenium Cerium Scandium Cesium Selenium Chromium Silver Cobalt Thallium Copper Thorium Gallium Tin Germanium Titanium Gold Tungsten Indium Uranium Iodide Vanadium Lanthanum Ytterbium Lead Yttrium Lithium Zinc Manganese Zirconium Molybdenum

logarithmically with depth. The metabolic activities of the microorganisms catalyze reactions within the groundwater that involve transferring electrons to form different compounds. Organic material can be converted to inorganic compounds such as carbon dioxide and water through this process. Such reactions are the basis for in situ bioremediation of organic contaminants. Ion Exchange and Adsorption Essentially all surfaces of aquifer materials are electrically charged. For instance sands are quartzitic materials generally carrying negative charges, but are often partly coated with mineral oxide (iron, manganese, aluminum) compounds that have a net positive charge. Ions in the groundwater attach (adsorb) to the charged surfaces, thereby altering the chemical make-up of the groundwater. Ion exchange refers to the preferential sorption due to electrostatic forces of multivalent ions over monovalent ions, that is reversible if the aqueous concentration of monovalent ions is high enough. Clay minerals have dense surface charges and are typically involved in ion exchange and adsorption. Ion exchange in clay minerals involves intra-particle sites, and exchange is associated with swelling or shrinking of the clay medium on the macroscale. For instance when a single bivalent ion is replaced by two monovalent ions, the clay particle swells. Hydrolysis

Given an adequate supply of calcite and carbonic acid, calcium will continue to dissolve into solution until an equilibrium state is reached.

As noted earlier, the polar structure of water facilitates reaction with chemical compounds to form new compounds. The replacement of ions in a compound with Hþ or OH
ions of water is termed hydrolysis. The chemical make-up of groundwater will influence the degree to which hydrolysis will occur. Some of these geochemical and biochemical reactions occur simultaneously, while others occur sequentially. The rates at which these reactions occur vary considerably, ranging from nearly instantaneously to slowly enough that the equilibrium is never reached. Knowledge about the processes is critical in predicting the way in which groundwater quality evolves and responds to treatment, and the rate of the change; efforts continue by scientists and engineers to accurately model the reactions occurring in the complex groundwater environment.

Oxidation and Reduction Reactions

GROUND WATER QUALITY ISSUES

Microorganisms, primarily bacteria, are ubiquitous throughout the subsurface environment, with population diversity and density typically decreasing

Groundwater contamination has a direct effect on the quality of drinking water for many people. More than fifty percent of the drinking water in the United States

Organic compounds (shallow) Humic acid Fulvic acid Carbohydrates Amino acids

Tannins Lignins Hydrocarbons

Organic compounds (deep) Acetate Propionate Source: Domenico and Schwartz,[2] modified from Davis and DeWiest.[3]

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Table 1 The dissolved constituents in potable groundwater classified according to relative abundance

486

is groundwater.[5] Contaminants to groundwater can originate from a ‘‘point source’’ (PS; single specific discharge point) or from a ‘‘non-point source’’ (NPS; diffuse source that contributes a contaminant, or contaminants, to the environment). PS of groundwater contamination include industrial waste discharges, leaking petroleum storage facilities, and municipal wastewater treatment plant discharges. NPS discharges are often associated with rainfall runoff and snowmelt events from agricultural operations, roadways and vehicle emissions, construction sites, mining operations, landfills, and logging activities. Other sources of NPS pollution include soil erosion (sediment transfer), failing onsite wastewater treatment systems, animal wastes in feedlot runoff, or animal waste holding pond overflows. Agricultural Wastes Agricultural operations contribute many constituents that contaminate the groundwater. The chemical pesticides, herbicides, and fertilizers applied to crops can reach the groundwater through land application and in rainfall runoff. Animal manure wastewater, which harbors human pathogens such as Cryptosporidium parvum oocysts, can similarly migrate to the groundwater. NPS contamination can result from irrigation using animal manure wastewater, as well as land application of waste solids or liquids for non-irrigation purposes. Industrial Wastes Global– Groundwater

The disposal of the chemical byproducts of industrial processes is regulated to varying degrees around the world. The wastes may not be adequately treated before being discharged to the environment, where they frequently migrate to groundwater. Additional sources of contamination are chemical spills and leaking storage tanks. Such sources include those from military and energy facilities, which often involve heavy metals and/or radionuclides in solution, as well as dissolved explosives and solid fuels for propellants. Municipal Wastewater The increase in synthetic chemical usage around the home results in the discharge of portions of these

© 2008 by Taylor & Francis Group, LLC

Groundwater: Quality

chemicals into the wastewater sewerage system. Treatment plants are designed to purify human wastewater, but are not designed to remove these additional chemicals, with the result that increasing concentrations of chemicals are bypassing the treatment process and are released into the environment, eventually being detected in groundwater. Synthetic Chemicals Increasingly, groundwater contamination results from the growing usage of synthetic chemicals. With over 65,000 synthetic chemicals in common use in the United States today, these chemicals are being detected in groundwater supplies with increasing frequency. Products that contain organic chemicals include solvents, pesticides, paints, inks, dyes, varnishes, and gasoline. The U.S. Environmental Protection Agency performed groundwater surveys in the 1990, that have confirmed the widespread presence of organic contaminants.[5] Groundwater Salinization In coastal regions, the pumping of groundwater from an aquifer can result in the intrusion of saline waters, thereby severely altering the quality of water. As groundwater mining increases, this is becoming an increasing problem in many regions as salinity can degrade the quality to the extent that it is no longer potable. Groundwater is a resource that is increasing in value for the support of human activities and life itself. Increasing efforts are required to slow the degradation of groundwater quality in much of the United States.

REFERENCES 1. Freeze, R.A.; Cherry, J.A. Groundwater; Prentice-Hall Inc.: New Jersey, 1979; 1–604. 2. Domenico, P.A.; Schwartz, F.W. Physical and Chemical Hydrogeology, 2nd Ed.; John Wiley & Sons, Inc.: New York, 1998; 1–506. 3. Davis, S.N.; DeWiest, R.J.M. Hydrogeology; John Wiley & Sons, Inc.: New York, 1966; 1–463. 4. Watson, I.; Burnett, A.D. Hydrology: An Environmental Approach; CRC Press, Inc.: Boca Raton, FL, 1995; 1–702. 5. http://www.epa.gov/seahome/groundwater/src/overview. htm (accessed August, 2001).

Groundwater: Quality and Irrigated Agriculture Stephen R. Grattan Hydrologic Sciences, Department of Land, Air and Water Resources (LAWR), University of California–Davis, Davis, California, U.S.A.

Irrigation water quality can have a profound impact on crop production inasmuch as irrigated agriculture can affect groundwater quality. Irrigated agriculture not only involves the application of water, which contains dissolved mineral elements, but is often coupled with other inputs such as fertilizers and pesticides. Many of these constituents can leach past the crop rootzone and pollute the underlying aquifer. Other chapters in the Encyclopedia will address impacts of irrigated agriculture on groundwater quality. The emphasis of this chapter is on groundwater quality, and its potential impacts on irrigated agriculture.

GROUNDWATER QUALITY All groundwater sources used for irrigation contain dissolved mineral salts, but the concentration and composition of the dissolved salts vary from one aquifer to another. Dissolved mineral salts form ions; either positively charged cations or negatively charged anions. The most common cations are calcium (Ca2þ), magnesium (Mg2þ), and sodium (Naþ) whereas the most abundant anions are chloride (Cl
), sulfate (SO2
4 ), þ and bicarbonate (HCO
3 ). Potassium (K ), carbonate
(CO2
3 ), nitrate (NO3 ), and trace elements also exist in groundwater supplies but most often concentrations of these constituents are comparatively low. On the other hand, some groundwater sources contain boron (B) at comparatively low concentrations but at levels that may be detrimental to certain crops. An understanding of the quality of water used for irrigation and its potential negative impacts on the crop, soil, and irrigation system is essential to avoid problems and optimize production. The salinity of the water is important because too much salt can reduce crop production while too little salt or certain compositions of salt (i.e., sodic waters) can reduce water infiltration, which indirectly affects the crop. Certain elements or combination of elements in the groundwater can be toxic to sensitive crops or pose a management or maintenance problem. More detailed information on water quality and impacts on agriculture can be found in Ref.[1]. For more information

on the nature and extent of agricultural salinity, see Ref.[2] or visit http://water.usgs.gov/nwis/gw for actual groundwater quality data in the United States. Characterizing Salinity There are two water quality parameters that characterize the salinity of the irrigation water: electrical conductivity (ECw) and total salt concentration or total dissolved solids (TDS). The units of TDS are usually in milligrams of salt per liter of water (mg L
1). This term is used by many commercial analytical laboratories and represents the total mg of salt that would remain after a liter of water is evaporated to dryness. Often, TDS is reported as parts per million (ppm), which is numerically equivalent to mg L
1. The higher the TDS, the higher is the salinity of water. Electrical conductivity is a much more useful term because the measurement can be made instantaneous in the field. Salts that are dissolved in water conduct electricity and therefore the salt content in the water is directly related to the ECw. Units of EC reported by labs are usually in decisiemens per meter (dS m
1) or millimhos or micromhos per centimeter (mmhos cm
1 or mmmhos cm
1). One mmho cm
1 ¼ 1000 mmmhos cm
1 ¼ 1 dS m
1. Often, a conversion between ECw and TDS is made based on guidelines from Ref.[3] (i.e., ECw  640 ¼ TDS) but caution is advised because this conversion is dependent on both salinity and composition of the water. The USDA-ARS Salinity Laboratory has a web site that has educational material, models, databases, and lists of publications on various chemical, physical, and phyto-biological aspects of salinity including a pdf version of Handbook 60 http://www. ussl.ars.usda.gov/. Characterizing Sodicity The sodicity or alkalinity of the groundwater is characterized on the basis of its Naþ relative to Ca2þ and Mg2þ concentration. Sodicity[4] refers to either the exchangeable Na percentage (ESP), or the sodium adsorption ratio (SAR) of the soil solution. The SAR ¼ Naþ/(Ca2þ þ Mg2þ)0.5 where ion concentrations 487

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INTRODUCTION

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Groundwater: Quality and Irrigated Agriculture

are millimolar and the ESP is the percentage of the soil’s cation-exchange-capacity (CEC) occupied by Naþ.

RELATIONS BETWEEN IRRIGATION WATER, SOIL SALINITY, AND LEACHING

Global– Groundwater

Salts can accumulate in the root zone from the irrigation water due to insufficient leaching. To prevent salt accumulation in the root zone from the irrigation water, the soil must be adequately leached. Leaching is the process of applying more water to the field than can be held by the soil in the crop root zone such that the excess water drains below the root system carrying salts with it. The more water that is applied in excess of the crop water requirement, the less the salinity in the root zone will be despite the fact that more salt has been added to the field. The term ‘‘leaching fraction’’ (LF) is used to relate the fraction or percent of water infiltrated to the field that actually drains below the root zone. Below are some useful relationships between the salinity in the irrigation water (ECw) and the average root zone salinity (ECe). The ECe is the electrical conductivity of the saturate soil paste (i.e., soil samples are saturated with distilled water, the soil water is then extracted, and the EC is measured on the extracted water). These relationships predict what would happen over the long-term if the LFs indicated are achieved, assuming steady-state conditions and a 40-30-20-10 root water extraction pattern where the top and bottom quarters of the root zone extracts 40% and 10% of the crops consumptive water use. LF 10% ECw  2:1 ¼ ECe LF 15
20%

ECw  1:5 ¼ ECe

LF 30% ECw ¼ ECe The leaching requirement is an attractive concept but has limitations. First, the ET of the crop is assumed to be independent of the average root zone salinity.[5] Thus, calculated crop water requirements will be high where the average root zone salinity exceeds the threshold salinity of the crop, which corresponds to a yield potential less than 100%. Second, the leaching requirement is based on steady-state conditions and does not account for the initial salinity status in the root zone. Finally, applying irrigation water to a field to achieve a given LF is very difficult, if not impossible, particularly with fine textured soils in climates with high evaporative demand. Nevertheless, in order to control

© 2008 by Taylor & Francis Group, LLC

salinity, leaching must occur whether it is achieved before the season, midway through the season, or at the end of the season.[1] In fields where salinity has increased in the root zone to damaging levels, ‘‘reclamation leaching’’ is recommended. Ref.[6] provides additional information on reclamation of soils. For more information on relations between irrigation water salinity, leaching, and root zone salinity—see Refs.[7,8].

IMPACT ON CROPS Salinity, caused by either too much salts in the groundwater supply and/or insufficient leaching, can directly affect the crop in two ways, by osmotic effects and by specific ion effects. The osmotic effects are responsible for growth reduction, the most common whole-plant response to salinity. Within limits, isosmotic concentrations of different combinations of salts cause nearly equal reductions in growth. On the other hand, specific ions such as Naþ, Cl
, and B may be particularly injurious to certain crops or under specific management practices. A detailed discussion of mechanisms of salt tolerance and injury can be found in Refs.[9,10] and references cited therein. Estimating Yield Potential Crops vary widely in their response to salinity. Some crops such as bean and onion are very sensitive to salinity while others such as cotton and asparagus are tolerant. The salt tolerance of a crop is best described by plotting its relative yield as a function of the average root zone salinity (ECe). This response curve is represented by two line segments; one, a tolerance plateau with zero slope and the second, a concentration-dependent line whose slope indicates the yield reduction per unit increase in ECe[11] (Fig. 1). Maas and Hoffman[11] assembled a table with salinity coefficients. The point where the first line segment meets the second line segment is referred to as the yield threshold coefficient (a). This represents the maximum soil salinity a crop can tolerate before its yield declines. The slope of the second line is the second salinity coefficient (b), which represents the percent decrease in yield per unit increase in ECe. Thus, the relative yield (%) ¼ 100
b(ECe
a). Additional salinity coefficients can be found in Ref.[12]. These salinity coefficients are particularly useful in predicting yield potentials based on either the average root zone salinity or based on the irrigation water itself by using the relations between ECw, ECe, and LF described earlier.

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489

Fig. 1 Divisions for classifying crop tolerance to salinity. Source: From Ref.[12].

It is important to emphasize that these are only guidelines and assume that all other factors such as fertility, irrigation scheduling, and pest control are managed to maximize crop performance. It is also important to note that most of the experiments that were used to generate these guidelines were conducted in the interior of California where the climate is hot and dry during the summer. Crops grown in the coastal regions or where the climate is milder will likely tolerate greater salinities than indicated in these publications. For more detailed information on relations between crop yields and salinity, see Refs.[1,12].

Cl or B in the soil water that a crop can tolerate before it develops symptoms of ion toxicity.[12] The irrigation method can affect crop sensitivity to water quality. With drip and furrow irrigation, chloride and sodium injury does not generally occur in most

In addition to salinity’s general osmotic effect, some crops, particularly tree and vines, are injured by certain elements, specifically Naþ, Cl
, and boron (B). These elements are absorbed by the root and move with the transpirational stream to the leaves where they concentrate. Thus, older leaves or older portions of leaves such as margins and tips transpire more than younger tissue and develop injury first. Injury usually begins as chlorosis and advances into necrosis as injury becomes more severe. Fig. 2 shows both Cl and B injury to tomato leaves. Although the injury is similar, Cl injury in most crops has more chlorosis (leaf yellowing) contiguous to the necrotic tissue whereas necrotic portions from B injury are often darker with a reddishbrown coloration. This additional injury complicates salt-tolerance in that the combined osmotic and specific-ion effects may affect the yield potentials of the crop more than the salt-tolerance guidelines would indicate. Tables are provided that list the maximum concentration of

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Global– Groundwater

Crop Toxicity to Specific Elements

Fig. 2 Progression of injury to tomato leaves due to boron toxicity (upper) and chloride toxicity (lower).

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vegetable and row crops unless salinity is severe. Under sprinkler irrigation, injury may develop on wetted leaves of susceptible plants such as peppers, potatoes, and tomatoes if the ECw exceeds 1.5 dS m
1 (see Ref.[12]). Injury occurs due to direct foliar absorption of salts. Susceptibility to leaf injury is related to leaf wettability, leaf morphology, and the rate of foliar salt absorption and not tolerance to soil salinity. Increased frequency of sprinkler irrigation is usually more damaging that increased duration. Some vegetable and row crops are sensitive to boron. Generally, leaf injury must be severe to cause reduced yields and crop quality. Long-term use of irrigation water containing more than 0.5
0.7 mg L
1 boron can reduce the yields of bean, onion, garlic, strawberry, broccoli, carrot, potato, and lettuce and greater than 2 mg L
1 can reduce yields of cabbage and cauliflower. Unlike most annual crops, tree and vine crops are generally sensitive to boron, chloride, and sodium toxicity. Tolerances vary among varieties and rootstocks. Tolerant varieties and rootstocks resist the uptake and accumulation of toxic ions in the stem and leaf tissue. Continued use of irrigation water with boron concentrations in excess of 0.75 mg L
1 can reduce the yields of grapes and many deciduous tree and fruit crops. This represents a threshold concentration and does not imply that irrigation water with boron at or slightly above this level cannot be used successfully. Chloride moves readily with the soil water and is taken up by the roots. It is then transported to the stems and leaves. Sensitive berries and avocado rootstocks can tolerate only up to 120 mg L
1 Cl while grapes can tolerate up to 700 mg L
1 or more. The ability of the tree to tolerate sodium varies considerably. Sodium injury on avocado, citrus, and stonefruit trees has been reported at concentrations as low as 115 mg L
1. Initially sodium is retained in the roots and lower trunk but after 3 or 4 yr the conversion of sapwood to heartwood apparently releases the accumulated sodium, which then moves to the leaves causing leaf burn. It is unclear how extensive sodium toxicity occurs because when injury is evident, levels of chloride are often high as well. Ref.[12] contains information on crops as they are affected by specific ion toxicity. Climate and soil factors affect crops response to specific-ion injury. Under cool, moist climatic conditions, higher concentrations of B, Cl, or Na can be tolerated. Hot dry weather on the other hand could cause more severe injury at a given tissue-ion concentration. In addition, soil conditions influence the time it takes for injury to occur. The finer the soil texture, the longer it will take for injury to occur. Furthermore, there is an evidence that salinity may reduce boron’s injurious effect so that plants can tolerate a higher

© 2008 by Taylor & Francis Group, LLC

Groundwater: Quality and Irrigated Agriculture

concentration of B than the guidelines indicate. For more information on boron, see Refs.[12,13]. Indirect Na Effects on Plants In addition to osmotic and specific-ion toxicity, sodic or saline–sodic groundwater may also induce an indirect effect such as Na-induced Ca deficiency. Ca deficiency in the crop maybe obvious such as whip-like appearances in young emerging leaves, blackheart in celery, blossom end rot in tomato and pepper, but are more likely to be subtle where visual symptoms are absent.[14] Such an interaction has been described in Refs.[10,14].

IMPACT ON SOILS Soil physical properties can be affected by irrigation with sodic or saline–sodic groundwater particularly when good quality water or rains follow.[15] Potential consequences include reduced infiltration and redistribution rates within the soil, poor soil tilth, and inadequate aeration resulting in anoxic conditions for roots. These negative impacts are enhanced with decreasing soil salinity and with increasing exchangeable Na (i.e., ESP). At the soil surface, infiltration rates and soil tilth are particularly sensitive to salt and exchangeable Na levels. The mechanical impact and stirring action of the irrigation water, or rain, combined with the freedom for soil particle movement at the soil surface, can result in low infiltration rates when the soil is wet, and hard, dense soil crusts when the soil is dry. Crusts can block the emergence of seedlings thereby reducing stand establishment (Fig. 3). Tillage of crusted soils can result in hard soil clods that are difficult to reduce in size when the clod is dry. Extensive tillage can be

Fig. 3 Reduced stand establishment in cotton in a field previously irrigated with saline–sodic water.

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491

Infiltration of Irrigation Water There are two water quality parameters that are currently used to assess irrigation water quality for potential water infiltration problems. These are the ECw and the SAR. Both a low salt content (low ECw) and high SAR can cause permeability or water infiltration problems even on sandy soils. A low ECw and/or high SAR can act separately or collectively to disperse soil aggregates, which in turn reduces the number of large pores in the soil. These large pores are responsible for adequate aeration and drainage. A negative effect from the breakdown of soil aggregates is soil sealing and crust formation. Table 1 provides guidelines that can be used to assess the potential likelihood of water infiltration problems based on ECw and SAR. Table 1 indicates that water infiltration problems are likely if the ECw is less than 0.3 dS m
1 regardless of the SAR. For example, if the ECw falls below 0.4 dS m
1, infiltration rates can drop to less than 0.1 in. hr
1 (2.5 mm hr
1). An infiltration rate of 2.5 mm hr
1 would require 30 hr for a full irrigation of 75 mm to infiltrate the soil. Thus, very high quality water can cause infiltration problems even when applied on soils with a high sand content. Soils may also be prone to water infiltration problems in late Fall and Winter months after high quality rainwater falls on fields previously irrigated with sodic or saline–sodic groundwater. For more information on soil response to saline and sodic conditions, see Refs.[15,16]. Fortunately, infiltration problems due to a low salt content or high SAR can easily be improved by the addition of amendments to either the irrigation water or soil that directly (e.g., gypsum) or indirectly (e.g., acidifying agents) supply free calcium (Ca2þ) to the soil water. When the irrigation water contacts gypsum, Table 1 Likelihood of potential water infiltration problems based on ECw and SAR Potential water infiltration problem SAR of irrigation or soil water

Unlikely if ECw is (dS m
1)

Likely if ECw is (dS m
1)

0–3

>0.6

1.0

2.0

3.0

5.0

DCs þ IC þ SC

Hydrologic– Irrigation

The terms new in Model 2 are IB, representing indirect (real external) benefits, SB denoting secondary (pecuniary external) benefits, IC standing for real external costs, and SC denoting secondary external costs. Secondary benefits, the multiplier effects arising from increased purchases of production inputs and consumption goods when a project comes into operation, are typically concentrated in the project region. They are normally measured by specialized economic models (such as regional interindustry models), which simulate the effects of an increment of resources on the economy. Secondary costs (SC) are the pecuniary benefits foregone when a public investment draws funds (via taxes) from the economy at large. Secondary costs typically spread throughout the national economy and are very difficult to measure. The conventional economic wisdom (embedded in public planning manuals) is that from the national accounting stance, secondary or pecuniary costs are at least as large or larger than secondary benefits. Hence, the two effects offset each other and except in special cases,

© 2008 by Taylor & Francis Group, LLC

secondary economic impacts can be ignored for national irrigation investment planning purposes.[1] Indirect costs and benefits, the other class of external effects, should also be incorporated into evaluations adopting a public accounting stance. Indirect benefits are seldom economically important, but indirect costs are typically very significant. Examples of indirect costs of irrigation water diversions include reduced downstream water supplies or adverse effects on water quality downstream for offstream (irrigators, industries, and households) and instream (hydroelectric power plants, recreational water users, and fish and wildlife habitat) water users.

EMPIRICAL EVIDENCE ON ECONOMIC IMPACTS OF IRRIGATED AGRICULTURE A number of sources suggest that the direct economic benefits of irrigation, even from the private accounting stance, are not as large as assumed by non-specialists or the lay public. One bit of evidence is that farmers are seldom able or willing to pay for public project costs, even if repayment requirements are but a small fraction of actual costs.[3] Econometric studies of land and water rights markets infer that direct benefits of irrigation investments are modest relative to costs.[4] River basin simulation models that adopt a public accounting stance by incorporating indirect costs show that indirect costs to instream water users (such as hydropower producers) may exceed the economic benefits of upstream crop irrigation.[5] Elsewhere, similar evidence is accumulating that when social costs are accounted for, net social benefits of public irrigation developments have been quite unimpressive. The large loss to Aral Sea fisheries and to regional environmental quality from diverting the inflow source for cotton production is one well-known example. Econometric studies in India and China report low rates of economic return to investments in irrigated agriculture (implying negative net social benefits when discounted at conventional interest rates) particularly as compared to the return on expenditures on agricultural research, on education, or on rural road construction.[6] If the public feasibility studies were correct in concluding that substantial net economic benefits would flow from water resource developments, regional economic studies of ex post impacts would be expected to show corresponding positive impacts on economic growth indicators. Several statistical studies of the role of water investments in regional economic growth in the United States conducted over two decades ago were unable to find statistically significant positive effects of water development on regional incomes.[7] More recently, a regional economic model of the Sacramento Valley (a California agricultural region

Irrigated Agriculture: Economic Impacts of Investments

comprising nearly 2 million irrigated acres) simulated the effect of hypothetical drought scenarios which would reduce water availability by up to 25%. Even the most drastic scenario, and measuring both direct and secondary effects, was predicted to reduce employment by only 300 jobs and reduce total regional income by less than 1%.[8] The large regional secondary (multiplier) effects from irrigation development sometimes assumed by non-economists are not substantiated on careful study. And, because labor-saving technologies have reduced the labor requirements in agriculture and related industries (dramatically so in the developed world), direct and secondary employment impacts of irrigated agriculture are found to be modest. These ex post studies have of necessity used private prices. If the data had permitted adjustments for subsidized product and input prices, and acknowledged the downstream indirect costs, the conclusions would be even more pessimistic from the social accounting stance.

CONCLUSION

output subsides. By making overoptimistic assumptions on crop productivity and prices, by ignoring the opportunity costs of certain inputs, or not properly accounting for public subsidies and external costs, public irrigation planning agencies have tended to systematically overstate net economic benefits of public investments in irrigated agriculture.

REFERENCES 1. Gittinger, J.P. Economic Analysis of Agricultural Projects; Johns Hopkins University Press (for the World Bank): Baltimore, 1982. 2. Young, R.A. Measuring Economic Benefits of Water Investments and Policies, Technical Report 338; World Bank: Washington, DC, 1996. 3. Wilson, P.N. Economic discovery in federally supported irrigation districts. J. Agric. Res. Econ. 1997, 22 (1), 61–77. 4. Faux, J.; Perry, G.M. Estimating irrigation water value using hedonic price analysis: a case study of Malheur county Oregon. Land Econ. 1999, 75 (3), 440–452. 5. Booker, J.F.; Young, R.A. Modeling intrastate and interstate markets for Colorado river water resources. J. Environ. Econ. Manag. 1994, 26 (1), 66–87. 6. Fan, S.; Hazell, P. Returns to public investments in the less-favored areas of India and China. Am. J. Agric. Econ. 2001, 83 (5), 1217–1222. 7. Howe, C.W. Effects of water resource development on economic growth: the conditions for success. Nat. Resour. J. 1976, 6, 939–956. 8. Lee, H.; Sumner, D.; Howitt, R. Potential economic impacts of irrigation-water reductions estimated for the Sacramento valley. Calif. Agric. 2001, 55 (2), 33–40.

Hydrologic– Irrigation

Many early public investments in irrigated agriculture likely yielded an adequate social return. However, conceptual and empirical reasons combine to make it difficult to avoid the inference that the net economic benefits of investments in irrigated agriculture over, say, the last half century, have not been large in the United States and even elsewhere. This conclusion is particularly firm when a social accounting stance is adopted, so that negative indirect effects are accounted for and impact measures are adjusted for input and

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Irrigated Agriculture: Endangered Species Policy Norman K. Whittlesey Ray G. Huffaker Department of Agricultural Economics, Washington State University, Pullman, Washington, U.S.A.

Joel R. Hamilton Department of Agricultural Economics, University of Idaho, Moscow, Idaho, U.S.A.

INTRODUCTION

Hydrologic– Irrigation

Irrigated agriculture in the western United States (the West) holds the most senior appropriative water rights allocated pursuant to state statutes, and accounts for about 90% of the consumptive water use in the West.[1] Appropriation of the dependable flow of regional rivers into irrigation has altered natural flow regimes to the detriment of aquatic habitats, and consequently has contributed to the listing of several species relying on aquatic habitats as endangered or threatened pursuant to the federal Endangered Species Act (ESA).[2] Notable examples are the listings of several anadromous salmon species in the Columbia River Basin,[3] and of waterfowl and fish species in the Platte River Basin.[4] The curtailment of state appropriative water rights pursuant to ESA-sanctioned species recovery plans has placed federal and state law on a collision course whose resolution will establish the legal parameters governing policy tradeoffs in allocating water between irrigated agricultural and endangered species protection. Our objective is to illuminate what these legal parameters might be. We begin with brief reviews of the ESA and of the prior appropriation doctrine that provides the foundation of state water law.

species, and the Secretary of the Interior—acting through the Fish and Wildlife Service (FWS)—lists all other species. The listing agencies are required to designate the species’ critical habitat, and to prepare ‘‘recovery plans’’ detailing strategies to revive populations to healthy levels. Species recovery receives top priority in the formulation of recovery plans if conflicts arise with construction, development, or other economic activities. Section 1536 directs federal agencies to consult with listing agencies to ensure that proposed federal actions do not jeopardize the continued existence of a listed (or proposed to be listed) species or adversely modify its critical habitat. If the proposed action is deemed to have an ‘‘incidental’’ impact, the listing agency can require that the consulting agency take ‘‘reasonable and prudent measures’’ to minimize the impact. A consulting agency is banned from making any ‘‘irreversible or irretrievable’’ action that would foreclose the implementation of such measures. Section 1538 bans the ‘‘taking’’ (e.g., harassment, killing, or capturing) of listed species, and applies to all persons within the jurisdiction of the United States, with exceptions for ‘‘incidental takings’’ (defined as takings that are ‘‘incidental to, and not the purpose of, the carrying out of an otherwise lawful activity’’).

THE ENDANGERED SPECIES ACT

THE PRIOR APPROPRIATION DOCTRINE

The Endangered Species Act (ESA) elevates the conservation of endangered species to the highest level of federal policy objectives, and sets forth a legal procedure affording them extensive protection. Section 1533 authorizes two federal agencies to ‘‘list’’ imperiled species as ‘‘endangered’’ (defined as species in danger of becoming extinct through all or a significant portion of their range) or ‘‘threatened’’ (defined as species likely to become endangered in the foreseeable future) based on solely biological criteria. The Secretary of Commerce—acting through the National Marine Fisheries Service (NMFS)—lists marine

Water allocation generally is governed by state law in the West. Variations of the prior appropriation doctrine provide the foundation of most western water law. Briefly, a person acquires the right to use some quantity of publicly owned water by diverting it to a beneficial use on a fixed tract of land (the ‘‘water duty’’). The priority of the right is established by the date of first diversion. During water shortages, the longest-term (senior) appropriators receive their full water duties until no water remains at the source. The water rights of shorter-term (junior) appropriators are curtailed completely. Water that is not beneficially

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used is forfeited and available for re-appropriation by another person (‘‘use it or lose it’’). The prior appropriation doctrine ideally protects water-right holders from encroachment by other water-right holders taking water out-of-priority or enlarging their rights in a manner not prescribed by statute. This protection depends on the security of complicated interrelationships or ‘‘use-dependencies’’ created among water users due to the fugitive nature of water resources. Actual consumption of water (‘‘consumptive use’’) in irrigation is often less than the full amount diverted from the stream (‘‘diversion’’). Unconsumed water may return to the stream (‘‘return flow’’). Return flows, along with natural stream flows, supply water available for appropriation by other irrigators, and thus constitute a portion of their water rights. Appropriators who modify water use from that prevailing when their water rights were granted may shift the timing, location, quantity, or quality of return/escape flows, and consequently may impair other use-dependent water rights.

CONFLICT The extent to which federal environmental programs such as the ESA authorize federal regulators to disrupt state-created appropriative water rights is controversial. At one extreme, some observers contend that such programs establish ‘‘federal regulatory rights’’ that empower the federal government to ‘‘cancel the historic de facto assignment of property rights in commons to exploiters and reassign them to the government as agent for the public generally.’’ (See Ref.[5], p. 3.) At the other extreme, some federal courts have held that the federal government must defer to state-created water rights in the absence of explicit congressional intent to pre-empt them.[6] So far, the federal government has not used the ESA as authority to establish a new brand of ‘‘federal regulatory rights.’’ However, it has curtailed state-created water rights under two sections of the law. Federal agencies supplying or distributing water to private irrigators have curtailed state-granted water rights for varying lengths of time in compliance with a Section 1536 consultation with the listing ESA agency. For example, the U.S. Forest Service shut down irrigation ditches operating on agency land in the Methow Valley in Washington State for much of the 1999 irrigation season.[7] In another example, the U.S. Bureau of Reclamation cut-off water to 90% of the 220,000 acres in the Klamath Project in Oregon for much of the 2001 irrigation season.[8] The potential for further curtailment of state-granted water rights under Section 1536 consultations is great because the Bureau of Reclamation is the largest supplier and manager of water in the West.[1]

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The federal government also has relied on Section 1538 (banning the taking of listed species by private parties) to threaten the curtailment of state-created water rights of irrigators using non-federally developed or delivered water. For example, NMFS officials warned the Methow Valley Irrigation District that water would be cut-off during the 2002 irrigation season for about 250 irrigators unless the district switched to a more efficient, fish-friendly means of distributing water.[9] The Methow Valley and Klamath Project water curtailments, and the ensuing losses to the local agricultural economy, understandably have generated substantial ill-will toward the federal government among irrigators, irrigation districts, rural communities, and state governments. For example, the Klamath Project curtailment is estimated to have cost farmers approximately $200 million in lost crops.[10] Business in local communities also has suffered.[10] The prospect of these losses drove about 100 irrigators to risk arrest when they ran an irrigation line to divert water around a canal head gate that the federal government had closed to protect listed fish in Upper Klamath Lake.[11] Could the federal government avoid such confrontation by evolving toward the extreme of deferring to state prior appropriation statutes to satisfy ESA mandates? Unfortunately, the prior appropriation doctrine is not designed to protect aquatic habitat, and thus would be an ineffective replacement for the legal protection that endangered species receive under the ESA.[12] Non-diversionary water uses were not recognized as beneficial uses when traditional appropriative rights were being locked into irrigated agriculture in the late 19th and early 20th centuries. Consequently, irrigated agriculture currently has priority regardless of how little water remains for non-diversionary uses. For example, Wilkinson noted that, under the prior appropriation doctrine, the most senior water-rights holders need not share the water with emerging new water needs, but can ‘‘with impunity, flood deep canyons and literally dry up streams, as has happened with some regularity.’’ (See Ref.[13], p. 21.) Are there policies available for protecting endangered aquatic habitats while mitigating adverse impacts on state-created water rights? Economists have long recommended water marketing as a means of shifting water from prior appropriative uses to competing private and public uses. Unfortunately, while most state water statutes permit public interest groups to purchase water rights for the purpose of augmenting instream flows, state protection of such rights from appropriation by other water-rights holders generally is difficult. Moreover, states impose moderate to severe limits on water transfers to protect third-party water-rights holders from impairment due to changes in return flows. Perhaps the best policy for accommodating endangered species

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protection and state-created water rights is the use of specialized water transfers designed to limit the extent and duration of impairment to use-dependent rights. Examples are ‘‘trial transfers’’ (transfers that can be modified or revoked if actual impairment results), ‘‘one-time temporary transfers’’ (transfers whose short-term nature makes injuries short-lived), and ‘‘contingent transfers’’ (transfers that occur intermittently and are triggered only by some predetermined contingency such as instream flow below some critical level).[12,14]

REFERENCES 1. Michelsen, A.M.; Taylor, R.G.; Huffaker, R.G.; McGuckin, J.T. Emerging agricultural water conservation price incentives. J. Agric. Resour. Econ. 1999, 24 (1), 222–238. 2. Endangered Species Act, 16 U.S.C. xx1531–1543. 3. Federal register. 1991, 56 (224), 58,619–58,624. 4. Ring, R. Saving the platte. High Country News 1999, 31 (2), 1.

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Irrigated Agriculture: Endangered Species Policy

5. Tarlock, D. The endangered species act and western water rights. Land Water Law Rev. 1985, 20 (1), 1–30. 6. California v. U.S., 438 U.S. 645, 1978. 7. Hicks, L. Ditch problems have no easy solutions. Methow Valley News 1999, September 2. 8. Bernard, J. Dozens trespass in irrigation protest. Spokane Spokesman-Rev. 2001, August 30. 9. Hanson, D. Feds, methow facing off in water dispute. Spokane Spokesman-Rev. 2001, September 3. 10. Hold Your Breath, Suckers: A Fish That Has Divided the West May Help Define Gale Norton’s Greenery. The Economist, February 9, 2002; 28–29. 11. Associated Press. Farmers Bypass Canal Head Gate to Water Crops. The Spokane Spokesman-Review, 2001, July 16. 12. Huffaker, R.G.; Whittlesey, N.; Hamilton, J.R. The role of prior appropriation in allocating water resources into the 21st century. Water Resour. Dev. 2000, 16 (2), 265–273. 13. Wilkinson, C. Crossing the Next Meridian; Island Press: Washington, DC, 1992. 14. Hamilton, J.R.; Whittlesey, N.; Halverson, P. Interruptible water markets in the pacific northwest. Am. J. Agric. Econ. 1989, 71 (1), 265–273.

Irrigated Agriculture: Historical View Lyman S. Willardson

INTRODUCTION There is no known record of the beginning of irrigated agriculture. It was most likely started on a very small scale by someone trying to keep a wilted plant alive by pouring water on the soil around its base. Then, ways were found to keep more plants supplied with water when they were remote from the water supply. However, carrying water from a spring or a stream to supply many plants is heavy work. By scraping small furrows from a stream to the plants, irrigation could be practiced with a greatly reduced labor input. The practitioners soon realized that they could produce more food by keeping an adequate supply of water available to their plants at all times. The availability of more food from irrigated plants meant that more people could live in a smaller area and communal living could be practiced. Communities could grow into cities and cities could grow into nations. When governments were organized, public resources were available to construct the necessary infrastructure to supply water to all suitable lands. It is certain that irrigation became a necessity as population increased in arid or semiarid areas. In many areas, the season of limited rainfall corresponds to the season of maturity of food crops. If the crops are short of water at that time, yields are severely depressed. It therefore became important to develop irrigation systems that could supply water to crops in seasons of rainfall shortage. With irrigation, it is possible to grow crops in areas of very low rainfall or areas where nearly all the precipitation falls in the non-growing season. Irrigation is a means of taking advantage of the productive capacity of suitably fertile soils which lack only adequate water for crop plants in their normal growing season.

IRRIGATION IN ANCIENT TIMES One of the earliest written records of irrigation practice was found in the Code of Hammurabi.[1] Various translations of the Code exist that include references to laws related to irrigation. Irrigation in Babylonian times was very important. One article in the law says ‘‘If the irrigator neglects to repair his dyke, or leaves his runnel open and causes a flood, he has to make

good the damage done to his neighbor’s crops, or be sold with his family to pay. The theft of a wateringmachine, water-bucket, or other agricultural implement was heavily fined.’’ This law was in effect in approximately 1750 B.C. Another famous historical irrigation development occurred between the Tigris and Euphrates rivers in what is modern Iraq. A very large civilization developed in that area and then disappeared. Originally, it was thought that the developing population denuded the watersheds and the canals filled with silt from erosion of the watersheds, making continued irrigation impossible. It is more likely that a rising water table in the area caused salinization of the soils being irrigated. The loss of food production on the saltaffected soils caused the civilization to disappear. Another possible explanation is that the area was overrun by conquerors who had no appreciation for the need to maintain the irrigation systems. The irrigation works were allowed to deteriorate until they could no longer feed the population. In a book, copyrighted in 1898 by King,[2] the author reported extensive irrigation developments in many areas of the world. He references a paper presented by Mr. Frederick S. Gipps before the Royal Society of New South Wales in 1887 claiming that the first authentic lake or reservoir was Lake Maeris. It was constructed by King Maeris or King Amenemhet III of the 12th Dynasty in 2084 B.C. Water was stored at the time of flooding on the Nile to relieve some flooding and was later released back into the river. Sesostris in 1491 B.C. built many canals in Lower Egypt for irrigation and transportation. Egypt claims the world’s oldest dam[3] built in about 3000 B.C. The Phoenicians, about 1100 B.C.[2] were irrigating the ‘‘African shore’’ and had gardens and large plantations ‘‘abounding in canals.’’ The Bible mentions many ditches in Second Kings 3:16–17. In more modern times, the Romans built extensive aqueducts to supply water to cities. They therefore had the technology necessary to develop and transport water for irrigation and food production. The valley of the Po River in Italy is currently irrigated with many old, if not ancient, canals. China[2] also has some ancient irrigation works on a grand scale. The Great Imperial Canal has a length of 539

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Department of Biological and Irrigation Engineering, Utah State University, Logan, Utah, U.S.A.

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more than 1000 km (650 mi) and connects two rivers. The canal even crossed some lakes on elevated dikes. One of the most significant democratic institutional arrangements in the history of irrigation is the ‘‘Tribunal of Waters,’’ which still exists, after more than 1000 yr, in the irrigation districts of eastern Spain.[4] Each Tribunal, which consists of locally elected canal presidents, meets at a fixed time every week to hear farmers’ complaints about water use offenders and applies appropriate sanctions. When the Spaniards began their conquest of Mexico and Peru, they found irrigation being practiced on a relatively large scale. In the ruins of older cities, they found evidence of irrigation canals that had long been abandoned. In Peru, in a district named Condesuyos, they found a comprehensive canal that passed through a number of basins and had a length of approximately 600 km. In recent historical times, as engineering and science progressed, irrigation was practiced extensively in many of the countries of Europe, not considered to be arid or even semiarid. Much of the irrigation water was applied to pastures. Water wheels were used to lift water out of streams. By 1800, the total irrigated area of the world was about 8 million ha.[5] The British, during their commonwealth period in the late 1800s, developed large irrigation systems in the areas that are now India and Pakistan. Water was diverted from rivers and carried in large canals for long distances. Many wells were also developed to irrigate localized areas. More than 2,000,000 ha were brought under irrigation. In Ceylon (Sri Lanka) at the same time, irrigation systems consisted of a system of tanks (reservoirs) that served as small an area as one farm. Runoff water from rainstorms was captured, stored, and used in times of water shortage, primarily for irrigation of paddy rice. Irrigation was just beginning in southeast Australia, which was under British rule during this same period. By 1900, the total irrigated land in the world was about 48 million ha.[5] Extensive irrigation in the United States of America began in the mid-1800s in the West. At that time, there was limited irrigation of small gardens by native Mexicans living in the southern area of California. When the Mormons migrated from the humid east and central part of the United States and the rainfed areas of Europe to the arid valleys of the Rocky Mountains, the settlers found themselves in a low rainfall environment that would not support agricultural crops without irrigation. With limited information about irrigation gained from explorers of the West, they began diverting water from perennial streams onto the soil to make it possible to plow, cultivate, and plant food crops. Diversion of water from natural streams became a common practice and new settlements were established wherever there was a dependable water supply.

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Irrigated Agriculture: Historical View

The arid west was settled rapidly and extensively and a modern civilization was quickly established, based on irrigated agriculture. Ancient civilizations grew as a result of irrigation. Western American civilization developed based on irrigation. Without irrigation, the arid western United States could only support a limited population of hunters and gatherers. As the number of settlers in the West increased, the competition for water increased and it became necessary to determine how the water should be divided equitably among the potential users. Some crops needed more water than others and some soils needed more frequent irrigation to keep plants growing properly. New laws had to be developed specifically for management of the limited water supply. The system of law defining Riparian Water Rights, common to humid areas, gave legal use rights to the landowner touching the streams. This law was not appropriate for water-short areas where the water had to be taken away from the stream to non-riparian land. A new system of water law, called Prior Rights, was developed. The doctrine of prior rights states that first in time of use is first in right and that beneficial use of the water determines the right. The first person to use the water has a legally superior right to use the water and can maintain that right as long as he uses the water beneficially. Since the land was essentially worthless without a water right, water rights normally were sold with the land. The establishment of the Land Grant University system by President Abraham Lincoln in 1862, gave emphasis to the development of agriculture. In the arid and semiarid areas of the country, agriculture depended on irrigation water management. Universities in those areas gave special emphasis to research on soil–plant–water relations. This information was very important to the effective and efficient use of water. In ancient times, water was applied to the soil until the irrigator felt that the soil was wet enough. He also learned that too much water was damaging to the plants. His only means of applying water was to carry water to the individual plants or bring the water from the source in a small ditch or furrow. He could also build a small dike around an area containing plants and flood irrigation water into the resulting basin. Rice can be irrigated by continually flooding it in a basin area. From ancient to modern times, the majority of the irrigation taking place in the world is by surface irrigation methods. Irrigation that uses the soil surface to transport as well as absorb the water is called surface irrigation. As a result of research, mostly at land grand universities, irrigation water can now be applied using mechanical systems. The water is transported in pipes and is applied to the soil using sprinklers or drip-irrigation systems. Surface irrigation, to be successful, requires a lot of hand labor and the good

Irrigated Agriculture: Historical View

judgment of the irrigator. Mechanized irrigation can be accomplished by complete automation, including the decisions of when to irrigate and how much water to apply. Irrigation on a large scale developed in the American West soon after the passage of the Reclamation Act in 1902. The U.S. Bureau of Reclamation of the U.S. Department of the Interior, backed by the U.S. Government financing, was able to build large dams that provided water for reclaiming many western desert lands. In recent years, controversy[6] has arisen over the environmental changes caused by using rivers to generate electric power, to supply water to remote cities in other drainage basins, and to make desert lands agriculturally productive, rather than leaving the rivers in their natural ecological state. Similar objections have not been raised in India, Pakistan, and China where there are even larger expanses of irrigated land.

CONCLUSION

that have significant areas under irrigation. In the year 1998, there were more than 271 million ha of land being irrigated in the world. At least 22 persons depend, in some degree, on the food produced on each hectare of land irrigated in the world. Irrigation plays a more important role in food production in the modern world than it did in the ancient world. During the 21st Century, most of the world’s increased food supply will have to come from the higher agricultural productivity of existing irrigated lands.

REFERENCES 1. Babylonian Law—The Code of Hammurabi. Encyclopedia Britannica 1910–1911, 11th Ed.; Yale Law School Avalon Project. 2. King, F.H. Irrigation and Drainage, Principles and Practice of Their Cultural Phases. 1912; The Macmillan Company: New York, 1913; 66–72. 3. Israelsen, O.W.; Hansen, V.E. Irrigation Principles and Practices; John Wiley and Sons: New York, 1962; 1–3. 4. Glick, T.F. Irrigation and Society in Medieval Valencia; The Belknap Press of Harvard University: Cambridge, MA, 1970; 386 pp. 5. Fukuda, H. Irrigation in the World—Comparative Developments; University of Tokyo Press: Tokyo, Japan, 1976; 329 pp. 6. Reisner, M. Cadillac Desert; Penguin Books: New York, 1986; 582 pp.

Hydrologic– Irrigation

There is an International Commission on Irrigation and Drainage headquartered in New Delhi, India. This organization maintains statistics about irrigation and encourages more efficient and effective use of irrigation water. There are 25 countries in Africa, 16 in the America, 29 in Asia and Oceana, and 28 in Europe

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Irrigated Agriculture: Managing toward Sustainability Wayne Clyma Water Management Consultant, Fort Collins, Colorado, U.S.A.

Muhammad Siddique Shafique Office for Project Services, United Nations, Lahore, Pakistan

Jan van Schilfgaarde U.S. Department of Agriculture (USDA), Fort Collins, Colorado, U.S.A.

INTRODUCTION

Hydrologic– Irrigation

Water management improvements in an irrigated valley improve productivity, reduce the environmental impacts of irrigation, enhance the environment of rivers, and usually provide additional water that can be used for other purposes. Long-standing traditional beliefs are that water cannot be saved by improving water management in an irrigated valley. These beliefs are shown to be based upon erroneous assumptions. Improving water management increases yields, area affected by waterlogging and salinity is reduced, return flows and salinity are lowered, and water saved in the reservoir improves the environment of the river when released. Organizations and individual actions need to be changed for successful water management improvements. Irrigated agriculture plays an important role in food production. Commonly, irrigated agricultural production averages nearly twice the production of rainfed agriculture per unit area. Actually, effective irrigated agriculture easily produces 3–5 times rainfed production. The reduced level of average production under irrigation is a measure of the inadequacy of performance of irrigation. The Food and Agricultural Organization (FAO)[1] has shown that water shortages are currently an issue in many countries. By 2030, these shortages will cause serious food shortages in many countries. Therefore, a key strategy for irrigated agriculture is to develop water conservation programs to conserve and enhance water supplies and substantially increase productivity worldwide.

IRRIGATION WATER MANAGEMENT STATUS Irrigation water management practices need significant improvement worldwide.[2,3] Fig. 1A shows an irrigated valley with major waterlogging and salinity problems caused by poor water management practices. Fig. 1B 542

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shows evaporation from standing water and excess evapotranspiration from crops supported from a high water table. Poor or non-functioning drainage systems also cause excess evapotranspiration as shown in Fig. 1C. Water use for irrigation is often 2– 4 times as much as good management achieves. Encroaching salinity and waterlogging remove millions of hectares from production each year. Worldwide, the irrigated area severely or moderately affected by waterlogging and salinity equals 30% of the irrigated area, and it increases by 1–2% per year.[1] Poor water management practices throughout an irrigation project cause the waterlogging and salinity. With high water tables, from 15% to 85% of the rainfall plus irrigation supplied can evapotranspire. Evaporation and evapotranspiration losses from waterlogged areas represent a major source of water that can be conserved. Thus, water management improvements to increase productivity, reduce the impacts of waterlogging and salinity, and conserve water supplies are urgently needed practices. Farm Water Management Assessment Crop yields under irrigated agriculture can be assessed using many different strategies. Comparing average yields to record yields for a country or an irrigated valley is one strategy for assessing the potential yields. Often, average yields are one-third to one-fifth and even less of the record yields. Causes of these lower yields are varied. Basic to yield improvement under surface irrigation is a precision-leveled field especially when level basins are the field system. Level basins should be within 15 mm of the average elevation for good water management and to achieve potential yields. Because fields are usually designed and leveled by farmers, most are not adequately leveled. Over application of water to fields or parts of fields is often the major factor causing waterlogging in irrigation projects. Farmers in Pakistan were thought to use lower than recommended levels of fertilizer. More careful evaluation showed

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Fig. 1 (A) Irrigated valley with waterlogged area and poorly defined surface drainage system. (B) Waterlogging and salinity is an integral part of irrigated agriculture in most projects and the water evaporated or evapotranspired is a non-beneficial use. (C) Drainage systems for many irrigation projects cause non-beneficial use and reduce return flows. Source: From Ref.[12].

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that the level of fertilizer used was near the optimum for the fields’ potential yields. Studies in several countries have shown that precision leveling (precision conventional leveling or laser leveling) with appropriate inputs contributed to many-fold yield increases while water requirements were reduced substantially, often by half or even more. Thus, water management improvements at the field level increase productivity while reducing the water required for irrigating the field. Water management and input improvements generally increase yields sufficiently to more than pay for the services when quality service is provided at an effective cost with credit available when needed. Productivity improvements and increases in the effective use of water supplies are often the highest priorities for a country.

Delivery of Water Supplies

Hydrologic– Irrigation

Field studies in irrigation projects around the world have shown that adequacy, dependability, and equity are important, but often unattained, goals of water delivery. Canals and watercourses usually do not provide a target discharge of necessary duration for farmers to irrigate adequately. Undependable water supplies can cause farmers not to plant any crop, or to use traditional seeds with little or no fertilizer instead of high yielding varieties with adequate fertilizer. They also may not follow appropriate cropping practices, and limit the use of adequate weed and pest controls.[4] Therefore, undependable water supplies often create greater constraints on productivity and potential productivity than just those caused by inadequate water. Assessments of productivity in canal commands in many countries have shown that yields under undependable, inadequate water supplies are fractions of adjacent fields with dependable, adequate water.[4] Farmers irrigate too frequently and apply too much water when supplies are undependable but available. They use the water when available because experience has taught them that it may not be available when they next need to irrigate. Frequent irrigations cause excess evapotranspiration and increase waterlogging. Inadequate water supplies reduce actual yields, and teach farmers that investments in inputs for higher yields are not profitable. Studies in India showed that the intensity of cash value crops was directly correlated with distance from the canal outlet with the type of crop grown determined by water supply.[5] In Nepal and Sri Lanka, when farmers were not adequately advised about expected water supplies, they planned for inadequate water supplies as described earlier. Every extensive water distribution system studied had head to tail inequities in the adequacy and

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Irrigated Agriculture: Managing toward Sustainability

dependability of water supplies. Inequities in water delivery have been measured in India, Egypt, Sri Lanka, Nepal, Pakistan, Thailand, Somalia, and the United States of America to name a few countries. Upper reaches of most delivery systems or branches with influential farmers often have three or more times the target water supply while those at the end of the canals, laterals, and watercourses receive half or zero of the target. Water Quality Management Irrigation water management in recent decades has focused on salinity management—now urgently needed. Irrigating a field supplies water to be used by the crop through evapotranspiration. Since the water changes to vapor as it leaves the soil or plant surface, the salts it carries remain in the soil. All irrigation leaves salts in the soil that must be controlled by adding additional water. The additional water travels through the rootzone and removes salts by a process called leaching. The excess water becomes deep percolation that goes to groundwater or to a substratum. Some excess water, carrying the leached salt, often travels to the river as return flow. Additional salts can be in the return flow because the groundwater was more saline to start with or because the substratum had excess salts that were added to the water. Sometimes, natural return flows to the river are not sufficient and rapid enough, and drainage must be provided to remove the excess water and salts. Erosion of the fields during irrigation may cause sediment pickup. Erosion damages the irrigated fields, and the sediment may damage lands and waterways where the sediment is deposited. Sediment that remains in suspension in the water limits the value of the water for reuse. Return flows from irrigation may also contain nitrogen, phosphorus, and other agrochemicals, used to control weeds or pests, from agricultural operations that contaminate the water for other uses. Small amounts of selenium, boron, or other elements toxic to plants (and humans) in low concentrations may also limit the value of the return flows for subsequent irrigation or other uses. Excess chloride, which causes foliage damage when the water is used in sprinkle irrigation, can be a serious problem especially on vegetable crops. Excess sodium in the irrigation water may cause difficulties with infiltration of the water into soils and thus limit the usefulness of the water. Reductions in water quality lower the value of water or increase the costs of using the water. Traditional Views of Water Management Water conservation is a widely misunderstood concept. Placed in an irrigated valley context, even greater

Irrigated Agriculture: Managing toward Sustainability

SUSTAINABLE WATER MANAGEMENT FOR IRRIGATED AGRICULTURE Sustainable water management must achieve effective water management improvements and manage to limit salinity impacts. Institutional and attitudinal changes are essential to achieve such improvements.

Sustainable Water Management A farmer irrigating a field is the focus of water management improvements. Reducing the volume of water required for irrigating the field is the objective. Water not supplied for irrigating the field is water saved. Water not released from a reservoir or diverted from the river is water conserved. When the reduced supply for irrigation results in reduced return flows to the river, then an appropriate volume of the saved water may be made available to replace the reduction in return flow. Water released to replace return flows was conserved because the water has a greater value since it was not used initially for irrigation. The remaining water is available for reallocation, whether in reservoir or groundwater storage, or is continuing flow in the river. Water available for reallocation can be allocated for other uses such as industrial, municipal, environmental, or even irrigation.

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Water available for reallocation comes from reduction of non-beneficial evapotranspiration within the irrigation project. The water conserved also can result from water supplies stored in groundwater systems or from returns to the river with major increases in salinity that materially reduces the value of the water. Water seriously contaminated when used for irrigation by one of the previously defined contaminants also can be conserved. Excess irrigation water that returns to salt sinks, such as the ocean or saline water bodies, also can be conserved. Water conservation accomplished by improving water management that reduces non-beneficial evapotranspiration, as an example, is now illustrated for a waterlogged irrigation project. Table 1 summarizes approximate data for an irrigation project in the central part of the Punjab in Pakistan.[7] Within the irrigation project, beneficial use comes from crop evapotranspiration. Non-beneficial use comes from areas waterlogged, areas with a high water table that increases evaporation at the soil surface, poorly leveled fields with low areas where standing water evaporates, and poor drainage systems. Water is available for evaporation at the potential rate when water stands on the land surface as in a waterlogged area. Total water supply is an important variable and rainfall is important particularly in monsoon climates. With the lower water supply (Table 1), waterlogging developed from irrigation in Pakistan over more than a 100 yr period because water losses persisted although the total water supply was less than potential evapotranspiration. Eighty-six percent of the lower water supply is lost to non-beneficial evapotranspiration (Table 1) when waterlogging developed. When water supplies were increased because dams were completed, waterlogging continued because beneficial and nonbeneficial uses did not exceed the water supply. With the high water supply, as much as 34% of the total water supply is lost (Table 1). The key point is that large volumes of the water supply are lost through non-beneficial evapotranspiration. These data do not Hydrologic– Irrigation

misunderstandings exist. First, many believe that improving low irrigation efficiencies automatically make large amounts of water available. They believe the conserved water is available for the improved farm, the irrigated valley, or other water uses as an additional water supply. This understanding is not valid when the excess water returns to the river. The value assigned to the water saved must be reduced by the value of the return flows for reuse. Second, another common misunderstanding, shared by the public and many professionals, is that water conservation has almost no place in irrigated valleys. They believe water can be conserved only when direct flows to salt sinks, such as saline water bodies and the ocean, are prevented. This view is supported by often erroneous assumptions that 100% of the excess irrigation water is available and 100% of this available water returns to the river as return flow.[6] The reduced value of the return flow caused by reductions in water quality is often not considered. This concept seriously hampers efforts to reduce the impacts of waterlogging and salinity and control the environmental effects of irrigation. A more balanced concept for achieving water conservation in an irrigated valley is the focus of the following section.

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Table 1 Depletion of total water supply from non-beneficial evapotranspiration from a waterlogged area in an irrigation project Total water supply (irrigation þ rainfall) (m3 m
2)

Depletion (%)

15

0.63

42

30

0.63

86

15

1.69

16

30

1.69

34

Waterlogged area (%)

Project area is 500,000 ha, annual potential ET is 1.88 m3 m
2, irrigation water supplies are 0.16 m3 m
2 and 1.22 m3 m
2, and rainfall is 0.47 m3 m
2.

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assess the impacts of unlevel fields, and ineffective drainage systems. Improving water management can achieve major results through water conservation by eliminating non-beneficial uses.

Water Quality Management

Hydrologic– Irrigation

Irrigated agriculture is not sustainable unless salinity is managed. A strategy for improving management of salinity should include reducing leaching fractions to a minimum.[8] This strategy is consistent with the water management improvement focus because when irrigation efficiencies are 40% or less, then attempting to target a 5% or less leaching fraction is not appropriate. Lower leaching fractions reduce the total salts that return to the river in return flows, and may sometimes precipitate some salts before they enter the river.[9] More careful control of leaching can precipitate salts below the rootzone and minimize the pick up of salts from saline strata. A useful strategy is to use good quality irrigation water to grow salt sensitive crops such as lettuce. Then, as salinity increases down an irrigated valley, to grow more salt tolerant crops such as wheat and cotton, and further down the valley even more tolerant crops such as barley.[8] Continuing to use water from the river for irrigation that increases in salinity reduces the return flows to the river and lowers the volume of saline water that must be managed for disposal. Evaporation ponds and pipelines to salt sinks such as the ocean are appropriate disposal alternatives.[8] These strategies for managing salinity are a key part of the water management improvements for increased water conservation. A critical need is a system for costing and valuing changes in water quality. Traditionally, reductions in water quality are neither assigned a cost nor are improvements in water quality assigned a value. Keller, Keller, and Seckler[10] suggested that return flow volumes be reduced by the volume of additional water required for leaching because of the salinity increases in the return flow. While this is an important step, the result is an inadequate approach for costing and valuing increases in salinity, and does not evaluate other important quality changes. A strategy for costing and valuing water quality changes is further described. The amount of water used to replace the reduced return flows from water conservation should be based upon an effective volume or value assessment. Clyma and Shafique[6] suggest that the effective value of the return flow be determined. Then, the reduction in return flow is replaced by a volume from storage that equals the reduction in effective value caused by water conservation, if any. Keller and Keller’s[10] approach does not consider reductions in yield or crop changes

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Irrigated Agriculture: Managing toward Sustainability

that result from increased salinity, or other reductions in value such as lifting costs for pumping. Clyma and Shafique[6] allow for increased value of the replacement water and decreases in value from salinity, energy costs, impacts of other contaminants, and other factors. An important consideration is the trade-offs between water stored in a reservoir, water in groundwater storage, and return flows that vary in amount and the time when available in the river. The valley water management strategy starts at the farm with improvements in water control that cause water applied to approach crop needs and minimum leaching requirements. Using minimum leaching requirements does reduce salt loads. Minimum return flows also reduce the total salt load in the river but may increase the concentration of salts in the return flows. Salinity increases down the river are reduced because of the lower total salts. Water releases from the upstream reservoir or remaining in the river provide additional flow volume for the river further reducing salinity in the river. Additional good quality water is made available for subsequent canal commands. When the return flow salinity reaches a critical level approaching zero value for the return flows, then disposal of the water from evaporation ponds or with a pipeline to the ocean or another salt sink can be considered. Such a management strategy approaches achieving a permanent irrigated agriculture. Institutional and Attitudinal Changes Farmers change their water management when they understand and experience the value of change. Supporting organizations and their professional personnel must appreciate the changes needed to enable them to support farmers in accomplishing such changes effectively. Institutions must modify their policies and programs to define and then support farmer needs for change. Then, they must support personnel that effectively provides the needed support to farmers.[11] Changing individuals and organizations—farmer, private, and local, county, state, and federal units—is difficult but can be successfully accomplished.[11] Changes to water laws will often be needed if water is to be conserved. Farmers must benefit financially when water conservation increases the effective water supply. Water banks, begun recently in some Western States in the United States, offer some of the changes needed. Water rights in many countries will need major redefinition.

CONCLUSIONS Water management improvements provide major opportunities for increasing productivity, reducing

Irrigated Agriculture: Managing toward Sustainability

the environmental impacts of irrigation, and increasing water supplies by accomplishing water conservation. Poorly managed water supplies and field irrigation limit productivity and create major increases in waterlogging and salinity. Water quality for irrigation decreases down a valley further reducing productivity. Water management improvements reduce the amount of water required to irrigate a field. Water not supplied to the field is water saved. When return flows are reduced because of improvements, water saved can be released to replace the return flows. The remaining water saved is water available for reallocation. It is available for a variety of uses such as industrial, municipal, environmental, or even expanded irrigation. Improvements in water management reduce waterlogging and salinity. Because leaching volumes are reduced, total salt returning to the river is often reduced. Water released to replace return flows further improves the quality of water in the river and provides more water for fish and wild life at an improved quality. Careful management may achieve a permanent irrigated agriculture.

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9.

REFERENCES

11.

12.

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1. FAO. Food Production: The Critical Role of Water; Food and Agricultural Organization: Rome, 1996. 2. Clyma, W. Changing irrigated agriculture for the new millennium. In National Irrigation Symposium, Proceedings of the Fourth Decennial Symposium, Phoenix, AZ, Nov 14–16, 2000; ASAE: St. Joseph, MI, 2000; 182–186. 3. Clyma, W.; Clemmens, A.J. Farmer management strategies for level basins using advance distance criteria. In National Irrigation Symposium, Proceedings of the Fourth Decennial Symposium, Phoenix, AZ, Nov 14–16,

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2000; Evans, R.G., Benham, B.L., Trooien, P.P., Eds.; ASAE: St. Joseph, MI, 2000; 573–578. Clyma, W.; Katariya, S.R.; Nelson, L.J.; Tomar, S.P.; Reddy, J.M.; Bakliwal, S.K.; Haider, M.I.; Mehta, U.R.; Lowdermilk, M.K. Diagnostic Analysis of Farm Irrigation Systems on the Gambhiri Irrigation Project, Rajasthan, India, Water Management Synthesis Report No. 17; Colorado State University: Fort Collins, CO, 1983; Vol. I–V. Jayaraman, T.K.; Lowdermilk, M.K.; Nelson, L.J.; Clyma, W.; Reddy, J.M.; Haider, M.I. Diagnostic Analysis of Farm Irrigation Systems in the MahiKadana Irrigation Project, Gujarat, India, Water Management Synthesis Report No. 18; Colorado State University: Fort Collins, CO, 1983. Clyma, W.; Shafique, M.S. Basin-Wide Water Management Concepts for the New Millennium, ASAE Paper No. 012051; ASAE: St. Joseph, MI, 2001; 1–16. Clyma, W. Evapotranspiration and irrigation water requirements for Pakistan. Bull. Irrig. Drain. Flood Control Res. Council 1973, 3 (2), 1–8. van Schilfgaarde, J.; Rhoades, J.D. Coping with salinity. In Water Scarcity, Impacts in Western Agriculture; Engelbert, E.A., Ed.; University of California Press: Berkeley, CA, 1984; Vol. 6, 157–179. Suarez, D.L.; Rhoades, J.D. Effect of leaching fraction on river salinity. J. Irrig. Drng. Div., ASCE 1977, 103 (IR2), 245–257. Keller, A.; Keller, J.; Seckler, D. Integrated Water Resource Systems: Theory and Policy Implications, Research Report 3; International Irrigation Management Institute: Colombo, Sri Lanka, 1996; 1–15. Dedrick, A.R.; Bautista, E.; Clyma, W.; Levine, D.B.; Rish, S.A. The management improvement program: a process for improving the performance of irrigated agriculture. Irrig. Drng. Syst. 2000, 14 (1–2), 5–39. Clyma, W.; Shafique, M.S. Water Saved in Irrigated Valleys by Improving Classical Efficiency; Colorado Institute for Irrigation Management, The Water Center: Fort Collins, CO, 2002.

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Irrigated Agriculture: Social Impacts Gaylord V. Skogerboe Utah State University, Ogden, Utah, U.S.A.

INTRODUCTION Ancient hydraulic civilizations had absolute power, including strong organizational coordination and complete control of resources, elaborate postal communication and intelligence networks for social control, along with the dominant religion being under the authority of the state. In sharp contrast, small indigenous farmer-managed hydraulic societies have existed for centuries in many countries. These irrigation systems are operated by using democratic principles wherein all farmers participate in managing their system. During the past century, the world’s irrigated land has increased fivefold, but there will be limited expansion in the future. These systems are commonly managed by government agencies. But, water scarcity will require that some of the irrigation water supplies be transferred to meet increasing municipal and industrial water needs. In order to feed growing populations, much higher levels of irrigation water management will be required. This will necessitate transferring these government-managed irrigated systems to farmers so that they become more self-reliant and innovative. To significantly increase crop yields with less water requires: 1) a clearly defined water rights system; and 2) sustainable farmers organizations. NATURE OF SOCIAL IMPACTS

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The degree of productivity in irrigated agriculture is highly dependent upon the degree of cooperation among farmers and with the individuals responsible for managing the irrigation system. In addition, each farmer is impacted by the actions occurring upstream, such as unreliable water deliveries and water theft. Farmers who are more self-reliant are more likely to maintain their irrigation facilities. Cooperation and independence also foster innovations that lead to increased agricultural productivity, possibly more employment and better health. The dominant factor impacting these traits is the type of social organization employed for managing an irrigation system. Worldwide irrigated agriculture has grown from 8 Mha in 1800 to 48 Mha a century later,[1] with the United Nations[2] estimating 255 Mha in 1995, which is most likely an overestimate. More importantly, the 548

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amount of irrigated land is expected to increase very little in the future. About 17% of the world’s agricultural land is presently irrigated, which accounts for about 40% of the world’s food production.[2] For the future, planners place heavy emphasis upon 75% or more of increased food production coming from irrigated agriculture. A good case has been made[3] for doubling the productivity of water in order to feed 8 billion people within the next three decades, while protecting the world’s ecosystems. With increasing water scarcity in many global locations, some of the present irrigation water supplies must be transferred to meet future urban and industrial water demands. This will necessitate much higher levels of water management, with a major emphasis on significantly improved social organization of irrigation systems in many parts of the world. CENTRALLY-ADMINISTERED HYDRAULIC CIVILIZATIONS Wittfogel[4] reports on the administrative management of numerous irrigation systems around the globe, but especially Asia, over many thousand of years. Smallscale irrigation is called a ‘‘hydraulic society,’’ while a large-scale and government-managed irrigation network is a ‘‘hydraulic civilization.’’ There are three paramount characteristics of a hydraulic civilization: 1) involves a division of labor; 2) intensifies cultivation; and 3) necessitates cooperation on a large scale. Hydraulic civilizations used corvee forced labor, which was conscripted on a temporary, but recurring, basis. In Imperial China, every commoner family was expected on demand to provide labor for hydraulic and other public services. The writings of India, as well as the Incas and Aztecs, indicate a similar claim on corvee labor. In terms of social control and natural resources development, the master builders of hydraulic civilizations had no equal in the non-hydraulic world because of control over the entire country’s labor and materials. The dispersed castles of Medieval Europe are clear evidence of feudal society, just as huge administrative cities and colossal palaces, temples and tombs of Asia, Egypt, and ancient America express the organizational coordination and resources mobilization of the hydraulic civilizations.[4]

Irrigated Agriculture: Social Impacts

INDIGENOUS FARMER-MANAGED HYDRAULIC SOCIETIES For many centuries, farmer-managed irrigation systems (FMISs) have existed at various locations around the world. In Asia, the systems in Nepal, Thailand, and the Philippines have been partially investigated. The social organizational arrangements are a sharp contrast with the despotic hydraulic civilizations described previously. Two-thirds of the irrigated agriculture in Nepal is farmer-managed; mostly, these thousands of systems are autonomous, self-governing entities ranging in cultivated area from 10 ha to 15,000 ha. A comparative study of 21 FMISs has been reported,[6] along with institutional arrangements consisting of social organization and property rights in water.[7] These irrigation organizations perform tasks of water acquisition, water allocation and distribution, resource mobilization (people and tools), system maintenance, decisionmaking, communication, and conflict resolution. Decisions regarding irrigation water management are made by the irrigators as a whole at their annual meeting, where the farmers review the performance

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of the previous year, audit and settle accounts, decide on the plan and program for each major task, and elect officeholders. An irrigation management committee is elected to carry out the decisions of the general body of irrigators. Remuneration to committee members often consists of cash or kind, but sometimes nothing. Resource mobilization (such as channel cleaning and replacement of low-cost structures that failed due to floods), may be based on the size of landholding, water shares, water outlet size, village units, or the number of households in the command area. Water allocation may be based on the size of landholding, labor contributions for maintenance, original investment, water shares, or the type of land. Water distribution needs intensive supervision, particularly when the water supply is barely sufficient to meet the crop needs. When traveling throughout Thailand, it is readily apparent that the best agriculture occurs on roughly 2000 small FMISs located in the North (two-thirds of cultivated area), where irrigation has been practiced for at least 700 yr. A 10-yr multidisciplinary study of five FMISs[8] shows there is a high degree of acceptance among the farmers of the water rules and regulations, which in earlier times were considered sacred because they provided rice for everyone; thus, water theft was considered a severe crime against society. In Thailand, the farmer leaders for a FMIS receive much respect because they are trusted by the farmers, which results from the leaders diligently doing what they promised, along with being very fair in their dealings, and placing a strong emphasis on bettering the community. These traits result[8] in equitable water distribution.

GOVERNMENT AGENCY-MANAGED IRRIGATION SYSTEMS From the mid-19th century through the 20th century, about three-fourths of the developed irrigated lands are administered by government irrigation agencies. Majority of these irrigation systems in developing countries are not properly maintained, so they are unable to increase agricultural productivity for feeding a growing population. Rehabilitation is often considered a remedy, but usually results in another costly cycle of improvement and decay with no long-term benefits.[9] There is a growing perception that these public irrigation agencies lack the incentives and responsiveness to improve management performance and that a management system which is more accountable to farmers will be more equitable and responsive. The argument can easily be made that farmers under these agency-managed irrigation systems (AMISs) are oppressed. Certainly, they have limited

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Administrators and officers were placed in all major settlements, which virtually everywhere assumed the character of government-controlled administrative and garrison towns. In addition, almost all hydraulic civilizations enhanced their power by elaborate systems of postal communication and intelligence, which became a formidable weapon of social control. The masters of the empire in China combined state roads and man-made waterways in establishing a postal and intelligence system that lasted for more than 2000 yr, but with some disruptions.[4] The government of a hydraulic civilization was an integral part of the irrigation management bureaucracy, with the dominant religion being closely attached to the state. Nowhere in hydraulic civilizations did the dominant religion place itself outside the authority of the state as a national or international autonomous church.[4] This formidable concentration of vital functions gave the government its genuinely absolutist power, where its rule was not effectively checked by non-governmental forces. Egypt was an important deviation. The central government imposed a tax on the farmers of 10–20% of their harvest, but the administration of the irrigation system remained local.[3] An observation[5] is that ‘‘Egypt probably survived for so long because production did not depend on a centralized state; the collapse of government or the turnover of dynasties did little to undermine irrigation and agricultural production at the local level.’’

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control of their destiny. They are not organized for administering their irrigation system, and they do not have meaningful water rights. Increasing agricultural productivity over time is highly dependent on farmers being empowered so that they become more self-reliant and innovative, thereby benefiting socially and economically from their ingenuity.

SELF-RELIANCE AND INNOVATION The degree of independence and innovation demonstrated by the farmers in an irrigation system is a good indicator of social impacts. A healthy agricultural environment relies on farmers being innovative, which in turn is dependent upon farmers being able to benefit socially and economically from their inventive behavior. A major goal of agricultural development should be to establish an institutional environment that strongly supports innovations by farmers. In order for farmers to become more confident, a highly participatory approach is required from all types of agricultural support services. Farmers must not only be treated as equals, but they must be recognized as the local experts. Thus, the attitudes and behavior by those individuals providing support services is crucial to successful agricultural development. The most underutilized resource for improving irrigated agriculture, the farmers, can only be effectively strengthened by using participatory approaches that strongly emphasize farmers first.[10] The most significant determinant of self-reliance will be the degree that farmers manage their own irrigation system. Farmers recognize the significance of controlling the entire canal network, including the canal headworks. This can be readily envisioned for relatively small irrigation systems, but farmer management is even more important for the much larger canal systems such as encountered in China, India, and Pakistan.

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INSTITUTIONAL EMPOWERMENT The major lesson from past hydraulic civilizations, numerous FMISs over many centuries, as well as Australia, Canada, and the American West in the last 150 yr, is that locally managed irrigation enterprises are to be preferred for long-term sustainability. The critical ingredients to highly productive agriculture for such systems are having the power to assess the beneficiaries for making improvements, along with water rights to encourage long-term investments. The success of irrigated agriculture requires fitting many pieces of the puzzle together, including social cohesion, but certainly institutional measures should lead, not follow technology, in this continual struggle for progress.

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Irrigated Agriculture: Social Impacts

CONCLUSION Entering the 21st century, there were about 25 countries experimenting with the transfer of irrigation system management from government agencies to farmers, which is encouraging. This is a time-consuming task requiring decades. The American West generally required three decades, or more, to develop effective irrigation institutions. The necessity for doubling the water productivity of irrigated agriculture over the next three decades is strongly dependent upon enhancing the social impacts by having a clearly defined water rights system in each irrigated region, as well as sustainable farmers organizations as measured by: 1) equitable water distribution throughout the irrigation system; and 2) farmers feeling free to report offenders (such as stealing water) and their organization is capable of applying sanctions.[11]

REFERENCES 1. Fukuda, H. Irrigation in the World—Comparative Developments; University of Tokyo Press: Tokyo, 1976. 2. United Nations Food and Agriculture Organization, 1996 Production Yearbook; FAO: Rome, 1997; Vol. 50. 3. Postel, S. Pillar of Sand—Can the Irrigation Miracle Last? W.W. Norton and Company: New York, 1999. 4. Wittfogel, K.A. Oriental Despotism: A Comparative Study of Total Power; Yale University Press: New Haven, CT, 1957. 5. Hassan, F.A. The dynamics of a riverine civilization: a geoarchaeological perspective on the Nile River Valley, Egypt. World Archaeol. 1997, 29 (1). 6. Pradhan, P. Patterns of Irrigation Organization in Nepal: A Comparative Study of 21 Farmer-Managed Irrigation Systems; Nepal No. 1 Country Paper, International Irrigation Management Institute: Colombo, Sri Lanka, 1989. 7. Martin, E.D.; Yoder, R. Institutions for Irrigation Management in Farmer-Managed Systems: Examples from the Hills of Nepal; Research Paper No. 5, International Irrigation Management Institute: Colombo, Sri Lanka, 1987. 8. Surarerks, V. Historical Development and Management of Irrigation Systems in Northern Thailand; Chareon Printing Ltd.: Bangkok, Thailand, 1986. 9. Skogerboe, G.V.; Merkley, G.P. Irrigation Maintenance and Operations Learning Process; Water Resources Publications LLC: Highlands Ranch, CO, 1996. 10. Chambers, R., Pacey, A., Thrupp, L.A., Eds. Farmers First: Farmer Innovation and Agricultural Research; Intermediate Technology Publications: London, 1989. 11. Skogerboe, G.V.; Bandaragoda, D.J. Towards Environmentally Sustainable Agriculture in the Indus Basin Irrigation System; Pakistan Report No. R-77, International Irrigation Management Institute: Lahore, 1998.

Irrigated Water: Market Role in Reallocating Bonnie G. Colby

INTRODUCTION Market transactions are an important strategy for responding to water scarcity and the conflicts among water users, communities, and governments that can be stimulated by water scarcity. This article outlines the policy and economic issues raised by water markets, drawing on several decades of experience with marketing water in the western United States.

WHY HAVE MARKETS DEVELOPED? Market acquisitions of irrigation water rights are increasingly common worldwide in regions where existing water supplies are fully appropriated and development of new supplies is costly. Those needing additional water must bid supplies away from current water users, primarily irrigators. In the American West, urban growth, environmental disputes, and Native American water claims, all create incentives for acquisition of agricultural water supplies and a few active regional markets have developed. While market acquisitions of additional water supplies often are essential to economic development, they also are the subjects of controversy and complex regulatory systems exist to govern market transactions in water. A water market consists of the interaction of individuals and organizations which buy, sell and lease water rights, use of water supplies, and access to water-related infrastructure (canals, pumps, reservoirs). The degree of market activity varies among and within the western United States in terms of numbers of buyers and sellers, frequency of transactions and prices. Only a few areas have well-developed water markets with many transactions occurring every year. In other areas, sales of water rights largely involve water exchanges among neighboring farmers, transactions occur sporadically and price information is difficult to obtain. The Southwest generally is perceived to be the most active region of the United States, with respect to market activity, although drought in the Pacific Northwest is stimulating transactions there.[1] In the western United States throughout the 1900s, water development projects diverted vast amounts of water from streams in order to irrigate crops.

Water quality, recreation, and wildlife benefits associated with water left instream were largely unacknowledged, as were Native American claims to water. Irrigation districts, farmers, ranchers, and towns were accorded property rights in the resource. Water supplies are renewed by nature in a stochastic and seasonal manner so that policymakers and water users cannot predict river flows far in advance. This uncertainty has prompted investment in infrastructure to store and convey surface water and to recharge groundwater so that supplies are available in a more predictable manner. Public and private expenditures to reduce variability in water supplies have been immense. Federal subsidies for irrigation projects in the western United States have covered approximately eighty percent of the capital costs of providing irrigation water to farm lands receiving water from federal projects.[2,3] The West’s economic transition from ranching, irrigated farming, and mining to urban growth, services, tourism, and industry has brought strong pressure to transfer water out of agriculture. Agriculture still accounts for 85–95% of water use in most western states, and the cost of reducing irrigated acreage so that water can be available for other uses generally is far less than the cost of developing new water supplies. Western U.S. cities pioneered water marketing by purchasing irrigated land, sometimes entire irrigation districts, to acquire water rights for urban development.[4] While urban growth still is the driving force behind water markets, water transfers to support wildlife, fisheries, and recreation have become more common.[5] Transfers have become more complex and innovative in order to respond to drought, and to environmental and community concerns.

PUBLIC GOODS AND EXTERNALITIES The term ‘‘public good’’ refers to resources characterized by non-excludability, meaning it is difficult or impossible to exclude those who do not pay from enjoying the benefits of the resource. Water for recreation and wildlife habitat provides public benefits for which beneficiaries cannot readily be charged a user fee. Streamflows also provide public good benefits 551

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Department of Agricultural and Resource Economics, University of Arizona, Tucson, Arizona, U.S.A.

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through dilution and water quality enhancement. Many individuals who benefit from streams and wetlands may be ‘‘free riders,’’ enjoying these resources but making no payments—because payments are not required. Due to non-excludability and free rider tendencies, market transactions alone are unlikely to ensure that adequate flows remain in streams to preserve habitat, water quality, and recreational opportunities. Therefore, public agencies sometimes assume the task of protecting streamflows. Water transfers can generate externalities, including reduced water supplies for other water right holders, diminished economic activity in areas from which water is taken, lower river flows and degradation of water quality, fish and wildlife habitat, and recreation. While water transfers create positive externalities in the area to which water is being moved, it is the negative impacts that create controversy and pressure to carefully regulate transactions. Western U.S. state laws specifically exclude some parties who may experience significant externalities from formally objecting to water transfer approval. In general, only water right holders can force their concerns to be accounted for. Recreationists and environmental advocates typically have little bargaining power in the regulatory process. Broader access to property rights in water and to the transfer approval process can allow a wider array of externalities to be considered.

MARKETS AND LITIGATION: COMPLEMENTARY FORCES

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Voluntary transfers of water are not the only mechanism used to move water out of agriculture. Complex legal proceedings, termed adjudications, are taking place in many areas to quantify and prioritize the competing claims of Native Americans, wilderness areas, cities, and farms. Litigation based on the Endangered Species Act, the Clean Water Act, federal reserved rights and the public-trust doctrine has successfully forced reallocation of water to enhance streamflows for recreation, fish, wildlife, and water quality. Voluntary and involuntary pressures for reallocation often work in a complementary manner. There is no incentive quite so effective in stimulating voluntary transfers as the looming threat of a protracted and costly court battle. The threat of judicial and administrative reallocations has provided impetus for numerous voluntary reallocations among parties embroiled in conflicts over water.[6]

DEVELOPING COST EFFECTIVE POLICIES The key challenge in developing policies to govern water markets is to utilize the flexibility that markets

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Irrigated Water: Market Role in Reallocating

offer, while protecting third parties and public interests that can be impaired by water transfers. The complex nature of water rights and the changing social values associated with water make instantaneous, faceless and standardized transactions in water improbable and undesirable. Nevertheless, market incentives should play a significant role in water allocation; to move water to uses where it generates higher economic returns and to give water users incentives for efficient water use. A ‘‘command and control’’ bureaucratic allocation system is undesirable due to its inflexibility as new demands arise and water values change. While government policies must play a primary role in evaluating proposed water transfers to prevent uncompensated third party impacts, bureaucracies should not dictate how much water must be used by whom and for what purpose. Every western U.S. state imposes conditions on water transfers and there is no ‘‘free market.’’ Market transactions sometimes resemble complex diplomatic negotiations rather than commodity exchanges. Regulatory policies generate uncertainties and costs for transferors and these costs sometimes are perceived as unnecessary impositions on the market. However, public policies should not necessarily seek to minimize the cost of reallocating water because appropriately structured transaction costs may facilitate efficient reallocation, by giving transacting parties an incentive to account for social costs of transfers.[7] Transaction costs are the costs of making a market system work. In western U.S. water markets, parties incur transaction costs in searching for water supplies, contacting willing buyers and sellers, ascertaining the characteristics of water rights, negotiating price, and obtaining legal approval for the proposed change in water use. This latter category of transactions costs can include attorneys’ fees, engineering and hydrologic studies, court costs, and fees paid to state agencies. Transaction costs incurred to comply with regulatory policies reflect the substantial and multiple economic benefits associated with water in various uses, benefits which can be impaired by a transfer. Transactions costs are an important issue in western water reallocation. If the costs of implementing a water transfer become too high, many beneficial transfers will not take place and water supplies will remain locked into suboptimal use patterns. On the other hand, the ability to impose transactions costs on those proposing to transfer water represents bargaining power in the water allocation process. Some transaction costs are necessary, justified by the need to better account for externalities and public goods. Transaction costs also reflect the absence of ‘‘free’’ information and the need for hydrologic, legal, and economic data to address externalities in an efficient

Irrigated Water: Market Role in Reallocating

manner (see Ref.[8], for a detailed discussion of balancing transactions costs and consideration of third-party impacts).

CONCLUSION

REFERENCES 1. Smith, R.; Ed. Water Strategist; StratEcon: Claremont, California, 2001. 2. Wahl, R. Markets for Federal Water; Resources for the Future: Washington, DC, 1989. 3. Anderson, T.L.; Snyder, P. Water Markets: Priming the Invisible Pump; Cato Institute: Washington, DC, 1997. 4. Saliba, B.C.; Bush, D.B. Water Marketing in Theory and Practice: Market Transfers Water Values and Public Policy; Westview Press: Boulder, 1987. 5. Colby, B.G. Benefits, costs and water acquisition strategies: economic considerations in instream flow protection. In Instream Flow Law and Policy; MacDonnell, L., Ed.; University of Colorado Press: Boulder, 1993; Chap. 6. 6. Colby, B.G.; McGinnis, M.; Rait, K. Mitigating environmental externalities through voluntary and involuntary water allocation: Nevada’s Truckee-Carson river basin. Nat. Res. J. 1991, 31 (4), 757–784. 7. Colby, B.G. Transaction costs and efficiency in western water allocation. Am. J. Agric. Econ. 1990, 72, 1184–1192. 8. National Research Council. Water Transfers in the West: Efficiency, Equity and the Environment; National Academy Press: Washington, DC, 1992.

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In summary, market transactions are an essential response to water scarcity. Without the flexibility provided through voluntary transfers, water supplies would remain locked into outdated patterns of use. Markets allow water to move permanently out of agriculture and to be leased to alleviate temporary scarcities (as during drought). Water scarcity creates tensions worldwide and voluntary transactions are one important strategy for addressing such conflicts. However, to provide flexibility and increased economic returns from regional water supplies, markets must be governed by policies that carefully weigh the advantages of a proposed transfer against externalities, impairment of public goods, and the concerns of affected communities and governments.

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Irrigated Water: Polymer Application Bill Orts Cereal Processing Research Unit, Western Regional Research Center, U.S. Department of Agriculture (USDA), Albany, California, U.S.A.

Robert E. Sojka Northwest Irrigation and Soils Research Laboratory, U.S. Department of Agriculture (USDA), Kimberly, Idaho, U.S.A.

INTRODUCTION

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In the past decade, water-soluble polyacrylamide (PAM) was identified as an environmentally safe and highly effective erosion preventing and infiltration enhancing polymer when applied in furrow irrigation water at 1–10 mg L 1, i.e., 1–10 ppm.[1–9] Various polymers and biopolymers have long been recognized as viable soil conditioners because they stabilize soil surface structure and pore continuity. The new strategy of adding the conditioner, high molecular weight anionic PAM, to irrigation water in the first several hours of irrigation implies a significant costs savings over traditional application methods, in which hundreds of kilograms per hectare of soil additives are tilled into the entire (15 cm deep) soil surface layer. By adding PAM to the irrigation water, soil structure is improved in the important 1–5 mm thick layer at the soil/water interface of the 25–30% of field surface contacted by flowing water.[7] In 1995, the U.S. Natural Resource Conservation Service (NRCS) published a PAM-use conservation practice standard for PAM-use in irrigation water.[10] A 3-year study[2] applying these standards showed that PAM at dosage rates of 1–2 kg ha 1 per irrigation eliminated 94% (80–99% range) of sediment loss in furrow irrigation runoff, while increasing infiltration 15–50%. Seasonal application rates using the NRCS standard typically total 3–5 kg ha 1. As PAM-use is one of the most effective and economical technologies for reducing soil-runoff, it has branched into stabilization of construction sites and road cuts, with formal statewide application standards set in Wisconsin and several southern states. Recent studies with biopolymers such as charged polysaccharides,[11–14] whey,[15] and industrial cellulose derivatives[11,14] introduce potential biopolymer alternatives to PAM.

polymers varying in chain length, charge type, charge concentration, and the number and types of side-group substitutions.[16–20] Typically, PAM for erosion control is a charged copolymer with one in five acrylamide chain segments replaced by an acrylic acid entity (Fig. 1), which generally exhibits a negative charge in water. Molecular weights of PAM used for irrigated agriculture range from 12 million g mol 1 to 15 million g mol 1 (over 150,000 monomer units per chain). As a result of its structure, PAM attracts soil particles via coulombic and Van der Waals forces.[11,17,21,22] Ionic bridging creates large stable aggregates of PAM and soil, in which charged entities on both the polymer and multiple soil particles are thought to interact with the aid of calcium counterions.[11,22–24] Chain bridging further stabilizes aggregates, whereby the long polymer chain spans between separate soil particles. Despite their large size, PAM copolymers used for erosion control are formulated to dissolve in water, although this sometimes requires vigorous agitation.

PAM Erosion Control Lentz and Sojka[2] reported a 94% reduction in runoff sediment loss over 3 yr using the NRCS application standard.[10] The 1995 NRCS standard calls for dissolving 10 ppm (or 10 g m 3) PAM in furrow inflow water as it first crosses a field—typically the first 10–25% of an irrigation duration—then halting PAM dosing when runoff begins. Under many circumstances, applying PAM continuously at 1–2 ppm for the full irrigation cycle can be equally effective, although continuous application at 0.25 ppm PAM was a third less effective.[25–27]

PAM and Infiltration POLYACRYLAMIDE The term polyacrylamide and acronym ‘‘PAM’’ are chemistry jargon for a broad class of acrylamide-based 554

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The infiltration rate of PAM-treated furrows on medium to fine textured soil is usually higher than untreated furrows—typically 15% higher than for untreated water on silt loam soils and up to 50% higher

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infiltration uniformity problems (runoff or runon), e.g., with center pivots on steep or variable slopes, have begun to use PAM. Testimonials claim that PAM-use improves stands because of reduced ponding, crusting and damping off (a plant seedling disease complex). Fig. 1 PAM: Poly(acrylamide-co-acrylic acid).

ENVIRONMENTAL IMPACT OF PAM [29]

on clays. Bjorneberg reported that in tube diameters >10 mm, the PAM–water viscosity did not rise sharply until the PAM concentration in the water was > 400 ppm. However, in small soil pores, ‘‘apparent viscosity’’ increases significantly, even at the low PAM concentrations used for erosion control.[30] Most likely, PAM infiltration effects are a balance between prevention of surface sealing and apparent viscosity increases in soil pores.[30–34] In medium to fine textured soils, maintenance of pore continuity via aggregate stabilization is more important. In coarse textured soils, where PAM achieves little pore continuity enhancement, infiltration effects are nil or even slightly negative, particularly above 20 ppm.[28] Because PAM prevents erosion of furrow bottoms and sealing of the wetted perimeter, water moves about 25% further laterally in silt loams compared to nontreated furrows.[1,2] This can be a significant water conserving effect for early irrigations. Farmers should take advantage of PAMs erosion prevention to improve field infiltration uniformity by increasing inflow rates two to threefold (compared to normal). This reduces infiltration opportunity time differences between inflow and outflow ends of furrows.[28,35] Sprinkler Application of PAM Farmers and agronomists are showing interest in PAM for sprinkler irrigation.[5,6,36–40] PAM may prevent runoff/runon problems and ponding effects on stand establishment and irrigation uniformity. Polyacrylamide sprinkler application rates of 2–4 kg ha 1 reduced runoff 70% and soil loss 75% compared with controls.[36] However, the effectiveness of sprinklerapplied PAM is more variable than for furrow irrigation because of application strategies and system variables that affect water drop energy, the rate of water and PAM delivery, and possible application timing scenarios. Multiple groups[6,36–40] report improved aggregate stability from sprinkler-applied PAM, leading to decreased runoff and erosion. Flanagan, Norton, and Shainberg[5,6] increased sprinkler infiltration with 10 ppm PAM, which they attributed to reduced surface sealing. Polyacrylamide effects under sprinkler irrigation have been more transitory, less predictable and have usually needed higher seasonal field application totals for efficacy. However, farmers with sprinkler

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The overriding environmental impact of PAM is reduced erosion-induced sediment runoff,[1,2] with corresponding reductions of entrained chemical residue reaching riparian waterways.[41–43] For example, PAM prevents yearly topsoil runoff of up to 6.4 tn acre 1[2] and at least three times that as on-field erosion.[34] Since toxic pesticides and herbicides are transported via soil sediment to open water and then eventually into the air there is an increasing need to prevent soil-runoff. Recently, PAM was shown to sequester biological and chemical contaminants of runoff, providing significant potential for reduced spread of phytopathogens, animal coliforms, and other organisms of public health concern.[44,45] The main environmental concerns in PAM-use revolve around polymer purity,[46,47] and issues related to biodegradation/accumulation;[48–53] i.e., since PAM degrades slowly, the long-term, unknown effects on organisms must be considered. Biological degradation of PAM incorporated into soil is about 10% per year.[50] However, low application rates and shallow surface application is thought to accelerate degradation via various pathways, including deamination, shear-induced chain scission, and UV photosensitive chain scission.[50–53] Even at 10% annual degradation, PAM accumulation is insignificant at these application rates. Sojka and Lentz[26] showed that only 1–3% of applied PAM leaves fields in runoff and that this is quickly adsorbed by entrained sediment or ditch surfaces. Barvenik[16,50] noted that anionic PAM is safe for aquatic organisms at surprisingly high concentrations, with LC50 > 50 times the inflow dosage rates. Water impurities further buffer environmental effects by quickly deactivating dissolved PAM. Care must be taken by PAM supplies to ensure polymer purity, since the acrylamide monomer (AMD) used to synthesize PAM is a neurotoxin. The EPA recently reviewed the use of PAM with USDA and PAM industry scientists, and concluded that the AMD concentrations of 10 yr) on a single site, some systems may have some ‘‘portability’’ included into the design in order to accommodate uncertainty in a local water source or land-lease agreements. These situations are not

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necessarily the norm, but can provide some unique and challenging design scenarios. The following equation represents a basic mass balance approach and is used to determine any one of the components given the values of the other three. ðQsys ÞðT c Þ ¼ CðAÞðI gr Þ

ð1Þ

where Qsys is either the flow rate of the irrigation system or the water system supply rate from the source (L/sec); Tc, the operational time (hr) of the system per cycle or period of time needed or desired to apply Igr; A, the size of the area to be irrigated (ha); Igr, the gross depth of irrigation water that is to be applied (mm) as a daily or seasonal amount and must correspond to Tc; and C, a constant of proportionality to

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561

Igr ¼

I net Esys

ð2Þ

Irrigation system efficiency, Esys, can range from as low as 20% to over 90% and characterizes water that is ‘‘lost’’ in the conveyance system (canals or pipes), the distribution system (sprinklers, emitters, orifices, etc.), and water that is lost in the field due to runoff and/ or deep percolation below the root zone of the crop. The net irrigation depth, Inet, represents the amount of water that is needed for and directly useable by the crop. This may be expressed as the amount of water to refill the soil profile from a certain deficit level to the field capacity level, it may represent a daily peak or design evapotranspiration depth, or it may represent a seasonal (or specific time period) depth of water that is to be applied. For example, the last condition may represent the amount of water that needs to be applied from a lagoon to a cropped land area in a certain window of time, Tc. It may also represent the result of a seasonal water balance: Inet ¼ ET c
P e
ASW

ð3Þ

where Inet is the seasonal net irrigation water requirement; ETc, the seasonal crop evapotranspiration; Pe, the seasonal effective precipitation; and ASW, the available water in the soil profile at the beginning of the irrigation period that can be used by the crop during the irrigation period. The quality of the water source must be assessed as to how physical, biological, and/or chemical constituents in the water may affect or interact with components of the delivery system (pump, pipes, valves, and emitters), the soil, and crop. Water treatment and amendment practices may need to be incorporated into the design to avoid clogging of certain irrigation components. The required level of treatment will depend on the quality of the water and the sensitivity of components to the various constituents in the water. Water quality from both groundwater and surface water sources can range from excellent to very poor, and typical quality concerns include suspended solids, dissolved solids, and biological organisms. Poor well screening and/or well development problems can result in suspended sand, silt, or clay particles. Surface water sources may have suspended particles of silt and/or clay, aquatic plants, small fish, algae, larvae, or other organic debris. While these physical constituents can generally be controlled with proper filtration,

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chemical treatment may be necessary to neutralize related organic growths. Dissolved solids such as calcium, iron, or other elements can precipitate under certain conditions and subsequently clog some microirrigation emitters. Biological growths include slimes (associated with iron and/or hydrogen sulfide), fungi, and algae. Such organic growths can grow within and clog pipelines, valves, and irrigation emission devices (sprinklers, drip emitters, etc.). Chemical treatment of the water is often necessary under these conditions in addition to filtration to prevent or ‘‘clean-up’’ these organic growths. Severe instances of several of the above water quality problems may require expensive remediation components and/or management practices to ensure proper and continual operation of the irrigation system. Because such conditions may result in a financially impractical design, or poor system performance or failure, a thorough assessment of the quality of the water source must be performed prior to completion of the final design. Recycled water sources (municipal wastewater, livestock wastewater lagoons, industry wastewater sources) should be thoroughly assessed for their physical, chemical, and biological constituents. Key concerns will include pH, salts, nitrogen, and phosphorus. Some ‘‘contaminated’’ water sources may contain heavy metals or organic compounds that may be of concern when applied to agricultural crops and fields. Water application and/or loading rates may need to be assessed with respect to the concentrations of certain key elements or compounds in the water. Allowable water application amounts may not be sufficient to meet peak or design crop water demands and supplemental ‘‘clean’’ water sources may be needed to augment the water supply.

FIELD AND CROPPING SYSTEM CHARACTERISTICS The site for the planned irrigation systems needs to be assessed for dimensions, topography, physical features, soil characteristics, and climatic characteristics. The size and shape of the field must be measured and include lengths of boundaries, interior angles of adjoining sides, and any on-site, physical obstructions (trees, power poles, buildings, etc.). Obstructions should be identified as to whether they can be removed. It is also beneficial to identify on-site or nearby electrical power sources. Because land slope and surface conditions influence the type of irrigation systems that can be used and the design of the selected system, those elements should be characterized. A contour map can be very helpful for these purposes.

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adjust and properly cancel the units in the other four variables (C ¼ 2.78 for these SI units). The gross irrigation depth is related to the net irrigation depth, Inet, with the irrigation system efficiency as:

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Soil characteristics should include physical and chemical assessments. Soil texture in the surface and subsoil components of the profile influence water holding capacity and can also influence rooting characteristics of the crop. Surface conditions should be assessed for infiltration rates and subsurface conditions should be assessed for high water table conditions or restrictive soil horizons. A chemical analysis of the soil should be conducted to evaluate the pH, salinity, and nutritional characteristics. Climatic conditions have several impacts on system selection and design. The utilization, selection, spacing, and placement of sprinklers and microsprinklers will be influenced by local wind characteristics of speed, duration, and direction. While most systems are designed to meet the evapotranspiration demands on the crop, some systems may have heat stress relief or cold protection incorporated into the design. For example, if a citrus irrigation system needs to be used for general crop evapotranspiration-based irrigation requirements and for freeze protection, then the entire field must be irrigated simultaneously rather than sequentially in zones. Thus the resulting pump system and pipe network must be substantially larger. Irrigation is used for a variety of crops and cropping systems that include traditional field crops (corn, cotton, etc.), tree crops (citrus, apples, cherries, etc.), vine crops (grapes and other berries), vegetable crops (tomatoes, melons, etc.), ornamental plants (flowers, shrubs, trees, etc.), and turf (landscapes, golf courses, and commercial production). These crops have various heights, plant densities, row spacing, plant spacing, sensitivity to water stress, bedding or soil tillage practices, artificial or natural mulches, and other cultural practices. These characteristics need to be considered by the designer and incorporated into the design. In addition, crop value and irrigated yield potential will also influence the type and complexity of the irrigation system.

Irrigation Design: Steps and Elements

Required pump head has three components that include elevation head, friction head, and pressure head. Elevation head refers to the vertical elevation difference between the pumping water level and the level of the highest irrigation system outlet (sprinklers, etc.). As water flows through the pipes and other fittings, friction reduces the energy level of the water and is characterized as friction head. Finally, the irrigation system will have a specified water pressure for proper operation of the discharge devices and this is characterized using pressure head. Because most waterdischarging devices provide different flow rates of water with respect to operational pressure, one of the design goals is to minimize pressure variations within the pipe network and to maintain variations within allowable design limits. Therefore, while the designer is sizing and configuring the main lines, header pipes, laterals, associated fittings and components, material costs, pipeline flow velocities, and pressure head variations due to elevation changes and friction head losses are also being computed and analyzed to achieve an economical, hydraulically balanced and uniform irrigation system.

CONCLUSION Irrigation system design involves an assessment of the intended use and desired goal for the system; the location, availability, and quality of the water source; physical and climatic characteristics of the site; and production system characteristics of the crop. The designer sizes and configures the pump system, pipelines, water discharge devices, and the associated fittings and components into a system that will uniformly distribute water to meet the desired goals within the economic, cultural, and physical constraints associated with the site, the crops, and the production system. The final design and installed system should be evaluated to ascertain proper performance and operation.

Hydrologic– Irrigation

WATER SUPPLY SYSTEM ACKNOWLEDGMENTS The water supply system includes a pump and a network of pipes, valves, and fittings (Fig. 1) to deliver water to the infield distribution system, which may be a center pivot system, solid-set sprinklers, microirrigation laterals, or other distribution devices. The pump has two primary purposes: 1) it moves water at a desired flow rate; and 2) it provides energy to the water. Eq. (1) was discussed and presented as a method to determine the required pump capacity (Qsys). The energy requirements of the pump are often referred to as ‘‘pump head’’ and expressed in units of a height (m) of a column of water.

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The author would like to acknowledge the following individuals for their mentorship, collaboration, and professional feedback, which have all contributed to his professional growth and development in the area of irrigation system design and management: Dr. Allen G. Smajstrla (deceased), Professor, Agricultural and Biological Engineering Department, University of Florida; Dr. Freddie Lamm, Professor, NW Research and Extension Center, Kansas State University; Dr. Dorota Haman, Professor, Agricultural and Biological Engineering Department, University of Florida;

Irrigation Design: Steps and Elements

Mr. James Prochaska, P.E., President, JNM Technologies, Inc.; Dr. Danny Rogers, Professor, Department of Biological and Agricultural Engineering, Kansas State University; Dr. Muluneh Yitayew, Associate Professor, Department of Agricultural and Biosystems Engineering, University of Arizona; and the many students, in the United States and abroad, who have participated in my irrigation system design courses, workshops, and seminars.

REFERENCES

5. Howell, T.A.; Stevenson, D.S.; Aljibury, F.K.; Gitlin, H.M.; Wu, I.P.; Warick, A.W.; Raats, P.A.C. Design and operation of trickle (Drip) systems. In Design and Operation of Farm Irrigation Systems, Jensen, M.E., Ed.; ASAE Monograph No. 3, ASAE: St. Joseph, MI, 1983; 663–720. 6. Keller, J.; Bliesner, R.D. Sprinkle and Trickle Irrigation; Van Nostrand Reinhold: New York, 1990; 652 pp. 7. Boswell, M.J. Micro-irrigation Design Manual; James Hardie Irrigation: El Cajon, CA, 1990. 8. Scherer, T.; Kranz, W.; Pfost, D.; Werner, H.; Wright, J.; Yonts, C.D. Sprinkler Irrigation Systems, Midwest Plan Service Publication, MWPS-30; Iowa State University: Ames, IA, 1999; 250 pp. 9. ASAE, Design and installation of microirrigation systems. ASAE Standards, ASAE EP405.1 DEC99; ASAE Standards: St. Joseph, MI, 2000. 10. ASAE, Polyethylene pipe used for microirrigation laterals. ASAE Standards, ASAE S435 DEC99; ASAE Standards: St. Joseph, MI, 2000. 11. ASAE, Field evaluation of microirrigation systems. ASAE Standards, ASAE EP458 DEC99; ASAE Standards: St. Joseph, MI, 2000. 12. ASAE, Design, installation and performance of underground thermoplastic irrigation pipelines. ASAE Standards, ASAE S376.2 JAN98; ASAE Standards: St. Joseph, MI, 2000. 13. ASAE, Graphic symbols for pressurized irrigation system design. ASAE Standards, ASAE S491 DEC99; ASAE Standards: St. Joseph, MI, 2000.

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1. Karmelli, D.; Keller, J. Trickle Irrigation Design, 1st Ed.; Rain Bird Sprinkler Mfg. Corp.: Glendora, CA, 1975; 133 pp. 2. Pair, C.H.; Hinz, W.H.; Frost, K.R.; Sneed, R.E.; Schiltz, T.J. Irrigation, 5th Ed.; The Irrigation Association: Arlington, VA, 1983; 686 pp. 3. Heermann, D.F.; Kohl, R.A. Fluid dynamics of sprinkler systems. In Design and Operation of Farm Irrigation Systems, Jensen, M.E., Ed.; ASAE Monograph No. 3, ASAE: St. Joseph, MI, 1983; 583–620. 4. Addink, J.W.; Keller, J.; Pair, C.H.; Sneed, R.E.; Wolfe, J.W. Design and operation of sprinkler systems. In Design and Operation of Farm Irrigation Systems, Jensen, M.E., Ed.; ASAE Monograph No. 3, ASAE: St. Joseph, MI, 1983; 621–662.

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Irrigation Districts and Similar Organizations David E. Nelson Irrigation Engineering, P.E., Billings, Montana, U.S.A.

Anisa Joy Divine Resources Planning and Management Department, Imperial Irrigation District, Imperial, California, U.S.A.

INTRODUCTION

Water User Associations

Irrigation canal systems are typically managed either by a government agency or by an organization run directly or indirectly by irrigators. Organizations run by irrigators are often referred to as water user associations (WUAs). WUAs can be further divided into two major types: irrigation districts that are similar to local governments, and canal companies that are typically non-profit corporations. Irrigation organizations deliver water; drain excess water; construct, maintain, and improve the system; and collect fees. They develop plans and budgets, hire and manage employees, and are increasingly involved in environmental stewardship. They may provide water to urban, industrial, commercial, and environmental users as well as farmers.

WUAs may be run directly by the users, or indirectly by people employed by the users or their representatives. A WUA can be created as a government entity (irrigation district), or a private canal company (typically a non-profit corporation). For legal recognition, an irrigation district or canal company may be required to have the support of more than half the landowners within its boundaries.

IRRIGATION ORGANIZATIONS Government Agencies

Hydrologic– Irrigation

Worldwide, roughly half of all irrigation canal systems are managed by government agencies.[1] Often, this managing agency also designed and constructed the original canal and drainage system. Such an agency may be staffed with professionally trained employees who understand the system and the principles underlying its construction. Although agency systems can be well managed, one or more of the following problems may develop. Agency employees, not being responsible to the users, may lack incentives to perform well. The agency may have more employees than needed. If insufficient fees or taxes are collected, the system may become a financial burden to the government. Sometimes income is diverted to activities other than managing the irrigation and drainage system. Finally, when an irrigation system is managed by a government agency, the irrigators often develop the attitude that ‘‘it is the government’s system’’ rather than ‘‘our system.’’ 564

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Irrigator-Controlled Government Entities This type of WUA has the legal status of a local government and, in the United States, is called an irrigation district. In the United States, irrigation districts are organized under state laws. State laws are adapted to that state’s conditions, and are often very specific as to how the district is to be run. Districts may establish their own bylaws, which further specify how they will be managed.

Irrigator-Controlled Canal Companies A canal or ditch company is typically formed as a nonprofit corporation under corporation laws. Irrigators own shares in the company, usually in proportion to the amount of irrigated land each owns. Canal company bylaws specify how the corporation will be managed. Canal companies generally have more flexibility in how they operate than irrigation districts because corporation law is less restrictive. Canal companies are similar to—or may even be—cooperatives because they are owned by and managed for the benefit of their members. Informal Associations Irrigators may also establish informal associations, without legal status. Informal associations use social

Irrigation Districts and Similar Organizations

Other Groupings Combinations of organizational types may be found on large irrigation systems. For example, a government agency may be responsible for the dam, storage reservoir, and main canal, with water user organizations managing secondary and tertiary canals that make up the delivery system and the drainage system. Sometimes WUAs establish a Joint Board of Control, comprised of representatives from each WUA, to manage the main canal. Such boards typically operate under a contract with specific provisions. Costs are shared by the participating organizations under the terms of a negotiated contract. WUAs may also contract with a private service company for system management. Irrigation organizations in the same region often form larger associations to share ideas, make plans, solve problems, and provide input regarding government policy.

FUNCTIONS OF IRRIGATION ORGANIZATIONS Water Delivery and Drainage Water delivery is the primary function of canal system management. Each irrigator should receive a fair share of the irrigation water. Some organizations measure water deliveries at each farm turnout, whereas others measure water only at main sections or major laterals. Efficiency and flexibility are also major objectives, so that water can be delivered to crops at the proper time and in proper amounts. A ditchrider patrols a section of a canal system and distributes water to the users in that section. Ditchriders are commonly required to keep written records of their water deliveries, to promote fairness in water distribution and to protect the organization from unfair complaints or lawsuits. Sometimes drainage water is managed by a separate agency. However, the water delivery organization usually has this responsibility.

Maintenance The physical system used to deliver irrigation water and to remove drainage must be adequately maintained.

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But frequently, this is not the case. When systems are built, maintenance needs are low for the first few years, so organizations are slow to develop good maintenance programs. Even after a program is established, maintenance is often postponed in an effort to remain within the organization’s budget. Three guidelines can help prevent this from happening: 1. Separate operating funds from maintenance funds. 2. Devote some staff entirely to maintenance. 3. Establish specific annual maintenance goals, such as rehabilitating or replacing a specific number of structures (based on the total number of structures and their average useful life), and cleaning a specific length of canals and drains.

Finances Ideally, all irrigation organizations should be financially self-supporting, but many rely on external sources of funds.[2] Usually, irrigators are charged assessment fees, which vary in proportion to the area they irrigate, the amount of water they receive, or a combination of both. If fees are not paid, water delivery is terminated. In some systems, voting is not allowed until all fees have been paid. Penalties may be charged for failure to pay fees on schedule. Systems managed by government agencies often have difficulty collecting irrigation assessments because irrigators regard the system as belonging to the government. In some cases, irrigators invest time and effort to obtaining government subsidies rather than paying their fair share, often resulting in systems that are inadequately financed and poorly maintained. Irrigation organizations sometimes keep some money in a separate reserve account for unexpected large expenditures. The amount typically ranges from about one-third to the full amount of the average annual operation and maintenance expenditures.

Oversight Irrigation organizations often conduct two or more meetings with the irrigators each year to discuss accomplishments, expenditures, and plans for the coming year. For WUAs, these meetings may be used for electing or selecting leaders, setting irrigation fees, deciding on rules and policies, and approving or modifying plans. The meetings also provide an opportunity for irrigation training programs.

Hydrologic– Irrigation

pressure or refusal of water to ensure cooperation and support. Irrigators on agency-managed systems may form informal associations to represent their interests and promote changes and improvements.

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Fair and clear voting procedures are necessary for an effective WUA. Some examples of voting methods are as follows: 1. One vote per member: This method is simple. But it may not be fair to give a small landowner’s vote the same weight as that of a large landowner because it can lead to domination of the organization by small landowners. In the United States, for example, small landowners are often not farmers, and may not be interested in the performance of the system. 2. One vote per unit of irrigated land or per share of stock owned: This proportionality is generally fairer than one-vote-per-member, but it makes voting more difficult. This is the most common system in the United States, and is favored because it better reflects the costs borne by individual irrigators.[3] In some situations, it can lead to domination by a few large landowners. 3. A structured voting system: In the United States, Oregon irrigation district law[4] provides one vote for irrigators with up to 40 acres (16 ha); two votes for 40–160 acres (16–65 ha); and three votes for more than 160 acres (65 ha). This simplifies voting, but still partially reflects the amount of irrigated land owned on the system.

Hydrologic– Irrigation

Voting is often done by secret ballot, particularly for election of directors or leaders.[3] This reduces the chance of a director being hostile to those not supporting him or her. Secrecy can be difficult to maintain when votes are proportional to the number of shares owned, or area of land irrigated. In this situation, colored ballots can help maintain confidentiality. For example, one color of ballot may count for one vote and another color may count for 10 votes. Each irrigator is given the proper number of ballots of each color to match one’s vote entitlement. Membership and voting power are usually limited to owners of irrigable land in the service area. However, where an organization also provides water for non-irrigation uses, all residents in the area served may be entitled to vote. Decisions are normally made by simple-majority vote, but a two-thirds majority may be required for borrowing money or changing bylaws. Elections are sometimes conducted by an outside organization, or at least monitored by outside observers, to reduce the chance of election fraud. The number of people on a leadership body or board of directors commonly varies from three to nine. Directors may be elected for staggered terms, so that only a portion of the board can be replaced in a

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particular election. This ensures that there are always some experienced members on the board. Once elected, the leaders or board members usually meet at least monthly to conduct business and to solve management problems. The board usually hires a manager to supervise day-to-day operation and maintenance. The board and/or the manager also hire ditchriders, maintenance workers, and other needed staff. Most managing organizations are primarily concerned with system operation and maintenance. Others get involved in associated activities such as technical assistance to the farmers, sale of seeds and fertilizer, and marketing of crops. Some organizations also manage the generation and sale of hydroelectric power. With increasing population and intensified use of water, managing organizations are increasingly involved in water conservation and environmental stewardship. Irrigation districts are often required to have outside supervision of elections and an annual independent audit of the district’s financial records, and all meetings and records must be open to the public. Another desirable but rare component is an outside audit of the water distribution records. As with all organizations, an outside evaluation of performance should occasionally be done, perhaps once every 5–10 years. The Bureau of Reclamation in the United States requires such evaluations at least once every 6 years.

REFERENCES 1. Diemer, G. World-Wide Inventory of Management Types in Irrigation; International Programme for Technology and Research in Irrigation and Drainage, Food and Agriculture Organization: Rome, February 2001. GRID Issue 17. 2. Lee, P.S. ICID Survey on Funding of Operation, Maintenance and Management of Irrigation and Drainage Projects; International Commission on Irrigation and Drainage, 2000; 20–21. ISBN 81-85068-75-5. 3. Wilkins-Wells, J. The Irrigation Enterprise Management Practice Study. (Management and Governance Topic, Board of Directors and Election Procedures Subtopic); Colorado State University, 1999; http://waterlab. colostate.edu/csuhome.htm. 4. Oregon Revised Statutes; Chapter 545-Irrigation Districts, Section 545.007; State of Oregon Legislative Counsel, Salem, OR 97301 USA.

BIBLIOGRAPHY 1. A good general reference on this subject is: Sagardoy, J.A., Bottrall, A., Uittenbogaard, G.O. Organization,

Irrigation Districts and Similar Organizations

3. State of Montana: http://data.opi.state.mt.us/bills/mca_ toc/85_7.htm 4. Tumalo Irrigation District in Oregon: http://www. tumalo.org/policys.htm 5. Bylaws, World Bank Guidelines: http://www.worldbank. org/wbi/pimelg/bylaw.htm 6. Regarding the development of water user organizations, see International Network on Participatory Irrigation Management; Washington, DC, USA (http://www.inpim.org).

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Operation, and Maintenance of Irrigation Schemes. FAO Irrigation and Drainage Paper No. 40. Food and Agriculture Organization of the United Nations, Rome, 1982. Section 3 of this publication is about irrigation organizations, and is also available at http://www. fao.org/docrep/X5647E/x5647e06.htm. 2. For other examples of irrigation district laws and bylaws, see: State of California, see Division 11: http://ceres.ca. gov/topic/env_law/water

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Irrigation Economics: Global Keith D. Wiebe Noel Gollehon Resource Economics Division, U.S. Department of Agriculture (USDA), Washington, District of Columbia, U.S.A.

INTRODUCTION Irrigation involves complex interactions among ecological, social, and economic processes at a variety of scales, with important implications for agricultural production, income generation, poverty reduction, and environmental quality. No simple measure can fully capture the global economic importance of irrigation. Nevertheless, a review of historic trends in agricultural demand and resource use indicates that irrigation has contributed to dramatic increases in global crop yields and production over the past four decades. Given projected trends in demand for agricultural commodities and in the availability and condition of land and other natural resources, irrigation will continue to play a critical role in the future. Improved management will be necessary, however, to balance public and private economic and environmental objectives. TRENDS IN DEMAND FOR AGRICULTURAL COMMODITIES

Hydrologic– Irrigation

Global demand for agricultural commodities has increased rapidly since the mid-20th century as a result of growth in population, income, and other factors. Based on continued growth in these factors, the Food and Agriculture Organization (FAO)[1] and the International Food Policy Research Institute[2] project that global demand for cereals will increase by 1.2–1.3% per year over the next several decades, while demand for meat will increase slightly faster. Most of the increased demand is projected to come from developing countries, especially in Asia (most of which are already highly dependent on irrigation). Although demand growth rates are slowing and remain within the range of crop production growth rates achieved over the past several decades, demands on natural resources— including water—will increase. TRENDS IN USE OF NATURAL RESOURCES

worldwide has increased by about 0.3% per year since 1961, to 1.5 billion ha in 1998. Growth has slowed markedly in the past decade, to about 0.1% per year, as a result of weak grain prices, deliberate policy reforms (in North America and Europe), and institutional changes (e.g., those in the former Soviet Union). The Food and Agriculture Organization estimates that an additional 2.7 billion ha currently in other uses are suitable for crop production, but this land is unevenly distributed and includes land with relatively low yield potential and/or significant environmental value. Therefore, cropland area is expected to expand only slightly over the next several decades. Genetic Resources About half of all gains in crop yields over the past century are attributable to genetic improvements through scientific plant breeding.[3] By the 1990s, most developing countries’ (and all the developed countries’) cropland in wheat, rice, and maize was planted to scientifically bred varieties. Gains from genetic improvements will continue in future, but likely at slower rates and increasing research costs.[3] Climate The Intergovernmental Panel on Climate Change,[4] representing a broad scientific consensus, projects that the earth’s climate will change significantly during the 21st century because of increasing concentrations of carbon dioxide (CO2) and other ‘‘greenhouse’’ gases in the atmosphere. Given the adjustments that farmers would likely make in response to these climatic changes, aggregate global crop production may not be dramatically affected, but regional impacts may be significant: agricultural production would tend to increase in temperate latitudes and decrease in the tropics due to projected changes in precipitation and temperature (and thus in the spatial and temporal distribution of water).

Land

Water

The Food and Agriculture Organization reports that the total area devoted to annual and permanent crops

Fresh water is abundant globally, but only a small portion—about 10,000 km3 yr 1—is renewable and

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available for human use. Furthermore, this portion is distributed unevenly between countries, within countries, and across seasons and years. Of this portion, about a third is currently withdrawn for human use.[5] Agriculture accounts for about 70% of water withdrawals worldwide, and over 90% of withdrawals in low-income, developing countries.[6]

TRENDS IN IRRIGATION The extent of irrigated cropland worldwide has grown at an average annual rate of 1.8% since 1961 (six times the rate of total cropland expansion), from 139 million ha in 1961 (10% of total cropland) to 274 million ha in 1999 (18% of total cropland).[7] Growth in irrigated area has been especially rapid in India, West Asia, North Africa, Latin America and the Caribbean, and about two-thirds of the world’s irrigated area is in Asia (Table 1). Irrigation expansion has slowed significantly in recent decades, from 2.2% per year during 1967–1982 to 1.5% per year during 1982–1995, due to declining cereal prices, the lower quality of land available for new irrigation, and the increasing economic, social, and environmental costs of large-scale irrigation systems.[2] The Food and Agriculture Organization[1] projects that irrigation expansion will slow further to an average increase of 0.6% per year through 2030. Water withdrawn for agriculture averaged about 1 m in depth over all irrigated area in 1990 in 118 countries studied by Seckler et al.[5] Wood, Sebastian, and Scherr[8] note that global estimates of irrigation efficiency (i.e., the proportion of water withdrawn for irrigation that is actually consumed by crops) average

about 43%, with most of the remainder being returned to the river or to the groundwater aquifer.

TRENDS IN AGRICULTURAL PRODUCTION Growth in publicly-funded surface irrigation and in largely privately-funded tubewell irrigation contributed significantly to the food production increases and real food price declines of the Green Revolution.[9] Food and Agriculture Organization data indicate that cereal yields have increased in developing countries by an average of 2.3% per year since the early 1960s. Some of this increase is due to increased use of irrigation water (along with fertilizer and scientifically bred crop varieties); in developing countries, cereal yields are more than twice as high in irrigated areas (3.8 Mg ha 1) as they are in rainfed areas (1.7 Mg ha 1 ). Irrigated cropland now produces 30–40% of the world’s crop output, including nearly two-thirds of all rice and wheat; at international agricultural prices for 1989– 1991, the irrigated share corresponds to a total value of roughly $400–530 billion per year.[8] Over time, however, subsidized water delivery (whether via public infrastructure or subsidized fuel for private tubewell operation) and inadequate property rights in water have led to excessive and inefficient exploitation of water resources in some countries.[10] Barker and van Koppen[9] argue that these trends will adversely affect food production in key grain-producing areas (including India and China) in the coming decades. Waterlogging and salinization of irrigated land threaten crop yields in some areas, and are likely to become an increasing problem in the absence of appropriate management.

Region

Total irrigated area (million ha)

Area growth rate (% yr 1)

Irrigation depth (m)

Irrigation efficiencya (%)

World

243.0

1.6

1.0

43

Asia

154.4

1.9

1.0

39

China

48.0

1.3

1.0

39

India

45.1

2.7

1.1

40

Other Asia

61.3

1.7

0.9

32

West Asia and North Africa

22.6

2.5

1.2

60

North America

21.6

0.9

0.9

53

Europe

16.7

0.6

0.9

56

Latin America and Caribbean

16.2

2.4

1.2

45

Sub-Saharan Africa

4.8

1.2

1.6

50

South Africa

1.3

0.8

1.2

45

Oceania

2.1

3.6

0.3

66

a

Irrigation efficiency is a complex concept, but is generally defined as the ratio of water actually used by crops (i.e., returned to the atmosphere via transpiration) to the gross amount of water extracted for irrigation use. Source: For further discussion, see Ref.[13]. From Ref.[8].

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Table 1 Irrigation indicators, 1990

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Meanwhile, population growth and the increasing cost of developing new sources of water will place increasing pressure on world water supplies in the coming decades. Even as demand for irrigation water increases, farmers face growing competition for water from urban and industrial users, and from demands to protect in-stream ecological functions by imposing minimum in-stream flows. In light of these conditions, Rosegrant et al.[2] argue that water will likely become a major constraint on increased food production and improved food security in many developing countries, especially in Central and Western Asia and in Africa. Seckler et al.[5] assert that in a growing number of countries, water has become the single most important constraint to increased food production.

OPTIONS FOR INCREASED SUPPLY AND IMPROVED MANAGEMENT OF WATER

Hydrologic– Irrigation

Water storage is a key component of strategies to overcome spatial and temporal variability in precipitation and river flows. National governments, multilateral agencies, and local communities have invested heavily in dams over the past century, but such investments are becoming increasingly expensive in financial, environmental, and political terms.[6] Groundwater has been withdrawn at rates in excess of recharge and degraded through contamination in many areas. Interbasin transfers may be appealing in some areas, but are characterized by the same costs that limit new investment in dams. Water recycling and desalination remain too costly for extensive use. Given limitations on increased supply, a variety of options for improved water management become important. Serageldin[11] notes that because of water’s unique characteristics, governments have generally assumed central responsibility for its management. In seeking to assure access by all, however, governments generally price water as though it were an abundant resource rather than a scarce one, thereby encouraging excessive use in many countries.[12] As the costs of excessive use are recognized, e.g., in terms of groundwater depletion and salinization, increasing attention is being paid to policies that address market and government failures and provide incentives for more efficient water use. Key among these are efforts to price water at levels that better reflect costs, and to establish tradable water rights. Policy and technology also play a role in changing management practices to improve water infiltration and moisture-holding capacity on agricultural lands. Seckler et al.[5] estimate that about half of the projected increase in global demand for water by the year 2025 can be met by reducing losses to evaporation

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Irrigation Economics: Global

and sinks, controlling salinity and pollution, reallocating water from lower-valued to higher-valued crops, and investing in genetic improvements that increase crop yields per unit of water. It is important to note, however, that reducing runoff or deep percolation to groundwater may affect the water supply for other water users or for environmental purposes, resulting in unintended and undesirable impacts.[13]

CONCLUSION In considering strategies to improve irrigation supplies and management, it is essential that the full costs and benefits to all affected parties are recognized. Ultimately, the extent to which crop production keeps pace with future increases in demand at acceptable economic and environmental cost will depend on the institutions, market incentives, policy measures, and investments in research and infrastructure that influence how water and other resources are used.

REFERENCES 1. FAO. Agriculture: Towards 2015/30 (Technical Interim Report); Food and Agriculture Organization of the United Nations: Rome, 2000. 2. Rosegrant, M.W.; Paisner, M.S.; Meijer, S.; Witcover, J. Global Food Projections to 2020: Emerging Trends and Alternative Futures; International Food Policy Research Institute: Washington, DC, 2001. 3. Byerlee, D.; Heisey, P.; Pingali, P. Realizing yield gains for food staples in developing countries in the early twenty-first century: prospects and challenges. In Food Needs of the Developing World in the Early TwentyFirst Century, Pontificiae Academiae Scientiarum Scripta Varia 97, Proceedings of the Study-Week of the Pontifical Academy of Sciences, Jan 27–30, 1999; 2000. 4. Intergovernmental Panel on Climate Change; website; http://www.ipcc.ch/index.html (accessed 28 February, 2002). 5. Seckler, D.; Amarasinghe, U.; Molden, D.; de Silva, R.; Barker, R. World Water Demand and Supply, 1990 to 2025; Scenarios and Issues, International Water Management Institute: Colombo, Sri Lanka, 1998. 6. Meinzen-Dick, R.S.; Rosegrant, M.W. Overview. In Overcoming Water Scarcity and Quality Constraints; Meinzen-Dick, R.S., Rosegrant, M.W., Eds.; 2020 Vision Focus 9; International Food Policy Research Institute: Washington, DC, 2001. 7. FAO. FAOSTAT online database http://apps.fao.org, Food and Agriculture Organization of the United Nations (accessed 15 February, 2002). 8. Wood, S.; Sebastian, K.; Scherr, S.J. Pilot Analysis of Global Ecosystems: Agroecosystems; International

Irrigation Economics: Global

11. Serageldin, I. Toward Sustainable Management of Water Resources; World Bank: Washington, DC, 1995. 12. Coyle, W.; Gilmour, B. Water supply in the APEC region: scarcity or abundance? Agricultural Outlook; USDA Economic Research Service (November): Washington, DC, 2001. 13. Aillery, M.; Gollehon, N. Irrigation water management. Agricultural Resources and Environmental Indicators, 2000; USDA Economic Research Service: Washington, DC, 2000; http://www.ers.usda.gov/Emphases/Harmony/ issues/arei2000/.

Hydrologic– Irrigation

Food Policy Research Institute and World Resources Institute: Washington, DC, 2000. 9. Barker, R.; van Koppen, B. Water Scarcity and Poverty; IWMI Water Brief 3; International Water Management Institute: Colombo, Sri Lanka, 1999. 10. Rosegrant, M.W. Dealing with water scarcity in the 21st century. In The Unfinished Agenda: Perspectives on Overcoming Hunger, Poverty, and Environmental Degradation; Pinstrup-Andersen, P., Pandya-Lorch, R., Eds.; International Food Policy Research Institute: Washington, DC, 2001.

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Irrigation Economics: United States Noel Gollehon Resource Economics Division, U.S. Department of Agriculture (USDA), Washington, District of Columbia, U.S.A.

INTRODUCTION Irrigation is the defining characteristic of crop production in the American West and an increasingly important feature of crop production in the Eastern United States. The irrigated cotton fields of the Southwest, corn farms of the Plains, and citrus groves of Florida, all attest to the magnitude, extent, and importance of irrigation. This article provides an overview of the contribution of irrigation to crop production in the United States. It also focuses on the sales value of crops produced, but provides a context for those values by first considering irrigated area and water use in irrigation. Readers will gain insight into irrigation’s importance both from an economic and a resource use perspective.

IRRIGATED AGRICULTURAL AREA

Hydrologic– Irrigation

The National Agricultural Statistics Service (NASS), USDA conducts the Census of Agriculture on a 5-yr interval.[1] The 1997 Census data are the latest available; with the next Census of Agriculture due to be collected in 2003 for the 2002 calendar year. The long history, consistent methodology, and statistical reliability of the Census data series makes these data especially useful for capturing irrigation trends. According to the 1997 Census of Agriculture,[1] 55.0 million acres of agricultural land were irrigated in the United States. This represents a new census-year high, with an additional 5.6 million acres (over 11%) over levels reported in the 1992 Census of Agriculture. The distribution of irrigated lands (both cropland and pastureland) shows that 78% (43 million acres) were located in the 19 Western states with the remaining 22% (12 million acres) in 31 Eastern states (Table 1). Some cropland is irrigated in all 50 states. In 1997, irrigated land area ranged from about 2500 acres in Vermont, New Hampshire, and Alaska to about 8.7 million acres in California. Irrigated areas have

Views expressed are the author’s and do not necessarily represent those of Economic Research Service or USDA. 572

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historically been concentrated in the West (89% of U.S. irrigated area in 1969) because arid conditions required irrigation to supplement inadequate growing season rainfall. The West still retains the bulk of the irrigated land, but irrigated area is expanding in the more humid East. Since 1969, irrigated land in the East has increased by almost the same number of acres as in the West, with a much faster rate of growth (187–23%). More recently (1987–1997), irrigated land in the West increased by about 5.3 million acres (14%) compared with 3.3 million acres (38%) in the East. Of the 55 million irrigated acres reported in the 1997 Census, there were 50 million acres of ‘‘Cropland Harvested,’’ and 5 million acres of ‘‘Pastureland and other land.’’ Nationally, irrigated cropland represented about 16% of all harvested cropland. In the West, irrigated cropland harvested comprises a greater share of total cropland acres (about 27%), representing about 76% of the nation’s total harvested irrigated cropland. While irrigation in the East accounts for the remaining 24% of the nation’s total, only a small share (7%) of the harvested cropland in the East was irrigated (Table 1).

WATER USED FOR IRRIGATION The U.S. Geological Survey, U.S. Department of the Interior, estimates both water withdrawals and consumptive use every 5 yr.[2] Estimates are made at a local level based on locally available information, including theoretical estimates of crop water use, crop area, delivery records of off-farm water suppliers, and details on conveyance losses, water application rates, and return flows. Three measures can be used to characterize water use for agricultural irrigation: withdrawals, applications, and consumptive use. Withdrawals represent total water diverted from surface water sources and extracted from groundwater aquifers.[2] Applications measure that portion of the water withdrawn that is delivered to the field, excluding off-field conveyance system losses and gains.[3] Water applications represent the portion of withdrawals that are directly under producers’ control and are thus impacted by on-farm irrigation management and technology choice decisions.

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Table 1 Irrigated area in the United States, by region, 1997 Region

Harvested cropland irrigated (million acres)

Pastureland irrigated (million acres)

Total irrigated area (million acres)

50.0 38.2 11.8

5.0 4.8 0.2

55.0 43.0 12.0

United States Western statesa Eastern states a

Consumptive use refers to that portion of water withdrawn and applied that is actually consumed for plant needs.[2,4] Consumptive use is usually estimated based on plant water requirement models, and does not include excess water lost to percolation, runoff, or evaporation, other than that required for plant growth. Measures of irrigation water use may be used to describe the impact of irrigation on hydrologic conditions. Withdrawals are the best indication of the water quantity impacts of irrigation water diversions. While withdrawn water that is not consumptively used may be available for future use, the location, quality, and timing of availability are often affected. Consumptive use is an indicator of the water quantity lost to the immediate hydrologic cycle. Irrigation withdrawals, as well as withdrawals for other out-of-stream uses, may be quantified and compared. None of these measures consider in-stream water uses, such as hydroelectric power generation, navigation, recreation flows, or flows to maintain ecosystems. In-stream uses may be more significant than off-stream uses in many locations, but specific quantities are difficult to measure. Irrigated agriculture withdraws and consumes the most freshwater of any economic sector in the United States Irrigation accounts for withdrawals of 150 million acre feet (maf ) nationally, almost 40% of total freshwater withdrawals (Table 2). When measured by consumptive use, irrigation uses about 91 maf of water, or more than 80% of the total consumptive use. Comparing across sectors, irrigation consumes about 60% of irrigation water withdrawn—a much greater share relative to the average consumption rate of 9% in other sectors.[2]

The water use picture varies substantially between the 19-state western and 31-state eastern regions. In the West, irrigated agriculture accounts for 133 maf or 75% of total freshwater withdrawals in the region. The average water quantity withdrawn per acre of irrigated crop and pastureland is 2.7 acre ft. When measured by consumptive use, irrigation uses about 79 maf of water, or almost 90% of total water consumed in the West. Roughly two-thirds of the irrigation water in the West is supplied from surface water sources with groundwater accounting for the remaining supply. California has more than double the irrigation withdrawals of any other State (32 maf), while Idaho, Colorado, and Texas—all have withdrawals greater than 10 maf. Of these four states, only Texas withdraws more groundwater than surface water.[2] By comparison, irrigation withdrawals in the 31 Eastern states account for 17 maf, or about 8% of total regional withdrawals. Withdrawal rates were substantially lower than in the West, at 1.3 acre ft per acre of irrigated crop and pastureland reflecting the greater natural precipitation in agricultural areas. Irrigation consumptive use (12 maf) accounts for 52% of total water consumed in the East. (Thermoelectric power generation, which withdraws a large quantity of water but consumes little, accounts for most Eastern withdrawals.) Groundwater is the primary source of irrigation water in the East. Arkansas withdraws the largest quantity of irrigation water (6.6 maf) among eastern states, primarily from groundwater sources. Florida (almost 4 maf) and Mississippi (almost 2 maf) are also major irrigation withdrawal states in the region.[2]

Table 2 Irrigation and water withdrawals and consumption in the United States, 1995 Region

Sector

Water withdrawals (maf)

Consumptive water use (maf)

United States

All Irrigation

382 150

112 91

Western statesa

All Irrigation

179 133

88 79

Eastern states

All Irrigation

203 17

24 12

a

Western states include HI, AK, WA, OR, CA, ID, NV, MT, WY, UT, CO, AZ, NM, ND, SD, NE, KS, OK, and TX. Source: Ref.[2].

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Hydrologic– Irrigation

Western states includes HI, AK, WA, OR, CA, ID, NV, MT, WY, UT, CO, AZ, NM, ND, SD, NE, KS, OK, and TX. Source: Ref.[1].

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Several factors will influence the extent of future increases or continuation of current withdrawal levels of water for irrigation use. Increasing demands from other sectors for both out-of-stream and in-stream use have recently limited irrigation water withdrawals in some areas, particularly under drought conditions. Increasing capital costs for new projects and recognition of environmental impacts have combined to limit large-scale new water developments to augment irrigation water supplies. It is unlikely that these trends will reverse, at least in the near future, implying that expansions in irrigated area will likely occur primarily through more efficient use of the water already dedicated to agricultural production.

VALUE OF IRRIGATION Crop sales reports from the Census of Agriculture provide the basis for current estimates of irrigated crop values. Individual Census of Agriculture farms were classified into one of three irrigation groups for each commodity group: only irrigated, only non-irrigated, and combined irrigated and non-irrigated. Irrigated commodity sales was calculated as the sum of the only irrigated farms plus an apportioned share of the

Irrigation Economics: United States

sales on combined farms in the 1997 Census of Agriculture.[1] Crop sales, which measure the value of commodities leaving the farm gate, also serve as an indication of economic activity associated with farming and related income flows through rural areas. Preferred measures of irrigation’s contribution to crop production would be profitability of irrigated agriculture or the direct value of total crop production, but those estimates are not available. The Census of Agriculture reports the amount of crop sales at the farm market gate, but does not capture the value of crops that are produced and consumed on-the-farm without entering a market channel, most prevalent with irrigated forage and feed crops used on the farm. Although this value adjustment is not known for 1997, the underestimation was about 15% of sales value in 1987.[5] Based on calculations from the 1997 Census of Agriculture information, there were 309 million acre of harvested cropland that produced crop sales of $98 billion. Irrigated crops occupied 16% of that area, but accounted for 49% of the total value of sales from U.S. farms and ranches (Fig. 1, top row). Average sales per harvested acre were $950 for irrigated cropland compared with $200 for non-irrigated cropland. Irrigated crop sales were highest for orchards, vegetables,

Hydrologic– Irrigation Fig. 1 Irrigated crop area and sales as a share of total, 1997. Source: From Ref.[1].

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Irrigation Economics: United States

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CONCLUSION This article examined three measures of irrigation: irrigated agricultural area, water used in irrigation, and the sales value of crops produced. By all three measures, irrigation is an important contributor to the value of crop agriculture. In 1997, irrigated lands produced 49% of crop sales on only 16% of the harvested crop area, providing an important input to most of the nation’s higher-valued crop production. Irrigation is also an important component in the nation’s hydrologic picture by virtue of the spatial extent and volume of water involved. In 1995, irrigation accounted for 40% of the nation’s water freshwater withdrawals from lakes, rivers, and aquifers. Irrigation accounted for over 80% of the total consumptive use by all sectors of the economy. In future increased competition for water will affect irrigated agriculture’s ability to withdraw and consume water at current levels.

REFERENCES 1. National Agricultural Statistics Service. 1997 Census of Agriculture, AC97-A-51; U.S. Department of Agriculture, 1999. 2. Solley, W.B.; Pierce, R.R.; Perlman, H.A. Estimated Use of Water in the United States in 1995, U.S. Geological Survey Circular 1200; U.S. Geological Survey, U.S. Department of the Interior, 1998. 3. National Agricultural Statistics Service. 1998 Farm and Ranch Irrigation Survey, AC97-SP-1 Vol. 3, Special Studies Part 1; U.S. Department of Agriculture, 1999. 4. Aillery, M.; Gollehon, N. Irrigation water management. In Agricultural Resources and Environmental Indicators, 1996–97, Agricultural Handbook No. 712; Economic Research Service, U.S. Department of Agriculture, 1997; 225–240. 5. Bureau of the Census, 1987, Census of Agriculture, AC87-A-51; U.S. Department of Commerce, 1989. 6. Gollehon, N. Water markets: implications for rural areas of the west. In Rural Development Perspectives; Economic Research Service, U.S. Department of Agriculture, 1999; Vol. 14 (2, August), 57–63.

Hydrologic– Irrigation

and nursery crops while irrigated cropland area was dominated by grain and forage crops, primarily corn for grain and alfalfa hay. In the West, the 1997 Census reported 142 million acre of harvested cropland that produced total crop sales of $45 billion. Irrigated crops in the West accounted for 27% of the area, but produced 72% of the total value of sales in the region (Fig. 1, middle row). The sales of Western irrigated crops totaled about $32 billion in 1997, or roughly one-third of all U.S. crop sales. Average sales per harvested acre in the West were $850 for irrigated and $122 for nonirrigated cropland. As was the case when examining the national values, irrigated crop sales were led by orchards, vegetables, and nursery crops while irrigated cropland area was dominated by grain and forage crops. In the East, the 1997 Census reported 167 million acres of harvested cropland with total crop sales of almost $53 billion. Irrigated crops in the east occupied only 7% of the harvested cropland area, but produced $15 billion or 29% of the region’s total value of sales (Fig. 1, bottom row). Average sales per harvested irrigated acre were greater than the national average at over $1200, while non-irrigated cropland averaged sales of $200 per acre. The greatest contribution to sales totals were made by irrigated nursery crops, orchard crops, and vegetables. Rice, soybeans, and corn for grain dominated irrigated cropland area in the region. The wide differences in crop sales values, coupled with the fact that most of the crop sales comes from high-valued crops, provides significant flexibility for irrigated agriculture to adjust to changes in water availability. Farmers can adjust to physical water shortages by improving irrigation technology and/or adjusting cropping choices to maintain production of the highervalued crops. This ability to substitute crops is an important response to water shortfalls. In addition, innovative water markets have increased the ability of farmers and water suppliers to transfer water, enabling maintenance of higher-valued crops during droughts.[6]

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Irrigation Management: Humid Regions Carl R. Camp Jr. Coastal Plains Research Center, Agricultural Research Service (USDA-ARS), U.S. Department of Agriculture, Florence, South Carolina, U.S.A.

Edward John Sadler Coastal Plains Soil, Water, and Plant Research Center, Agricultural Research Service (USDA-ARS), U.S. Department of Agriculture, Florence, South Carolina, U.S.A.

James E. Hook NESPAL, University of Georgia, Tifton, Georgia, U.S.A.

INTRODUCTION

Hydrologic– Irrigation

Irrigation management includes deciding how much irrigation water to apply, and when to start and stop the irrigation. For any management decision, the choice of operation depends on what one wants to do. The simple answer may appear to be ‘‘put on some water,’’ but the choice is often more complex. For instance, one can attempt to maximize net return, minimize operating costs (especially labor), maximize yield, optimize limited water supply, minimize environmental risk, or optimize production under a limited irrigation system capacity. All of these may be constrained by regulations. In general, water supply and irrigation costs control the economics, so the best result is obtained by maximizing yield on all irrigated land, usually called the land-limiting case. In simple terms, one irrigates to avoid crop water stress. Important irrigation system parameters for consideration include the irrigation application rate per unit area, the total system supply rate, and for moving systems, the velocity. At a given pumping rate, moving machines cover their entire irrigated area in a given return time. If more application depth is required, the system can be set for a slower velocity, which trades increasing depth for longer operating times. For solid-set systems, including sprinklers and drip, increasing application depth is achieved by operating the system longer. All these considerations apply for both arid and humid areas, and are covered briefly here so that the reader may interpret the article without referring elsewhere. For the following, the discussion concentrates on the particular case of humid areas, contrasting with the conventional, more-arid case.

CONTEXT OF HUMID AREAS Humid area climate differs from arid area climate in several ways. First is the defining characteristic, 576

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humidity. In humid areas, the dew point often equals the early morning air temperature, unlike most arid areas, and the difference in vapor pressure deficit affects crop temperature. Along with higher humidity comes, generally, more clouds. These reduce the total daily solar radiation, which reduces the evaporative demand. Finally, humid areas generally receive more rain than arid areas. The possibility of rain occurring just after an irrigation can complicate an irrigation manager’s decision in two important ways beyond applying unnecessary water. One is the risk of waterlogging a crop and causing damage by lack of aeration. The other is risk of leaching nutrients or other chemicals. On the other hand, during periods without rain, the weather in humid areas can be similar to that in arid areas. Rain-free days are generally less cloudy, which then means more nearly clear-sky solar radiation and higher evaporative demand. Also, air temperature may be higher and humidity lower than averages, which include the cooler wet days. Similarity between humid and arid regions during periods of drought, which can occur in as little as two rainless weeks, creates additional challenges for irrigation managers in the humid region. Strategies optimized assuming the next rainfall is imminent can fail if the next rain occurs four or six weeks later. Another consequence of higher rainfall amounts in humid areas is a radically different water supply system than in arid regions. There are few large water projects with objectives to provide irrigation water, and no extensive water districts to manage the allocation. Therefore, most water supplies must be farmerdeveloped. Historically, these were farm ponds retaining runoff or streams from either of which farmers pumped directly. Where groundwater was available, wells were added to provide backup to ponds, and where extensive aquifers existed, irrigation expanded using high-capacity wells. Since they were developed individually, farmer water supplies were not regulated, or if so, minimal information was required for permits.

Irrigation Management: Humid Regions

IRRIGATION SCHEDULING EMPLOYED IN HUMID AREAS As mentioned earlier, in the usual (land-limiting) case, one achieves the optimum economic return by maximizing yield on all irrigated land, which is achieved by irrigating to avoid crop stress. This can be done in several ways, all of which have been used in humid areas of the eastern United States. One can sense the water status of the crop, measure soil moisture, or compute the soil water balance. There are many variations on these three approaches. Even the fixed timeclock control of lawn and other turf irrigation systems (if adjusted properly) is an attempt to maintain soil moisture in a range suitable for plant health. Plant Stress Methods The most well-known indicators of plant water stress are visual: rolled or drooping leaves and color change. However, by the time these conditions occur in crops, yield has already been reduced. Therefore, scientists looked for earlier indicators of water stress. One early stress measurement is the infrared thermometer, which is a non-contact device and is sensitive to longwave (10 mm) radiation, that measures the average temperature in the field of view. For theoretical reasons, the difference between the canopy and the air temperature is the important measure, but the vapor pressure deficit is an important factor in the interpretation of the temperature difference. In humid areas, the canopy temperature may be somewhat higher relative to the air temperature than in arid areas. Both air temperature and vapor pressure deficit are taken into account with the crop water stress index, or CWSI, but research in the humid southeastern United States indicates that additional work is needed before this method can be widely applied. Other plant stress monitors have been proposed and are included here for reference. Near-infrared (NIR, 1 mm) photography and remote sensing have been used in research environments, but other stresses, such as disease or poor nutrition, can also cause NIR responses. Some research has monitored leaf water potential using the pressure chamber or the leaf press. Sap flow devices have been used to measure water movement into plants, particularly for perennial

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plants. Since water flow is in response to plant water needs and soil water supply, the device can detect periods when these are limited. A recent report from Israel suggested that minute changes in leaf thickness could be sensed as an indication of plant water status. All of these devices have been useful in research for assessing plant water stress, but expense and/or complexity have limited their application in production. Successful use of any plant water status measure depends on being able to identify some trigger point at which to initiate irrigation. For the infraredthermometer-based CWSI method, some have determined responses to initiation at one value or another, say 0.5 in the range 0 (no stress) to 1.0 (complete stress). However, as mentioned earlier, using any absolute value of the CWSI in humid areas requires additional research or possibly local calibration. In addition, scheduling irrigation based on observing stress is subject to an inherent limitation in that it cannot successfully predict what time in the future one needs to irrigate. In practice, experience can overcome this limitation. These local calibrations may be avoided using an innovative approach to initiating irrigation using an indicator of the variation in soil water that exists across a field. In this approach, an infrared thermometer is read as it is moved across a field, as from the window of a moving vehicle. Irrigation is triggered when the variation in temperature in the series of measurements exceeds a certain amount. Basically, this approach uses the driest area of the field as an indicator; when it gets dry, the rest of the field would not be far behind, so irrigate soon. Because the air temperature, vapor pressure, and other factors are all reasonably constant during the scan, this method can use the actual crop temperature.

Soil Moisture Methods Research has shown that plants can extract water from soil when it is held somewhere between the field capacity, which happens after free drainage following rain and is between 0.01 MPa and 0.03 MPa soil water potential, and the permanent wilting point, usually assumed to be at 1.5 MPa soil water potential. These concepts have been debated, but use has shown them to be useful approximations. Most of the water contained in the soil is held between the field capacity and 0.1 MPa. For this reason, tensiometers, which can measure water between 0 MPa and 0.08 MPa, can monitor soil water over a range important to irrigation. Researchers in the southeastern United States have employed tensiometers, with irrigation being triggered when the potential at 0.3-m depth gets drier from 0.02 MPa to 0.05 MPa. Important considerations

Hydrologic– Irrigation

As a direct result of this history of irrigation development, little knowledge exists regarding the water withdrawals, irrigation capabilities, or area irrigated in most humid areas. A useful case study of the difficulties caused by this lack of information can be found in southwestern Georgia.[1]

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include the depth, the position of the sensor relative to the plant row and roots, and also the crop species. Tensiometers must be serviced periodically to remove air and ensure that they have not gotten out of range, which causes unpredictable readings. They must also be monitored frequently in sandy soils because the water removed from the soil in a day might cause the readings to go out of range. Electrical resistance devices have been embedded in the root zone to measure soil water potential indirectly, using the known relationship between water content and electrical resistance of gypsum. Advantages of these devices are low maintenance and adaptability for reading with simple meters or data loggers. With experience, managers can use these simple devices to indicate that the soil is becoming too dry for continued plant growth. The other measure of soil moisture is water content. As mentioned earlier, the water content and potential are related through the water-holding capacity function, which may differ for each soil and soil layer. If this relationship is known, soil water content (SWC) sensors can be used to sense soil moisture for irrigation purposes, by determining the water content corresponding to the field capacity and wilting point. Water content can be expressed per unit volume or per unit weight. The only practical method that produces a value per unit weight is the gravimetric technique, in which a sample is weighed, dried, and weighed again. If this technique is used, the mass of soil per unit volume in the original state, or bulk density, must be known to convert to a volume basis. Knowing the water content on a volume basis adds to the irrigation manager’s tools because the difference between the water content and the wilting point is an indication of how much water remains, and the difference between field capacity and the water content is how much can be applied at the time of the measurement. Clearly, soil moisture measurements must represent the water content of the effective root zone for this technique to be useful, and the root zone thickness changes with type and usually increases with age of the crop.

Water Balance Techniques The checkbook-type water balance method has been known for nearly 50 yr. This direct analog to a bank checkbook uses rain and irrigation as credits and evapotranspiration (ET) as a debit to maintain a water content between the field capacity and wilting point for the root zone. One can adjust the rainfall for runoff and drainage below the root zone. Availability of ET data has been the main problem using this technique in the southeastern United States. Evaporation pans have

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Irrigation Management: Humid Regions

been used, with research testing whether screened or open pans are most reliable. A physical model of the checkbook method has been implemented using an evaporation pan directly. For this, a calibrated scale is placed on the pan (usually a screened pan) with indications for a full and an empty rooting zone. An inexpensive, recent implementation uses a large washtub specially fitted with a float attached to a flag visible from some distance. When the water level has dropped to a point equivalent to the soil water refill amount, the flag passes a preset mark and irrigation is indicated. An overflow hole is set at a level representing full SWC. Since the device is placed in the field of sprinkler irrigated crops, it receives both irrigation and rainfall, filling to the overflow mark with excessive rain or simply adding to the pan as rain and irrigation partially refill the soil and pan. A computer model of the water balance is simply an automated version of the manual and physical methods mentioned earlier. Usually, ET is calculated from weather station data for temperature and solar radiation, but if ET data were available in published reports, it could be entered. Some computer-based methods use the best available information, starting with measurements, then calculations from weather station data, then calculations from forecast weather, and for predictions beyond the forecast period, historical data. Fig. 1 illustrates the water balance technique. Note the increased water-holding capacity of the profile as the root zone expanded during the season. Up to 85 days after planting, the irrigation in this case was scheduled to successfully control the SWC above the CL line. After that time, the SWC fell below the control limit, and the computer program flagged the line with triangles to indicate the need for irrigation.

Common Considerations In all of the previous soil-based methods, some decision must be made about the allowable range of soil moisture. Seldom does an irrigator want to allow the soil moisture to drop very close to the wilting point; most trigger points are approximately 30%– 50% depletion of available water-holding capacity, for the sake of insurance. Conversely, if irrigation fills the soil rooting zone to field capacity, and rain falls soon after, then the root zone can be subject to aeration problems, and this free water (rain) is lost through runoff or drainage. This choice of management-allowed depletion depends on the probability of rainfall and the likely amount that could be tolerated or used. In this regard, humid-area management is different than arid-area management. The range over which the manager can control soil moisture may need to be restricted to allow for the higher chance of rain.[2]

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Fig. 1 Water balance technique illustrated. Source was computer-based method in Camp and Campbell, 1988. The lines labeled UL and LL are the upper and lower limits of available water within the root zone. The line CL was the irrigation control point, here at 50% managementallowed depletion. The line SWC is the computed SWC, with triangles flagging the need to irrigate. Solid bars indicate irrigation; open bars indicate rain.

Comparisons A multi-state study of irrigation scheduling methods concluded that, if properly employed, tensiometers, evaporation pans, and computerized water balance methods, all could be used in the southeastern United States. Tensiometers had one advantage in that they were fairly universal, requiring little calibration. On the other hand, they required significant labor for reading and maintenance. Evaporation pans were also labor-intensive. Computerized water balance models were data-intensive and occasionally needed adjustment of the SWC to eliminate accumulated errors, but were much more amenable to forecasting future irrigation needs.

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TECHNICAL ABSTRACT During the long history of irrigation, management of irrigation systems, which includes deciding how much irrigation to apply and when to do it, has been the subject of much study. Most of this work has been done in primarily arid areas, where the development of irrigation started earlier. However, current trends include increasing irrigated areas in humid regions, for which the contrasting climatic conditions require correspondingly different management techniques. In addition to having higher humidity, humid areas are generally more cloudy (lower solar radiation and thus lower evaporative demand), receive more rainfall, and tend to be cooler on average. However, during even short droughts, the conditions may be quite similar to those in arid regions. Dynamic weather complicates the management of irrigation systems in humid regions, forcing managers to trade off the possibility of rain against the need to leave storage space for potential rain by controlling a relatively narrow range of managementallowed depletion. Doing so can be achieved more easily using frequent, light irrigations instead of lessfrequent, heavy ones commonly used in arid regions. Case studies of irrigation management in the southeastern United States serve to illustrate the common management methods, which include tensiometers, evaporation pans, and computer-based water balances. Continuing trends of increasing irrigated area and increasing interest in precision agriculture may combine to focus on spatially variable irrigation management in humid regions. Interpretive Summary Irrigation management includes deciding when to apply irrigation, and also how much to apply. Making

Hydrologic– Irrigation

These requirements, a narrow range for deficit irrigation and inherent limits of the soil water storage, support using frequent, small irrigations rather than less-frequent, large ones. This can lead to a higher fraction of water evaporated directly from the soil surface, which is wet more frequently in the case of sprinkler irrigation, and to increased disease incidence when susceptible crops are frequently wetted. The combination of these considerations brings more interest in buried drip irrigation, which can, if so designed, irrigate frequently, yet keep the soil surface mostly dry. All measurements represent the area where the measurements are made, but spatial variation will cause any measurement to be unrepresentative of the entire area in most fields. Assuming the field is irrigated the same throughout, how many measurements are needed to represent the entire field? There is no simple answer to this question, nor is there one for the trade offs when distinctly dissimilar soils exist all in one irrigation management unit.

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these choices in humid regions is somewhat more complicated than in arid ones, primarily because of the possibility of receiving rain shortly after an irrigation. Besides being wasteful, this possibility also carries a risk of drainage and runoff carrying nutrients to groundwater or streams. Managing irrigation to save some room in the soil for possible rain requires a careful balance between crop needs and soil capacity, which can be limited by sandy soils or shallow rooting depths. Management methods leave storage space for potential rain by controlling a relatively narrow range of management-allowed depletion. Doing so in humid regions can be achieved more easily using frequent, light irrigations instead of less-frequent, heavy ones commonly used in arid regions. Case studies of irrigation management in the southeastern United States showed that common methods, which include tensiometers, evaporation pans, and computer-based water balances, can all work. Increases in irrigated area and interest in precision agriculture may combine to focus on spatially variable irrigation management in humid regions.

CONCLUSION At the current time, two trends in irrigation are apparent. While they may also exist elsewhere, they are somewhat recent in the southeastern United States.

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Irrigation Management: Humid Regions

The first is the simultaneous increase in irrigated area and increased competition with nonfarm users for water resources. This leads to both a less-than-optimal water supply and higher valuation of the water resource. Therefore, additional questions arise. If one cannot irrigate all the land, where should the water be used to greatest advantage? Should it be used only on high value crops? Should it be applied suboptimally to all the land? Or should the second trend, interest in precision (site-specific) agriculture, be extended to irrigation, so that each individual soil in a field could be irrigated optimally, or less-productive soils be left rainfed while productive soils are irrigated optimally? These questions are currently of increasing interest to researchers. Should the southeastern drought of 1998–2002 continue, they will likely be of increasing interest as well to producers.

REFERENCES 1. Hook, J.E. Water Crisis in the Humid Southeast: Implications for Farm Irrigation; Univ. Georgia NESPAL: Tifton, GA, 1999. 2. Camp, C.R.; Sadler, E.J.; Sneed, R.E.; Hook, J.E.; Ligetvari, F. Irrigation for humid areas. In Management of Farm Irrigation Systems; Hoffman, G.J., Howell, T.A., Solomon, K.H., Eds.; Am. Soc. Agric. Eng.: St. Joseph, MI, 1990; 549–578, Chap. 15.

Irrigation Management: Tropics R. Sakthivadivel Hilmy Sally

INTRODUCTION

CLIMATE AND CROPS

The tropics refer to ‘‘that part of the world located between 23.5 degrees north and south of the equator.’’ The tropics make up 38% of the earth’s land surface (approximately 5 billion ha) and 45% of the world’s population, estimated at 6.1 billion in 2000,[1] live there. About 75 countries, most of them ‘‘developing,’’ lie wholly or mostly in the tropics. The data and statistics that will be presented in this article are based on entire countries. Hence, even though some parts of southern China are within the latitudinal definition of the tropics, China has not been included. On the other hand, India, Bangladesh, Mexico, and Brazil have been taken into account although parts of these countries lie outside the boundaries. With this provision, the major irrigated countries such as China, Iran, Pakistan, the Russian Federation, Turkey, and the United States of America fall outside the tropics. This article will first present the climate and other key features of the tropics, with special emphasis on water, irrigation, and food production. Trends in irrigated agriculture and changes in land–water–people balances are then shown. Finally, strategies and conditions to promote effective and sustainable management of irrigation and water resources are discussed. The need to adopt a holistic approach, integrating the technical, social, and institutional aspects of irrigation and water resources management, is highlighted.

Tropical temperatures remain fairly constant throughout the year with a mean monthly temperature variation of 5 C or less between the average of the three warmest and the three coldest months.[2] Annual rainfall varies from 0 mm to 10,000 mm, decreasing with increasing latitude, with a high year-to-year and monthly variability. Indeed, the climate of the region is largely determined by the distribution of rainfall rather than the total amount. Three main zones can be delimited on this basis: 1) permanent humid zone, experiencing 9.5–12 mo of rain, located very close to the equator, and occupying roughly one-fourth of the tropics; 2) seasonally humid zone (4.5–9.5 mo of rain), covering one-half of the tropics and where most crops (including rice in the monsoon region of Asia) are grown; 3) semiarid zone, receiving 2–4.5 mo of rainfall and providing good growing conditions for crops such as maize and cotton. Although agriculture is the main economic activity in the tropical regions, the proportion of cultivated lands (about 10%) is virtually the same as in the temperate region. But tropical soils tend to be more fragile and failure to replace soil nutrients can ultimately undermine their productive capacity. Rice, cassava, corn, wheat, sorghum, and millet are among the main crops grown in the tropics. Ninety percent of rice production comes from tropical Asia.

IRRIGATION SYSTEMS AND RESOURCE ENDOWMENTS 

The main countries include: Angola, Bangladesh, Benin, Bolivia, Brazil, Burkina Faso, Burundi, Cambodia, Cameroon, Central African Republic, Chad, Colombia, Congo (Democratic Republic), Congo (Republic of), Costa Rica, Co ^te d’Ivoire, Cuba, Djibouti, Dominican Republic, Ecuador, El Salvador, Eritrea, Ethiopia, Gabon, Gambia, Ghana, Guatemala, Guinea, Guinea Bissau, Haiti, Honduras, India, Indonesia, Kenya, Korea (North), Korea (South), Liberia, Madagascar, Malawi, Malaysia, Mali, Mauritania, Mauritius, Mexico, Mozambique, Myanmar, Namibia, Nicaragua, Niger, Nigeria, Oman, Panama, Papua New Guinea, Paraguay, Peru, Philippines, Re´union, Rwanda, Sao Tome & Principe, Senegal, Sierra Leone, Somalia, Sri Lanka, Sudan, Swaziland, Tanzania, Thailand, Togo, Uganda, Uruguay, Venezuela, Vietnam, Yemen, Zambia, Zimbabwe.

Irrigation systems in the tropics are subject to both shortage and surplus of water at different times, posing special problems for managing irrigation water efficiently and productively. Differences in rainfall regimes can have a significant impact on irrigation management and irrigation performance. For example, in a situation where rainfed cropping is possible, farmers may cultivate extents of land beyond what the irrigation system had been designed to support. This would complicate management in the event of a drought because all the crops could need water irrespective of whether they were authorized for irrigation 581

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International Water Management Institute (IWMI), Colombo, Sri Lanka

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or not; while in more favorable rainfall conditions, little or no irrigation water may be needed even for authorized crops. In contrast, farmers in arid areas would plan their activities in expectation of irrigation rather than depending on rainfall. As irrigation is more predictable and the demands relatively stable, management is simplified. Producers in such low-rainfall areas know what to expect by way of water supply in the rainy season and would accordingly limit the extent of high-value, water sensitive crops with the balance area under low-value crops or kept fallow. If there is ample rainfall, it will benefit all the planted crops. But if the rains fail, low-value crops would probably be lost, whereas high-input crops would survive due to irrigation. Access to alternate sources of water such as a well could allow high value crops over the whole farm. Given the costs associated with pumping, producers would also tend to make effective use of the surface water supply (subject to any legal constraints) and rainfall. Indeed, how well irrigation systems make use of rainfall will have decisive implications for water management and for efficiency of water use. Land and labor endowments can also affect irrigation development and management, and the relative prosperity of a region. The eastern and western Gangetic plains in India, lying in two different climatic zones, provide a good example. Population density in the eastern Gangetic plain has always been higher than in the west, with most suitable land farmed. Higher population pressure has resulted in extension of cultivation to less favorable areas with an attendant decline in farm size. But the persistence of yield-limiting constraints means that even in a good year there is little surplus. In contrast, in the western

Irrigation Management: Tropics

Gangetic plain, irrigated agriculture has greatly expanded, cropping patterns changed, and higher yields are obtained. This illustrates the fact that favorable shifts in the relative endowments of land and labor in association with technologies that improve the reliability and predictability of agricultural conditions can greatly enhance regional and national productivity.

TRENDS IN IRRIGATED AGRICULTURE Population and its growth are crucial factors that drive water development strategies whether for food production, or for domestic or industrial purposes. In this section we will examine the trends in irrigation, food production and food consumption, and assess the impacts on water availability and water use in the tropics. Fig. 1 shows the past and projected trends in population growth and per capita internally renewable water resources (IRWR) in the tropics for the 60-year period 1965–2025. Between 1965 and 1995 the population has nearly doubled and the per capita IRWR halved. If population growth in the tropics follows the United Nations (UN) medium projection path, this will result in a further 20% decrease in IRWR. The per capita IRWR available in 2025 is projected to be 5000 m3 (kiloliters), which is still considered enough to meet the water needs of each person in the tropics. However, this aggregate figure masks substantial spatial and temporal variations between countries and regions on one hand, and between different times in the year on the other. Table 1 shows that in 1995, the average per capita calorie supply in the tropical countries was 2456 kcal.

Hydrologic– Irrigation Fig. 1 Trends in population and internally renewable water resources in the tropics. Source: Adapted from Ref.[3].

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Assuming that a calorie supply of 2700–3000 kcal per person per day is needed to meet most of the nutritional requirements of people in developing countries, this indicates that a substantial number of poor people in this region suffer from nutritional deficiencies. Table 2 shows the production and productivity statistics for the same 30-year period between 1965 and 1995. In 1995, the total cereal consumption was 529 million metric tons (Table 1, against a total cereal production of only 470 million metric tons (Table 2, indicating a production deficit of 59 million metric tons. According to IWMI,[3] this shortfall would increase to 98 million metric tons to meet a targeted calorific requirement of 2747 kcal in 2025, even though this is still lower than the global average. The above analysis brings to light the urgent need to increase cereal production in the tropics in the next few decades to meet the food and nutritional requirement of its people. Table 1 also shows that although the total amount of cereal consumed increased by 147% during the 30-year period 1965–1995 (at an average annual growth rate of 3.1%), there has not been a commensurate increase in per capita calorie supply, which has recorded an overall increase of only 19% in the same period. From Table 2 it can be observed that cereal production in the tropics has increased by 137% in the 30 yr since 1965 (at nearly 2.9% per year), largely as a result of the near doubling of both irrigated area and yield. While the cereal irrigated area in the tropics is only 30% of the cultivated area, it provides nearly 50% of the cereal production. In terms of the water resources situation, the total water diverted in the tropics is 1057 km3, of which the total primary water diverted is only 685 km3. The rest is return flow. This 685 km3 represents only 8% of potentially utilizable water resources (PUWR) of the tropics. But 89% of this water is used for irrigation. Hence, improving the productivity performance of existing irrigation systems must be given high priority

in efforts to overcome the food and nutritional deficits confronting the tropics. Possible irrigation management strategies to help achieve this goal are discussed in the following section.

STRATEGIES FOR IMPROVING AND SUSTAINING IRRIGATED AGRICULTURE While irrigated agriculture has made significant contributions to the food security of growing populations, concerns have also been expressed about its performance and some of its less desirable consequences: the poor returns to irrigation investments, the environmental impacts (such as waterlogging and salinization) brought about by poor design and operation of irrigation schemes, and the lack of attention paid to the needs of the poorer sections of society. So, the challenge is to find ways of improving and sustaining food security and livelihoods that make optimum use of available resources and do not degrade the productive capacity of land and water. While increasing cropped area, either by expansion of irrigation facilities or increasing crop intensity remains a possibility, there must also be emphasis on improving land and water productivity through the adoption of appropriate agricultural and water management practices. Measures will include: developing high yielding crop varieties, promoting innovative low-cost techniques for water harvesting and water application, conjunctive use of surface and groundwater, and implementation of institutional and policy reforms to enable integrated water resources management at the basin level, recognizing the multiple uses and users of water. The Basin Perspective and Water Savings

Table 1 Total population, calorie supply, and cereal consumption in the tropics Population

Per capita calorie supply

Cereal consumption

Year

Population (million)

Annual growth (%)

Total (kcal)

Annual growth (%)

Total (M Mt)

Annual growth (%)

1965

1302

—

2061

—

214

—

1975

1665

2.5

2114

0.3

289

3.1

1985

2099

2.3

2277

0.7

399

3.3

1995

2574

2.1

2456

0.8

529

2.9

Growth

97.6%

2.3%

0.6%

147%

3.1%

1965–1995 Source: Adapted from Ref.[3].

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19%

Hydrologic– Irrigation

It is generally recognized that the river basin is the appropriate unit of analysis to assess water availability,

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Table 2 Cereal production, cereal harvested area, cereal yield, and net irrigated area Cereal Production Year

Total (M Mt)

Annual growth (%)

Area Total (Mha)

Yield

Annual growth (%)

Average (ton/ha)

Irrigated area

Annual growth (%)

Net area (Mha)

Annual growth (%)

1965

198

—

220

—

0.90

—

47

—

1975

269

3.1

240

0.9

1.12

2.2

60

2.5

1985

370

3.2

258

0.7

1.43

2.5

76

2.4

1995

470

2.4

279

0.8

1.68

1.7

96

2.3

Growth

137%

2.9%

0.8%

87%

2.1%

27%

104%

2.4%

1965–1995 Source: Adapted from Ref.[3].

Hydrologic– Irrigation

water use, and thereby, the scope for water savings. Essentially, water saving means diverting water from non-beneficial or less beneficial uses and making it available for other more productive uses. For example, flows to saline sinks or unrecoverable water bodies can be minimized through interventions that reduce irrecoverable deep percolation and surface runoff. Similarly, the pollution caused by the movement of salts into recoverable irrigation return flows can be reduced by minimizing the passage of these flows through saline soils or saline groundwater. Decreasing non-beneficial depletion of water can also be achieved by: 1) reducing evaporation from water applied to irrigated fields by adopting appropriate precision irrigation technologies such as drip irrigation, or agronomic practices such as mulching, or by changing the crop planting dates to match the period of lower evaporative demand; 2) reducing the evaporation from fallow land; 3) decreasing the area of free water surfaces; 4) decreasing the amount of non- or less-beneficial vegetation, and controlling weeds; and 5) by diverting saline or otherwise polluted water directly to sinks without having to dilute it with fresh water. Projects are quite often justified on the basis of water savings. But this is misleading because, the commonly used term ‘‘irrigation efficiency’’ ignores water recycling and reuse, phenomena that are prevalent in many irrigation systems. Therefore, one has to be careful in assuming that apparent water savings at field level will automatically result in real water savings. Only proper basin-level analysis will reveal if there are uncommitted outflows available and whether water savings are really possible.[4]

Role of Storage Different annual rainfall regimes result in different levels of availability of fresh water resources. In the

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permanent humid climatic zone there is water surplus but because the rainfall distribution is regular, the run-off is quite stable and there are few problems with floods. In many parts of the seasonally humid and semiarid climates, river run-off is irregularly distributed within the year. Heavy rain alternates with dry spells resulting in alternating flood and drought periods. This is particularly serious in monsoon Asia and in the semiarid areas of Africa and India. In such areas, suitable measures to capture and store excess water for irrigation, industry, and domestic use must be adopted. In Sri Lanka and southern India, this has been done over thousands of years through the construction of small storage reservoirs (called ‘‘tanks’’) by building a bund at a strategic location in a catchment area. In fact, much of the growth in irrigation in the last three decades has been made possible by water development projects ranging from multipurpose storage reservoirs to extensive groundwater extraction from underground aquifers. Storage, whether in reservoirs, small tanks, farm ponds, or groundwater aquifers helps to match water demand and supply especially in drier periods, in the face of spatial and temporal variations in natural water supply. A recent development in India has been the concept of ‘‘watershedbased systems for resource conservation, management and use,’’ which involves the optimum use of precipitation through improved water, soil, and crop management. This is accomplished through improving infiltration of rainfall into the soil, run-off collection, and by recovery from wells after deep percolation resulting in the improvement and stabilization of agriculture in the watershed. As stated in the preceding section, planning for storage is best done on the basis of water resources analysis in a basin perspective. One of the first steps is to ascertain whether the basin in question is open, closed, or semiclosed.[5] Current water use and productivity in the basin must be assessed to determine the extent to which increased demands for irrigated

agricultural production can be met by increasing water productivity, and the degree to which increased demands will require increased consumption of water. Then plans to capture and use any uncommitted discharge from open or semiclosed basins can be drawn up. Combinations of small and large surface water storage and groundwater recharge are generally the best systems where they are feasible. In monsoon Asia, research and development are needed on how to manage water under monsoonal conditions, particularly on how to develop effective irrigation management responses to rainfall. Increasing the Productivity of Water Not only is per capita water availability decreasing in many developing countries but agriculture’s share of water is also declining while water demands are increasing from the industrial and urban sectors. When managing water for agriculture, especially in areas where water rather than land is the limiting resource, it is useful to shift the focus from increasing the productivity of land to increasing the productivity of water. That is, to identify and adopt agricultural and water management practices that achieve more output per unit of water consumed. On one hand, these will include selecting crops or crop varieties that are less water-consuming, or which yield higher physical or economic productivity per unit of water, and improved land preparation and fertilization practices. On the other hand, techniques such as deficit, supplemental, or precision irrigation that allow better control, timing, and reliability of water supplies will enable farmers to apply limited amounts of water to their crops in the time and amount that help realize optimum crop response to water. Any water thereby freed up can, in turn, be reallocated to other uses with potentially dramatic increases in overall economic productivity of water. Such techniques do not always imply high-tech options but will include simple bucket and drum kits for drip irrigation, pitcher irrigation, small sprinkler systems, level basins, as well as conventional drip and sprinkler systems. Innovative and affordable water management systems such as these are especially important for small farmers in situations where rainfall is limited and uncertain, and where one or two irrigation applications can have a big impact on crop yields and household food security and income. Sustainable Management of Groundwater In many tropical countries with high levels of rural poverty, groundwater development offers major opportunities for promoting food and improving livelihoods.

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Affordable innovations in manual irrigation technologies such as the treadle pump (costing US$ 12–25 per unit and which can be operated even by children) have dramatically improved poor people’s access to groundwater in Bangladesh, Eastern India, and Nepal.[6] The capital requirements to develop groundwater irrigation are generally low and its productivity higher compared to surface irrigation. It offers farmers irrigation water ‘‘on-demand’’ and responds slower to drought. Farmers also tend to exercise more care in using it because of the costs involved in lifting water, thus maximizing application efficiencies. The undoubted benefits of groundwater development have to be balanced with the risks of overexploitation and contamination. One of the most serious effects of groundwater depletion is seawater intrusion in coastal aquifers as in the Tamilnadu coast near Madras in India and in the Saurashtra coast of the Western Indian State of Gujarat. While there are only rough estimates of the amount of unsustainable groundwater use, the 1–3 m per year decline in water tables occurring in pump intensive areas of India and China clearly highlights the gravity and magnitude of the problem. While reducing pumping for irrigation is an obvious response, this will have adverse effects on the outputs from this highly productive form of agriculture and ultimately affect the food security of the concerned countries. More desirable solutions are groundwater recharge and increasing water productivity to achieve the same production with less water. Regulating groundwater overdraft is a far more complex and difficult issue compared with stimulating groundwater use where it is abundant. Enforcing groundwater laws in developing regions present formidable difficulties given the sheer numbers involved (for e.g., the total number of private tubewells in South Asia is thought to exceed 20 million, largely unregistered and unlicensed, and growing at a rate of 1 million per year). The challenge therefore is to identify appropriate institutional and legal frameworks compatible with the local environment through a careful learning process approach to combat the problem of groundwater overdraft.

Effective Irrigation Management Institutions and Policies Sound institutions and policies are vital for effective irrigation management and increasing food production while sustaining land and water quality, especially in light of growing demands and competition for water from other sectors. Irrigation management must therefore be viewed within the overall context of an integrated and sustainable approach to water resources

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management that is sensitive to the requirements of all uses and users of water. This not only entails the formulation of adequate laws, institutions, and policies, but also the development of the requisite organizational capacity and skills for enforcement and regulation. Key institutional attributes include the demarcation of the roles, rights and responsibilities of the various actors in the water sector, the promotion of new forms of public and private partnerships for investment, operation and maintenance, and the emergence of financially self-reliant service delivery organizations that are responsive and accountable to water users. In fact, the overarching concern must be to ensure meaningful participation of all stakeholders in the whole gamut of planning, operation, maintenance, and management of irrigation schemes.

Improving Irrigation Services

Hydrologic– Irrigation

The level of irrigation services provided to farmers is influenced by the physical design of the system, its operation and maintenance, as well as the underlying institutional environment. When farmers are provided with reliable irrigation services, they would be more likely to invest in improved technologies and practices, generally resulting in increased production, higher incomes, and improved irrigation performance. But it is useful to remember that, in general, more flexible and sophisticated technology will require levels of management capability that are not always readily available in rural environments. Thus, in seeking to provide a stable and predictable water supply to farmers, it is quite important to find ways and means of doing so that are commensurate with the available skills and resources. The dilemma of rigidity vs. flexibility in irrigation design, and the interactions between design and system management have been discussed in detail by Horst.[7] In discussing the large public irrigation schemes of monsoon Asia, Burns[8] has pointed out that structuring such systems to formalize the allocation of sporadic wet season scarcity, while protecting the civil works from producer damage, is a necessary task. He further states that unless these schemes are run as regulated and monitored public utilities, actual system performance will bear little resemblance to design intentions due to the rent-seeking activities of system designers, operators, and individual farmers. Ensuring reliable irrigation services also implies the establishment of: 1) clear rules and agreements between providers and users giving details of the nature of the service and the compensation arrangements for providing and receiving such services; 2) mechanisms for monitoring and control of obligations; 3) modalities

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Irrigation Management: Tropics

for conflict-resolution; and 4) procedures for modifying and updating agreements. These aspects take on added significance in light of the progressive disengagement of public agencies from irrigation management with attendant transfer of responsibilities to the beneficiaries.

CONCLUSION Feeding the world’s population presents a formidable challenge: there are more mouths to feed, less arable land available, increasing competition for water resources, and major concerns about deteriorating ecosystems. Irrigated agriculture makes undeniable contributions to the food security of people in the tropics, providing nearly 50% of its cereal production. But there is a gap between supply and demand and it is urgent to find sustainable ways to improve the performance of the irrigation sector and to satisfy the nutritional requirements of a growing population. The management of natural resources, especially water, takes on added significance as population increases and changes in resource utilization place greater pressures on land and water. With fewer opportunities to expand irrigated areas by the development of new systems, and growing demands and competition for water from other sectors, the emphasis must shift to practicing more effective methods of management with particular focus on improving the productivity of water. A vital prerequisite is for irrigation management to be viewed within an overall framework of an integrated, holistic approach to water resources management. This is best done on the basin scale, taking into account the needs of all uses and users but also considering the conjunctive use of surface water, groundwater, and rainfall. Given the multiple facets of irrigation development and management, a piecemeal and uni-dimensional focus on individual components is bound to produce outputs that fall short of expectations. For instance, concentrating only on the technological aspects of an innovation while neglecting other aspects such as maintenance, institutional support, training, and skills development, will yield disappointing overall results, however, well that individual component has been addressed. Hence, the need to take a holistic view and move towards service-oriented irrigation management with meaningful participation of all interested parties.

REFERENCES 1. http://www.census.gov/ipc/www/popwnote.html (accessed February 2002).

Irrigation Management: Tropics

6. Shah, T.; Alam, M.; Dinesh Kumar, A.; Nagar, R.K.; Mahendra Singh Pedaling Out of Poverty; Social Impact of a Manual Irrigation Technology in South Asia, Research Report 45; International Water Management Institute: Colombo, Sri Lanka, 2000. 7. Horst, L. The Dilemmas of Water Division: Considerations and Criteria for Irrigation System Design; International Water Management Institute: Colombo, Sri Lanka, 1998. 8. Burns, E.R. Irrigated rice culture in monsoon Asia: the search for an effective water control technology. World Dev. 1993, 212 (5), 771–789.

Hydrologic– Irrigation

2. Sanchez, P.A. Properties and Management of Soils in the Tropics; John Wiley & Sons: New York, 1976. 3. IWMI. Water for Rural Development: Background Paper on Water for Rural Development; International Water Management Institute: Colombo, Sri Lanka, 2001. 4. Molden, D.J.; Sakthivadivel, R. Water accounting to assess use and productivity of water. Int. J. Water Resour. Dev. 1999, 15 (1/2), 55–71. 5. Keller, A.; Sakthivadivel, R.; Seckler, D. Water Scarcity and the Role of Storage in Development, Research Report 39; International Water Management Institute: Colombo, Sri Lanka, 2000.

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Irrigation Return Flow and Quality Kenneth K. Tanji Department of Land, Air and Water Resources (LAWR), University of California–Davis, Davis, California, U.S.A.

Ramo´n Aragu¨e´s Agronomic Research Service, Diputacion General de Aragon, Zaragoza, Spain

INTRODUCTION

Hydrologic– Irrigation

The return flows from irrigated agriculture (i.e., Irrigation Return Flows, IRF) are considered the major diffuse or ‘‘non-point’’ contributor to the pollution of surface and groundwater bodies.[1] This off-the-farm discharge (‘‘off-site’’ contamination) is inevitable since irrigated agriculture cannot survive if salts and other constituents accumulate in excessive amounts in the crop’s root zone (‘‘on-site’’ contamination), and so they must be reached and exported with the drainage waters.[2] Thus, the major task concerning the viability and the long-term sustainability of irrigated agriculture is the attainment of a proper balance for optimizing crop production while minimizing both the ‘‘on-site’’ and the ‘‘off-site’’ environmental damages or impacts and, ultimately, finding an acceptable disposal of the IRF.[3,4] As a consequence of this increasing ‘‘off-site’’ environmental problem, water pollution standards and emerging policies regulating the discharge of the IRF are being implemented in developed countries. The key policies for mitigating the negative environmental impacts of irrigation are incorporated in the Water Pollution Control Act in United States,[5] and in the Nitrates, Habitats and Environmental Impact Assessment, and Water Framework directives in European Union.[6] The degree of the ‘‘off-site’’ irrigation-induced pollution depends on the hydrogeological characteristics of the irrigated land and substrata, the agricultural production technologies used, and the water supply and drainage conveyance systems.[1] This article reviews these issues in IRF, describes the main components and chemical constituents of IRF, and summarizes recommended management practices aimed at reducing the off-site water quality impact from irrigated agriculture.

The water removal subsystem (i.e., the IRF) may be divided into the surface drainage, consisting of the overflow or bypass water and surface runoff or tailwater, and the collected subsurface drainage components. Since IRF are mixtures of these components, their proportions determine the final quality of IRF. Table 1 summarizes the expected water quality changes of the three IRF components (overflow, tailwater, and subsurface drainage) relative to the quality of the applied irrigation water. Overflow is the result of operational spill waters from distribution conveyances that are directly discharged into the drainage system and its quality is generally similar to that of the irrigation water (Table 1). Tailwater is the portion of the applied irrigation water that runs off over the soil and discharges from the lower end of the field directly into the drain system. Because of its limited contact and exposure to the soil surface, its quality degradation is generally minor. Even so, these waters may increase slightly in salinity and may pick up considerable amounts of sediments and associated nutrients (phosphorus in particular) as well as water-applied agricultural chemicals such as pesticides and nitrogen fertilizers (anhydrous ammonia in particular) (Table 1). Subsurface drainage is the portion of the infiltrating water that flows through the soil and is collected by the under drainage system. Because of its more intimate contact with the soil and the dynamic soil–plant–water interactions, its quality degradation is generally substantial. These subsurface drain waters carry any anthropogenic chemicals present in a soluble form in the soil water as well as any salts and other soluble elements present in the soil and parent geologic material and intercepted shallow groundwaters. The salinity and agrochemicals in subsurface drainage are the primary source of pollution associated with irrigated agriculture (Table 1).

COMPONENTS OF IRF WATER QUALITY CONSTITUENTS IN IRF Fig. 1 gives a schematic diagram of a typical irrigationcrop-soil-drainage system, composed of the water delivery, the farm, and the water removal subsystems.[2] 588

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Irrigation return flows provide the vehicle for conveying the pollutants to a receiving stream or groundwater

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Fig. 1 Idealized sketch showing the diversion of irrigation water through a main canal, its distribution through a lateral, and its application to croplands. The three main components of the irrigation return flows (IRF) to the river channel are shown. The deeper groundwater zone, a second receiving water system, is not shown.

reservoir. It is therefore necessary to characterize their most important water quality constituents (namely, inorganic salts, agrochemicals and trace elements) and to develop management strategies aimed at alleviating their detrimental effects on the receiving water bodies. Salts Salts are a major quality factor since they can restrict the municipal, industrial and agricultural uses of water and can dramatically decrease the productivity and sustainability of irrigated agriculture in arid zones.

The primary source of dissolved mineral salts (also referred to as salinity) is the chemical weathering of rocks, minerals, and soils. Salinity is reported in terms of total dissolved solids (TDS in mg L
1) or Electrical Conductivity (EC in dS m
1 at 25 C). The main solutes contributing to salinity are the cations calcium (Ca), sodium (Na), and magnesium (Mg), and the anions chloride (Cl), sulfate (SO4), and bicarbonate (HCO3). These solutes are reactive in waters and soil solutions participating, among others, in cation exchange and mineral solubility. The excessive accumulation of Na (i.e., sodicity, generally expressed by the Sodium Adsorption Ratio or SAR) in the soil solution and exchange complex may impair poor soil physical

Table 1 Quality parameters of the three irrigation return flow (IRF) components and their expected quality changes as related to the quality of the irrigation water

Quality parameters General quality degradation

Overflow

Tailwater

Subsurface drainage

0

þ

þþ

Salinity

0

0, þ

þþ

Nitrogen

0

0, þ, þþ

þþ, þ

Phosphorus Oxygen demanding organics Sediments

0, þ

þþ

0,
, þ

0

þ, 0

0,
,



0, þ,


þþ





Pesticide residues

0

þþ

0,
, þ

Trace elements

0

0, þ

0,
, þ

Pathogenic organisms

0

0, þ


,



0: Negligible quality changes expected. þ,
: Expected to be slightly higher (i.e., pick up), lower (deposition). þþ: Expected to be significantly higher due to concentrating effects, application of agricultural chemicals, erosional losses, pick up of natural geochemical sources, etc.

: Expected to be significantly lower due to filtration, fixation, microbial degradation, etc.

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Components of IRF

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properties and is a critical factor in the sustainability of irrigated soils.[7] Growing plants extract water through evapotranspiration and leave behind most of the dissolved salts, increasing its concentration in the soil water (‘‘evapoconcentration effect’’). Irrigation also adds to the salt load in IRF by leaching natural salts arising from weathered minerals occurring in the soil profile, or deposited below (‘‘weathering effect’’).[2] As a consequence of both effects, it follows that the salinity and chemical composition of IRF depend basically on the characteristics of the irrigation water, the soil and subsoil, and the hydrogeology, as well as on the management of the irrigation water or Leaching Fraction (LF) defined as the fraction of infiltrated water that percolates out of the root zone. Thus, high LFs promote the weathering effect and the salt load carried out with the IRF (i.e., increased ‘‘off-site’’ pollution) whereas low LFs promote the evapoconcentration effect and the concentration of salts in the crop’s root zone (i.e., increased ‘‘on-site’’ pollution). In conclusion, the mass of salts or salt loading in IRF depends mainly on the salinity of the irrigation water, the minerals present in the soil and subsoil, and the water management (LF). The salt loading values may vary widely, from values similar to those of the irrigation water to values one order of magnitude higher. Thus, typical salt loading values in IRF from arid-land irrigated agriculture vary between 2 Mg ha
1 yr
1 and 20 Mg ha
1 yr
1.[1,2,7] The quantification of salt loading is critical to ascertain the ‘‘offsite’’ contamination of irrigated agriculture, since the prediction of the resultant salt concentration in a body of water after mixing with the IRF requires knowledge of the mass of salts (i.e., concentration and flow) in each contributing body.

Nitrogen Hydrologic– Irrigation

Nitrogen can be in either the organic or the inorganic (ammonium, nitrate and nitrite) form. Organic N is predominant in surface drainage (although it is not usually an issue in arid areas), whereas inorganic N is predominant in subsurface drainage water. Although nitrite is considered more hazardous than nitrate, it is in general a transient form of N present in small quantities. Nitrate is thus the dominant form of N in IRF and should be the focus of the water quality evaluation.[3] High nitrate (NO3) concentrations in IRF are a major concern since they may cause eutrophication (excessive algal growth) and hypoxia (decline in dissolved oxygen from decay of algae) problems. When nitrate is ingested in substantial amounts by humans

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Irrigation Return Flow and Quality

and animals, it may cause methemoglobinemia (bluebay like symptoms from oxygen starvation exhibited by infants and elderly) and certain cancers.[7] Thus, USEPA has set the maximum allowable concentration of nitrate in public water supplies at 45 mg L
1,[5] whereas the European Union has limited it to 50 mg L
1.[6] The three major sources of nitrate found in IRF are leaching from croplands, land disposal of urban sewage, and concentrated animal feeding (beef feedlots, dairies, swine, chicken houses) wastes. The potential for nitrate leaching is a function of soil type, weather conditions and crop management system. In general, the higher the N application rate, the greater the amount of N available to be lost, since fertilizer N recovery by harvested crops averages about 50% and tends to be even lower when high N application rates are used. In addition, mineralization of organic N, followed by nitrification of NH4 may also increase the N losses.[4] Drainage has a large influence upon losses of nitrogen. The N loss from poorly drained soils is generally much less than from soils with improved drainage systems. As previously indicated, much of the N transported in surface runoff is organic N associated with the sediment, although the amount lost is usually small and poses little threat to the environment except in pristine waters. On the other hand, nitrate concentrations in subsurface drainage water are much higher and variable, depending on the N fertilization rates and time of applications, and on water and soil management.[4]

Phosphorus Phosphorus (P), present in both organic and inorganic forms, is a relevant water quality constituent in IRF because of its contribution to eutrophication of surface waters. Most of the P in surface drainage is in particulate (i.e., sediment and organic matter-bound) form whereas most of the P in subsurface drainage water is in soluble phosphate form. The release of P depends on such biogeochemical processes as adsorption/desorption of phosphate, precipitation/dissolution of inorganic P forms, and mineralization of organic P forms.[7] Phosphorus in subsurface drainage waters is typically low in concentration because of its strong adsorption in arid zone soils. Thus, although P discharge from agricultural fields vary considerably, it is usually in the range of 0.2 kg P ha
1 yr
1 to less than 3 kg P ha
1 yr
1. Even though P loading in IRF is minor, the P concentrations measured in many agricultural IRF may be orders of magnitude above the soluble (10 mg P L
1) and total

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Table 2 Summary of recommended management practices at the water delivery, farm, and water removal subsystems to reduce off-site water quality impacts from irrigated agriculture Water delivery subsystem Designed to meet the farm water requirements while reducing undesirable water losses Canal lining and/or closed conduits and reservoir lining: prevent seepage losses, phreatophyte ET losses, soil waterlogging, and groundwater recharge; improve irrigation water quality (i.e., suspended solids). Installation of flow measuring devices: water control; appropriate water charges and penalties; reduce bypass losses; attain high water-conveyance efficiencies. Construction of regulation reservoirs at the irrigation district level to increase flexibility in water delivery. Implement an efficient institutional framework, service-oriented besides its regulatory character; scheduled maintenance programs. Farm subsystem Designed to maintain or increase crop productivity while improving source control Improve cultural practices: rate and timing of fertilizers; slow-release fertilizers; fertigation; pest control; seeding and tillage practices. Adopt less environmentally damaging agricultural practices: integrated management systems; mixed cropping practices; organic farming. Increase irrigation application efficiency and uniformity: proper design of the farm irrigation layout; choice of irrigation system; optimum irrigation scheduling; reduce evaporation through mulching and reduced tillage. Minimize the Leaching Fraction according to the leaching requirement of crops: reduce drainage volume; maximize mineral precipitation; minimize pick up of salts. Provide training and technical services to farmers; eliminate institutional constraints. Water removal subsystem Designed to improve sink control and minimize loading in IRF Constraints in disposal of IRF to meet quality objectives in the receiving water body. Reuse for irrigation drainage waters, municipal wastewaters and sewage effluents; integrated on-farm drainage management (i.e., on-farm cycling of drainage waters through biological materials-agroforestry systems). Ocean and inland (i.e., evaporation ponds; solar evaporators; deep well injection) disposal of drainage waters. Design and management of drainage systems: include water quality as a design parameter; depth and distance of placement of drains; integrated drain flow and irrigation management; crop water use from shallow watertables (i.e., subirrigation); controlled drainage (i.e., management of the water level in the drainage outlet); reduce nitrate effluxes by maintaining a high water table to increase denitrification losses. Pumping and disposal of groundwater to reduce intercepted groundwater by the drainage network.

Physical, chemical, and biological treatment of drainage waters: particle removal; adsorption, air stripping; desalination (membrane processes and distillation); coagulation and flocculation; chemical precipitation; ion exchange; advanced oxidation processes; biofiltration (irrigation of specific crops that accumulate large quantities of undesirable constituents such as Se, Mo, B, NO3, etc.); algal–bacterial treatment facilities (removal of NO3 and Se).

(20 mg P L
1) critical levels assumed to accelerate the eutrophication of freshwater aquatic ecosystems.[5] Pesticide Residues Pesticide contamination in IRF is of concern in some agricultural areas, although it is in general less significant than the salinity or nitrogen pollution problem.[3]

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Pesticides used in irrigated agriculture include herbicides, insecticides, fungicides, and nematicides. These various types make it difficult to assess their potential impacts on water quality. Pesticide concentrations in surface drainage are usually much greater than those in subsurface drainage due to the filtering action of the soil. Thus, the total loss of pesticides via subsurface drainage is usually 0.15% or less of the amount

Hydrologic– Irrigation

Flowing of surface drainage water through vegetated filters and riparian vegetation (removal of sediments and sedimentsassociated contaminants), flowing of subsurface drainage water through riparian zones (removal of nitrate due to plant uptake and denitrification); flowing of drainage water through constructed wetlands (sink for sediment, nutrients, trace elements, and pesticides).

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applied, whereas losses via surface drainage can be up to 5% or more.[4] The environmental fate of pesticides is quite complex. Chemical-specific properties influence the reactivity of pesticides. Pesticides can be degraded by microbes, chemical and photochemical reactions, adsorbed on to soil organic matter and clay minerals, lost to the atmosphere through volatilization, and lost through surface runoff and leaching.[4] Once a pesticide enters into the soil, its fate is largely dependent on sorption (evaluated by use of a sorption coefficient based on the organic carbon content of soils) and persistence (evaluated in terms of the half-life or the time it takes for 50% of the chemical to be degraded or transformed). Pesticides with low sorption coefficient (such as atrazine, DBCP, and aldicarb) are likely to leach readily, whereas pesticides with long half-lives (such as DDT, lindane, and endosulfan) are so persistent that many of them banned various decades ago are still found in stream sediments or are now being detected in the groundwaters.[7]

Trace Elements

Hydrologic– Irrigation

High concentrations of trace elements in soils and waters pose a threat to agriculture, wildlife, drinking water, and human health. The trace elements of most importance, documented as pollutants associated with irrigated agriculture, are barium (Ba) and lithium (Li) (alkali and alkali earth metals), chromium (Cr), molybdenum (Mo), and vanadium (V) (transition metals), arsenic (As), boron (B), and selenium (Se) (non-metals), and cadmium (Cd), copper (Cu), lead (Pb), mercury (Hg), nickel (Ni), and zinc (Zn) (heavy metals).[3] Those trace elements such as As, Cd, Hg, Pb, B, Cr, and Se are especially harmful to aquatic species because of biological magnification.[5] Due to the generally narrow window between deficiency and toxicity of trace elements, it is essential to have an adequate information on their concentrations in soils and waters. The sources of trace element contamination may be divided into natural (i.e., geologic materials) and agricultural-induced (i.e., fertilizers, irrigation waters, soil and water amendments, animal manures, sewage effluent and sludge, and pesticides). Increases in trace element concentrations in surface runoff are generally not expected, whereas the presence of trace elements in groundwaters is influenced by the nature of the sources, the speciation and reactivity of the trace elements, and the mobility and transport processes. Thus, high concentrations of trace elements in subsurface drainage water appear to be strongly associated with the geologic setting of the irrigated area and may be affected by the same processes that affect the soil and groundwater salinity.[3,7]

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Irrigation Return Flow and Quality

An illustrative example of trace element contamination is the selenium toxicosis of waterfowl at Kesterson reservoir (California, U.S.A.), a terminal evaporation pond for drainage waters high in Se (300 ppb average) originating from the Moreno shale, a geologic formation of the Coast Range Mountains in the west side of the San Joaquin Valley.[7]

MANAGEMENT OPTIONS TO REDUCE OFF-SITE WATER QUALITY IMPACTS FROM IRRIGATED AGRICULTURE The basic idea behind the control of irrigation-induced environmental problems is the change in focus from a ‘‘water resource development’’ to a ‘‘water resource management’’ approach. This new ‘‘thinking’’ involves both policy changes, such as reducing the applied water through economic and regulatory policies (i.e., water metering, water pricing, licenses and time-limited abstraction permits), and developing farmer’s incentives for promoting best management practices (i.e., compensation and agri-environment payments for irrigated crops), and a variety of technical measures.[5,6,8] Since a detailed description of the technical measures is too lengthy for this article, Table 2 summarizes some of the recommended strategies aimed at reducing the off-site water quality impacts from irrigated agriculture. However, it should be cautioned that these measures should be applied in a ‘‘case-by-case’’ basis, since some of them could aggravate the ‘‘on-site’’ pollution problems. Typical examples will be (i) the ‘‘minimum leaching fraction concept,’’ that could promote soil sodification and structural stability problems due to the precipitation of calcium minerals such as calcite and gypsum; (ii) the reuse of drainage water for irrigation, which is only sustainable if it is of sufficient good quality; and (iii) the disposal of drainage water in evaporation ponds, which may eventually lead to other environmental problems. The reader is referred to the references given at the end of the entry for further information on the myriad of technical management options developed in the last decades and on details of their advantages and limitations.

REFERENCES 1. Law, J.P.; Skogerboe, G.W.; Eds. Irrigation Return Flow Quality Management, Proceedings of National Conference, Fort Collins, Co, May 16–19, 1977, 451 pp. 2. Yaron, D.; Ed.; Salinity in Irrigation and Water Resources, Civil Engineering No. 4; Marcel Dekker: New York, 1981; 432 pp.

Irrigation Return Flow and Quality

6. Institute for European Environmental Policy. The Environmental Impacts of Irrigation in the European Union; Report to the Environment Directorate of the European Commission: London, 2000; 138 pp. 7. Tanji, K.K.; Ed. Agricultural Salinity Assessment and Management, ASCE Manuals and Reports on Engineering Practices No. 71; American Society of Civil Engineers New York, 1990; 619 pp. 8. Food and Agriculture Organization of the United Nations, Control of Water Pollution from Agriculture, Ongley, E.D., Ed.; FAO Irrigation and Drainage Paper No. 55; FAO: Rome, 1996; 85 pp.

Hydrologic– Irrigation

3. Food and Agriculture Organization of the United Nations, Management of Agricultural Drainage Water Quality; Madramootoo, CA., Johnston, W.R., Willardson, L.S., Eds.; FAO Water Report No. 13; FAO: Rome, 1997; 94 pp. 4. Skaggs, R.W.; van Schilfgaarde, J.; Eds. Agricultural Drainage, Agronomy No. 38, ASA, CSSA and SSSA, Inc.: Madison, WI, 1999; 1328 pp. 5. National Research Council, Soil and Water Quality, an Agenda for Agriculture, Batie, S.A. Chair; Committee on Long-Range Soil and Water Conservation, National Academy Press: Washington, DC, 1993; 516 pp.

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Irrigation Sagacity (IS) Charles M. Burt Ken H. Solomon BioResource and Agricultural Engineering Department, California Polytechnic State University, San Luis Obispo, California, U.S.A.

INTRODUCTION Throughout the Western United States, water rights are granted for reasonable and beneficial purposes. Engineers require an irrigation performance parameter that embodies the reasonable and beneficial standard to assess irrigation systems, practices, and competing uses, whether against each other, or against benchmark targets. Irrigation sagacity (IS), initially proposed by Solomon,[1] is such a parameter. The term sagacity comes from sagacious, meaning wise or prudent. IS is fundamentally different from irrigation efficiency (IE), long used to quantify beneficial use of irrigation water.[2] Water is used beneficially if it contributes directly to the agronomic production of the crop. However, due to physical, economic, or managerial constraints, and various environmental requirements, some degree of non-beneficial use is generally reasonable. IS goes beyond IE to incorporate quantification of reasonable uses: those uses that may not contribute to agronomic production, but are nonetheless justified under the particular circumstances at hand.

BENEFICIAL USES

Hydrologic– Irrigation

Both IE and IS credit those portions of the irrigation water that are judged to be beneficially used. Examples of beneficial uses include: crop evapotranspiration (ET), water harvested with the crop, water used for salt control (leaching), climate control, seedbed preparation, softening the soil crust for seedling emergence, and ET from beneficial plants (windbreak, cover-crop, habitat for beneficial insects). Evaporation during regular and reclamation leaching, and evaporation during necessary irrigations are beneficial, because an agronomic objective is achieved during those events. Examples of non-beneficial uses at the farm level include: overirrigation due to non-uniformity, uncollected tailwater, deep percolation beyond that needed for salt removal, unnecessary evaporation from wet soil outside cropped area, spray drift beyond field boundaries, and evaporation associated with excessively frequent irrigations. At the irrigation district level, non-beneficial uses include: spills, seepage, 594

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evaporation from canals or reservoirs, and ET from non-beneficial plants such as weeds and phreatophytes.

REASONABLE USES IS quantifies that portion of irrigation water going to sagacious (either beneficial or reasonable) uses.[1,2] Reasonable uses are those that, while not directly benefiting agronomic production, are nonetheless reasonable under prevailing economic and physical conditions. Examples of reasonable, though not beneficial, water uses include the following. Losses that cannot be economically avoided are considered reasonable. For example, canal seepage may be reasonable if canal seepage rates are low and it is not economical to line the canal to avoid that seepage. No irrigation system can be designed to apply water with perfect uniformity, so some deep percolation due to non-uniformity is reasonable. Losses tied to technical requirements may be reasonable. Reservoirs in the distribution system add flexibility and reduce canal spills. Evaporation from such reservoirs constitutes a reasonable use. Microirrigation systems generally require filtration, and filters need to be flushed periodically. Filter flush water may be a reasonable use. If sprinkler irrigation is the appropriate technology, spray evaporation and wind drift losses are an inevitable consequence of using that technology to irrigate, and hence are a reasonable use. Losses due to the uncertainties associated with many aspects of water management may be reasonable. Exactly how much water is held in the soil? Exactly how much crop ET since the last irrigation? Exactly how much water is necessary for maintenance leaching? In the face of such uncertainties, it is reasonable for the farmers to err on the side of overapplication, so some deep percolation due to uncertainty may be reasonable. Losses that contribute toward environmental goals may be reasonable. If canal seepage feeds a wetland or wildlife habitat area in a timely manner, that seepage may be deemed a reasonable use. (Even though feeding a wetlands/habitat area meets environmental goals, it is not considered a beneficial use because it

does not directly aid the production of the crop being irrigated.) If tailwater blends with drainage water to meet water quality standards in receiving waters, then that tailwater may be a reasonable use. Non-sagacious uses (neither beneficial nor reasonable) are those uses without economic, practical, or other justification. An example of a non-sagacious use is wet soil and spray evaporation associated with excessively frequent irrigations. No agronomic objective is served by irrigating more frequently than needed, and it is difficult to imagine an economic justification for doing so. Hence, these losses are without justification. They are unreasonable and non-sagacious.

CONSERVATION IMPLICATIONS It is a common misunderstanding that (1 IE) represents the fraction of the applied irrigation water that is wasted, and therefore, the fraction that may be conserved or reallocated. However, as noted above, some degree of non-beneficial use is generally reasonable, so the potential for conservation and reallocation consists only of water uses that are both non-beneficial and unreasonable.

DETERMINATION OF SAGACITY As with other irrigation performance parameters, application of the IS concept requires that boundaries be specified, flows into and out of the bounded area be quantified, fractions of the irrigation water flowing to various destinations be estimated, and judgments be made about whether those fractions are beneficial or reasonable. Whereas the determination of beneficial use involves only an agronomic criterion—direct contribution to the agronomic production of the crop— the determination of reasonable use involves more varied criteria. Feasibility Identifying a particular non-beneficial water use as unreasonable requires that an alternate practice be identified that uses less water, and is practically, technically, economically, and environmentally feasible. Practical feasibility considers physical constraints such as limitations due to climate, soil, terrain, water delivery schedules, or water travel time. Required resources, which can include labor (sufficient quantity and with suitable experience), infrastructure (maintenance of specialized equipment, extension advice on proposed crops, etc.), and information (precise knowledge, facts, data available when needed), are available.

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Even after identifying the benefits of a new practice, there will be a lag time before implementation is possible. Decision-makers need to be convinced, approvals obtained, plans drawn, and financing arranged. Thus, a realistic time schedule for implementation is also required. Technical feasibility has not only hardware but software and operational aspects. Equipment must be available, affordable, and perform reliably in an agricultural environment. It must satisfy requirements for accuracy and precision of flow, time or other quantities to be measured or controlled. Local, farm scale demonstration projects may be necessary to prove that equipment and plans are reliable, and operations feasible. A phased transition into any new practice should be planned. Economic feasibility is an obvious but complex test. It is not enough to compare the costs of operating one way to the costs of operating another way. The proposition facing a farmer is to change from one practice to another. The costs involved in abandoning an old practice and adopting a new one, or converting from the old to the new, may well be greater than the cost if the new practice were started from scratch in a new operation. Further, even if the annualized cost of an alternate practice is favorable, it may not be possible for farmers to implement it unless additional resources such as financing and credit are available. Economic feasibility must also consider risk. It is not reasonable to ask farmers to undertake a large risk to actualize the potential of a small benefit. Economic feasibility must include plans and mechanisms for properly allocating the costs of alternate practices to those who will ultimately reap the benefit. This may be particularly difficult in the case of alternate practices whose ultimate beneficiary is the environment, because it is often not clear who ‘‘ought’’ to pay on behalf of an environmental common good. Environmental feasibility requires that the alternative practice must be environmentally benign or beneficial, or that the costs of any required environmental mitigation are considered. If no alternate practice using less water meets all four feasibility tests, the current practice is reasonable. The current practice is unreasonable if a feasible alternate using less water exists. The amount of unreasonable (non-sagacious) use due to the current practice is the difference between the current use and the (reduced) use of the preferred alternate.

CONCLUSION The results of a sagacity determination may vary with location, geographic scale, or time. Because sagacity

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includes economics, which can change as markets and prices do, sagacity can change with place and time. Technology and the availability of resources are also factors influencing sagacity that can change with place and time. Results of the various feasibility checks can depend on scale. To a farmer, the district’s water delivery policies and schedule are given. At the district level, these things may be considered adjustable. Districts can and should consider options that individual farmers cannot consider. Economics and the ability to absorb risk change with scale as well. What may not be economical to an individual farmer could be economical to a district, region, or to another competing water user, if there is a way for them to share in the costs as well as the benefits. While individual farmers are less able to bear risks due to uncertainty or reduced water use, the shift to a district or societal level offers the potential to ‘‘average’’ individual outcomes and pool risks. So sagacity is very much a site-, scale-, and timespecific quantity. Therefore, a necessary preliminary to

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Irrigation Sagacity

the determination of sagacity described above is to specify the boundaries and geographic extent of study area, the time frame for economic and technological determinations, and the perspective for feasibility checks (individual farmer, district, region, or society). For a more complete discussion of IS and its application, the reader is referred to Ref.[3].

REFERENCES 1. Solomon, K.H. Technical memorandum; Center for Irrigation Technology, California State University: Fresno, CA, 1993; 1–2. 2. Burt, C.M.; Clemmens, A.J.; Strelkoff, T.S.; Solomon, K.H.; Bliesner, R.D.; Hardy, L.A.; Howell, T.A.; Eisenhauer, D.E. Irrigation performance measures— efficiency and uniformity. J. Irrig. Drain. Eng. 1997, 123 (6), 423–442. 3. Solomon, K.H.; Burt, C.M. Irrigation sagacity: a measure of prudent water use. Irrig. Sci. 1999, 18, 135–140.

Irrigation Scheduling: Plant Indicators (Field Application) David A. Goldhamer

INTRODUCTION While there has been extensive research in measuring plant water status and its impact on plant processes and aspects of crop production, relatively little work has been published on giving specific protocols for using plant-based measurements in irrigation scheduling. This is likely the result of plant water status being very dynamic since the plant is coupled to both its soil water and atmospheric surroundings. Thus, no single value can be used to indicate the onset of water stress. Plant water status changes diurnally and over the season, and its dynamic nature makes it difficult to identify threshold values for practical use. Plant water measurements, by themselves, mean little if they are not considered relative to the equivalent measurements representing fully irrigated plants in the same environment. This has been accomplished by developing ‘‘reference’’ or ‘‘baseline’’ values, representing the behavior of plants under non-limiting soil water supply.

APPLICATIONS OF PLANT INDICATORS FOR THE MANAGEMENT OF WATER STRESS IN IRRIGATION It is well documented that virtually all plant processes are affected by a reduction in plant water status.[1] Plant water deficits decrease leaf growth and thus, leaf area. With field and row crops, where the goal is to achieve a full canopy as soon as possible, early season water deficits translate into lower field photosynthesis and ultimately lower total biomass production. Thus, with crops harvested for biomass, such as alfalfa and silage corn, any water stress will reduce yield. The linear nature of the classical production function relating yield to crop water use illustrates this fact clearly.[2] The effect of water deficits on production of crops where only the reproductive organ is harvested is much less straightforward. One example is cotton; an indeterminate row crop. Severe water stress can reduce leaf area, photosynthesis, the number of fruiting sites, and thus reduce lint yield.[3] On the other hand, mild water stress, enough to significantly reduce vegetative growth, did not reduce yield.[4] There are few

documented cases of water stress increasing crop yields. Fereres[5] reported that mild water deficits increased yields of cotton and sorghum over those under full irrigation. This was attributed to the increase in harvest index due to a greater partitioning of assimilate from vegetative to reproductive sinks. Chalmers, Mitchell, and van Heek[6] reported that regulated deficit irrigation (RDI) significantly reduced unwanted vegetative growth in peach and consequently, an increase in harvest fruit size, again due to greater assimilate partitioning. It should be noted that others have tried to reproduce these results, specifically, increased fruit size in response to water stress, and failed.[7–9] However, these experiments occurred under different soil and atmospheric conditions and with different cultivars. There are far more instances reported of water stress improving some aspect of fruit quality than fruit size. For example, Goldhamer et al.[9] found that RDI can significantly reduce peel creasing in navel oranges without negatively affecting other yield components, thus increasing grower profit. The interactions between irrigation and pest and disease management offer opportunities for the use of plant indicators for beneficial purposes. While water stress is usually associated with higher insect pressures, there are cases where water stress had beneficial effects. Leigh et al.[10] found that lygus bug levels in cotton were reduced by 50% by the imposition of water stress compared to fully irrigated plants. Goldhamer et al.[11] found that epicarp lesion on pistachio nuts, believed to be the result of feeding by leaf-footed plant bugs, was significantly reduced in severely stressed trees. Reduced insect pressures in response to water stress are usually attributed to a less favorable feeding environment due to higher canopy temperatures. However, high temperatures are also related with greater pest pressures due to higher insect development rates.[12] Teviotdale et al.[13] noted that preharvest water deficits in almond trees significantly reduced the fungal disease of hull rot in almonds. Further work resulted in a recommendation that water stress be imposed for a two-week period about 1 mo prior to harvest not to be less than a predawn leaf (C) water potential of 1.5 MPa.[14] Although this approach provided a great amount of disease control, the authors advised that water stress 597

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could reduce kernel size (3–5%) and thus, the grower must decide on whether the impact of the disease would be worse than that of water stress.

providing a table showing stem C for a range of relative humidity and air temperature conditions (Table 1). Additional work had identified periods of the season, which were most stress tolerant[16] resulting in specific, recommended protocols of desired stem C over the season (Table 2). These protocols have been adopted by numerous prune growers in California. One short coming of plant-based scheduling approaches is that they do not provide quantitative information on how much water should be applied at each irrigation as is possible with soil water monitoring and atmospheric-based methods. Thus, protocols for using plant C should somehow address this issue. Here, both measurement and irrigation frequency are important; the more frequent the monitoring, the greater is the opportunity to adjust the irrigation amount. It is difficult to conduct frequent manual stem C monitoring, and this measurement cannot be automated currently. There is little chance of coupling these manual measurements to irrigation electronic controllers. Goldhamer and Fereres[17] presented protocols based on experimental data for using trunk diameter measurement to schedule irrigations in almond trees. They suggested maximum daily trunk shrinkage (MDS) as the stress indicator parameter. These data were normalized for vapor pressure deficit (VPD) using reference values obtaining either from fully irrigated trees or relationships previously developed between VPD and fully irrigated trees. They used the term ‘‘signal’’ (ratio of measured to reference value) to represent the stress magnitude and suggested target signals for irrigation scheduling based on how much stress the grower desired during the season. If the measured signal consistently exceeds the target threshold, the irrigation rate is increased by 10% for the next cycle.

INTERPRETATION OF MEASUREMENTS FOR IRRIGATION DECISION MAKING The fact that plant species and plant processes differ in their sensitivity to water stress complicates the issue of applying indicators of plant water status to irrigation management. For example, predawn water C of leaves in fully irrigated pistachio tress in the central valley of California is from 0.8 MPa to 1.0 MPa compared with 0.15 MPa to 0.20 MPa for walnut trees in the same environment. Mild to moderate water stress from mid-May to early July has a little impact on any pistachio yield component while it reduces the size of harvested walnuts. In order to use plant C in production agriculture, it is of paramount importance that both accurate measurements of normalized water stress and how they impact yield be known for successful application of any plant-based scheduling program. This two-phase knowledge is rare. Nevertheless, some workers have developed plant-based protocols that have achieved varying degrees of success in on-farm water management. Examples of the most promising of these approaches for tree crops are highlighted in the following discussion. Shackel et al.[15] proposed using stem C for irrigation scheduling of prunes. This crop is an ideal candidate for plant-based scheduling that accurately identifies stress in that it is a dried product and thus, lower fruit hydration at harvest resulting from water deficits during the season is actually beneficial. The influence of evaporative demand was addressed by

Table 1 Values of midday stem C in MPa expected for fully irrigated prune trees under different air temperature and relative humidity conditions Air relative humidity (RH) (%) Temperature ( C)

10

20

30

40

50

60

70

6.9

0.68

0.65

0.62

0.59

0.56

0.53

0.50

9.7

0.73

0.70

0.66

0.62

0.59

0.55

0.52

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12.4

0.79

0.75

0.70

0.66

0.62

0.58

0.54

15.2

0.85

0.81

0.76

0.71

0.66

0.61

0.56

18.0

0.93

0.87

0.82

0.76

0.70

0.64

0.58

20.8

1.02

0.95

0.88

0.82

0.75

0.68

0.61

23.6

1.12

1.04

0.96

0.88

0.80

0.72

0.65

26.3

1.23

1.14

1.05

0.96

0.87

0.78

0.68

29.1

1.36

1.26

1.15

1.04

0.94

0.83

0.73

31.9

1.51

1.39

1.26

1.14

1.02

0.90

0.78

Source: Adapted from Ref.[15].

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Table 2 Suggested target levels of midday stem C in MPa during the growing season in prunes Month Period Early-

March 0.6

April

May

June

July

0.8

0.9

1.1

1.2

August 1.3

September 1.5

Mid-

0.6

0.8

0.9

1.0

1.2

1.3

1.4

Late-

0.7

0.9

1.0

1.1

1.2

1.4

1.5

Source: Adapted from Ref.[16].

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plant-based monitoring technique performs best under a given set of conditions.

OVERCOMING BARRIERS FOR GROWER ADOPTION While scientists may disagree over which plant-based monitoring approach provides the most sensitive indicator of water stress and how stress impact yield, onfarm personnel responsible for irrigation scheduling have much simpler question that requires an affirmative answer for method adoption to occur: ‘‘Will it make their jobs easier?’’ A secondary issue is whether it is cost effective and the time profitably used. It is believed that the current state-of-the-art in plant-based monitoring is stem C for the following reasons: 1) it is grounded in proven scientific principles; and 2) it has been adopted by some growers. Its primary shortcoming is that the measurement is manually taken, requiring the technician to make trips to the field. Moreover, the measurements must be made within a 2 hr or so period around midday. Additional negatives are that a relatively bulky pressure chamber must be hauled to the field (although a lighter ‘‘pump-up’’ version is now commercially available), and data processing is manual. Continuously recorded trunk diameter measurements overcome the logistical problems and labor requirements of the stem C measurements. Since they are electronically recorded, graphical representation, which is a powerful incentive for grower adoption, is relatively easy. Electronically recorded measurements also facilitate data analysis. One can envision algorithms in a computer/controller that not only derive indicator parameters, such as MDS, but also generate and execute system-operating times. Additionally, the rapid availability and ease of data transfer on from local to global networks offer the prospect of irrigation control and consultation from remote locations. To be sure, there are trade-offs associated with using trunk diameter measurements in irrigation scheduling. The data are most variable (‘‘noisy’’) than corresponding stem C values.[18] The special LVDT mounts currently cost

Irrigation– Journals

If the measured signal is below the threshold, the irrigation rate is lowered by 10%. Their goal was to have the actual signal oscillate as close as possible around the selected threshold level. This approach was recently validated in a field study comparing different target threshold values. Goldhamer et al.[18] conducted an analysis of the sensitivity of these scheduling approaches to progressively greater water deficits on mature peach trees under high-frequency irrigation. Reference values were determined using control trees under non-limiting soil moisture conditions (irrigation based on a weighing lysimeter). Stem C and MDS measurements during both the deficit irrigation period and when full irrigation was reintroduced to the stressed trees tracked each other well (Fig. 1A, B). Both stem C and MDS were equal in identifying the onset of stress but after the first few days, the MDS signal was clearly higher than the stem C signal for the remainder of the stress period (Fig. 1C). Following the return to full irrigation, both indicator signals remained well-above, 1.0, reflecting the fact that the soil water reservoir was not refilled. In addition to signal strength, the variability of the indicator values will determine the usefulness of the signal in irrigation scheduling. There have been reports that MDS is more variable than SWP in tree crops.[17,19,20] High measurement variability (‘‘noise’’) would require more tree measurements to decrease the uncertainty, increasing the monitoring costs. Goldhamer et al.[18] found that MDS variability was higher than that of SWP under mild to moderate stress but lower than MDS under severe water stress (Fig. 1D, E). During the entire stress range, SWP variability remained unchanged. The signal/noise ratio, which integrates both the indicator strength and variability, was equal for both stem C and MDS under mild to moderate stress (Fig. 1F). In other words, even though the MDS signal was higher than stem C in this stress range, the increased variability reduced the usefulness as a scheduling indicator. However, under severe stress, the MDS signal/noise ratio was about five times higher than the peach stem C value. This example clearly illustrates that a multitude of factors must be taken into account in evaluating which

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Fig. 1 For mature peach trees under both full (Control) and deficit irrigation (Stress) through July-31 followed by a return to full irrigation: A) stem water potential (SWP); B) maximum daily trunk shrinkage (MDS); C) MDS and SWP signal strength (Stress/Control); D) MDS coefficient of variation (noise); E) SWP coefficient of variation (noise), and F) MDS and SWP ‘‘signal/noise’’ ratio. Vertical bars are two standard errors. Source: From Ref.[18].

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$US 40–80 and LVDTs are $US 150–250. Repairing electronic devices is beyond the ability of most on-farm personnel. Additionally, dataloggers located in the field require periodic trips by a technician to download data unless wireless communication has been established. The decreasing availability and increasing cost of water for agriculture will be incentives to improve irrigation management that growers will be unable to ignore in the future. Using plant-based methods to quantify stress and adjusting irrigation schedules

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accordingly should become more prevalent, especially in high water cost areas and with high value crops. They can be used as stand-alone methods or more likely, to augment and fine tune the more widely available types of atmospheric-based approaches. Indeed, a combination of scheduling techniques may prove optimal, especially for cropping situations where early season stress is desirable. For example, the quality of wine grapes has been shown to improve with deficit irrigation prior to veraison (berry color change).

Irrigation Scheduling: Plant Indicators (Field Application)

Williams (personal communication) recommend that leaf water C be maintained at 1.0 MPa (mild stress) early in the season and then irrigated at 60% ETc from veraison to harvest. Imposing early season stress based on irrigating at certain percentages of ETc is difficult since it is necessary to accurately characterize and account for the amount of water available in the soil moisture reservoir. Early season plant-based monitoring overcomes this problem. As growers become more sophisticated in their irrigation programs, especially those that utilize water stress beneficially (RDI), plant-based monitoring, whether they taken manually or recorded electronically, will become a more valuable tool for use in production agriculture.

REFERENCES

10. Leigh, T.F.; Grimes, D.W.; Dickens, W.L.; Jackson, C.E. Planting pattern, plant population, irrigation, and insect interactions in cotton. Environ. Entomol. 1974, 3, 492–496. 11. Goldhamer, D.A.; Phene, B.C.; Beede, R.; Scherlin, L.; Brazil, J.; Kjelgren, R.K.; Rose, D. Water management studies of pistachio: I. Tree performance after two years of sustained deficit irrigation. California Pistachio Industry Annual Report 1985–86; 1986; 104–112. 12. Van de Urie, M.; McMutry, J.A.; Huffaker, C.B. Ecology of tetranychid mites and their natural enemies: a review. III. Biology, ecology, pest status, and host– plant relations of tetranychids. Hilgardia 1972, 41, 343–432. 13. Teviotdale, B.L.; Michailides, T.J.; Goldhamer, D.A.; Viveros, M. Reduction of almond hull rot disease caused by Rhizopus Stolonifer by early termination of preharvest irrigation. Plant Dis. 1995, 79 (4), 402–405. 14. Teviotdale, B.L.; Goldhamer, D.A.; Viveros, M. Effects of deficit irrigation on hull rot disease of almond trees caused by Monilinia fructicola and Rhizopus stolonifer. Plant Dis. 2001, 85 (4), 399–403. 15. Shackel, K.A.; Ahmadi, H.; Biasi, W.; Buchner, R.; Goldhamer, D.; Gurusinghe, S.; Hasey, J.; Kester, D.; Krueger, B.; Lampinen, B.; McGourty, G.; Micke, W.; Mitcham, E.; Olson, B.; Pelletrau, K.; Philips, H.; Ramos, D.; Schwankl, L.; Sibbett, S.; Snyder, R.; Southwick, S.; Stevenson, M.; Thorpe, M.; Weinbaum, S.; Yeager, J. Plant water status as an index of irrigation need in deciduous fruit trees. HortTechnology 1997, 7 (1), 23–29. 16. Shackel, K.; Lampinen, B.; Southwick, S.; Goldhamer, D.; Olson, W.; Sibbett, S.; Krueger, W.; Yeager, J. Deficit irrigation in prunes: maintaining productivity with less water. HortScience 2000, 35, 30–33. 17. Goldhamer, D.A.; Fereres, E. Irrigation scheduling protocols using continuously recorded trunk diameter measurements. Irrig. Sci. 2001, 20, 115–125. 18. Goldhamer, D.A.; Fereres, E.; Cohen, M.; Girona, J.; Mata, M.; Soler, M.; Salinas, M. Comparison of continuous and discrete plant-based monitoring for detecting tree water deficits and barriers to grower adoption for irrigation management. Acta Hort. 2000, 537, 431–445. 19. Ginestar, C.; Castel, J.R. Utilitzacion de Dendrometro Como Indicadores de Estres Hidrico en Mandarinos Jovenes Regados por Poteo. Riegos Drenajes XXI 1996, 89, 40–46. 20. Naor, A. Midday stem water potential as a plant water stress indicator for irrigation scheduling in fruit trees. Acta Hort. 2000, 537, 447–454.

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1. Hsiao, T.C. Plant responses to water stress. Annu. Rev. Plant Physiol. 1973, 24, 519–570. 2. Vaux, H.J.; Pruitt, W.O. Crop-water production functions. In Advances in Irrigation; Hillel, D., Ed.; Academic Press: New York, 1983; Vol. 2, 61–93. 3. Turner, N.C. Adaptation to water deficits: a changing perspective. Aust. J. Plant Physiol. 1986, 13, 175–190. 4. Grimes, D.W.; Yamada, H.; Dickens, W.L. Functions for cotton (Gossypium Hirsutum L.) production from irrigation and nitrogen fertilization variables: I. Yield and evapotranspiration. Agron. J. 1969, 61, 769–773. 5. Fereres, E. Variability in adaptive mechanisms to water deficits in annual and perennial crop plants. Bull. Soc. Bot. Fr. Actual Bot. 1984, 131, 17–32. 6. Chalmers, D.J.; Mitchell, P.D.; van Heek, L.A.G. Control of peach tree growth and productivity by regulated water supply, tree density, and summer pruning. J. Am. Soc. Hort. Sci. 1981, 106, 307–312. 7. Girona, J.; Mata, M.; Goldhamer, D.A.; Johnson, R.S.; DeJong, T.M. Patterns of soil and tree water status and leaf functioning during regulated deficit irrigation scheduling in peach. J. Am. Soc. Hort. Sci. 1993, 118 (5), 580–586. 8. Marsal, J.; Girona, J. Relationship between leaf water potential and gas exchange activity at different phenological stages and fruit loads in peach trees. J. Am. Soc. Hort. Sci. 1997, 122 (3), 415–421. 9. Goldhamer, D.A.; Salinas, M.; Crisosto, C.; Day, K.R.; Soler, M.; Moriana, A. Effects of regulated deficit irrigation and partial root zone drying on late harvest peach tree performance. Acta Hort. 2002.

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Irrigation Scheduling: Plant Indicators (Measurement) David A. Goldhamer Kearney Agricultural Center, University of California, Parlier, California, U.S.A.

INTRODUCTION

The Pressure Chamber

Even though optimizing plant biomass or fruit production is the goal of irrigation, the plant is rarely the primary focus in irrigation scheduling techniques. Atmospheric techniques, where the plant is considered only tangentially when evapotranspiration (ETc) is estimated, and soil moisture monitoring with a wide variety of instruments are by far the dominant scheduling approaches in use today. The plant sits midway in the soil, plant, and atmospheric continuum and is the integrator of both the water status of the soil and the atmosphere. Moreover, virtually all plant processes that ultimately affect productivity are directly or indirectly linked to plant water status.[1] Although there are a variety of methods to measure or infer plant water status, few references in the literature propose the use of directly measured or inferred plant water status measurements in irrigation scheduling.[2–5] The primary reason for this may involve the difficulties in interpreting plant water status measurements due to their interactions with evaporative demand and crop specific physiological factors. On-farm use of plant-based scheduling is exceedingly low. However, the increasing importance of agricultural water productivity and recent advancements in equipment and sensors for plant-based monitoring focus renewed interest in this scheduling approach.

The pressure chamber requires that a leaf be excised, placed, and sealed in a chamber with the cut petiole end sticking out, and then the chamber is pressurized with nitrogen gas until xylem sap just appears at the end of the petiole.[8] The ‘‘balancing pressure’’ created by the compressed gas in the chamber is, under reasonable assumptions, a measure of the leaf C. There are a variety of techniques used and precautions recommended when using the pressure chamber to determine leaf C. Of particular importance is to minimize water loss from the leaf between excision and placement in the chamber. This can be accomplished by covering the leaf with damp cheesecloth or a small plastic bag prior to excision and placing the leaf/cloth/bag combination in the chamber. Some pressure chamber operators blow into the plastic bag just before placing it over the leaf in order to create high enough humidity to minimize transpiration. The rate that the chamber is pressurized also can influence the reading.[9] Hsiao[6] recommends pressurizing at a rate of less than 0.1 MPa sec 1 at the beginning of the measurement and 0.02 MPa sec 1 as the balancing pressure is approached. Turner[10] suggested a pressurization rate of 0.025 MPa sec 1. Fast pressurization rates can also cause adiabatic heating in the chamber. This also can be controlled by using the aforementioned rates of pressurization and by covering the leaf with a plastic bag. A variant in measuring leaf C is stem C. Here, an interior, shaded leaf is covered by a small plastic bag overlaid with aluminum foil for a period of time prior to excision. Shackel et al.[4] suggested that this period be a minimum of 2 hr while Fulton et al.[11] indicate that transpiration ceases with 15 min of bagging. The elimination of transpiration results in an equilibrium in C between the leaf and the adjacent stem and presumably in a tree, the trunk. Thus, stem C should be higher than leaf C, less coupled to the aerial environment, more representative of the whole tree water status, and less variable than leaf C measurements, that are influenced by stomatal behavior and leaf shade

MEASURING PLANT WATER STATUS

Irrigation– Journals

The most common parameter used for characterizing the water status of plants is the water potential (C). There are a variety of instruments that have been used to measure plant water potential that are covered in detail by Hsiao.[6] The instrument that best fit the requirements for use in irrigation programming is the pressure chamber. In addition, there are other techniques best suited for laboratory conditions such as thermocouple psychrometry[7] and the Shardakov dye method.[6]

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history. Naor[12] found that stem C was a better plant water stress indicator than predawn or midday leaf C. Leaf and stem C change with time over the day. Highest values occur at predawn, become more negative during the morning, sometimes reaching a ‘‘plateau’’ for 2–3 hr just after solar noon, and then increasing in the late afternoon and evening. Thus, readings are usually taken during the plateau period for day-to-day comparisons. Indirect evaluation of plant water status may be accomplished by numerous plant-based parameters that are related to plant C. Many of these parameters have logistical and operational advantages over direct measurement techniques. Following is a description of those considered most relevant for irrigation programming.

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fruit size oscillations’ relations to C as well as simply using fruit growth as a scheduling indicator. Linear variable displacement transducers (LVDTs) and strain gauges are the most common types of organ size measuring devices. Since daily stem diameter oscillations are very small (generally less than 300 mm in mature fruit tree trunks), care must be taken to prevent temperature effects on the sensors and mounting hardware. Measures include shading the instruments and using mounting materials with low thermal expansion properties. Stem diameter oscillations are identified by continuously recording organ size using dataloggers. Data are downloaded manually in the field or transmitted by cellular phone or radio signals to computers.

Canopy Temperature

Current models of steady-state porometers can accurately measure stomatal conductance, which is directly related to photosynthesis, and linked to plant water status. However, water deficits need to be moderate to severe to cause stomatal closure; thus, this parameter is not considered as a sensitive indicator for use in irrigation management.

Plant Organ Size Variations Both herbaceous and woody plant stems undergo diurnal oscillations in size due to both hydration and growth.[13] In the short term, size variations due to changing levels of hydration within various plant tissues greatly exceed those resulting from growth.[14] It is generally agreed that the living cells of the phloem, cambium, and parenchyma that surround the xylem provide most of the stored water contributing to size variations.[15] While Irvine and Grace[16] believe that contractions occur within the xylem in response to varying xylem Cp, the consensus is that xylem tissues are almost totally rigid and contribute very little (10–30%) to daily stem size variations.[15,17] It is generally agreed that stem diameter fluctuations are directly related to changes in plant water status.[18,19] While most of the attention in using plant organs as indicators of water status has been on stems, leaf, and fruit size daily fluctuations also occur. However, there is no known commercial use of these techniques. In the case of leaves, this may be due to having to calibrate thickness against an independent measure of C for each leaf.[20] Recent advancements in the sizemeasuring sensors and mounting hardware for large, well attached fruits bode well for future research in

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This technique relies on the fact that water-stressed plants undergo stomatal closure and consequently higher canopy temperatures due to lower transpirational cooling. The development of the infrared thermometer (IRT) made the canopy temperature measurement possible without physically contacting the plant and signaled the start of a high level of research on using the measurement in irrigation management.[21] Theoretical analysis[22] and experimental work[23] evolved the concept of the crop water stress index (CWSI). The CWSI is based on the temperature difference between the canopy (Tc) and surrounding air (Ta), normalized for the vapor pressure deficit of the air. It is calculated based on the ratio of where the Ta value falls between equivalent measured Tc values for a fully irrigated canopy (lower baseline experimentally determined) and severely stressed canopy (upper baseline empirically determined) under equivalent evaporative demand conditions. Protocols for use involve irrigating as to not exceed CWSI threshold values during specific periods of the season. The CWSI method has been tested primarily on field and row crops. The technique requires that canopy temperature be accurately determined with the IRT and thus, soil within the view of the instrument must be avoided, although a method for correcting for soil effects has been developed.[24] This method is not sensitive to detect water deficits so mild that do not reduce crop transpiration.

Expansive Growth The growth rate of leaves and stems is one of the most sensitive of all plant processes to water stress.[1] With indeterminate crops such as cotton, growth rate evaluation is likely the most popular, and certainly the

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oldest, plant method used to time irrigations. This is facilitated by the fact that any reduction in the distance between the growing terminal of the main stem and the reddish color associated with mature main stem tissue is visually very apparent. In addition to visual observations of growth, sensors may be placed on vegetative or reproductive organs to quantify growth. Goldhamer and Fereres[5] reported that this is a viable technique in young peach trees. Regardless of whether the approach is visual or mechanical, physiological and environmental factors other than water status can affect expansive growth. For example, growth with determinate crops ceases shortly after the onset of anthesis.

Sap Flow Sensors using both heat pulse[25] and heat balance[26] techniques have been developed to estimate transpiration in individual plants. Both techniques involve a heat source placed on the stem or trunk of the plant and then thermocouples either inserted into the conducting tissue or placed along the surface of the stem or trunk. One major difference in the techniques is that the heat pulse measures flow velocity and an estimate of the cross sectional area of flow is required to calculate transpiration. Quantifying flow cross sectional area is usually done by visually evaluating conducting tissue, which in the case of trees, requires a core taken with a trunk boring tool. This is difficult and can result in significant errors.[27] Nevertheless, sap flow measurements can be qualitatively useful in identifying differences in transpiration between plants of similar size and the same species. The primary drawback with using sap flow measurement for irrigation scheduling is the same as with stomatal conductance and canopy temperature—transpiration is not affected under mild stress levels likely to affect expansive growth. Many techniques have been developed in plant physiological research to characterize the water status of plants, but only a few, such as those discussed above, have promise in the development of relevant applications for irrigation scheduling.

REFERENCES Irrigation– Journals

1. Hsiao, T.C. Plant responses to water stress. Annu. Rev. Plant Physiol. 1973, 24, 519–570. 2. Grimes, D.W.; Yamada, H. Relation of cotton growth and yield to minimum leaf water potential. Crop Sci. 1982, 22, 134–139. 3. Peretz, J.; Evans, R.G.; Proebsting, E.L. Leaf water potentials for management of high frequency irrigation on apples. Trans. ASAE 1984, 84, 437–442.

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4. Shackel, K.A.; Ahmadi, H.; Biasi, W.; Buchner, R.; Goldhamer, D.; Gurusinghe, S.; Hasey, J.; Kester, D.; Krueger, B.; Lampinen, B.; McGourty, G.; Micke, W.; Mitcham, E.; Olson, B.; Pelletrau, K.; Philips, H.; Ramos, D.; Schwankl, L.; Sibbett, S.; Snyder, R.; Southwick, S.; Stevenson, M.; Thorpe, M.; Weinbaum, S.; Yeager, J. Plant water status as an index of irrigation need in deciduous fruit trees. HortTechnology 1997, 7 (1), 23–29. 5. Goldhamer, D.A.; Fereres, E. Irrigation scheduling protocols using continuously recorded trunk diameter measurements. Irrig. Sci. 2001, 20, 115–125. 6. Hsiao, T.C. Measurements of plant water status. In Irrigation of Agricultural Crops; Stewart, B.A., Nielsen, D.R., Eds.; American Society of Agronomy: Madison, WI, 1990; 30, 243–279. 7. Boyer, J.S. Thermocouple psychrometry in plant research. Proceedings of the International Conference on Measurement of Soil and Plant Water Status, Utah State University: Logan, UT, July 1987; Vol. 3, 26–30. 8. Scholander, P.F.; Hammel, H.T.; Hemmingsen, E.A.; Bradstreet, E.D. Hydrostatic pressure and osmotic potential in leaves of mangroves and some other plants. Proc. Natl Acad. Sci. U.S.A. 1964, 52, 119–125. 9. Naor, A.; Peres, M. Pressure-induced rate affects the accuracy of stem water potential measurements in deciduous fruit trees using the pressurechamber technique. J. Hortic. Biotechnol. 2001, 76 (6), 661–663. 10. Turner, N.C. Techniques and experimental approaches for measurement of plant water status. Plant Soil 1981, 58, 339–366. 11. Fulton, A.; Buchner, R.; Olson, W.; Shackel, K. Rapid equilibration of leaf and stem water potential under field conditions in almonds, walnuts, and prunes. HortTechnology 2001, 11 (4), 609–615. 12. Naor, A. Midday stem water potential as a plant water stress indicator for irrigation scheduling in fruit trees. Acta Hortic. 2000, 537, 447–454. 13. Kozlowski, T.T. Diurnal variations in stem diameter of small trees. Bot. Gaz. 1967, 123, 60–68. 14. Kozlowski, T.T. Shrinking and swelling of plant tissues. In Water Deficits and Plant Growth; Kozlowski, T.T., Ed.; Academic Press: New York, 1972; Vol. 3, 1–64. 15. Brough, D.W.; Jones, H.G.; Grace, J. Diurnal changes in water content of the stems of apple trees, as influenced by irrigation. Plant Cell Environ. 1986, 9, 1–7. 16. Irvine, J.; Grace, J. Continuous measurements of water tensions in the xylem of trees based on the elastic properties of wood. Planta 1997, 202, 455–461. 17. Ueda, M.; Shibata, E. Diurnal changes in branch diameter as indicator of water status of Hinoki cypress, Chamaecyparis obtusa. Trees 2001, 15, 315–318. 18. Klepper, B.; Browning, V.D.; Taylor, H.M. Stem diameter in relation to plant water status. Plant Physiol. 1971, 48, 683–685. 19. Panterne, P.; Burger, J.; Cruiziat, P. A model of the variation of water potential and diameter within a woody axis cross-section under transpiration conditions. Trees 1998, 12, 293–301.

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surface-air temperature and spectral vegetation index. Remote Sens. Environ. 1994, 49, 246–263. 25. Cohen, Y.; Fuchs, M.; Green, G.C. Improvement of the heat pulse method for measuring sap flow in the stem of trees and herbaceous plants. Agronomie 1981, 9, 321–325. 26. Sakuratani, T. A heat balance method for measuring water flow in the stem of intact plant. J. Agric. Meteorol. 1981, 37, 9–17. 27. Cohen, M.; Goldhamer, D.A.; Fereres, E.; Girona, J.; Mata, M. Assessment of peach tree responses to irrigation water deficits by continuous monitoring of trunk diameter changes. J. Hortic. Sci. Biotechnol. 2001, 76 (1), 55–60.

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20. McBurney, T. The relationship between leaf thickness and plant water potential. J. Exp. Bot. 1992, 43 (248), 327–335. 21. Ehrler, W.L.; Isdo, S.B.; Jackson, R.D.; Reginato, R.J. Wheat canopy temperature: relation to plant water potential. Agron. J. 1978, 70 (2), 251–256. 22. Jackson, R.D.; Idso, S.B.; Reginato, R.J.; Pinter, P.J. Canopy temperature as a crop water indicator. Water Resour. Res. 1981, 17, 1133–1138. 23. Idso, S.B. Non-water-stressed baselines: a key to measuring and interpreting plant water stress. Agric. Meteorol. 1981, 29, 213–217. 24. Moran, M.S.; Clarke, T.R.; Inoue, Y.; Vidal, A. Estimating crop water deficit using the relation between

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Irrigation Scheduling: Remote Sensing Technologies Stephan J. Maas Texas Tech University, Lubbock, Texas, U.S.A.

INTRODUCTION Remote sensing is a method of quantifying physical characteristics of an object through measurements that do not require physical contact with the object. The physical characteristics of the object are determined from measurements of electromagnetic radiation, often in specific wavelengths, reflected or emitted by the object’s surface. Although we usually associate remote sensing with satellites or aircraft, some types of remote sensing involve measurements made with groundbased sensors. Researchers have suggested that there are two main characteristics that favor the use of remote sensing as part of a procedure for scheduling the irrigation of agricultural crops. First, remote sensing provides a quantification of the degree of crop water stress derived from measurements made directly on the crop canopy. This is in contrast to inferring crop water stress from measurements of properties of the soil, such as soil water content or soil water potential, in which the crop is grown. Second, remote sensing imagery can show the detailed variability of crop water stress within a field. When field conditions are variable, basing an irrigation strategy on a limited number of measurements may lead to over- or underwatering some parts of the field. Remote sensing imagery can provide a depiction of the variability in crop water stress across a field with a spatial resolution much greater than what can typically be achieved through conventional field measurements, such as water content or leaf water potential.

APPROACHES

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Attempts to use remote sensing in irrigation scheduling have concentrated on the observation of two physical characteristics of the plant canopy: leaf temperature and leaf water content. Each of these characteristics has given rise to distinct approaches in measuring and utilizing remote sensing data in this application. Leaf Temperature The temperature of the leaf surfaces in a plant canopy is the result of the balance between the energy gained 606

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from the surrounding environment and lost to the surrounding environment.[1] During the daytime, when most irrigation-related remote sensing observations are made, the energy balance of a plant canopy can be expressed, Rab þ C ¼ E þ Rem In this expression, Rab is the longwave and shortwave radiant energy absorbed by the leaf canopy from the sun, the sky, and surrounding soil and plant surfaces. C is the sensible heat gained or lost to the surrounding air through convection. If the surrounding air is warmer than the plant canopy, the canopy will gain heat energy from the air. The opposite is true if the surrounding air is cooler than the plant canopy. E is the latent heat energy lost by the plant canopy through transpiration, i.e., the evaporation of water from the leaves. Rem is the longwave radiant energy emitted by the plant canopy, and is a function of the temperature of the leaf surfaces, Rem ¼ esT 4 In this expression, e is the emissivity of the canopy (approximately 1 for leaves), s is the Stefan– Boltzmann constant, and T is the absolute temperature of the leaf canopy (in K). It is through this expression that remote sensing systems can measure leaf temperature, by measuring the longwave radiant energy emitted by the plant canopy. In the energy balance for the plant canopy, the magnitude of E is determined by the ambient micrometeorological conditions (air temperature, humidity, and wind speed) and the degree to which the leaf stomata are open. As crop plants deplete the soil water below the level necessary for optimum growth, the stomata close to restrict the further loss of water from the plants through evaporation. As the magnitude of E is reduced through stomatal closure, the magnitude of Rem must correspondingly increase to maintain the energy balance. This increase is observed through remote sensing as an increase in leaf temperature. Several different approaches have been described for using remote sensing observations of leaf temperature in irrigation scheduling. The most well-documented of these are described in the following three sections.

Irrigation Scheduling: Remote Sensing Technologies

As in most organisms, the biochemical reactions leading to the growth of crop plants are controlled by enzymes. The temperature dependence of enzyme function helps establish the growth rate of a crop plant in a given environment. If the environment is too warm or too cool, enzyme activity will be inhibited and the potential rate of growth will be reduced. A range of temperature has been identified for each of several crops within which the activities of many important enzymes are at optimum levels.[2] This range is called the Thermal Kinetic Window (TKW). In arid and semiarid regions, leaf temperatures may exceed the range specified by the TKW during part of the day, particularly if soil water is limited.[3] Under these circumstances, irrigating the crop can cause leaf temperatures to fall within the TKW by increasing the loss of heat energy from the canopy through evaporation (Fig. 1). Maintaining leaf temperatures within the TKW by this strategy insures that biochemical reactions proceed at optimum rates. Field studies in the Texas High Plains demonstrated that cotton yields could be maximized by irrigating the crop when the observed leaf canopy temperature exceeded 28
C.[4,5] Canopy temperatures were sensed with infrared thermometers, the signals from which could be used to control an automated irrigation system. A refinement of this procedure involved using the time that the crop was above 28
C (the Temperature–Time Threshold, or ‘‘TTT’’) to control irrigation application.[6] Using a TTT of 4 hr resulted in crop yields equaling those achieved with shorter values of TTT, but with less irrigation.

Fig. 1 Diurnal air temperature and foliage temperatures of irrigated and dryland cotton. The solid horizontal lines delimit the TKW and the dashed line illustrates the temperature providing optimum enzyme function. Source: From Ref.[3].

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This concept was commercialized in a system called the Biologically Identified Optimal Temperature Interactive Console, or ‘‘BIOTIC.’’[7] In BIOTIC (Fig. 2), the leaf canopy temperature is measured with an infrared thermometer, and the time that the canopy temperature is above a specified threshold is accumulated by the control unit. When this time accumulation exceeds another predetermined threshold, and taking into account ambient humidity conditions, a signal is sent to turn on the irrigation system. A limitation to BIOTIC is that, early in the growing season when plants are small, measurements of leaf canopy temperature made by the infrared thermometer may be confounded by the higher temperature of the soil surrounding the plants. This might result in the irrigation system being turned on more often than necessary. This problem disappears when the plants reach a size to completely fill the field of view of the infrared thermometer. Since the time accumulation used in BIOTIC is based on essentially continuous measurements of canopy temperature, this approach is limited to ground-based, in-field sensor systems.

Crop Water Stress Index In environments with a high evaporative demand, the temperature of a fully irrigated crop canopy is typically below the ambient air temperature during the day. Thus, the difference between canopy and air temperature has been recognized as an indicator of crop water

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Fig. 2 A BIOTIC system set up in a cotton field. (Photo courtesy of J. Mahan, USDA-ARS.)

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stress. Theoretical and experimental work led to the development of the Crop Water Stress Index, or ‘‘CWSI.’’[8,9] The CWSI expresses the degree to which the evapotranspiration (ET) of the crop approaches the maximum possible value of ET determined by ambient environmental conditions (the potential ET, or ‘‘PET’’), CWSI ¼ 1  ðET=PETÞ In practice, PET can be calculated from ambient meteorological conditions or estimated from measurements of ET from a well-watered grass or alfalfa surface. CWSI is related to the difference between the canopy temperature Tc and air temperature Ta through the expression, CWSI ¼ ½ðT c  T a Þmin  ðT c  T a Þobs   ½ðT c  T a Þmin  ðT c  T a Þmax 

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In this expression, (Tc  Ta)obs is the observed temperature difference, and (Tc  Ta)min and (Tc  Ta)max are the minimum and maximum differences to be expected based on ambient environmental conditions. Empirically derived expressions relating (Tc  Ta)min and (Tc  Ta)max to the ambient saturation vapor pressure deficit have been reported for a number of crops.[10] A limitation of the practical application of CWSI is that its strict derivation does not include soil temperature effects. For aircraft or satellite observing systems, which are usually pointed straight down at a field, the measured surface temperature will be a combination of plant canopy temperature and soil temperature when the canopy does not completely cover the soil surface. An adaptation of the CWSI has been developed that accounts for incomplete vegetation cover.[11] In this approach, called the ‘‘Vegetation Index/Temperature(VIT) Trapezoid,’’ measured surface minus air temperature is plotted vs. a measure of ground cover derived from remote sensing observations in the visible and near-infrared wavelengths. This point should lie within a trapezoid, the vertices of which represent the surface–air temperatures of a wellwatered complete canopy, a severely water-stressed complete canopy, a saturated bare soil surface, and a completely dry bare soil surface (Fig. 3). In this figure, the ratio of the line segments AC/AB represents a Water Deficit Index (WDI) analogous to CWSI, but accounting for the degree of vegetation cover. In practice, the ordinate in the VIT Trapezoid is usually evaluated in terms of a vegetation index, like the SoilAdjusted Vegetation Index,[12] that is derived from remote sensing observations and is proportional to vegetation cover.

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Fig. 3 The VIT Trapezoid. Source: From Ref.[11].

The use of WDI in detecting crop condition in support of irrigation management was demonstrated at the Maricopa Agricultural Center in Arizona.[13,14] It was concluded from these studies that satellite observations were too infrequent to support operational irrigation scheduling based on WDI. Aircraft observations could be obtained with sufficient frequency for this application. To accommodate infrequent remote sensing observations, the VIT Trapezoid was utilized in conjunction with a mathematical model of daily crop growth and water use. In this approach, the model (called ‘‘ProBE’’) provides a daily description of the water status of the crop based on weather and soil conditions, while the model simulation is calibrated through the evaluation of the actual water status of the crop on days with remote sensing observations.[15,16] A desirable feature of this approach is the capability to predict crop water status beyond the current day using weather forecasts or climatological data. In this way, the onset of water stress can be anticipated prior to its occurrence, allowing advance preparations to be made for irrigating the crop.

Spatial Variability in Canopy Temperature When crop plants in a field are adequately watered, their transpiration rates are determined more by the evaporative demand of the ambient atmosphere than by soil conditions. As the plants deplete the soil water in the field to the point of becoming stressed, their transpiration rates become more dependent on soil conditions. As soil conditions typically are more spatially variable across a field than atmospheric conditions, one would expect to see more spatial variability in observed canopy temperature for a water-stressed crop than for an adequately watered crop.

Irrigation Scheduling: Remote Sensing Technologies

Leaf Water Content The interaction of electromagnetic radiation with water contained in plant tissues provides another mechanism for possibly detecting crop water status using remote sensing. Leaf water content is observable in leaf reflectance measurements particularly in nearinfrared wavelengths (0.8–2.5 mm), where a systematic decrease in leaf reflectance is noted with decreasing leaf water content.[19] Several strong water absorption bands (particularly at 1.45 mm and 1.95 mm) are observable in leaf reflectance spectra in the near infrared. Radar backscatter at microwave frequencies in the range 5–10 GHz is also affected by plant canopy and soil water content.[20] While leaf water content can be observed using remote sensing, there are several factors that limit its potential effectiveness in irrigation scheduling. First, leaf transpiration rate is physically related to leaf water potential, not to leaf water content. Thus, there is no direct connection between the observed remote sensing data and crop water status, as is the case with leaf temperature. Second, because leaf water is contained within plant tissue, remotely sensed leaf water content is affected by not only the water content of the tissue, but also how much tissue is present in the observation. Thus, remote sensing observations at near-infrared or microwave wavelengths cannot unambiguously discriminate between the amount of vegetation and the water status of the vegetation. In part because of these reasons, there are currently no widely recognized approaches for

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irrigation scheduling based on remotely sensed leaf water content.

CONCLUSION Several irrigation scheduling approaches based on remotely sensed plant canopy temperature have been demonstrated to be operationally feasible. The TKW approach can be effective in maintaining crop canopy temperatures within the range conducive to optimal enzyme activity and growth. The WDI can compensate for incomplete crop ground cover in remote sensing observations of surface temperature, and can be used with a crop model to predict the onset of water stress. Other procedures involving spatial temperature variability and leaf water content await further development and testing.

REFERENCES 1. Campbell, G.S.; Norman, J.M. An Introduction to Environmental Biophysics, 2nd Ed.; Springer: New York, 1998; 223–246. 2. Burke, J.J.; Mahan, J.R.; Hatfield, J.L. Crop-specific thermal kinetic windows in relation to wheat and cotton biomass production. Agron. J. 1988, 80 (4), 553–556. 3. Burke, J.J.; Hatfield, J.L.; Wanjura, D.F. A thermal stress index for cotton. Agron. J. 1990, 82 (3), 526–530. 4. Wanjura, D.F.; Upchurch, D.R.; Mahan, J.R. Evaluating decision criteria for irrigation scheduling of cotton. Trans. ASAE 1990, 33 (2), 512–518. 5. Wanjura, D.F.; Upchurch, D.R.; Mahan, J.R. Automated irrigation based on threshold canopy temperature. Trans. ASAE 1992, 35 (1), 153–159. 6. Wanjura, D.F.; Upchurch, D.R.; Mahan, J.R. Control of irrigation scheduling using temperature–time thresholds. Trans. ASAE 1995, 38 (2), 403–409. 7. Upchurch, D.R.; Wanjura, D.F.; Burke, J.J.; Mahan, J.R. Biologically-Identified Optimal Temperature Interactive Console (BIOTIC) for Managing Irrigation. U.S. Patent 5,539,637, July 23, 1996. 8. Jackson, R.D.; Idso, D.B.; Reginato, R.J.; Pinter, P.J., Jr. Canopy temperature as a crop water stress indicator. Water Resour. Res. 1981, 17, 1133–1138. 9. Idso, S.B.; Jackson, R.D.; Pinter, P.J., Jr.; Reginato, R.J.; Hatfield, J.L. Normalizing the stress-degree-day parameter for environmental variability. Agric. Meteorol. 1981, 24, 45–55. 10. Idso, S.G. Non-water-stressed baselines: a key to measuring and interpreting plant water stress. Agric. Meteorol. 1982, 27, 59–70. 11. Moran, M.S.; Clarke, T.R.; Inoue, Y.; Vidal, A. Estimating crop water deficit using the relation between surface–air temperature and spectral vegetation index. Remote Sens. Environ. 1994, 49, 246–263. 12. Huete, A.R. A soil-adjusted vegetation index (SAVI). Remote Sens. Environ. 1988, 27, 47–57.

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Studies involving corn in the U.S. Great Plains suggest that irrigation should be applied when the range of six measurements of canopy temperature made within a field exceeds 0.7
C.[17] Researchers in Arizona have developed a ‘‘Histogram-derived Crop Water Stress Index’’ (HCWSI) that measures the departure of the distribution of measured canopy temperatures in a field from a normal distribution.[18] This approach is based on the observation that the distribution of canopy temperature measurements becomes skewed as the crop becomes stressed. An advantage of scheduling irrigation based on this approach is that the variability in observed temperature, and not the precise measurement of temperature, is used as an indicator of the onset of stress. Thus, uncalibrated thermal images of a field might be sufficient for this application. Previous studies have been conducted on fields where the crop completely covers the soil surface. Observed surface temperature variability resulting from the variability in crop ground cover might seriously confound the measurements used in this approach.

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13. Moran, M.S. Irrigation management in Arizona using satellites and airplanes. Irrig. Sci. 1994, 15, 35–44. 14. Moran, M.S.; Clarke, T.R.; Qi, J.; Pinter, P.J., Jr. MADMAC: a test of multispectral airborne imagery as a farm management tool. Proceedings of the 26th International Symposium on Remote Sensing of Environment, Vancouver, BC, Canada, Mar 25-29, 1996; ERIM: Madison, WI, 1996. 15. Moran, M.S.; Maas, S.J.; Pinter, P.J., Jr. Combining remote sensing and modeling for estimating surface evaporation and biomass production. Remote Sens. Rev. 1995, 12, 335–353. 16. Moran, M.S.; Maas, S.J.; Clarke, T.R.; Pinter, P.J., Jr.; Qi, J.; Mitchell, T.A.; Kimball, B.A.; Neale, C.M.U. Modeling/remote sensing approach for irrigation scheduling. Proceedings of the International Conference on Evapotranspiration and Irrigation Scheduling,

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17.

18.

19.

20.

San Antonio, TX, Nov 3–6, 1996; ASAE: St. Joseph, MI, 1996. Clawson, K.L.; Blad, B.L. Infrared thermometry for scheduling irrigation of corn. Agron. J. 1982, 74, 311–316. Bryant, R.B.; Moran, M.S. Determining crop water stress from crop temperature variability. Annual Research Report; U.S. Water Conservation Laboratory, USDA-ARS: Phoenix, AZ, 1999; 124–127. Gausman, H.W. Plant Leaf Optical Properties in Visible and Near-Infrared Light; Texas Tech University Press: Lubbock, TX, 1985; 24–30. Moran, M.S.; Vidal, A.; Troufleau, D.; Qi, J.; Clarke, T.R.; Pinter, P.J., Jr.; Mitchell, T.A.; Inoue, Y.; Neale, C.M.U. Combining multifrequency microwave and optical data for crop management. Remote Sens. Environ. 1997, 61, 96–109.

Irrigation Scheduling: Soil Water Status Philip Charlesworth Davies Laboratory, Commonwealth Scientific and Industrial Research Organisation (CSIRO), Townsville, Queensland, Australia

Richard J. Stirzaker Commonwealth Scientific and Industrial Research Organisation (CSIRO), Canberra, Australian Capital Territory, Australia

INTRODUCTION There are three methods for matching irrigation with crop water requirements. The first is to measure how much water the soil contains. The second is to monitor some attribute of the plant that is related to water deficits. The third is to calculate how much water the atmosphere can extract from a well-watered crop. This article is about the first method, irrigation management by soil water status. Successful irrigation by this method requires more than just the ability to measure soil water status. We need to know how to relate measurements of soil water status to the amount and timing of irrigation, and how this ultimately affects crop yield.

water is held more strongly in smaller pores or closer to the soil particles themselves. To obtain water, the roots must be in contact with the water films around the soil particles and in effect, the roots must be ‘‘drier’’ or exert more ‘‘pull’’ on the water than the soil. The size of this ‘‘pull’’ can be expressed in energy terms and is called soil water or matric potential. It is usually measured in kilopascals (kPa) and this gives a measure of the force needed to extract water from the soil matrix. When the soil is wet, little force is needed, as it dries, more force or pull is needed. A soil water retention curve (SWRC) is used to convert the amount of water in a soil (volumetric water content) to its availability (energy status as given by the matric potential). As the clay content of a soil increases, the SWRC curve is displaced towards higher water contents (Fig. 1).

DEFINITIONS OF SOIL WATER STATUS MEASURING SOIL WATER STATUS Field monitoring of soil water potential began in the 1930s with the development of the tensiometer.[2] Routine, non-destructive measurements of soil water content were made possible by the development of the neutron scattering technique.[3] The last 20 yr have seen the rapid development of new tools for measuring soil water content, particularly measurements based on time domain reflectometry, capacitance, and heat dissipation.[4] A description of 25 commercially available products for measuring matric potential and soil water content and their mode of operation has been produced.[5]

USING SOIL WATER MEASUREMENTS During irrigation the soil water potential rises close to 0 kPa. If the application rate exceeds the infiltration rate of the soil, then the soil water potential rises to 0 kPa and water ponds on the soil surface. Immediately after irrigation large pores drain rapidly, so the wetted 611

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Soil consists of a range of different sized particles from fine clays (2 mm). Water adheres to the surface of these particles, so soils with a finer texture (more clay) can store more water than soils with a sandier texture. The soil particles are also arranged so as to produce aggregates and pores or voids, giving the soil the property termed structure. Pores with a diameter in the range of 0.5–50 mm are important for storing water. Pores larger than these are normally air-filled and water in pores smaller than this is usually not available to plants. For the purposes of irrigation, the water status of the soil is usually expressed as the volume of water in a given volume of soil. Thus, if a cubic meter of soil contained 300 L of water the volumetric water content would be 0.3 m3 of water per cubic meter of soil ð0:3 m3w m
3 s Þ or 30%. Since rain and irrigation are measured in depths (mm) it is often easier to express 0.3 m3 m
3 as a depth equivalent, i.e., 300 mm of water in 1-m depth of soil. Plants can easily extract water from wet soil because the water is held in large pores. As soil becomes drier,

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Fig. 1 Soil water retention curves for a range of soils from A—sand, to F—heavy clay. Source: From Ref.[1].

depth of soil increases and the average water content of the topsoil falls. The rapid drainage phase is usually completed within 2 days and the soil water content at this stage is termed the drained upper limit (DUL) (Fig. 2). In reality, drainage continues indefinitely, although at slower and slower rates, so the DUL is not an intrinsic property of the soil. However, the drainage rate at the DUL is generally very much lower than the other source of water loss from soil,

Irrigation Scheduling: Soil Water Status

evaporation, so for practical irrigation purposes DUL is a convenient measure of the full point. A plant will extract water from the full profile until the soil is so dry that the plant wilts, even if the relative humidity around the leaves is near 100%. The soil water content at this stage is called the lower limit (LL). The amount of water between the DUL and LL is called plant available water (PAW), a term first used in 1949.[6] The main assumptions, errors, and remedies in deriving these terms have been summarized.[7] The terms ‘‘Field Capacity’’ and ‘‘Wilting Point’’ are also used to describe the range of soil water availability. In practice, water becomes increasingly less available to plants between the DUL and LL. Some studies have shown that growth can be sustained until 70–80% of the PAW has been consumed.[8] For practical purposes a more conservative value of 0.5 PAW is often recommended as the depletion amount below DUL at which the profile should be refilled. This amount is referred to as readily available water (RAW). The idea of having this conservative refill point is to avoid the possibility of growth and yield reductions from the drying conditions in commercial crops. The amount of water readily available to a crop is calculated as the RAW multiplied by the rooting depth. Table 1 gives examples of the average values for the water contents at DUL and the LL and the RAW over 1-m depth of soil for a range of soil textures. When considering the water availability, a soil water potential of 0 kPa indicates the soil would be saturated or waterlogged. Most soils have sufficient air for root function once the soil water potential drops to

Irrigation– Journals Fig. 2 The change in water content during an irrigation event followed by 10 days of drying. The soil water content falls rapidly immediately after irrigation when drainage dominates, and slows after the readily available water has been transpired. (Assumes constant evapotranspiration.)

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Irrigation Scheduling: Soil Water Status

613

Sand

Loam

Clay

Saturation

0.4

0.5

0.6

DUL

0.06

0.29

0.41

LL RAW for 1 m root zone

0.02

0.05

0.2

20 mm

120 mm

105 mm

Source: Adapted from Ref.[9].


5 kPa. Once the soil dries to
50 kPa, many plants will experience some water stress. By
1500 kPa, only small amounts of water can be extracted from the soil and most plants will wilt.

WATER DEFICIT AND PLANT GROWTH The calculation of RAW above is a useful approximation but belies much of the complexity of soil plant water relations. In reality the rate of water uptake needed to sustain rapid growth is determined by the atmospheric conditions, the leaf area, rooting distribution of the crop and the soil type, and water status.[10–12] More recently, evidence is accumulating that chemical signals produced by roots growing in drying soil affects plant growth. Stomata may close and growth rates fall before there is any detectable change in leaf water potential.[13] These signals could also lead to the root distribution changing to exploit wetter regions of the soil. Such adaptation may be limited by both soil strength and crop growth phase. The majority of crops are most sensitive to water deficits at the time of flowering and fruit set and more tolerant during the vegetative and maturation stages.[14]

OLD VS. NEW CONCEPTS IN IRRIGATION The calculation of readily available water represents the maximum extraction of water before the crop yield may decline. Such a concept has its roots in flood irrigation and sprinkler irrigation systems where pipes had to be moved from field to field. Flood irrigation is best suited to applying large volumes of water at infrequent intervals, and the method specified how long that interval could be. In the case of sprinkler irrigation, a long interval cuts down the cost associated with moving pipes. The advent of drip and micro irrigation and center pivot or lateral move sprinkler systems has changed the focus away from refill points. Since these systems allow irrigation to be performed at virtually anytime, there is no need to approach the refill point and the

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associated dangers of drying the soil out too much. The aim is to keep the soil near the full point by applying water daily, or at most, weekly. Drip and micro irrigation only wet part of the root zone, so the wetted volume of soil is much smaller than for flood or sprinkler. This means irrigation management must be more precise, as the reduced amount of stored water increases the risk to crop health of, for example, equipment failure. In many cases, these systems have been shown to produce higher yields. Micro irrigation also entails special problems related to the placement of soil water sensors with respect to the emitters.[15] Interpretation of soil water content measurement is also more difficult for frequent irrigation, where plant uptake, redistribution, and drainage of water can be occurring simultaneously. Few irrigators understand that water can be moving into a layer of soil at the same rate as it is moving out, and make the mistake of interpreting a flat water content vs. time trace as evidence of no drainage. There is also a much greater understanding of the way in which plants respond to water stress at different growth stages. For example, the practice of regulated deficit irrigation (RDI) can save up to 60% of the seasonal water requirement with little effect on fruit yield or quality.[16] The deficit is allowed to develop after flowering and fruit set at the time vegetative growth is at its maximum, and removed when the fruit size starts to increase. A variant of RDI is partial root zone drying, where half the root zone of a perennial crop is irrigated and the other half allowed to dry.[17] Irrigation is alternated such that the previously dry half is reirrigated and the previously wet half allowed to dry. In this way, half the roots are well watered and half experience drying soil. Roots in the drying soil produce hormonal signals that reduce vegetative growth and provide a more favorable balance between vegetative and fruit production. Again yields are unaffected by the stress or even increased. Of even greater importance is the impact of these controlled stresses on fruit quality, particularly in the wine industry.

ADOPTION BY IRRIGATORS Irrigation management by soil water status is a method that has been promoted for over 50 yr. However, despite decades of extension work, the uptake by irrigators remains disappointingly low. There are several reasons for this. In most situations water is not a major percentage of the variable costs and many irrigators baulk at the time and expense of collecting, interpreting, and implementing the information soil water sensors provide. In practical terms the cost to the individual farmer of overirrigation is less than the

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Table 1 Representative volumetric water contents (m3 m
3) and the readily available water in 1 m of soil for soils of three textures

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Irrigation Scheduling: Soil Water Status

penalty of under irrigating, and there is no doubt large quantities of irrigation water are wasted as a result.[18] Water treatment, algal bloom control, salinity are all examples of off-site impacts of overirrigation, which are generally not included in the cost–benefit analysis. Point to point variability in soil water content within a field is also a major disincentive for soil water monitoring. This variability is due to changes in soil properties, plant growth, and non-uniformity of water application. To properly account for such variation requires the installation of far more equipment than is practicable.[19] The problem of variability makes the atmosphere-based methods of irrigation scheduling more attractive. However, errors in this method, particularly related to the estimation of leaf area and root distribution, means that some combination of atmospheric and soil based measurement will provide the most robust feedback system. Irrigation scheduling by soil water status should show further improvements through the development of new soil water measuring technology, and particularly the software associated with them that simplifies the interpretation of data for the irrigator.

14.

REFERENCES

15.

1. Williams, J. Physical properties and water relations. Soils: An Australian Viewpoint; CSIRO Division of Soils: Melbourne, 1983. 2. Richards, L.A.; Neal, O.R. Some field observation with tensiometers. Soil Sci. Soc. Am. Proc. 1936, 1, 71–91. 3. Gardner, W.; Kirkham, D. Determination of soil moisture by neutron scattering. Soil Sci. 1952, 73, 391–401. 4. White, I.; Zegelin, S.J. Electric and dielectric methods for monitoring soil-water content. In Handbook of Vadose Zone Characterization & Monitoring; Wilson, L.G., Everett, L.G., Cullen, S.J., Eds.; Lewis Publishers: Boca Raton, 1995. 5. Charlesworth, P.B. Irrigation Insights No. 1—Soil Moisture Monitoring; National Program for Irrigation

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6.

7. 8.

9.

10. 11. 12. 13.

16.

17.

18.

19.

Research and Development; CSIRO Publishing: Melbourne, Australia, 2000. Veihmeyer, F.J.; Hendrickson, A.H. Methods of measuring field capacity and wilting percentages of soils. Soil Sci. 1949, 68, 75–94. Ritchie, J.T. Soil water availability. Plant Soil 1981, 58, 327–338. Meyer, W.S.; Green, G.C. Water use by wheat and plant indicators of available soil water. Agron. J. 1980, 72, 253–257. Marshall, T.J.; Holmes, J.W.; Rose, C.W. Soil Physics, 3rd Ed.; Cambridge University Press: Cambridge, UK, 1996. Slayter, R.O. Plant–Water Relationships; Academic Press: London, 1967. Gardner, W.R. Dynamic aspects of water availability to plants. Soil Sci. 1960, 89, 63–73. Philip, J.R. Plant water relations: some physical aspects. Annu. Rev. Plant Physiol. 1966, 17, 245–268. Passioura, J.B. The yield of crops in relation to drought. In Physiology and Determination of Crop Yield; Boote, K.J., Bennett, J.M., Sinclair, T.R., Paulsen, G.M., Eds.; American Society of Agronomy: Madison, 1994; 343–359. Rudich, J.; Kalmar, D.; Geizenber, G.C.; Harel, S. Low water tensions in defined growth stages of processing tomato plants and their effects on yield and quality. J. Hortic. Sci. 1977, 52, 391–399. Or, D.; Coelho, F.E. Flow and uptake patterns affecting soil water sensor placement for drip irrigation management. Trans. ASAE 1996, 39 (6), 2007–2016. Goodwin, I.; Jerie, P.; Boland, A. Water saving techniques for orchards in Northern China. Water Is Gold—Irrigation Association of Australia National Conference and Exhibition Proceedings, Irrigation Association of Australia: Sydney, May 19–21, 1998. Stoll, M.; Loveys, B.; Dry, P. Hormonal changes induced by partial rootzone drying of irrigated grapevine. J. Exp. Bot. 2000, 51, 1627–1634. Stirzaker, R.J. The problem of irrigated horticulture: matching the bio-physical efficiency with the economic efficiency. Agrofor. Syst. 1999, 45, 187–202. Schmitz, M.; Sourell, H. Variability in soil moisture measurements. Irrig. Sci. 2000, 19, 147–151.

Irrigation Scheduling: Water Budgeting Brian Leib Claudio Osvaldo Stockle Biological Systems Engineering, Washington State University, Pullman, Washington, U.S.A.

The basic purpose of irrigation in agriculture is to supply plants with sufficient water to obtain optimum harvestable crop yield and quality. For this purpose, irrigation events must be scheduled to provide the right amount of water at the appropriate time. Water stored in the soil explored by plant roots is the main source of water supply for crop uptake, mainly used to meet evaporation losses from plant surfaces. Soil water budgeting is therefore a primary irrigation scheduling approach for agricultural crops. Why Irrigation Scheduling? Irrigation scheduling consists of determining when and how much irrigation water to apply to crops growing in the field or greenhouses. The purpose is to supply plant water needs to meet given crop yield and quality targets. Late and/or insufficient irrigation may lead to undesirable crop water stress and yield/quality reduction. Excess irrigation generates undesired water percolation in the soil profile beyond the reach of roots. This not only represents a loss of water otherwise available for plant use, but percolating water also transports nutrients, pesticides, and other chemicals into deeper soil layers, eventually reaching groundwater.

WATER BUDGETING The approach to be used for proper irrigation scheduling depends on the method of irrigation. High frequency methods such as drip irrigation do not rely much on soil storage of water. Irrigations are applied to meet crop water use, typically on a daily basis, utilizing a small volume of the soil potentially available for root water extraction. Other irrigation methods such as furrow or move-set sprinkler irrigation rely on the soil profile explored by roots as water storage. Plants draw water from this storage until a point where replenishment by irrigation is required. The speed of storage depletion by crop water uptake (and direct soil surface evaporation), the soil water content

at any given time, and the critical soil water content (allowable depletion) at which irrigation is required are the central elements of irrigation scheduling by water budgeting. The general procedure for irrigation scheduling based on water budgeting was summarized in the early 1970s and can be easily implemented in a simple computer program or an electronic spreadsheet.[1] Irrigation scheduling decision support systems that implement computerized water budgeting by linking real-time weather information and soil and crop databases are now commonly available.[2] The daily soil water depletion within the soil profile effectively explored by roots is calculated as: Di ¼ Di
1 þ ðET
Pe
IRÞi

ð1Þ

where Di is soil water depletion on day i, Di
1 is soil water depletion on day i
1, ET is evapotranspiration (water loss by crop transpiration plus soil surface water evaporation) on day i, Pe is effective precipitation (precipitation depth that infiltrates into the soil) on day i, and IR is net irrigation (water depth actually stored in the root zone) on day i. All quantities are expressed as water depth in mm or in. Fig. 1 illustrates the water balance process described by Eq. (1). Soil water depletion is calculated relative to the upper limit of the soil storage of plant available water (PAW). Soil water may fluctuate from total dryness to saturation, where all porosity is filled with water. The PAW soil storage encompasses a fraction within these two water status extremes. The upper limit corresponds to the water depth equivalent after the soil profile has been fully irrigated and subsequently drained until the drainage rate becomes negligible (24–72 hr after irrigation, depending on soil texture). This is also referred to as the soil field capacity. The lower limit (also known as permanent wilting point) corresponds to the soil water storage level at which plants can no longer remove soil water. The PAW soil storage is the difference between the upper and lower limits. When the soil water content is equal to the upper limit water content, Di ¼ 0. When Di ¼ Do, the allowable depletion for a given crop, an irrigation 615

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INTRODUCTION

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Irrigation Scheduling: Water Budgeting

the topsoil. Evapotranspiration depletes the soil moisture in the soil profile explored by roots. Crop ET can be determined experimentally. However, for irrigation scheduling purposes, ET is normally calculated from weather data. The method consists of the use of empirical equations to calculate ET from a reference crop (ETo), typically defined as a short healthy grass surface, 12-cm high, fully covering the ground and with ample water supply. Evapotranspiration for the crop of interest (ETc) is obtained by multiplying ETo by a crop coefficient (Kc) that fluctuates throughout the growing season based on crop canopy characteristics and its ability to cover the ground. A large number of equations to calculate ETo are available.[4] Evaluations performed have shown the Penman–Monteith equation,[5] a biophysically based formulation, to be suitable for applications across climatic conditions.[6] A complete description of procedures to calculate ETo, Kc, and ETc is given by Allen et al.[7]

Soil Water Content

Fig. 1 Soil water balance in irrigation scheduling.

event should be scheduled. Do corresponds to a water content between the upper and lower limit of PAW. The refill point is closer to the upper limit as the sensitivity of crops to water stress and the atmospheric demand for water evaporation increase. The value of Do also depends on the size of the PAW soil storage. For more information on soils and irrigation scheduling see Ref.[3]. Evapotranspiration

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Soil water evaporation is the process of transformation of liquid water to vapor and subsequent removal from the soil surface. Crop transpiration consists of the transformation of liquid water into vapor inside the plant tissues, and the subsequent removal of vapor to the atmosphere, mainly through stomata. For plants growing under typical field conditions, these two processes occur simultaneously, a phenomenon referred to as evapotranspiration (ET). The ET rate (mm or in per unit time) depends on the energy supply, vapor pressure gradient, and wind as well as on the rate of water supply to plant roots (ultimately to the evaporation sites at the substomatal cavities) and

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Soil water sensors can be used in conjunction with a water balance Eq. (1) to help schedule irrigation. Sensors can be used to verify the soil water content estimated from a water balance, to restart a water balance at a known value of soil water content, or to evaluate the upward or downward trend in soil water content from irrigation designed to replace the loss of water from ET. It is important to understand instrument calibration when using a soil water sensor in conjunction with a water balance. Sensor calibration can vary in different soils types.[8,9] However, the relative accuracy of sensors can be useful in irrigation scheduling without the need for absolute accuracy via calibration.[10] Irrigation scheduling is most successfully implemented when ET driven water balances and soil water sensors function as independent checks to compensate for the inaccuracies of both methods. Fig. 2 shows how irrigation was scheduled by calculating the soil water depletion from both a water balance Eq. (1) and soil water sensors. There are several methods of measuring soil water content and how each type of sensor works is briefly described below. The tensiometer uses a porous ceramic tip in direct contact with the soil to measure soil tension. The granular matrix sensors and resistance blocks measure the change in electrical resistance that occurs as soil water moves in and out of the sensor in response to the surrounding soil moisture. The neutron probe counts the number of neutrons that collide with the hydrogen in water. Tensiometers, resistance sensors, and neutron scattering[11] have a fairly long history of use in irrigation scheduling. Most of the new sensors available on the market today measure

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617

the dielectric constant of the soil. The dielectric constants are of air, mineral soil and water is 1, 3–7, and 80, respectively. Therefore, the overall dielectric constant of bulk soil will vary predominantly in relationship to soil water content. One way to determine the soil’s dielectric constant is by measuring the change in a radio wave’s frequency as it passes through the soil known as frequency domain reflectometry (FDR) or RF capacitance.[12] Another, way to determine the dielectric constant of the soil is to measure the reflectance pattern of a voltage pulse that is applied to a wire guide known as time domain reflectometry (TDR).[13]

CONCLUSION Crop, soil, and weather information can be used to calculate components of the soil water balance, which can be used for irrigation scheduling (when and how much irrigation water to apply). Soil water budgeting is a robust approach, widely used in irrigated agriculture. Soil water calculations must be complemented with measurements of water content in the soil profile effectively explored by plant roots. A large array of sensors is available to measure soil water.

REFERENCES 1. Jensen, M.E.; Wright, J.L.; Pratt, B.J. Estimating soil moisture depletion from climate, crop and soil data. Trans. ASAE 1971, 14, 954–959. 2. Leib, B.G.; Elliott, T.V.; Mathews, G. WISE: a weblinked and producer oriented program for irrigation scheduling. Comput. Electron. Agric. 2001, 33 (1), 1–6, http://www.elsevier.nl/locate/issn/01681699. 3. James, L.G. Principles of farm irrigation system design; John Wiley & Sons: New York, 1988.

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4. Jensen, M.E.; Burman, R.D.; Allen, R.G. Evapotranspiration and Irrigation Water Requirements, ASCE Manuals and Reports on Engineering Practices No. 70; American Society of Civil Engineers: New York, 1990. 5. Monteith, J.L. Evaporation and environment. In 19th Symposia of the Society for Experimental Biology; University Press: Cambridge, 1965; 19, 205–234. 6. Allen, R.G.; Jensen, M.E.; Wright, J.L.; Burman, R.D. Operational estimates of reference evapotranspiration. Agron. J. 1989, 81, 650–662. 7. Allen, R.G.; Pereira, L.S.; Raes, D.; Smith, M. Crop Evapotranspiration: Guidelines for Computing Crop Water Requirements, Irrigation and Drainage Paper No. 56, FAO: Rome, 1998. 8. Yoder, R.E.; Johnston, D.L.; Wilkerson, J.B.; Yoder, D.C. Soil water sensor performance. Appl. Eng. Agric. 1998, 14 (2), 121–133. 9. Hansen, B.R.; Peters, D. Measuring Soil Moisture, Proceedings of the 19th Annual International Irrigation Show, San Diego, CA, Nov 1–3, 1998; The Irrigation Association: Falls Church, VA, 1998, 103–110. 10. Leib, B.G.; Matthews, G. The Relative Accuracy of Soil Moisture Sensors Used in Washington State, Paper for the 1999ASAE/CSAE-SCGR Annual International Meeting, Toronto, Canada, Jul 18–22, 1999; American Society of Agricultural Engineers: St. Joseph, MI; Paper No. 992270. 11. Phene, C.J.; Reginato, R.J.; Itier, B.; Tanner, B.R. Sensing irrigation needs. In Management of Farm Irrigation Systems; Hoffman, G.J., Howell, T.A., Solomon, K.H., Eds.; American Society of Agricultural Engineers: St. Joseph, MI, 1990; 207–264. 12. Ley, T.W.; Stevens, R.G.; Topielec, R.R.; Neibling, W.H. Soil Water Monitoring and Measurement; PNW 475, a Pacific Northwest Publication; Washington State University: Pullman, WA, 1994; 28–29. 13. Topp, G.C.; Davis, J.L.; Annan, A.P. Electromagnetic determination of soil water content: measurement of coaxial transmission lines. Water Resour. Res. 1980, 16, 574–582.

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Fig. 2 Example of a water balance used with soil water sensors in irrigation scheduling.

Irrigation Systems: Drip I. Pai Wu College of Tropical Agriculture and Human Resources, University of Hawaii, Honolulu, Hawaii, U.S.A.

Javier Barragan Department d’Enginyeria Agroforestal, University of Lleida, Lleida, Spain

Vince Bralts Agricutural and Biological Engineering, Purdue University, West Lafayette, Indiana, U.S.A.

INTRODUCTION A drip irrigation system is a form of localized irrigation that delivers water directly into the root zone of a crop. When properly designed and managed, a drip irrigation system can eliminate surface runoff, minimize deep seepage, and achieve high uniformity of water distribution and irrigation application efficiency. The development of drip irrigation in the late 1960s marked a period of tremendous improvement in irrigation science and technology in which water use is done more beneficially for agricultural production. With the increasing consequence of limited water resources and the increasing need for environmental protection, drip irrigation will play an even more important role in the future. Drip irrigation systems can be used for many different types of agricultural crops, including fruit trees, vegetables, pastures, specialty crops such as sugarcane, ornamentals, golf course grasses, and high economic value crops grown in greenhouses. An understanding of drip irrigation systems, irrigation scheduling, crop response, and economic ramifications will encourage greater use of drip irrigation in future agricultural production.

UNIFORMITY OF WATER APPLICATION AND DESIGN CONSIDERATIONS

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The desired uniformity of water application and the specific crops to be grown guides the creation of drip irrigation systems. There are two types of drip irrigation uniformity: system uniformity and spatial uniformity in the field. The consistency of system distribution of water into the field describes the system uniformity. The spatial uniformity is the regularity of water distribution considering overlapping emitter flow and translocation of water in the soil. For drip 618

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irrigation systems designed for trees with large spacing, the system uniformity is equal to the water application uniformity in the field. For high-density plantings, the emitter spacing should be designed considering overlapped wetting patterns and the spatial uniformity in the field. The uniformity of a drip irrigation system depends primarily on the hydraulic design, but must also consider the manufacturer’s variation, temperature effects, and potential emitter plugging. The effect of water temperature is generally negligible when using turbulent flow emitters. A combination of proper filtration and turbulent emitters can control emitter plugging. When grouping a number of emitters together as a unit, such as those designed to irrigate an individual plant’s root system, the uniformity of water application with respect to the plant will improve. Many expressions have been used to describe uniformity. The system uniformity, or emitter flow uniformity, can be expressed as the range or variation of water distribution in the field. This term was initially used for hydraulic design of drip irrigation systems given that the minimum and maximum emitter flows could be calculated and determined.[1] When more emitter flows are used or more samples are required for determining variation or spatial uniformity in the field, the Christiansen uniformity coefficient (UCC)[2] and coefficient of variation (CV), which is the ratio of standard deviation and the mean, are used. Each of the uniformity expressions are highly correlated with one other. HYDRAULIC DESIGN OF DRIP IRRIGATION SYSTEMS Once selection of the type of drip irrigation emitter is complete, the hydraulic design can be made to achieve the expected uniformity of irrigation application.

Irrigation Systems: Drip

619

qvar ¼

qmax
qmin qmax

ð1Þ

where qmax is the maximum emitter flow and qmin is the minimum emitter flow. Based on these data, other uniformity parameters such as UCC and CV can also be determined. There is a strong correlation between any two of the three uniformity parameters in the hydraulic design of drip irrigation systems, thus any one of the uniformity parameters can be used as a design criterion. This correlation also justifies using the simple emitter flow variation qvar for hydraulic design. The emitter flow variation qvar is converted to the CV when it is combined with the manufacturer’s variation of emitter flow. The total emitter flow variation caused by both hydraulic and manufacturer’s variation can be expressed by,[5] CVHM ¼

qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi CV2H þ CV2M

ð2Þ

where CVHM is the coefficient of variation of emitter flows caused by both hydraulic and manufacturer’s variation; CVH and CVM are the coefficients of variation of emitter flows caused by hydraulic design and manufacturer’s variation, respectively. The design criterion for emitter flow variation qvar for drip irrigation design is arbitrarily set as 10.0– 20.0%, which is equivalent to a CV, from 0.033 to 0.076, or 3.0–8.0%. Based on the research of last 30 yr, the manufacturer’s variation of turbulent emitters is maintained only in a range 3.0–5.0%, expressed by CV. When this variation is combined with emitter flow variation caused by hydraulic design with a range 3.0–8.0% in CV, the total variation determined by the equation above will be limited to a CV of less than 10.0%. This variation illustrates that the drip irrigation systems are designed to achieve high uniformity and irrigation application efficiency.

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Economic return can also be the basis of design criteria for drip irrigation. A new set of design criteria for drip irrigation was developed,[6] based on achieving an expected economic return with various water resources and environmental considerations (Table 1).

DRIP IRRIGATION FOR OPTIMAL RETURN, WATER CONSERVATION, AND ENVIRONMENTAL PROTECTION When the uniformity of a drip irrigation system is designed with a UCC of 70.0%, 30.0% or less in CV, the irrigation application is expressed as a straight-line distribution,[7,8] as shown in Fig. 1. This figure was plotted using percent of area (PA) against a relative irrigation depth, X, which is the ratio of required irrigation depth to mean irrigation application. The straight-line distribution in the dimensionless plot can be specified by a minimum value, a, a maximum value, (a þ b), in the X-scale and a slope b, where b specifies the uniformity of water application.[9] When a drip irrigation system is designed with fixed uniformity, it is possible to determine the sloped straight line with known value of a and b. A value (X) can then be selected between value a and (a þ b) and plotted (Fig. 1). The triangle formed above the horizontal line (X) results in an irrigation deficit and yield reduction. The triangle below the horizontal line results in over-irrigation and deep seepage. An important irrigation scheduling parameter, the relative irrigation depth, (X) indicates how much irrigation water is applied. The effectiveness of drip irrigation is shown not only by the high uniformity of the drip irrigation system, but also by the irrigation requirement and the strategy of irrigation scheduling. As illustrated in Fig. 1, the irrigation scheduling parameter (X) affects the areas of over-irrigation and water deficit conditions in the field and is directly Table 1 Design criteria for uniformity of drip irrigation system design Design consideration

CV (%)

UCC (%)

Water is abundant and no environmental pollution problems

30–20

75–85

Water is abundant but with environmental protection considerations

20–10

80–90

Limited water resources but with no environmental pollution problems

25–15

80–90

Considerations for both water conservation and environmental protection

15–5

85–95

Irrigation– Journals

The hydraulic design of a drip irrigation system involves designing both the submain and lateral lines. Early research in drip irrigation hydraulic design concentrated mainly on the single lateral line approach,[1,3,4] but in 1985 Bralts and Segerlind developed a method to design a submain unit. The hydraulic design is based on the energy relations in the drip tubing, the friction drop, and energy changes due to slopes in the field. Direct calculations of water pressures along a lateral line or in a submain unit are made by using an energy gradient line approach.[1] All emitter flows along a lateral line and in a submain can be determined based on their corresponding water pressures. Once the emitter flows are determined, the emitter flow variation, qvar is expressed by

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Fig. 1 A linear water application model for drip irrigation.

related to the economic return. Practically speaking, the X parameter is selected in a range from a to (a þ b), as shown in Fig. 1. Three typical irrigation schedules can be expressed by X and are as follows: X ¼ a

X ¼ X0

X ¼ (a þ b)

This schedule is a conventional irrigation schedule, which is based on the minimum emitter or minimum water application. The field is fully irrigated and whole field is over-irrigated except the point of minimum irrigation application. For an optimal return there is a value of X for the irrigation scheduling parameter between a and (a þ b). This irrigation schedule is based on the maximum emitter flow or maximum irrigation application. The whole field is under deficit condition except the point of maximum water application. There is no deep percolation.

Irrigation– Journals

An optimal irrigation schedule for maximum economic return was determined[9] based on cost of water, price of the yield, and damage such as environmental pollution and groundwater contamination caused by over irrigation. Different irrigation strategies require differ-

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ent amounts of water application. Water conservation and environmental protection are realized by comparing any two of the irrigation strategies.[10]

CONCLUSION Drip irrigation is an irrigation method that can distribute irrigation water uniformly and directly into the root zone of crops. It is one of the most efficient irrigation methods and can be designed and scheduled to meet the water requirement of crop and produce maximum yield in the field. When the drip irrigation system is designed with high uniformity, the slope b of the straight line of water application function (Fig. 1) can be controlled to achieve the desired variation. In this case the conventional irrigation schedule, X ¼ a, optimal irrigation schedule, X0, which is a location between a and (a þ b), and the irrigation schedule for environmental protection, X ¼ a þ b, are in close proximity. This closeness shows that the drip irrigation system can achieve optimal economic return, water conservation, and environmental protection.

REFERENCES 1. Wu, I.P.; Gitlin, H.M. Design of Drip Irrigation Lines; HAES Technical Bulletin 96, University of Hawaii: Honolulu, HI, 1974; 29 pp.

Irrigation Systems: Drip

7. Seginer, I. A note on the economic significance of uniform water application. Irrig. Sci. 1978, 1, 19–25. 8. Wu, I.P. Linearized water application function for drip irrigation schedules. Trans. ASAE 1988, 31 (6), 1743–1749. 9. Wu, I.P. Optimal scheduling and minimizing deep seepage in microirrigation. Trans. ASAE 1995, 38 (5), 1385–1392. 10. Barragan, J.; Wu, I.P. Optimal scheduling of a microirrigation system under deficit irrigation. J. Agric. Eng. Res. 2001, 80 (2), 201–208.

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2. Christiansen, J.E. The uniformity of application of water by sprinkler systems. Agric. Eng. 1941, 22, 89–92. 3. Howell, T.A.; Hiler, E.A. Trickle irrigation lateral design. Trans. ASAE 1974, 17 (5), 902–908. 4. Bralts, V.F.; Segerlind, L.J. Finite elements analysis of drip irrigation submain unit. Trans. ASAE 1985, 28, 809–814. 5. Bralts, V.F.; Wu, I.P.; Gitlin, H.M. Manufacturing variation and drip irrigation uniformity. Trans. ASAE 1981, 24 (1), 113–119. 6. Wu, I.P.; Barragan, J. Design criteria for microirrigation systems. Trans. ASAE 2000, 43 (5), 1145–1154.

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Irrigation Systems: History Wayne Clyma Water Management Consultant, Fort Collins, Colorado, U.S.A.

INTRODUCTION

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Irrigation as a technology probably has existed since 6000 B.C. rather than 3000 B.C. as determined from recorded history. Irrigation started with individual farmers experimenting with carrying water to individual plants or cropped areas and expanded to diversions from streams or building bunds for flooded areas. Surface irrigation began the process of irrigation, but sprinkle and microirrigation have evolved and expanded to present day management with computer managed controls and precise application of water to crops often on a daily basis. Prehistoric man has survived from the food provided from fruits, grains, vegetables, and animals as early as 10,000 B.C. Farming or crop cultivation is believed to have started about 8000 B.C. The first irrigation (Table 1) in Mesopotamia and Northern Africa is believed to have begun as early as 6000 B.C. This early date perhaps more nearly represents the beginning of irrigation rather than the beginning of large scale irrigation.[1] These dates reflect anthropological assessments of the beginning of irrigation rather than the first recorded history or physical evidence of the existence of canal irrigation. The recorded history of large-scale irrigation is reviewed more thoroughly by Willardson. Thailand, and the Yangtze Valley in China are estimated to have started irrigation about 5000 B.C. while the Indus Valley in what is now Pakistan started irrigation about 3500 B.C. Irrigation in Europe is believed to have begun as early as 4500 B.C. while irrigation started about 2500 B.C. in Japan at the influence of China. North and South America, represented by Mexico, started irrigating in about 1500 B.C. Many historians believe irrigated agriculture supported, but also required the development of advanced civilizations. Other historians believe that declines in irrigated agricultural capability led to the decline of civilizations. Others believe the decline of civilizations caused the decline of irrigation.

INVENTING IRRIGATION Irrigating crops likely started as a trial and process with perhaps some container used to carry water to individual fruit trees or small patches of grain or 622

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vegetables. The Pueblo Indians in New Mexico are believed to have irrigated all their corn by such a method. Studies in Mesopotamia showed that early irrigation efforts involved building canals that took water from the river, which was the irrigation supply source, to areas where trees and crops were growing. The form that these early field units took has never been described to my knowledge. Irrigation from the Nile River in Egypt is believed to have started by planting crops in the area where flood waters had receded. Subsequently, dikes were formed to control both entry of water to an area and the time an area was inundated. Studies of areas where farmers have developed small diversion canals to divert water from streams suggest farmers used small basins to control the water supplied to crops. Informal inspections of ancient sites in India suggest hand labor was used to carry and apply water to small areas in a castle when irrigation was needed. Informal reviews of the evolution of irrigation in the North America, South America, Asia, and Africa suggest that initial diversions of water through canals were used to irrigate crops by wild flooding. In the United States, India, and Chile, wild flooding was accomplished by spreading water over large areas without the use of dikes to control the spread of the water. For the United States and Chile, because of the larger field equipment, larger units for farming were used. No such goal was identified in India, but apparently transfer of water control concepts from other irrigated areas never occurred. In Afghanistan, Ethiopia, India, Kenya, Nepal, Somalia, Pakistan, Sri Lanka, and Thailand, small bunded units were constructed to control the water applied to crops. These small units used wild flooding, not the level basins as are frequently assumed, because lateral and longitudinal slopes, and high and low areas occurred in each basin. Even most rice fields are not level basins because within the basin they often have 15 cm or more ranges in elevation. In Egypt, small bunded units (often less than 10 m2) are used. The sizes of the units are believed to be related to the small, variable flow rates that are available for irrigation from Shadoofs, Archimedes’ screws, and sakias (water wheels) using human or animal power. Small units were also used in India where animal power was used for lifting. These units used oxen and a water bag to lift water and supply water to

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Area

Years

B.C.

Middle East Mesopotamia Northern Africa

6,000 6,000

Asia Yangtze Valley in China Thailand Indus Valley in now Pakistan

5,000 5,000 3,500

Europe

4,500

Japan

2,500

North and South America

1,500

irrigate a field. The crops were frequently opium poppies and the high cash value seemed to result in more precise leveling and greater care in growing the crop. Studies in Pakistan have shown that farmers adjust the size of their basin to the flow rate available. Farmers changing the size of field as available flow rates vary has also been observed in other countries. Farmers value fields that are level. Observations and farmer insights repeatedly find that farmers invest considerable labor attempting to level their fields precisely. Those same observations show that precision measuring instruments, surveying levels, or laser guided equipment, are necessary to level basins precisely.

EVOLUTION OF IRRIGATION SYSTEMS Surface irrigation systems were the first widely used method of irrigation. When soils and topography limited the effectiveness of surface irrigation systems, then sprinkle irrigation was invented. Initial sprinkle systems were pipes with holes to allow water to spray on adjacent crops. When shortages of water and need for precise timing and amounts of water became dominant considerations, trickle irrigation systems were developed and used in farm fields. Some equipment for trickle irrigation was adapted from greenhouse systems in England. Other initial trickle irrigation systems were buried pipes with holes next to the plant row or circular pipes placed around a tree. Both sprinkle and trickle irrigation systems have many additional sophisticated improvements since their initial invention. Both sprinkle and trickle irrigation (drip or microirrigation as more recently named) have the distinct advantage of being suited to non-leveled land and to soils not suited for surface irrigation. They also remove the soil, in a large part, as the hydraulic transport media. Pipes are used to transport the water largely eliminating seepage losses from canals and

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‘‘over irrigation’’ at the ‘‘head end’’ of fields and runoff from ‘‘tail ends’’ of fields. In addition, these newer irrigation technologies are better suited to applying smaller application volumes per unit land area reducing longer irrigation intervals typically associated with surface irrigation. All pressure irrigation systems required appropriate maintenance and management or their advantages of higher potential performance are lost.

Evolution of Surface Irrigation Systems As irrigation projects were observed around-the-world, a sequence for evolvement of field surface irrigation systems was identified. Just as irrigation development follows a sequence, the types of field irrigation systems often follow a sequence. This sequence of evolution of surface systems is as follows: Wild flooding ! Border ditch ! Graded borders and furrows ! Level basins Initial irrigation efforts are focused on supplying water when drought or lack of adequate rainfall results in low or undependable crop yields. Providing an irrigation water source is initially focused on methods for delivering water to the field. Usually delivery of water is accomplished through a canal from a lake or river by gravity to the field. Field application of water is accomplished by simply diverting water from the source onto the field. Without leveling of the field surface nor dikes to guide the water across the field, wild flooding is the result as shown in Fig. 1. When flow rates are small and large equipment does not dictate larger field units, small bunded units are used with channels constructed to each bunded unit. In rice

Irrigation– Journals

Table 1 Beginnings of irrigated agriculture in major areas of the world

Fig. 1 Wild flooding in surface irrigation of a field. (Water Management Synthesis II Project, Colorado State Univ., Fort Collins, CO.)

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Irrigation Systems: History

irrigation, channels are often omitted and flow is from bunded unit to bunded unit. Rice irrigation attempts to achieve a continuously flooded condition for weed control not because rice plants must be continuously flooded. Non-rice crops cannot grow with continuous flooded conditions. Therefore, supplying water by allowing it to flow from bunded unit to bunded unit is usually not successful for non-rice crops. Farmers in many countries, including the United States, often repeatedly try to grow non-rice crops with basin to basin flooding with no or limited success. The small bunded unit prevalent in many countries, often incorrectly called level basins, are wild flooded as shown in Fig. 2. Wild flooding is still practiced because within the bunded unit lateral and longitudinal slopes with high and low areas exist. With time, the surface conditions of the field are improved and channels are constructed to carry water into the field to ensure water is available to all the cropped area of the field. In the mountain west of the United States, these field irrigation systems are called border ditch irrigation systems. Illustrated in Fig. 3, water is carried into the field by ditches, bunds are constructed to guide water down the field, but water is distributed within each bunded unit by wild flooding. In countries with small, variable flows available for irrigation, the small basins for wild flooding are essentially a variation of border ditch irrigation systems. With land leveling and smoothing, bunds are constructed down the prevailing slope without any lateral slope between the bunds, and an appropriate flow of water is introduced at the upper end (Fig. 4). These field systems are graded border irrigation systems. They improve the uniformity of water distribution over

the field compared with wild flooding. The flow rate must be adjusted according to the slope and intake rate of the soil or excessive runoff will occur. Continuous flow must be provided until sufficient infiltration is achieved to meet crop requirements. Thus, effective management of graded border irrigation systems by farmers is complex and difficult. Farmers often insist that only trained irrigators or irrigators with experience can manage graded systems. Graded furrows fall in the same category as graded borders. They are the first development from wild flooding. Furrows are directed downslope between row crops with each furrow or alternate furrows supplied an appropriate flow rate as shown in Fig. 5.

Fig. 2 Border ditch irrigation by wild flooding of a field using small bunded units and channels to deliver water within the field. (Water Management Synthesis II Project, Colorado State Univ., Fort Collins, CO.)

Fig. 4 Graded border irrigation systems with bunds to guide the water down the slope and cross slopes eliminated. (Water Management Synthesis II Project, Colorado State Univ., Fort Collins, CO.)

Fig. 3 Border ditch irrigation of larger field units using channels to deliver water within the field by wild flooding the field. (Water Management Synthesis II Project, Colorado State Univ., Fort Collins, CO.)

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Fig. 5 Furrow irrigation of fields with graded furrows, contour furrows, and level furrows. Source: From Ref.[2].

Furrows may also be constructed on contour to limit cross slopes. Flow rates must be adjusted to meet the infiltration requirements of the furrow adequately with sufficient remaining flow to advance down the furrow. If the furrow flow rate is too large, erosion and excessive runoff are the result. If the furrow flow rate is too low, excessive deep percolation may occur at the upper end of the field and the lower end of the field may not receive enough water. Thus, management of graded furrows is complex as are graded borders. Furrows are more adaptable than borders in the sense that they can be constructed on fields with cross slopes if the furrow flow from irrigation or rainfall can be controlled not to exceed furrow capacity. When one furrow breaks, cross slope erosion can be a serious and damaging condition. Furrows can also be constructed in level basins or fields with no slope and result in level furrows. Furrows allow the placement of crops on ridges such that the plants are not covered by water. Vegetables and many field crops produce higher yields when grown on ridges. Level basins are the final targets of evolution in surface irrigation systems (Fig. 6). They provide the management advantage that farmers add water to the basin within a wide range of flow rates for a time such that the target volume of water is applied. If the basins are appropriately designed and managed, then farmers can use advance distance criteria to apply target amounts of water to the level basin without water measurement. Also, advance distance criteria can be used to shut off the water application to a field when a target amount of water is applied. Farmers aroundthe-world use advance distance criteria to apply water to fields, but the basins are not adequately level nor designed to apply target amounts of water to a field. Therefore, the technology is available and farmers are already attempting to use the technology. Using the available technology to apply target amounts

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Fig. 6 Level basin irrigation of a field with all slopes removed and precisely leveled. (Water Management Synthesis II Project, Colorado State Univ., Fort Collins, CO.)

of water to a field would achieve quantitative water management with level basins.[3] Level basins came into use in United States when laser leveling was developed. The large basins, often as large as 8 ha, require large flow rates and the very precise leveling achieved with laser-controlled leveling equipment (Fig. 7). Surface irrigation systems are varied and flexible. They are the most widely used method of irrigation. Level basins can achieve high levels of performance (higher than sprinkle and near the same level as trickle systems) with advantages for farmer management and sometimes system cost. Soils and topography limit the applicability of surface irrigation systems because uniform slopes (level or graded) and medium to lower intake soils of uniform texture are required for

Fig. 7 Laser controlled leveling, for level basins in United States up to 8 ha in size, is now used to accomplish precision land leveling in surface irrigation around-the-world. (Courtesy of Natural Resource Conservation Service, USDA; www.nrcs. usda.gov.)

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effective performance. Higher intake rate soils can be irrigated with surface irrigation, but smaller units are required. Surface irrigation systems are applicable to many irrigated areas and are often the most advantageous systems. They often require the lowest investment to become operational, but unless automated, may involve the highest labor costs. Irrigation was advanced by the coming of mechanical scrappers to form canals and ditches. In addition, canal lining with various materials from concrete to butyl rubber has attempted to reduce canal seepage losses of water. However, canal maintenance remains costly. Also, in the past few years, canal water level control, through Supervisory Control and Data Acquisition (SCADA) systems using computers to remotely access data, and then control canal gates and flow structures, has become widely used in United States to replace less reliable manual and incremental controls. Laser leveling both surveys and precisely levels fields for level basins and other surface systems. More recently, the use of ‘‘surge flow’’ for graded furrows, where flow is ‘‘on and off ’’ has been used to reduce runoff losses and provide greater uniformity in the time for infiltration along the furrow length. Surge flow valves are now solar powered and even provide the ability to apply fertilizer into the ending irrigation set times. Cablegation systems have also been developed to automate both on-farm canal and pipeline delivery systems.

Sprinkle Irrigation Systems

Irrigation– Journals

Sprinkle irrigation systems were developed to supply water to crops independent of the transmission capabilities of the soil. Higher intake soils, variable textured soils, and uneven land surfaces limited the appropriate use of surface irrigation systems. The first patented sprinkle irrigation system was developed in 1884.[4] These initial systems used controlled heads to distribute water from pipes. Many initial, key developments in sprinkle irrigation were developed by farmers with manufacturers taking over the development and refining the concept. The Rain Bird impact sprinkler was developed by a farmer and then refined by the Rain Bird Company. These initial sprinkle systems were moved by hand (Fig. 8). Other important developments were quick coupling thin walled pipes, rubber gaskets that supported portable, quick coupling pipes, and aluminum pipe that became economical and available after World War II.[4] The first mechanical move sprinkle systems were wheel line sprinklers of the lateral move type developed in the 1930s. They use wheels on pipes to irrigate the width of a field in one pass (Fig. 9). These sprinkle systems replaced the expensive solid set sprinklers and the

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Irrigation Systems: History

Fig. 8 Hand move sprinkler lateral for irrigation of a field. (Courtesy of Natural Resource Conservation Service, USDA; www.nrcs.usda.gov.)

labor intensive hand move systems commonly used. Mechanical move systems were also often more efficient and required less labor than surface irrigation systems. Center pivots were developed first by a Colorado farmer, Frank Zybach, that used a water drive mechanism. They use wheels and arches to support pipes that travel in a circle as shown in Fig. 10 to irrigate usually square fields. Thus, often the corners of the field are not irrigated. Dr. William Splinter of the University of Nebraska considered the center pivot sprinkle system ‘‘the most significant mechanical innovation since the replacement of draft animals by the tractor.’’[4] Center pivot sprinkle systems now dominate the area irrigated by sprinklers with twice as much area irrigated compared with other sprinkle methods. Large areas (such as shown in Fig. 11) are irrigated in circles with center pivots. Linear move systems provide the ability to irrigate rectangular fields with the equipment capabilities of center pivots (Fig. 12). System costs and management issues are the major detriments to sprinkle systems. Sprinkle irrigation equals about half the area irrigated in United States.[5] Mechanical move sprinkle systems have been adopted because of their reduced labor requirements,

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Fig. 9 Side roll sprinkler systems for irrigation of a field. (Courtesy of Natural Resource Conservation Service, USDA; www.nrcs.usda.gov.)

Currently, the ability to control individual applicators or heads, or groups of applicators offers the potential to use site-specific management or ‘‘prescription’’ management of small blocks (10s of m2). Each block can be managed independently to supply water, fertilizer, or chemicals as required while minimizing any wastes and possibly reducing any environmental impact. These are important developments for the future growth of precision agriculture. Traveling sprinkle systems use giant single guntype sprinklers that travel on a chassis down the length of a field and covers an area as much as 400 ft in diameter (Fig. 13). Nozzles operate at 80–100 psi. A cable

Irrigation– Journals

their adaptability for irrigating rolling topography, and their effectiveness in fields with sandier soils or mixed soil types. Irrigation is accomplished at a higher level of efficiency, but design efficiencies are often misleading. Duke, Heermann, and Dawson[6] found that all the center pivot units in some 60 units evaluated in Colorado could economically have their actual performance improved. Management improvement potential also existed. Recent advances in center pivot systems include controls that can automate system operation allowing remote controls via radio, cell phones, or infrared (line of sight) linkages from computers or other devices.

Fig. 10 Center pivot irrigation system for irrigation of a field. (Courtesy of Natural Resource Conservation Service, USDA; www.nrcs.usda.gov.)

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Fig. 13 Traveling sprinkle systems commonly use large guns and pull themselves across the field using a cable and winch. (Courtesy of Dr. Harold Duke, ARS, USDA retired.)

booms from 60 ft to 120 ft long are used to cover a large area similar to a boom sprinkler. Because of the high application rates, traveling sprinkle systems are commonly used for supplemental irrigation on sandy soils. Trickle Irrigation Systems

Fig. 11 Aerial view of an area irrigated by center pivot irrigation systems. (Courtesy of Natural Resource Conservation Service, USDA; www.nrcs.usda.gov.)

winds upon a drum on the self-powered chassis to move the unit down the field. A long hose is used to supply water to the unit from detachable hookups to an underground supply line. Sometimes rotating

Trickle irrigation systems include drip, spray, bubbler, and subsurface applications of water to crops. Some have suggested[4] that microirrigation is a more appropriate term. The concept is to apply or make available at each plant or tree the required amount of water for the root zone. Additional area is not irrigated with a reduction in water losses from evaporation and evapotranspiration from weeds. A schematic diagram of a trickle system is shown in Fig. 14. The first known application of trickle irrigation was in 1860, and used an underground tile line where drainage was accomplished during part of the year, and water was supplied for irrigation during other times.[7] Other major developments include the use of trickle

Irrigation– Journals Fig. 12 Linear move irrigation systems have equipment similar to center pivots but move from one end of the field to the other as side roll systems operate. (Courtesy of Natural Resource Conservation Service, USDA; www.nrcs.usda.gov.)

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Fig. 14 Schematic diagram of a trickle irrigation system. (Soil Conservation Service, 1984, p. 7–8.)

Irrigation Systems: History

systems in greenhouses in England during the 1940s.[7] Dr. Symcha Blass, according to Howell,[4] working with greenhouses in England after the war, improved the trickle irrigation systems. Then he transferred the technology to Israel during the 1950s to grow crops in the Negev desert including the use of highly saline waters. Trickle irrigation then spread to Australia, United States, South Africa and to other parts of the world. Trickle irrigation development in United States started with avocado orchards in California after individuals observed trickle irrigation systems in Israel. A company called Drip-Eze started the manufacturer of emitters for use in trickle irrigation systems.[4] Row crops, primarily vegetables, using trickle irrigation started in New York as new plastic products became available.[4] Trickle irrigation systems are used for crops that have widely spaced plants, e.g., orchards and vineyards as shown in Fig. 15. High valued vegetable crops are also widely adapted to trickle irrigation systems where careful water control allows increases in yield and improvements in the quality of the produce. Tomatoes are irrigated with an underground trickle system in

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Fig. 16. Water scarce areas often adapt to trickle irrigation because reduced water requirements result. Because irrigation can be accomplished daily and soil water can be maintained at a high level, irrigation with saline water can also be successful. The key component of the trickle irrigation system is filtration system. Fine particles, and chemical and bacterial clogging of lines and emitters can rapidly cause irrigation system failure if not carefully managed. Failure of the trickle irrigation system can be catastrophic if the annual crop fails and even the orchard or vineyard dies. Trickle irrigation is used to irrigate about 1,250,000 ha or 4.9% of the irrigated area in United States and irrigates a much higher percentage of the irrigated area in other countries.[8] Israel and other countries have major areas irrigated by trickle irrigation systems.

CRITICAL CONCEPTS IN IRRIGATION Irrigation as a technology 6000 or more years ago changed society. Civilizations on most continents are considered to have flourished because food production under irrigation allowed education, war, artisans, and a ruling class to develop because everyone no longer needed primarily to focus on finding his family’s food. Labor losses from allowing individuals to attend school, participate in a war, and build buildings, monuments, or artistic items are major drains on the food supply. Control of a food supply allows a ruling class to sustain power. Critical concepts in irrigation have supported the continuing important role of irrigation. Water supply management is important in irrigation.

Fig. 15 Trickle irrigation system of grapes showing limited area of water application. (Courtesy of Natural Resource Conservation Service, USDA; www.nrcs.usda.gov.)

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Use of flood waters for irrigation was an important step. Use of channels to divert water for irrigation extended the area that could be irrigated and the time when water was available. Creation of dams to control water and make it available during periods of low flow in the river was another important concept. Dams have been constructed for irrigation to divert water from streams and to store water almost from the first beginnings of irrigation. Dams for irrigation developed the west starting in the late 19th century. Other countries followed the practice with irrigation expanding by 2% per year during the 1960s and 1970s. Costs of irrigation development and concerns about the environmental impacts of irrigation have reduced that expansion to near 1% during the past several decades.[9] Dams also provided flood control along rivers and a power source from early water wheels to modern

Irrigation– Journals

Water Supply

630

Irrigation Systems: History

Fig. 16 Trickle irrigation of tomatoes using an underground trickle system. (Courtesy of Agricultural Research Service, USDA, www.ars.usda.gov.)

hydroelectric turbines. Recently, in the northwestern United States, dams have raised concern for stream ecology and fish habitats. Occasionally, dams are being removed to satisfy these environmental and societal reasons. Egypt expanded irrigation along the Nile River greater than many other irrigated valleys because they lifted water by the Archimedes’ screw and Shadoof. Pumps provided increased water supply flexibility during the 20th century by lifting water from streams and pumping water from wells. Individual control of water supply by a farmer through pumping is a major advantage in many irrigated areas. Construction of dams, diversions, and canals became a major part of irrigation development. Managing the delivery of water supplies effectively to meet farm water requirements still needs major improvements. During the 1970s, the importance of effective farm water management facilities began to be recognized. Inadequate farm water management, also constrained by water delivery management, is still a major constraint to effective irrigation in many projects.

Management Concepts

Irrigation– Journals

Irrigation scheduling[10] as part of a strategy for water management was one of the most important concepts developed in irrigation. Clyma[11] reviewed status of irrigation scheduling after three decades of application and concluded that the potential of the concept had not been achieved. First, a major part of the technical effort had focused on accurately estimating evapotranspiration for the growing crop. Management decisions were also focused on when and how much to

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irrigate. Processes for deciding how accurately to apply the target amount of water, and monitoring management decisions of when, how much, and how to irrigate are usually not a part of the irrigation scheduling process. Therefore, irrigation scheduling is not an adequate management process. Perhaps the future will define and apply irrigation scheduling as a strategy for a complete and successful management process. Farmers commonly do not measure the amount of water applied to a field. Therefore, excess applications of water, and waterlogging and salinity are the common result. Level basins are potentially a major part of surface irrigation systems in the future. A design and management concept, initially defined by Wattenburger and Clyma[12,13] and refined further and integrated into a computer design program,[14] allows farmers to apply a target amount of water to a level basin when the water advances a specified distance into a field.[3] The basin must be precisely leveled and farmers around the world already use advance distance as their criteria for irrigating a field. The technology just needs professionals to supply the needed assistance and appropriate target amounts of water will be applied to each field a farmer irrigates. Perhaps this important concept will be supplied as an urgently needed technology to farmers in the next decade.

CONCLUSION Irrigation as a technology probably started in 6000 B.C. rather than the 3000 B.C. suggested from recorded history and anthropological evidence of major structures. The intervening time involved the evolution of cities and capability to construct structures such that

Irrigation Systems: History

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ACKNOWLEDGMENTS The comments, suggestions, and additions of Dr. Terry Howell in his review of this material provided clarifications, insights and improvements and are gratefully acknowledged.

ARTICLE OF FURTHER INTEREST Irrigated Agriculture: Historical View, p. 539.

REFERENCES 1. Steward, J.H.; Adams, R.M.; Collier, D.; Palerm, A.; Wittfogel, K.A.; Beals, R.L. Irrigation Civilizations: A Comparative Study; Social Science Section, Department of Cultural Affairs, Pan American Union: Washington, DC, 1995. 2. Salazar, L. Water Management on Small Farms: A Training Manual for Farmers in Hill Areas; Water Management Synthesis Proj., Colorado State University: Fort Collins, CO, 1983; 50 pp. 3. Clyma, W.; Clemmens, A.J. Farmer management strategies for level basins using advance distance criteria. In National Irrigation Symposium, Proceedings of the Fourth Decennial Symposium, Phoenix, AZ, Nov 14–16, 2000; Evans, R.G., Benham, B.L., Trooien, P.P., Eds.; Am. Soc. Agric. Engrs.: St. Joseph, MI, 2000, 573–578. 4. Howell, T. Drops of life in the history of irrigation. Irrig. J. 2000, 50 (1), 8–10, 13–15. 5. Martin, D. The adoption of center pivot irrigation. Irrig. J. 1999, 49 (3), 8–10. 6. Duke, H.R.; Heermann, D.F.; Dawson, L.J. Appropriate depths of application for scheduling center pivot irrigations. Trans. ASAE 1992, 35 (5), 1457–1464. 7. Keller, J.; Karmeli, D. Trickle Irrigation Design; Rain Bird Sprinkler Manufacturing Corporation: Glendora, CA, 1975. 8. 2000 annual irrigation survey continues steady growth. Irrig. J. 2001, 51 (1), 12–30, 40–41. 9. Sandra, P. Redesigning irrigated agriculture. In National Irrigation Symposium, Proceeding of the Fourth decennial Symposium, Phoenix, AZ, Nov 14–16, 2000; Evans, R.G., Benham, B.L., Trooien, T.P., Eds.; Am. Soc. Agric. Engrs.: St. Joseph, MI, 2000, 1–12. 10. Jensen, M.E. Scheduling irrigations using computers. J. Soil Water Conserv. 1969, 24 (8), 193–195. 11. Clyma, W. Irrigation scheduling revisited: historical evaluations and reformation of the concept. In Evapotranspiration and Irrigation Scheduling, Proceedings of the International Conference, San Antonio, TX, Nov 3–6, 1966; Camp, C.R., Sadler, E.J., Yoder, R.E., Eds.; Am. Soc. Agric. Engrs.: St. Joseph, MI, 1966, 626–631.

Irrigation– Journals

evidence of irrigation and historical references to irrigation could evolve. Field irrigation systems used by farmers have not been defined from anthropology to my knowledge. Informal information suggesting that Pueblo Indian farmers in Southwest Colorado used pots to irrigate individual plants of corn does suggest a starting point. Observations of field irrigation around the world suggest that wild flooding was the initial surface irrigation system used by farmers. Channels were subsequently constructed to carry water into the field to improve distribution and border ditch irrigation evolved. Graded borders and graded furrows were the next improvements with ridges to control water flow across a field. In all the previous systems, flow rate, infiltration rate for the soil, and slope must be balanced to allow infiltration time for adequate irrigation and management was difficult. Level basins remove all slopes and allow farmers to apply the required amount of water to each field volumetrically. Sprinkler irrigation systems were developed to allow irrigation of soils with high intake rates, variable soil textures, and uneven land surfaces. Many developments in sprinkler irrigation were invented by farmers, but manufacturers continue to improve existing systems and develop new system components. Mechanical move systems were developed to reduce the labor of hand move systems and the expense of solid set systems. Mechanical move systems were also commonly more efficient than surface systems and required less labor. Trickle irrigation systems supply water only to the root zone of plants and greatly reduce water requirements for irrigation. Trickle systems also approach 100% efficiencies with good design and management. Labor is greatly reduced because most trickle systems are automated. Yields often increase substantially and the quality of the produce often improves significantly. Trickle systems are expensive, and without good management may fail and cause the loss of the growing crop—a financial disaster. Water supply management by constructing dams was an important conceptual development in irrigation. Pumps also added greater flexibility and access to adequate water supplies for farmers. Irrigation scheduling after three decades of use does not support adequate management of irrigation systems. Perhaps better management will increase the effectiveness of irrigation scheduling in the future. Appropriate design, precision land leveling of each field, and management of level basins using advance distance criteria should be the future of surface irrigation. Farmers already use advance distance criteria to manage water application. Combined with appropriate design and construction, target amounts of water can be applied to each field with a major improvement in water management.

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12. Wattenburger, P.L.; Clyma, W. Level basin design and management in the absence of water control, part I: evaluation of completion-of-advance irrigation. Trans. ASAE 1989, 32 (2), 838–843. 13. Wattenburger, P.L.; Clyma, W. Level basin design and management in the absence of water control, part II: design method for completion-of advance irrigation. Trans. ASAE 1989, 32 (2), 844–850. 14. Clemmens, A.J.; Dedrick, A.R.; Strand, R.J. BASIN 2.0: A Computer Program for the Design of Level-Basin Irrigation Systems; WCL

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Irrigation Systems: History

Rept. #19;U.S. Water Phoenix, AZ, 1995.

Conservation

Laboratory:

BIBLIOGRAPHY 1. Soil Conservation Service. Trickle irrigation, chapter 7, section 15. National Engineering Handbook; U.S. Dept. Agric., Chapter 7, Section 15, Natl. Eng. Handbook; Soil Conservation Service, U.S. Government Printing Office: Washington, DC, 1984; 7–8.

Irrigation Systems: Subsurface Drip Carl R. Camp Jr. Coastal Plains Research Center, Agricultural Research Service (USDA-ARS), U.S. Department of Agriculture, Florence, South Carolina, U.S.A.

Freddie L. Lamm Research and Extension Center, Kansas State University, Colby, Kansas, U.S.A.

Subsurface drip irrigation (SDI) is generally defined as the application of water below the soil surface through emitters, with discharge rates in the same range as drip irrigation.[1] While this definition is not specific regarding depth below the soil surface, most SDI laterals are installed at a depth sufficient to prevent interference with surface traffic or tillage implements, and to provide a useful life of several years as opposed to annual replacement of surface or near-surface drip laterals. Development of drip irrigation accelerated with the availability of plastics following World War II, primarily in Great Britain, Israel, and United States. SDI was part of drip irrigation development in the United States beginning about 1959, especially in Hawaii and California. While early drip irrigation products were relatively crude by modern standards, SDI devices were being installed in both experimental and commercial farms by the 1970s. As drip irrigation products improved during the 1970s and early 1980s, surface drip irrigation grew at a faster rate than SDI, probably because of emitter plugging problems and root intrusion. However, interest in SDI increased during the early 1980s, increased rapidly during the last half of the 1980s, and continues today, especially in areas with declining water supplies, with environmental issues related to irrigation, and where wastewater is used for irrigation. Initially, SDI was used primarily for sugarcane, vegetables, tree crops, and pineapple in Hawaii and California. Later, SDI use was expanded to other geographic areas and to agronomic and vines crops, including cotton, corn, and grapes. SDI has the advantage of multiple-year life, reduced interference with cultural practices, dry plant foliage, and a dry soil surface. Multiple-year life allows amortization of the entire system cost over several years, often more than ten. If all system components are installed below tillage depth, surface cultural practices can be accomplished with minimal concern for system damage. Dry soil surfaces can reduce weed growth

in arid climates and may reduce evaporation losses of applied water. Because the plant canopy is not irrigated, the foliage remains dry, which may reduce incidence of disease. SDI is also very adaptable to irregularly shaped fields and low-capacity water supplies that may provide design limitation with other irrigation systems. The major disadvantages of SDI include system cost, difficulty in locating and repairing system leaks and plugged emitters, and poor soil surface. Most system components are installed below the soil surface and are neither easy to locate nor directly observable. In a properly designed and managed SDI system, the soil surface should seldom be wet. Consequently, seed germination, especially for small seeds, can be very difficult. SDI systems offer considerable flexibility, both in design and operation. For example, SDI systems can apply small, frequent water applications, often multiple times each day, to very specific sites within the soil profile and plant root zone. Fertilizers, pesticides, and other chemical amendments can be applied via the irrigation system directly into the active root zone, often at a modest increase in equipment cost. In many cases, the operational cost may be less than that for applying these chemicals via conventional surface equipment.

SYSTEM DESIGN Site, Water Supply, and Crop Design of subsurface drip systems is similar to that of surface drip systems, especially with regard to hydraulic characteristics.[2] Specific crop and soil characteristics are used in the design process to select emitter spacing and flow rate, lateral depth and spacing, and the required system capacity. Emitter properties and lateral location are influenced by soil properties such as texture, soil compaction, and soil layering because these affect the rate of water movement through the soil profile and the subsequent wetting pattern for each emitter.

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The water supply capacity directly affects the design of a SDI system. The size of the irrigated field or zone is often determined by the water supply capacity. For example, in some humid areas, high-capacity wells are not available but multiple low-capacity wells can be distributed throughout a farm. Fortunately, the design of SDI systems can be economically adjusted to correspond to the field size and shape, to the available water supply capacity, and to other factors. Water supply quality should be tested by an approved laboratory before proceeding with system design. This information is needed for the proper design and management of the water filtration and treatment system. Some water supplies require frequent or intermittent injection of acids and/or chlorine. Other saline and/ or sodic water supplies may require treatment or special management. As water supplies become more limited, treated wastewater is becoming an increasingly important alternative water supply that can be applied through SDI systems. Camp[3] listed several reports that emphasized water supplies (saline, deficit, and wastewater) for SDI systems. The SDI system is usually designed to satisfy peak crop water requirements, which vary with specific site, soil, and crop conditions. When properly designed and managed, SDI is one of the most efficient irrigation methods, providing typical application efficiencies exceeding 90%. In comparison with other methods of irrigation, reported yields with SDI were equal to or greater than those with other irrigation methods. Generally, water requirements with SDI are similar or slightly lower than those with other irrigation methods. In some cases, water savings of up to 40% have been reported.[3] However, unless more specific information is available, it is usually best to use standard net water requirements for the location when designing SDI systems.

Lateral Type, Spacing, and Depth

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SDI lateral depth for various cropping systems is normally optimized for prevailing site conditions and soil characteristics.[3] Where systems are used for multiple years and tillage is a consideration, lateral depths vary from 0.20 m to 0.70 m. Where tillage is not a consideration (e.g., turfgrass, alfalfa) depth is sometimes less (0.10–0.40 m). Lateral spacing also varies considerably (0.25–5.0 m), with narrow spacing used primarily for turfgrass and wide spacing used for vegetable, tree, or vine crops. In uniformly spaced row crops, the lateral is usually located under either alternate or every third midrow area (furrow). For crops with alternating row spacing patterns, the lateral is located about 0.8 m from each row, usually in the narrow spacing of the pattern.

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Irrigation Systems: Subsurface Drip

The lateral should be installed deep enough to prevent damage by tillage or injection equipment but shallow enough to supply water to the crop root zone without wetting the soil surface. Generally, laterals in SDI systems are placed at depths of 0.1–0.5 m, at shallower depths in coarse-textured soils and at slightly deeper depths on finer-textured soils. The selection of emitter spacing and flow rate are influenced by crop rooting patterns, lateral depth, and soil characteristics. It is also desirable to select an emitter spacing that provides overlapping subsurface wetted zones along the lateral for most row crops. For wider spaced crops such as trees and vines, emitters are normally located near each plant and may have wider spacings that do not provide overlapping patterns. Lateral spacing is determined primarily by the soil, crop, and cultural practice, and should be narrow enough to provide a uniform supply of water to all plants.

Special Requirements Site topography must be considered in system design and selection of components as with any irrigation system, but SDI is suitable for most sites, ranging from flat to hilly. For sites with considerable elevation change, especially along the lateral, pressurecompensating emitters should be used. Two special design requirements for SDI systems, which are significantly different from those for surface drip systems, are the needs for flushing manifolds and air entry valves. Flushing manifolds are needed to allow frequent flushing of particulate matter that may accumulate in laterals. Air relief valves are needed to prevent aspiration of soil particles into emitter openings when the system is depressurized. These valves must be located in sufficient number and at the higher elevations for each lateral or zone to prevent negative pressures within the laterals. Emitter plugging caused by root intrusion is a major problem with some SDI systems, but can be minimized by chemicals, emitter design, and irrigation management. Chemical controls include the use of herbicides, either slow-release compounds embedded into emitters and filters or periodic injection of other chemical solutions (concentrated and/or diluted) into the irrigation supply. Periodic injection of acid and chlorine for general system maintenance can also modify the soil solution immediately adjacent to emitters and reduce root intrusion. In some cases, emitters plugged by roots may be cleared via injection of higher concentrations of chemicals, such as acids and chlorine. Emitter design may also affect root intrusion. Smaller orifices tend to have less root intrusion but are more susceptible to plugging by particulate matter. Some emitters are constructed with physical barriers to root

Irrigation Systems: Subsurface Drip

SYSTEM COMPONENTS Pumps, Filtration, and Pressure Regulation Pump requirements for SDI are similar to those for other drip irrigation systems, meaning water must be supplied at a relatively low pressure (170–275 kPa) and flow rate in comparison to other irrigation methods. Because of the flushing requirement for SDI systems, a flow velocity of about 0.3 m sec 1 must be achievable, either by reducing the zone size while using the same pumping rate or by increasing the pumping rate without changing the zone size. Water filtration is more critical for SDI systems than for surface drip systems because the consequences of emitter plugging are more severe and more costly. Generally, the better the water quality, the less complex the filtration system required. Surface and recycled or wastewater supplies require the most elaborate filtration systems. However, good filtration is the key to good system performance and long life, and should be a major emphasis in system design. Filtration systems range from simple screen filters for relatively clean water to more elaborate and complex disc and sand media filters for poorer quality water. The pressure regulation requirement in SDI systems is similar to that in surface drip systems. When nonpressure-compensating emitters are used on relatively flat areas, pressure is typically regulated within the system supply lines (main and/or submain) using pressureregulating valves. When pressure-compensating emitters are used, typically on more hilly terrain, the pressure within the system supply lines is controlled at a higher, but more variable, pressure that is within the recommended input pressure range for the emitters used. Water pressure should be monitored on a regular basis at the pump or supply port and at

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various locations throughout the SDI system, especially at the both ends of laterals. Laterals and Emitters Many types of driplines have been used successfully for SDI and most have emitters installed as an integral part of the dripline. This is accomplished by one of three methods: 1) molded indentions created during the fusing of dripline seams; 2) prefabricated emitters welded inside the dripline; or 3) circular prefabricated in-line emitters installed during extrusion. Regardless of the emitter used, dripline wall thickness and expected longevity must be considered along with other design factors in selecting the lateral depth. Flexible, thin-walled driplines typically are installed at shallow depths and normally have a shorter expected life. Thicker-walled, flexible driplines have been used successfully for several years provided they are installed deep enough to avoid tillage, cultivating, and harvesting machinery, but shallow enough to prevent excessive deformation or permanent collapse of the dripline by machinery or soil weight. Rigid tubing with thicker walls can be installed at deeper depths without deformation, and is often used on perennial crops or on annual crops for longer time periods (>10 yr). Some driplines are impregnated with bactericides or other chemicals to reduce the formation of sludge or other material that could plug emitters.

Chemical Injection Subsurface drip systems offer the potential for precise management of water, nutrients, and pesticides if the system is properly designed and managed. The marginal cost to add chemical injection equipment is generally competitive with other, more conventional application methods. Water and fertilizers can be applied in a variety of modes, varying from multiple continuous or pulsed applications each day to one application in several days. Choice of application frequency depends upon several factors, including soil characteristics, crop requirements, water supply, system design, and management strategies. If labeled for the purpose, some systemic pesticides and soil fumigants can be safely injected via SDI systems. Use of the SDI system for chemical applications has the potential to minimize exposure to workers and the environment, to reduce the cost of pesticide rinse water disposal, and improve precision of application to the desired target (root pests). Injection of other chemicals, such as acids and chlorine, is often required to clean and maintain emitters in optimum condition. However, a high level of management with system

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intrusion. Root intrusion appears to be more severe when emitters are located along dripline seams, which can be an area of preferential root growth. However, root intrusion problems appear to be greater for emitters, driplines, and porous tubes that are not chemically treated. Irrigation management can also be used to influence root intrusion by controlling the environment immediately adjacent to the emitter. High frequency pulsing that frequently saturates the soil immediately surrounding the emitter can discourage root growth in that area for some crops but not others. Conversely, deficit irrigation sometimes practiced to increase quality or maturity, or to control vegetative growth, can increase root intrusion in lower rainfall areas because of high root concentrations in the soil zone near emitters.

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automation and feedback control is required to minimize chemical movement to the ground water when chemicals are used. Air Entry and Flushing Air entry valves must be installed at higher elevations in SDI systems to prevent the emitter from ingesting soil particles that could plug emitters when the system is depressurized. Typically, air entry valves are located in water supply lines near the head works or control station, and in both the supply and flushing manifolds. In some cases, such as turf or pasture, air entry valves may be installed below the soil surface and enclosed within a protective box. Flushing valves installed on the flushing manifold are required to control periodic system flushing.

OPERATION AND MAINTENANCE Operation

Irrigation– Journals

SDI systems can be operated in several modes, varying from manual to fully automated. Overall, SDI systems are probably more easily automated than many other types of irrigation. One reason is that most are controlled from a central point using electrical or pneumatic valves and controllers that vary from a simple clock system to microprocessor systems, which are capable of receiving external inputs to initiate and/or terminate irrigation events. Irrigation scheduling is as important for SDI systems as for any other type of irrigation. Choosing to initiate an irrigation event and how much water to apply during each event depends on crop, soil, and irrigation system type and design. Factors that affect those decisions include soil water storage volume, sensitivity of the crop to water stress, irrigation application rate, weather conditions, and water supply capacity. Camp[3] discussed several irrigation scheduling methods that have been used successfully with SDI. However, the important point is that a sciencebased scheduling method can conserve the water supply and increase profit. If seed germination and seedling establishment and growth are critical, especially in arid climates when initial soil water content is not adequate, either sprinkler or surface irrigation is often used for germination. However, the need for two systems increases cost and decreases economic return. If subsurface drip is used for germination, an excessive amount of irrigation is often required to wet the seed zone for germination, which could result in excessive leaching and off-site

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Irrigation Systems: Subsurface Drip

environmental effects as well as increased cost. Surface wetting can also occur when the emitter flow rate exceeds the hydraulic conductivity of the soil surrounding the emitter, but wetted areas are often not uniform. Because salts tend to accumulate above the lateral, high salt concentrations may occur between the lateral and soil surface in arid areas where rainfall is not available to leach the salts downward. Salts may also be moved under the row when laterals are placed under the furrow.[4] Supplemental sprinkler irrigation may be required in some areas to control salinity if precipitation is inadequate for leaching during several consecutive years. Maintenance Often, SDI systems must have a long life (>10 yr) to be economical for lower value crops. Thus, appropriate management strategies are required to prevent emitter plugging and protect other system components to ensure proper system operation. Locating and repairing/replacing failed components is much more difficult and more expensive with SDI systems than with surface systems because most system components are buried, difficult to locate, and cannot be directly observed by managers. Consequently, operational parameters such as flow rate and pressure must be measured frequently and used as indicators of system performance. Good system performance requires constant attention to maintain good water quality, proper filtration, and periodic system flushing to remove particulate matter that could plug emitters. Periodic evaluation of SDI system performance in relation to design performance can identify problems before they become serious and significantly affect crop yield and quality.

CONCLUSION Although there is general consensus that use of SDI is increasing, this growth is difficult to document. A recent survey of irrigation in the United States reported 156,070 ha of SDI, which is about 0.6% of the total irrigated area of 25,501,831 ha.[5] Use of SDI should increase in the future, depending primarily upon the economic and water conservation benefits in comparison to other irrigation methods. As water supplies become more limited, the high application efficiency and water conserving features of SDI should increase its application. Also, SDI offers potential advantages such as reduced odors and exposure to pathogens when using recycled domestic and animal wastewater. The SDI technology offers the capability

Irrigation Systems: Subsurface Drip

to precisely place water, nutrients, and other chemicals in the plant root zone at the time and frequency needed for optimum crop production. With proper design, installation, and management, SDI systems can provide excellent irrigation efficiency and reliable performance with a system life of 10–20 yr.

REFERENCES

2. EP405.1. Design and installation of microirrigation systems. ASAE Standards, 49th Ed.; ASAE: St. Joseph, MI, 2001; 903–907. 3. Camp, C.R. Subsurface drip irrigation: a review. Trans. ASAE 1998, 41 (5), 1353–1367. 4. Ayars, J.E.; Phene, C.J.; Schoneman, R.A.; Meso, B.; Dale, F.; Penland, J. Impact of bed location on the operation of subsurface drip irrigation systems. In Microirrigation for a Changing World, Proc. Fifth International Microirrigation Congress, Orlando, FL, April 2–6, 1995; Lamm, F.R., Ed.; ASAE: St. Joseph, MI, 1995; 141–146. 5. Anonymous. 1999 Annual Irrigation Survey. Irrig. J. 2000, 50 (1), 16–31.

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1. S526.2. Soil and water terminology. ASAE Standards, 49th Ed.; ASAE: St. Joseph, MI, 2001; 970–990.

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Irrigation: Deficit M.H. Behboudian College of Sciences, Institute of Natural Resources, Massey University, Palmerston North, New Zealand

INTRODUCTION Water is the most precious environmental resource and is increasingly in demand mainly because of population growth, increased industrialization, and deterioration of water quality. Because irrigation of agricultural lands uses at least 75% of the world’s fresh water,[1] even a relatively minor reduction in irrigation water could substantially increase its availability for other purposes. Managing demand rather than trying to explore new supplies seems to be a realistic goal for irrigated agriculture. Deficit irrigation (DI) and partial rootzone drying (PRD), which is a variation of DI, are two water-saving irrigation strategies that could be employed for various crops with varying measures of success depending on the circumstances. In this presentation, I define both DI and PRD and briefly explore their potential for irrigated agriculture.

EXPLORING DI

Irrigation– Journals

Deficit irrigation is a system of managing soil water supply to impose periods of predetermined plant or soil water deficit that can result in some economic benefit. It involves giving less water to the plant than the prevailing evapotranspiration (ET) demand at selected times during the growing season. ET is a combination of evaporation (E) from the soil and transpiration (T) from the plant. Irrigation attempts to replace either all or part of ET. The term regulated DI (RDI) is normally used in the literature to denote DI of trees early in the season, before rapid fruit growth starts. Late-season RDI has also been used in some occasions to improve fruit quality.[2] The term DI is used by some authors to mean ‘‘no irrigation.’’ Here we take DI to be synonymous with RDI because it implies partial replacement of ET for achieving a predetermined plant/soil water deficit. Irrigation water in DI is given to the entire rootzone. The concept of RDI was first introduced in Australia and was used as a management strategy to control vigor in high-density plantings of ‘‘Golden Queen’’ peach[3] and ‘‘Bartlet’’ pear.[4] In these experiments, controlled water deficit was established in the plant during the period of rapid shoot and slow fruit growth. During RDI, trees were irrigated with less water than 638

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ET with sufficient water being made available to the plant just as the fruit started their rapid growth phase. The concept of RDI was subsequently explored in countries other than Australia including the United States. Application of DI is more feasible in fruit trees, especially in deciduous fruit trees, than in herbaceous plants. The physiological basis of this application rests on three concepts: functional equilibrium between roots and shoots, the phenological separation of shoot and fruit growth, and the ability of fruit to restart rapid growth once irrigation is resumed. A functional equilibrium exists between growth of roots and shoots. For example, in peach trees in a given environment there is a constant relationship between the relative growth rates of the top and of the roots even though the allocation of dry matter toward the above- and below-ground portions of the tree changes markedly. This suggests that a particular ratio of roots to shoots is developed in a given environment. Restricting root development in fully grown trees by orchard management techniques, such as DI, can thus be used to reduce vegetative vigor, which has the secondary benefit of increasing flower production, bloom density, and allocation of dry matter to fruit. The phenological separation of shoot and fruit growth that occurs in certain cultivars of some deciduous fruit crops is another important factor allowing the application of DI. This separation allows the timely application of DI to check undesired vegetative growth through a reduction of plant water status. Fruit are assumed to be less affected by water deficit than are shoots because fruit are stronger sinks and accumulate large quantities of soluble solids over the season. This should therefore make feasible the use of DI in species whose shoot and fruit growth overlap. The third physiological concept forming the basis of DI is the ability of fruit to restart rapid growth once irrigation is resumed. Sometimes the previously irrigated fruit may briefly grow at a faster rate than well-watered fruit as shown for peach.[3] This compensatory growth has been attributed to active osmotic adjustment during DI. Deficit irrigation will have the following advantages if applied judiciously. It will have a positive impact on environmental quality. Although well-drained soils are suitable for establishment of orchards, they also tend to facilitate the leaching into groundwater. Of special concern is leaching beyond rootzone of nitrates,

Irrigation: Deficit

PARTIAL ROOTZONE DRYING Partial rootzone drying is an irrigation protocol where at each turn of irrigation only a part of the rootzone receives water and the other part would be allowed to dry to a predetermined level. This has proven effective in inducing regulated deficit in several horticultural crops including grapevine.[5] In theory, this system works because roots are only active in moist soil. Therefore by inducing ‘‘dry spots’’ within the rootzone the effective rooting volume is reduced. The theory behind PRD centers on the role that chemical signals, primarily abscisic acid, originating from roots in the drying soil play in the control of shoot growth and transpiration. Stimulation of these signals through PRD can result in reduced stomatal conductance and therefore less transpiration. Photosynthesis will be reduced less than transpiration does. Ideally, vegetative growth and total plant water use will be reduced while maintaining crop yield and improving fruit quality as shown in a number of studies.[6,7] However, the theoretical potential of PRD will still have to be realized with further experimentation in the field. The PRD may be effective in limiting leaching and also reducing evaporation of irrigation water from the soil surface. It can also realize the other advantages mentioned for DI in the above. Partial rootzone drying can be achieved by the use of trickle irrigation or by careful placement of other water emitters. Dry and Loveys,[5] who introduced PRD for the first time on grapevine in Australia, showed its effectiveness in the control of vegetative growth and the enhancement of berry quality. Research has generally shown that DI improves fruit quality in comparison to full irrigation. For apple, this has been in terms of higher fruit concentrations

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of soluble solids, sugars, some aroma volatiles; brighter color in red cultivars; higher firmness; better storability; and lower weight loss in storage. There has been less research on PRD and some results are available for apple, tomato, and hot pepper carried out in this laboratory. Apple under PRD has not shown significant improvement in fruit quality compared to fully irrigated fruits[8], while both DI and PRD have shown to improve some fruit quality attributes in processing tomato including higher concentration of soluble solids.[7] For PRD in hot pepper, water balance and fruit volume improved compared to DI, but the opposite was true for total soluble solid concentration and skin color.[6] CONCLUSION Reduced irrigation (RI) is a necessity now given the dwindling water supplies and increasing demand for water worldwide. Deficit irrigation is a better-known RI technology whose advantages have been realized for various crops especially for deciduous fruits. Partial rootzone drying is a more recently applied RI technology and has been tried on only a few crops so far. Future prospects call for more research on the application of DI, and especially of PRD, particularly in areas where water is either scarce or too expensive for crop production. REFERENCES 1. Wallace, J.S.; Gregory, P. Water resources and their use in food production systems. Aquatic Sci. 2002, 64, 363–375. 2. Behboudian, M.H.; Mills, T.M. Deficit irrigation in deciduous orchards. Hortic. Rev. 1997, 21, 105–131. 3. Mitchell, P.D.; Chalmers, D.J. The effect of reduced water supply on peach tree growth and yields. J. Am. Soc. Hortic. Sci. 1982, 107, 853–856. 4. Mitchell, P.D.; Jerie, P.H.; Chalmers, D.J. The effects of regulated water deficit on pear tree growth, flowering, fruit growth and yield. J. Am. Soc. Hortic. Sci. 1984, 109, 604–606. 5. Dry, P.R.; Loveys, B.R. Factors influencing grapevine vigour and the potential for control with partial rootzone drying. Aust. J. Grape and Wine Res. 1998, 4, 140–148. 6. Dorji, K.; Behboudian, M.H.; Zegbe-Dominguez, J.A. Water relations, growth, yield, and fruit quality of hot pepper under deficit irrigation and partial rootzone drying. Scientia Hortic. 2005, 104, 137–149. 7. Zegbe-Dominguez, J.A.; Behboudian, M.H.; Lang, A.; Clothier, B.E. Deficit irrigation and partial rootzone drying maintain fruit dry mass and enhance fruit quality in ‘‘Petropride’’ processing tomato (Lycopersicon esculentum, Mill). Scientia Hortic. 2003, 98, 505–510. 8. Van Hooijdonk, B.M.; Dorji, K.; Behboudian, M.H. Responses of ‘‘Pacific Rose’’ TM apple to partial rootzone drying and to deficit irrigation. Eur. J. Hortic. Sci. 2004, 69 (3), 104–110.

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biocides, and dissolved mineral salts. Deficit irrigation in conjunction with a reduced use of biocides and nutrients may help prevent groundwater contamination and it will adhere to the environmental protection legislation that exists in some countries. Deficit irrigation will also save water and decreases irrigation cost and total cost of crop production considering the lesser input of fertilizers that are often applied with irrigation water (fertigation). Early-season application of DI in deciduous orchards will decrease vegetative growth resulting in reduced costs of summer and winter pruning. Except for possible reduction of fruit size, research on deciduous trees and herbaceous plants has indicated an improvement in fruit quality that can be listed as another advantage of DI. The possible reduction of fruit size, and yield, has led to the exploration of another DI technique, PRD, which is discussed below.

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Irrigation: Efficiency Terry A. Howell Conservation and Production Research Laboratory, Agricultural Research Service (USDA-ARS), U.S. Department of Agriculture, Bushland, Texas, U.S.A.

INTRODUCTION Irrigation efficiency is a critical measure of irrigation performance in terms of the water required to irrigate a field, farm, basin, irrigation district, or an entire watershed. The value of irrigation efficiency and its definition are important to the societal views of irrigated agriculture and its benefit in supplying the high quality, abundant food supply required to meet our growing world’s population. ‘‘Irrigation efficiency’’ is a basic engineering term used in irrigation science to characterize irrigation performance, evaluate irrigation water use, and to promote better or improved use of water resources, particularly those used in agriculture and turf/landscape management.[1–4] Irrigation efficiency is defined in terms of: 1) the irrigation system performance, 2) the uniformity of the water application, and 3) the response of the crop to irrigation. Each of these irrigation efficiency measures is interrelated and will vary with scale and time. Fig. 1 illustrates several of the water transport components involved in defining various irrigation performance measures. The spatial scale can vary from a single irrigation application device (a siphon tube, a gated pipe gate, a sprinkler, a microirrigation emitter) to an irrigation set (basin plot, a furrow set, a single sprinkler lateral, or a microirrigation lateral) to broader land scales (field, farm, an irrigation canal lateral, a whole irrigation district, a basin or watershed, a river system, or an aquifer). The timescale can vary from a single application (or irrigation set), a part of the crop season (preplanting, emergence to bloom or pollination, or reproduction to maturity), the irrigation season, to a crop season, or a year, partial year (premonsoon season, summer, etc.), or a water year (typically from the beginning of spring snow melt through the end of

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The use of trade, firm, or corporation names in this publication is for the information and convenience of the reader. Such use does not constitute an official endorsement or approval by the United States Department of Agriculture or the Agricultural Research Service of any product or service to the exclusion of others that may be suitable. This work was done by a U.S. Government employee and cannot be copyrighted. It may be reproduced freely for any non-commercial use. This article previously published in the Encyclopedia of Soil Science 2002 Marcel Dekker, Inc. 640

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irrigation diversion, or a rainy or monsoon season), or a period of years (a drought or a ‘‘wet’’ cycle). Irrigation efficiency affects the economics of irrigation, the amount of water needed to irrigate a specific land area, the spatial uniformity of the crop and its yield, the amount of water that might percolate beneath the crop root zone, the amount of water that can return to surface sources for downstream uses or to groundwater aquifers that might supply other water uses, and the amount of water lost to unrecoverable sources (salt sink, saline aquifer, ocean, or unsaturated vadose zone). The volumes of the water for the various irrigation components are typically given in units of depth (volume per unit area) or simply the volume for the area being evaluated. Irrigation water application volume is difficult to measure, so it is usually computed as the product of water flow rate and time. This places emphasis on accurately measuring the flow rate. It remains difficult to accurately measure water percolation volumes groundwater flow volumes, and water uptake from shallow groundwater.

IRRIGATION SYSTEM PERFORMANCE EFFICIENCY Irrigation water can be diverted from a storage reservoir and transported to the field or farm through a system of canals or pipelines; it can be pumped from a reservoir on the farm and transported through a system of farm canals or pipelines; or it might be pumped from a single well or a series of wells through farm canals or pipelines. Irrigation districts often include small to moderate size reservoirs to regulate flow and to provide short-term storage to manage the diverted water with the on-farm demand. Some on-farm systems include reservoirs for storage or regulation of flows from multiple wells.

Water Conveyance Efficiency The conveyance efficiency is typically defined as the ratio between the water that reaches a farm or field

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irrigations (irrigation sets). Application efficiency includes any application losses to evaporation or seepage from surface water channels or furrows, any leaks from sprinkler or drip pipelines, percolation beneath the root zone, drift from sprinklers, evaporation of droplets in the air, or runoff from the field. Application efficiency is defined as Ea ¼ 100

and that diverted from the irrigation water source.[1,3,4] It is defined as Ec ¼ 100

VF Vt

ð1Þ

where Ec is the conveyance efficiency (%), Vf is the volume of water that reaches the farm or field (m3), and Vt is the volume of water diverted (m3) from the source. Ec also applies to segments of canals or pipelines, where the water losses include canal seepage or leaks in pipelines. The global Ec can be computed as the product of the individual component efficiencies, Eci, where i represents the segment number. Conveyance losses include any canal spills (operational or accidental) and reservoir seepage and evaporation that might result from management as well as losses resulting from the physical configuration or condition of the irrigation system. Typically, conveyance losses are much lower for closed conduits or pipelines[4] compared with unlined or lined canals. Even the conveyance efficiency of lined canals may decline over time due to material deterioration or poor maintenance.

where Ea is the application efficiency (%), Vs is the irrigation needed by the crop (m3), and Vf is the water delivered to the field or farm (m3). The root zone may not need to be fully refilled, particularly if some root zone water-holding capacity is needed to store possible or likely rainfall. Often, Vs is characterized as the volume of water stored in the root zone from the irrigation application. Some irrigations may be applied for reasons other than meeting the crop water requirement (germination, frost control, crop cooling, chemigation, fertigation, or weed germination). The crop need is often based on the ‘‘beneficial water needs.’’[5] In some surface irrigation systems, the runoff water that is necessary to achieve good uniformity across the field can be recovered in a ‘‘tailwater pit’’ and recirculated with the current irrigation or used for later irrigations, and Vf should be adjusted to account for the ‘‘net’’ recovered tailwater. Efficiency values are typically site specific. Table 1 provides a range of typical farm and field irrigation application efficiencies[6–8] and potential or attainable efficiencies for different irrigation methods that assumes irrigations are applied to meet the crop need. Storage Efficiency Since the crop root zone may not need to be refilled with each irrigation, the storage efficiency has been defined.[4] The storage efficiency is given as Es ¼ 100

Vs V rz

ð3Þ

where Es is the storage efficiency (%) and Vrz is the root zone storage capacity (m3). The root zone depth and the water-holding capacity of the root zone determine Vrz. The storage efficiency has little utility for sprinkler or microirrigation because these irrigation methods seldom refill the root zone, while it is more often applied to surface irrigation methods.[4]

Application Efficiency

Seasonal Irrigation Efficiency

Application efficiency relates to the actual storage of water in the root zone to meet the crop water needs in relation to the water applied to the field. It might be defined for individual irrigation or parts of

The seasonal irrigation efficiency is defined as

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ð2Þ

Ei ¼ 100

Vb VF

Irrigation– Journals

Fig. 1 Illustration of the various water transport components needed to characterize irrigation efficiency.

Vs VF

ð4Þ

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Irrigation: Efficiency

Table 1 Example of farm and field irrigation application efficiency and attainable efficiencies Field efficiency (%) Irrigation method

Farm efficiency (%)

Attainable

Range

Average

Attainable

Range

Average

75 85 85 80 90

50–80 60–90 65–95 50–80 80–95

65 75 80 65 85

70 85 85 75 80

40–70 — — — —

65 — — — —

80 80 75

60–85 60–85 55–75

75 75 65

80 80 80

60–90 60–85 60–80

80 80 70

85 95 98

75–90 75–95 80–98

80 90 95

85 85 95

75–90 75–95 80–98

80 90 92

95 90

75–95 70–95

90 85

85 90

80–98 75–95

90 85

95 95 95

70–95 75–95 70–95

85 90 85

95 95 95

75–95 75–95 70–95

85 90 85

80 85

50–80 60–80

65 75

80 85

50–80 65–85

60 70

Surface Graded furrow w/tailwater reuse Level furrow Graded border Level basins Sprinkler Periodic move Side roll Moving big gun Center pivot Impact heads w/end gun Spray heads wo/end gun LEPAa wo/end gun Lateral move Spray heads w/hose feed Spray heads w/canal feed Microirrigation Trickle Subsurface drip Microspray Water table control Surface ditch Subsurface drain lines a

LEPA is low energy precision application. Source: From Refs.[6,7,11].

where Ei is the seasonal irrigation efficiency (%) and Vb is the water volume beneficially used by the crop (m3). Vb is somewhat subjective,[4,5] but it basically includes the required crop evapotranspiration (ETc) plus any required leaching water (Vl) for salinity management of the crop root zone.

Leaching requirement (or the leaching fraction) The leaching requirement,[9] also called the leaching fraction, is defined as Lr ¼

Vd ECi ¼ VF ECd

ð5Þ

Irrigation– Journals

where Lr is the leaching requirement, Vd is the volume of drainage water (m3), Vf is the volume of irrigation (m3) applied to the farm or field, ECi is the electrical conductivity of the irrigation water (dS m
1), and ECd is the electrical conductivity of the drainage water (dS m
1). The Lr is related to the irrigation application efficiency, particularly when drainage is the primary

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irrigation loss component. The Lr would be required ‘‘beneficial’’ irrigation use (Vl  LrVi), so only Vd greater than the minimum required leaching should reduce irrigation efficiency. Then, the irrigation efficiency can be determined by combining Eqs. (4) and (5)
 Vb ð6Þ þ Lr Ei ¼ 100 VF Burt et al.[5] defined the ‘‘beneficial’’ water use to include possible off-site needs to benefit society (riparian needs or wildlife or fishery needs). They also indicated that Vf should not include the change in the field or farm storage of water, principally soil water but it could include field (tailwater pits) or farm water storage (a reservoir) that wasn’t used within the time frame that was used to define Ei.

IRRIGATION UNIFORMITY The fraction of water used efficiently and beneficially is important for improved irrigation practice.

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643

Christiansen’s Uniformity Coefficient Christiansen[10] proposed a coefficient intended mainly for sprinkler system based on the catch volumes given as  CU ¼ 100

1
ð

P 
jÞ jX
x P X

ð7Þ

where CU is the Christiansen’s uniformity coefficient in percent, X is the depth (or volume) of water in each of the equally spaced catch containers in mm
is the mean depth (volume) of the catch or ml, and x (mm or ml). For CU values >70%, Hart[11] and Keller and Bliesner[8] presented " CU ¼ 100 1


 s 
2 0:5 x

p

# ð8Þ

where s is the standard deviation of the catch depth (mm) or volume (ml). Eq. (8) approximates the normal distribution for the catch amounts. The CU should be weighted by the area represented by the container[12] when the sprinkler catch containers intentionally represent unequal land areas, as is the case for catch containers beneath a center pivot. Heermann and Hein[12] revised the CU formula [Eq. (8)] to reflect the weighted area, particularly intended for a center pivot sprinkler, as follows:

CUðH&HÞ


P  39 2
P    PV i Si  > S i V i
=  7> Si 6 6 7 P ¼ 100 1
4 5> > ðV i Si Þ > > ; : 8 > >
70%, then the DU and CU are essentially equal.[13] The USDA-NRCS (formerly, the Soil Conservation Service) has widely used DUlq (p ¼ 1/4) for surface irrigation to access the uniformity applied to a field, i.e., by the irrigation volume (amount) received by the lowest one-quarter of the field from applications for the whole field. Typically, DUp is based on the postirrigation measurement[5] of water volume that infiltrates the soil because it can more easily be measured and better represents the water available to the crop. However, the postirrigation infiltrated water ignores any water intercepted by the crop and evaporated and any soil water evaporation that occurs before the measurement. Any water that percolates beneath the root zone or the sampling depth will also be ignored. The DU and CU coefficients are mathematically interrelated through the statistical variation (coefficient of variation, s=
x, Cv) and the type of distribution. Warrick[13] presented relationships between DU and CU for normal, log-normal, uniform, specialized power, beta- and gamma-distributions of applied irrigations.

Emission Uniformity For microirrigation systems, both the CU and DU concepts are impractical because the entire soil surface is not wetted. Keller and Karmeli[14] developed an equation for microirrigation design as follows
1=2

EU ¼ 100½1
1:27ðC vm Þn




q  m
q

 ð11Þ

where EU is the design emission uniformity (%), Cvm is the manufacturer’s coefficient of variability in emission device flow rate (1/h), n is the number of emitters per plant, qm is the minimum emission device flow rate
is the (1/h) at the minimum system pressure, and q

Irrigation– Journals

The uniformity of the applied water significantly affects irrigation efficiency. The uniformity is a statistical property of the applied water’s distribution. This distribution depends on many factors that are related to the method of irrigation, soil topography, soil hydraulic or infiltration characteristics, and hydraulic characteristics (pressure, flow rate, etc.) of the irrigation system. Irrigation application distributions are usually based on depths of water (volume per unit area); however, for microirrigation systems they are usually based on emitter flow volumes because the entire land area is not typically wetted.

644

Irrigation: Efficiency

mean emission device flow rate (1/h). This equation is based on the DUlq concept,[4] and includes the influence of multiple emitters per plant that each may have a flow rate from a population of random flow rates based on the emission device manufacturing variation. Nakayama, Bucks, and Clemmens[15] developed a design coefficient based more closely on the CU concept for emission device flow rates from a normal distribution given as C Ud ¼ 100ð1
0:798ðC vm Þn
1=2 Þ

ð12Þ

where CUd is the coefficient of design uniformity in percent and the numerical value, 0.798, is
0:5 2 p from Eq. (8). Many additional factors affect microirrigation uniformity including hydraulic factors, topographic factors, and emitter plugging or clogging.

Irrigation Water Use Efficiency The previous discussion of WUE does not explicitly explain the crop yield response to irrigation. Water use efficiency is influenced by the crop water use (ET). Bos[3] defined a term for WUE to characterize the influence of irrigation on WUE as WUE ¼

IWUE ¼

WUE ¼

Yg ET

ð13Þ

Irrigation– Journals

where WUE is water use efficiency (kg m
3), Yg is the economic yield (g m
2), and ET is the crop water use (mm). Water use efficiency is usually expressed by the economic yield, but it has been historically expressed as well in terms of the crop dry matter yield (either total biomass or aboveground dry matter). These two WUE bases (economic yield or dry matter yield) have led to some inconsistencies in the use of the WUE concept. The transpiration ratio (transpiration per unit dry matter) is a more consistent value that depends primarily on crop species and the environmental evaporative demand,[17] and it is simply the inverse of WUE expressed on a dry matter basis.

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ð14Þ

where WUE is irrigation water use efficiency (kg m
3), Ygi is the economic yield (g m
2) for irrigation level i, Ygd is the dryland yield (g m
2; actually, the crop yield without irrigation), ETi is the evapotranspiration (mm) for irrigation level i, and ETd is the evapotranspiration of the dryland crops (or of the ET without irrigation). Although Eq. (14) seems easy to use, both Ygd and ETd are difficult to evaluate. If the purpose is to compare irrigation and dryland production systems, then dryland rather than non-irrigated conditions should be used. If the purpose is to compare irrigated regimes with an unirrigated regime, then appropriate values for Ygd and ETd should be used. Often, in most semiarid to arid locations, Ygd may be zero. Bos[3] defined irrigation WUE as

WATER USE EFFICIENCY The previous sections discussed the engineering aspects of irrigation efficiency. Irrigation efficiency is clearly influenced by the amount of water used in relation to the irrigation water applied to the crop and the uniformity of the applied water. These efficiency factors impact irrigation costs, irrigation design, and more important, in some cases, the crop productivity. Water use efficiency (WUE) has been the most widely used parameter to describe irrigation effectiveness in terms of crop yield. Viets[16] defined WUE as

ðY gi
Y gd Þ ðETi
ETd Þ

ðY gi
Y gd Þ IRRi

ð15Þ

where IWUE is the irrigation efficiency (kg m
3) and IRRi is the irrigation water applied (mm) for irrigation level i. In Eq. (15), Ygd may be often zero in many arid situations. CONCLUSION Irrigation efficiency is an important engineering term that involves understanding soil and agronomic sciences to achieve the greatest benefit from irrigation. The enhanced understanding of irrigation efficiency can improve the beneficial use of limited and declining water resources needed to enhance crop and food production from irrigated lands.

REFERENCES 1. Israelsen, O.R.; Hansen, V.E. Irrigation Principles and Practices, 3rd Ed.; Wiley: New York, 1962; 447 pp. 2. ASCE. Describing irrigation efficiency and uniformity. J. Irrig. Drain. Div., ASCE 1978, 104 (IRI), 35–41. 3. Bos, M.G. Standards for irrigation efficiencies of ICID. J. Irrig. Drain. Div., ASCE 1979, 105 (IRI), 37–43. 4. Heermann, D.F.; Wallender, W.W.; Bos, M.G. Irrigation efficiency and uniformity. In Management of Farm Irrigation Systems; Hoffman, G.J., Howell,

Irrigation: Efficiency

5.

6.

7.

8. 9.

11. Hart, W.E. Overhead irrigation by sprinkling. Agric. Eng. 1961, 42 (7), 354–355. 12. Heermann, D.F.; Hein, P.R. Performance characteristics of self-propelled center-pivot sprinkler machines. Trans. ASAE 1968, 11 (1), 11–15. 13. Warrick, A.W. Interrelationships of irrigation uniformity terms. J. Irrig. Drain. Eng., ASCE 1983, 109 (3), 317–332. 14. Keller, J.; Karmeli, D. Trickle Irrigation Design; Rainbird Sprinkler Manufacturing: Glendora, CA, 1975; 133 pp. 15. Nakayama, F.S.; Bucks, D.A.; Clemmens, A.J. Assessing trickle emitter application uniformity. Trans. ASAE 1979, 22 (4), 816–821. 16. Viets, F.G. Fertilizers and the efficient use of water. Adv. Agron. 1962, 14, 223–264. 17. Tanner, C.B.; Sinclair, T.R. Efficient use of water in crop production: research or re-search? In Limitations to Efficient Water Use in Crop Production; Taylor, H.M., Jordan, W.R., Sinclair, T.R., Eds.; Am. Soc. Agron., Crop Sci. Soc. Am., Soil Sci. Soc. Am.: Madison, WI, 1983; 1–27.

Irrigation– Journals

10.

T.A., Solomon, K.H., Eds.; Am. Soc. Agric. Engrs.: St. Joseph, MI, 1990; 125–149. Burt, C.M.; Clemmens, A.J.; Strelkoff, T.S.; Solomon, K.H.; Bliesner, R.D.; Hardy, L.A.; Howell, T.A.; Eisenhauer, D.E. Irrigation performance measures: efficiency and uniformity. J. Irrig. Drain. Eng. 1997, 123 (3), 423–442. Howell, T.A. Irrigation efficiencies. In Handbook of Engineering in Agriculture; Brown, R.H., Ed.; CRC Press: Boca Raton, FL, 1988; Vol. I, 173–184. Merriam, J.L.; Keller, J. Farm Irrigation System Evaluation: A Guide for Management; Utah State University: Logan, UT, 1978; 271 pp. Keller, J.; Bliesner, R.D. Sprinkle and Trickle Irrigation; The Blackburn Press: Caldwell, NJ, 2000; 652 pp. U.S. Salinity Laboratory Staff. Diagnosis and Improvement of Saline and Alkali Soils; Handbook 60; U.S. Government Printing Office: Washington, DC, 1954; 160 pp. Christiansen, J.E. Irrigation by Sprinkling; California Agric. Exp. Bull. No. 570; University of California: Berkeley, CA, 1942; 94 pp.

645

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Irrigation: Frost Protection and Bloom Delay Robert W. Hill Biological and Irrigation Engineering Department, Utah State University, Logan, Utah, U.S.A.

Richard L. Snyder Department of Land, Air and Water Resources (LAWR), and Atmospheric Sciences, University of California–Davis, Davis, California, U.S.A.

INTRODUCTION Warm weather in the early spring initiates fruit tree bud development and blossoming. In many fruitproducing areas of the world, subsequent cold, frosty nights may kill the buds or blossoms because of ice crystals forming in the plant tissues. This leads to reduction in harvestable fruit or even total loss of the crop. Irrigation can be used to offset the detrimental effects of freezing fruit through freeze protection or sprinkling for bloom delay. Winter Rest

Irrigation– Journals

In the fall, a deciduous tree loses its leaves and enters a condition known as winter rest. The tree is incapable of growth during this period and fruit buds cannot grow until the rest period has been completed. After rest is completed, which occurs sometime between mid-winter and early spring, depending upon the climate, changes begin occurring in the buds that will eventually cause blossoming and leafing of the tree. When the required accumulation of chill units has been achieved for a particular fruit variety, trees are ready to begin their normal spring growth. (Chill units accumulate when the temperature is below a set threshold temperature measured in  C-days.) From this time on, the growth of the tree and development of the fruit is a function of temperature (measured in growing degree hours). The most critical time from a standpoint of freeze damage is from end of rest to full bloom. The rate of bud development depends upon the temperature environment of the buds after the completion of rest. If the early spring temperatures are consistently cool, blossoming is delayed. However, when spring temperatures are considerably above normal, bud development accelerates and the trees blossom early. An average temperature of 1.7 C (3 F) above normal for 1 week will decrease the time to bloom by about 1 day. If early bud development is followed by a sudden cold period, the freeze damage to fruit can be very serious. The extent of damage depends on how cold the temperature gets and how long it stays cold. 646

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The critical or lethal temperature is also a function of the development state of the fruit buds. Fruit growers have attempted to heat their orchards as a means of combating the cold, damaging temperatures. Heating is costly, may pollute the air, and is not always effective. TYPES OF FROST EVENTS (FROM REF.[1]) Advection Frost An advection frost occurs when cold air blows into an area to replace warmer air that was present before the weather change. It is associated with moderate to strong winds, no temperature inversion, and low humidity. Often, temperatures will drop below 32 F (0 C) and stay there all day. Advection frosts are difficult to protect against. Radiation Frost Radiation frosts are characterized by clear skies, calm winds, and temperature inversions. Radiation frosts occur because of heat losses in the form of radiant energy. Under clear, nighttime skies, more heat is radiated away from an orchard than it receives, so the temperature drops. The temperature falls faster near the radiating surface causing a temperature inversion to form (temperature increases with height above the ground). If you measure high enough, the temperature will reach the point where it begins to decrease with height (a lapse condition). The level where the temperature profile changes from an inversion to a lapse condition is called the ceiling. A weak inversion (high ceiling) occurs when the temperatures aloft are only slightly higher than near the surface. When there is a strong inversion (low ceiling), temperature increases rapidly with height. Most frost protection methods are more effective during low-ceiling, strong-inversion conditions. Energy Transfer Energy or heat transfer determines how cold it will get and the effectiveness of protection. The four methods

Irrigation: Frost Protection and Bloom Delay

of energy transfer are radiation, conduction, convection, and latent heat. Understanding these heat transfer mechanisms is extremely important for good frost protection management. Radiation Radiation is electromagnetic energy transfer. A good example of radiation is sunlight. Because it is very hot, considerable energy is radiated from the sun to Earth. Although much cooler object on Earth also radiates energy to its surroundings. Normally, if an object radiates more energy than it receives from other sources, it will cool. Conduction Conduction is heat transfer through matter where the objects do not move. A good example is the transfer of heat through a metal rod of one end placed in a fire. The heat is transferred by conduction to the other end of the rod. Conduction is important in soil heat transfer and hence frost protection.

647

protected from frost by irrigation.[4] Flooding involves the distribution of large amounts of relative warm water throughout the orchard. This warms the air through radiant energy from water. The costs of flooding may be similar to sprinkling. ‘‘The major drawbacks to this method (flooding) is the large amount of water needed, the reduction of the quality of this water during use and the lack of tolerance by the crop to being inundated by water for long periods of time. The volume of water needed is large because the field must be covered as much as possible to achieve maximum radiation. In averaging this over the ditches, furrows, and growing surfaces, the minimum would be about 0.3 ha-m (one acre foot per acre). Plants usually can tolerate standing water for short periods of time, but the periods of time required for protection during freezes usually far exceeds this tolerance. The time of inundation is much longer than the actual period of freezing temperatures due to the time it takes to totally inundate the field prior to freezing temperatures and remove the water after the freeze. The reduction in water quality can be high depending on the recent applications of herbicides, fertilizers, fungicides, etc. These three factors have led to the reduced use of flooding for freeze protection in most crops.’’

Convection

Latent Heat When water condenses, cools, or freezes, the temperature of the environment around the water rises because latent heat is changed to sensible heat. Latent heat is chemical energy stored in the bonds that join water molecules together and sensible heat is heat you measure with a thermometer. When latent heat is changed to sensible heat, the air temperature rises.[2] When ice melts, water warms, or water evaporates, sensible heat is changed to latent heat and the air temperature falls.

IRRIGATIONS FOR FRUIT BUD PROTECTION FROM COLD TEMPERATURES Irrigation water can be used for warming the fruit bud environment to mitigate the potential damaging effects of cold temperatures.[3] Strawberries have also been

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Sprinkler for Blossom Delay Sprinkling for blossom delay, developed at Utah State University, retards the time of fruit bud development until the major danger of spring freeze is past. This is accomplished with overhead sprinklers that utilize evaporative cooling to delay bud development and growth. The system requires an overhead sprinkling system with automatic controls to turn the system on and off at the desired temperature. If springtime comes early, after the tree has completed its winter rest, the system is activated on warm afternoons whenever the air temperature rises above 7.3 C (45 F). The water strikes the buds and then evaporates and this evaporation process keeps the buds cool and delays their development. Along the Wasatch Front of Northern Utah, where the tests were first conducted,[5] it was found that apple tree bloom could be delayed 2 to 3 weeks, and prevent over 80% of the major freeze damage that has occurred in the past. If the bud delay is combined with iceencasement freeze protection the chances for crop failure due to late spring freezes is even less. Sprinkling as a method of bloom delay for freeze protection has other advantages: it reduces fuel consumption and pollution of the environment. The same sprinkling system can be used to protect the trees against the late spring freezes and can also be used for irrigation during the summer growing season.

Irrigation– Journals

Convection is the process where heated matter physically moves from one place to another and takes heat with it. Air heated by smudge pots is an example of convection because the air, warmed by the heaters, rises and mixes with colder air in the orchard to raise the temperature. Smudge pots also radiate heat to nearby trees but the main protection comes from convection.

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Irrigation: Frost Protection and Bloom Delay

The amount of evaporative cooling that takes place on bare limbs depends upon 1) the temperature of the tree buds; 2) the difference in vapor pressure between the bud surface and the air; and 3) the rate at which evaporated water is removed from the boundary layer by diffusion or wind currents. Therefore, for maximum cooling, with the least amount of water application, it is necessary to completely wet the buds periodically and allow most of the water to evaporate before rewetting.

Sprinkling for Freeze Protection (Ice-Encasement) Sprinkling for freeze protection differs from sprinkling for bud delay in that the sprinkling takes place during cold temperatures.[6–8] A combination of the two methods can considerably decrease the chances for late spring freeze damage. Freeze protection is provided by overhead sprinkling when water turns to ice. This change in its phase is accompanied by the release of heat from the freezing ice-water film. As long as there is a mixture of ice and water on the buds, its temperature will remain near 0 C and it will not freeze. There are several decisions that must be made when considering whether sprinkling can be used as a method of freeze protection. They are: 1. Can the trees withstand the potential ice load? 2. If yes, when should I begin sprinkling to insure protection? 3. When can I safely discontinue? The determination of whether the trees can withstand the ice load or not depends on: 1. How the trees have been pruned. 2. The duration of the time trees must be sprinkled. 3. The amount of water that must be applied to maintain a mixture of ice and water on the buds. 4. The amount of wind and the depression of the wet bulb below the expected minimum temperature.

Irrigation– Journals

Sprinkling must begin at least by the time the wet bulb temperature drops to within 2 C (4 F) above the lethal temperature of the buds. The wet bulb temperature is the controlling temperature in sprinkling for freeze protection. Sprinkling must continue until the wet bulb temperature is back above the lethal temperature by the same amount. This 2 C (4 F) limit is required at the time of beginning sprinkling to allow the buds time to get thoroughly wet before the critical temperature is reached. On the discontinuing phase,

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the threshold is a safety factor because changing winds may bring in drier air and drop the wet bulb a few degrees after sprinkling has been discontinued and cause damage to the buds. This plus safety factor is a must on both ends of the sprinkling program. Even a higher margin may be advisable if the predicted minimum temperature is below 7 C (20 F). If the required amount of water cannot be made available, then it is better not to begin sprinkling at all. Insufficient water will allow bud temperatures to drop to the wet bulb, which will be below the expected minimum and cause additional damage to the buds. There are several considerations in the design of sprinkler systems for freeze protection which are different from installations used for bloom delay, irrigation, or both. The requirements for freeze protection are: 1. Capability of varying the sprinkler output over a wider range than is required for either of the other applications. 2. Capability of supplying different amounts of water in various parts of the orchard at the same time. 3. Capability of operating the sprinklers at below freezing temperatures without the sprinklers icing up. (Once sprinkling is begun it must be continued at an adequate rate until the wet bulb temperature is at least 1 C (2 F) above the lethal temperature of the buds.) Variable Water Requirements for Protection in a Windy Orchard Water requirements for freeze protection in a windy orchard are quite different than protection under relatively calm conditions. In the latter case, the water requirements are relatively uniform over the entire orchard, but under windy conditions, the wind is continually bringing air with a low wet bulb temperature into the upwind portion of the orchard. This dry air requires more water to maintain the mixture of ice and water on the buds of the trees than the air further downwind where the evaporated moisture has been added to the air to increase the wet bulb temperature and thus decrease the moisture requirements for protection. Sprinkler Application Rates When sprinklers are used for freeze protection, the two main factors to consider are 1) the application rate required for protection and 2) the proper time to start and stop the sprinklers. Considerably more energy is removed by evaporation than is supplied by cooling and freezing of an

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649

Table 1 Application rates for overhead sprinklers for frost protection of grapevines Temperature ( C) 1.7

Wind speed (m/sec)

30-sec rotation (mm/hr)

60-sec rotation (mm/hr)

30-sec rotation (l m 1 ha 1)

60-sec rotation (l m 1 ha 1)

0.0–0.5

2.0

2.5

334

418

3.3

0.0–0.5

2.8

3.3

468

551

5.0

0.0–0.5

3.8

4.3

635

718

1.7

0.9–1.4

2.5

3.0

418

501

3.3

0.9–1.4

3.3

3.8

551

635

0.9–1.4

4.6

5.1

768

852

5.0 [1]

Source: From Ref. .

equal quantity of water. Actually, in order to break even, about six times as much water must be cooled and frozen than evaporated. Fortunately, evaporation rates are relatively low during freeze nights, and sufficient water can usually be frozen to supply more heat from cooling and freezing than is lost to evaporation. However, a higher application rate is needed to compensate for greater evaporation on nights with stronger wind speeds and lower humidity. Application Rate Requirements

REFERENCES 1. Snyder, R.L. Principles of Frost Protection; Atmospheric Science, University of California: Davis, 2000. 2. Miller, F. College Physics; Harcourt Brace Jovanovich, Inc.: New York, 1977. 3. Evans, R.G. The Art of Protecting Grapevines from Low Temperature Injury, Proceedings of the Amer. Soc. Enol. and Vitic; 50th Anniversary Meeting, Seattle, Washington, June 19–23, 2000; 13 pp. 4. Hochmuth, G.J.; Lacascio, S.J.; Kostewicz, S.R.; Martin, F.G. Irrigation method and rowcover use for strawberry freeze protection. J. Am. Soc. Hortic. Sci. 1993, 188 (5), 575–579. 5. Griffin, R.E.; Anderson, J.L.; Ashcroft, G.L.; Richardson, E.A.; Seeley, S.D.; Walker, D.R.; Hill, R.W.; Alfaro, J.F. Reducing Fruit Losses Caused by Low Spring Temperatures; Final Rep. Proj. 652-366-084, Four Corners Reg. Comm. Utah Agric. Exp. St., Utah State University: Logan, UT, 1976. 6. Parsons, L.R.; Wheaton, T.A. Microsprinkler irrigation for freeze protection: evaporative cooling and extent of protection in an advective freeze. J. Am. Soc. Hortic. Sci. 1987, 112 (6), 879–902. 7. Parsons, L.R.; Wheaton, T.A.; Faryna, N.D.; Jackson, J.L. Elevated microsprinklers improve protection of citrus trees in an advective freeze. HortScience 1991, 26 (9), 1149–1151. 8. Rieger, M.; Davies, F.S.; Jackson, L.K. Microsprinkler irrigation and microclimate of young orange trees during freeze conditions. HortScience 1986, 21 (6), 1372–1374.

Irrigation– Journals

The application rate required for overplant sprinkling depends on the sprinkler rotation rate, wind speed, and the dew point temperature.[1] The wind speed and dew point temperatures are important because the evaporation rate increases with the wind speed and with decreasing dew point temperatures (a measure of water vapor content of the air). Sprinkler rotation rates are important because the temperature of wet plant parts initially rises as the water freezes and releases heat, but then it falls to near the wet-bulb temperature due to evaporation before the plant is hit again with another pulse of water. Often the wet-bulb temperature is below the critical damage temperate, so damage can result if there is too much time between hitting the plants with a pulse of water. The idea is to rewet the plants frequently so that the interval of time when the plant temperature is below the critical damage temperature is short. Generally, the rotation rate should not be longer than 60 sec; and 30 seconds is better. Sprinkler application rate recommendations for grapevines are given in Table 1. Application rates for other tall crops are similar. Distribution

uniformity and good coverage of the plants with water are important. Application rates are somewhat lower for low-growing crops because it is easier to obtain good wetting of the vegetation when it is shorter.

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Irrigation: Impact on River Flows Robert W. Hill Biological and Irrigation Engineering Department, Utah State University, Logan, Utah, U.S.A.

Ivan A. Walter Ivan’s Engineering, Inc., W.W. Wheeler, Denver, Colorado, U.S.A.

Irrigation– Journals

INTRODUCTION

Hill[5] defined crop depletion as:

The practice of irrigation necessitates developing a water source, conveying the water to the field, application of the water to the soil and collection and reuse or disposal of tailwater and subsurface drainage. These processes alter river basin hydrology and water quality in space and time. To sum up the effect of irrigation on a watershed in a word, it would be: DEPLETION. In hydrologic studies it is common engineering practice to quantify the impact upon the stream(s) from which the irrigation water is diverted. The impact upon the stream is actually of two kinds: 1) diversions that decrease the streamflow and 2) return flows that increase the streamflow. The engineering term used to describe the overall impact is ‘‘streamflow depletion’’ which means the net reduction in streamflow resulting from diversion to irrigation uses. Actual stream depletions are a function of many factors including the amount and timing of diversions, the type of diversion structure (well vs. ditch), crops grown, soil type, depth to groundwater, irrigation method, irrigation efficiency, properties of the alluvial aquifer, area irrigated, and evapotranspiration of precipitation, groundwater, and irrigation water.

Dpl ¼ Et
SMco
Pef

where Dpl is estimated depletion for a given site or sub-basin; Et is calculated crop water use; SMco is moisture which is ‘‘carried over’’ from the previous non-growing season (October 1–April 30) as stored soil water in the root zone available for crop water use subsequent to May 1; and Pef is an estimate of that portion of precipitation measured at an NWS station during May–September, which could be used by crops. The carry-over soil moisture (SMco) was estimated by assuming that 67% of adjusted precipitation from October through April could be stored in the root zone. If this exceeded 75% of the available soil waterholding capacity of the average root zone in the subbasin, the excess was considered as lost to drainage or runoff and not available for crop use. Growing season precipitation was considered to be 80% effective in contributing to crop water use. The effectiveness factor of 80% allowed for precipitation depths throughout a sub-basin that might differ from NWS rain–gage amounts. It also included a reduction for mismatches in timing between rainfall events and irrigation scheduling.

Depletion

HYDROGRAPH MODIFICATION

Depletion, in this context, is the consumptive abstraction of water from the hydrologic system as a result of irrigation. It is in addition to consumptive water use that would have occurred in the unmodified natural situation. As an example, waters of the Bear River Basin of Southern Idaho, Northern Utah, and Western Wyoming, because it is an interstate system, are administered by a federally established commission under the authority of the Bear River Compact.[1] Depletion is the basis, in the compact, for allocating Bear River water use among the three states. It is defined by a ‘‘Commission Approved Procedure’’ which includes consideration of land use and incorporates an equation for estimating depletion based on evapotranspiration. In a study for the commission,

Diversion of significant amounts of water from rivers and streams for irrigation and subsequent return flows alters the shape and timing of downstream hydrographs. In watersheds where mountain snowmelt provides the irrigation supply, such as in the Western United States, diversion during the spring runoff attenuates the peak flow rate while later return flows extend the flow duration into late summer and early fall.

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ð1Þ

Reservoir Storage Storage of water in reservoirs can significantly modify the natural stream hydrograph depending on the

Irrigation: Impact on River Flows

timing and quantity of the storage right. Irrigators with junior rights may only be able to store during time periods with low irrigation demand, such as during the winter, or during peak flow periods. Reductions of stream flow during the winter time may have considerable impact on downstream in-stream flows. Whereas, storage during periods of peak runoff may not affect minimum in-stream flow needs, but could deposit considerable amounts of sediment in the reservoir. Irrigation Return Flows

minus additional abstractions. Additional abstractions include incidental consumptive use from water surfaces as in open drains, along with non-crop vegetation. The timing of return flow varies from nearly instantaneous (recaptured tailwater) to delays of weeks and months or perhaps longer with deep percolation subsurface drainage. In a hydrologic model study of the Bear River Basin[4] delay times between diversion and subsequent appearance of the return flow at the next downstream river gage varied from 1.5 months to as long as 6 months. The delay appeared to be related to sub-basin shape and size. Irrigation Methods Four general irrigation methods are used: surface, subsurface, sprinkler, and trickle (also known as low flow or drip). Surface methods include wild or controlled

Irrigation– Journals

Irrigation return flows are comprised of surface runoff and/or subsurface drainage that becomes available for subsequent rediversion from either a surface stream or a groundwater aquifer downstream (hydrologically) of the initial use. Reusable return flow can be estimated as irrigation diversion minus crop related depletions

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Fig. 1 Comparison of basin efficiencies between surface and sprinkler irrigation methods with four return flow reuse cycles.

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flooding, furrow, border-strip, and ponded water (basin, paddy, or low-head bubbler). Hand move, wheel move, and center pivot are examples of sprinkler irrigation. Trickle irrigation includes point source emitters, microspray, bubbler, and linesource drip tape (above or below ground). Whereas the efficiency of surface irrigation is dependent upon the skills and experience of the irrigator, the performance of trickle and sprinkler systems is more dependent on the design. Generally, the more control that the system design (hardware) has on the irrigation system performance, the higher the application efficiency (Ea) can be. Thus, typical wheel move sprinklers have higher Ea values than surface irrigation, but lower values than for center pivots or trickle, assuming better than average management practices for each method. The impact on river flows can be quite different among the various irrigation methods. The nature of furrow and border surface irrigation generally produces tail water runoff, which can be immediately recaptured and reused, as well as deep percolation, which may not be available for reuse until after a period of time. Tailwater is essentially eliminated and deep percolation reduced with sprinklers (Fig. 1) compared to conventional surface irrigation. Whereas, with drip methods, deep percolation can be further reduced. The reduction of deep percolation implies increased salt concentration in the root zone leachate, but, perhaps significant reduction in salt pick-up potential from geologic conditions.

Irrigation Efficiencies Although a full discussion of the several variations of irrigation efficiency is beyond the scope herein, two terms will be defined and discussed. More complete discussions relating to irrigation efficiencies and water requirements are given elsewhere.[6–9,13] Keller and Bliesner[9] give a particularly thorough presentation of distribution uniformity and efficiencies. Application efficiency (Ea): Ea ¼ 100 Volume of water stored in the root zoneðVs Þ  Volume of water delivered to farm or fieldðVf Þ

Irrigation– Journals

Distribution uniformity: The distribution uniformity is a measure of how evenly the on-farm irrigation system distributes the water across the field. The definition of DU is: DU ¼ 100 Average of the lowest 25% of infiltrated water depth  Average of all infiltrated water depths across the field

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Irrigation: Impact on River Flows

On-farm or field application efficiencies can be affected by the distribution uniformity and vary widely for both surface and sprinkle irrigation methods. This is largely due to difference in management practices, appropriateness of design in matching the site conditions (slope, soils, and wind), and the degree of maintenance. In addition, for a given system uniformity, the higher the proportion of the field that is adequately irrigated (i.e., infiltrated water refills the soil water deficit) the lower will be the application efficiency. This is due to greater deep percolation losses in the overirrigated portions of the distribution pattern. Some values determined in recent Utah field evaluations are: Observed Method

High (%) Low (%) Typical (%)

Surface irrigation Ea Tailwater Deep percolation Sprinkler irrigation

72 55 65

24 5 20

50 20 30

Ea Evaporation Deep percolation

84 45 37

52 8 8

70 12 18

The Ea for a particular field may vary greatly during the season. Cultivation practices, microconsolidation of the soil surface and vegetation will alter surface irrigation efficiency both up and down from the seasonal average. Seasonal and diurnal variations in wind, humidity, and temperature will also affect sprinkle application efficiencies.

BASIN IRRIGATION EFFICIENCY The actual irrigation efficiency realized for several successive downstream fields where capture and reuse of return flows is experienced is higher than the Ea of an individual field. This notion of ‘‘Basin Irrigation Efficiency’’[12,13] is illustrated in Fig. 1. This simple example comparison of surface and sprinkle methods assumes four reuse cycles. In each of the five ‘‘fields’’ Et is assumed to be 50 units. The surface runoff is captured for reuse on the next field. All of the irrigationrelated evaporation is assumed ‘‘lost’’ as well as 5 units of deep percolation. After the fifth field, all surface and subsurface flows are lost. The basin efficiency for surface is 78%, which is the same as for sprinkle. The surface irrigation basin efficiency increase is dependent upon the surface return flow reuse, which is 20 units in this example. However, the depletion is greater for sprinkler due to the extra evaporation. In a Colorado

Irrigation: Impact on River Flows

ENVIRONMENTAL CONCERNS The process of evapotranspiration, or crop water use, extracts pure water from the soil water reservoir, which leaves behind the dissolved solids (salts) contained in the applied irrigation water. The ‘‘evapoconcentration’’ of salts is an inevitable result of irrigation for crop production. As stated by Bishop and Peterson:[2] ‘‘ . . . Other uses add something to the water, but irrigation basically takes some of the water away, concentrating the residual salts. Irrigation may also add substances by leaching natural salts or other materials from the soil or washing them from the surface. Irrigation return flow is a process by which the concentrated salts and other substances are conveyed from agricultural lands to the common stream or the underground water supply . . . ’’

Water Quality Implications for Agriculture Irrigated agriculture is dependent upon adequate, reasonably good quality water supplies. As the level of salt increases in an irrigation source, the quality of water for plant growth decreases. Since all irrigation waters contain a mixture of natural salt, irrigated soils will contain a similar mix to that in the applied water, but generally at a higher concentration. This necessitates applying extra irrigation water, or taking advantage of non-growing season precipitation, to leach the salts below the root zone.

Salt Loading Pick-Up Water percolating below the root zone or leaking from canals and ditches may ‘‘pick-up’’ additional salts from mineral weathering or from salt–bearing geologic formations (such as the Mancos shale of Western Colorado and Eastern Utah). This salt pick-up will increase the salt load of return flows and consequently increase the salinity of receiving waters. In the Colorado River Basin in the United States and Mexico salinity is a concern because of its adverse effects on agricultural, municipal, and industrial users.[10] The Salinity Control Act of 1974 (Public Law 93-320) created the Colorado River Basin Salinity

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Control Program to develop projects to reduce salt loading to the Colorado River. Salinity control projects include lining open canals and laterals (or replacing with pipe) and installing sprinklers in place of surface irrigation for the purpose of decreasing salt loading caused by canal leakage and irrigated crop deep percolation. Recently selenium in irrigation return flow has become a concern[3] and may also be reduced by salinity reduction projects.

In-Stream Flow Requirements Diversions in some reaches in some Western United States streams are ‘‘dried up’’ immediately downstream of diversion structures during times of peak irrigation demand. This condition eliminates any use of the reach for fisheries and other uses which depend on in-stream flow. In some instances, negotiated agreements with senior water rights users have allowed for bypass of minimal amounts of water to sustain the fishery or habitat, and for control of tailwater runoff to reduce agricultural related chemicals in the receiving water.

REFERENCES 1. Bear River Commission. Amended Bear River Compact, S.B. 255, 1979 Utah Legislature Session. 1979. 2. Bishop, A.A.; Peterson, H.B. (team leaders). Characteristics and Pollution Problems of Irrigation Return Flow. Final Report Project 14-12-408, Fed. Water Pollution Control Adm. U.S. Dept. of Interior (Ada, Oklahoma). Utah State University Foundation, Logan, Utah. May 1969. 3. Butler, D.L. Effects of Piping Irrigation Laterals on Selenium and Salt Loads, Montrose Arroyo Basin, Western Colorado; U.S. Geological Survey Water-Resources Investigations Report 01-4204 (in cooperation with the U.S. Bureau of Reclamation): Denver, Colorado, 2001. 4. Hill, R.W.; Israelsen, E.K.; Huber, A.L.; Riley, J.P. A Hydrologic Model of the Bear River Basin; PRWG72-1, Utah Water Resource Laboratory, Utah State University: Logan, Utah, 1970. 5. Hill, R.W.; Brockway, C.E.; Burman, R.D.; Allen, L.N.; Robison, C.W. Duty of Water Under the Bear River Compact: Field Verification of Empirical Methods for Estimating Depletion. Final Report; Utah Agriculture Experiment Station Research Report No. 125, Utah State University: Logan, Utah, January 1989. 6. Hoffman, G.J.; Howell, T.; Solomon, K. Management of Farm Irrigation Systems; The American Society of Agricultural Engineers: St. Joseph, MI, 1990. 7. Jensen, M.E. Design and Operation of Farm Irrigation Systems; ASAE Monograph, American Society of Agricultural Engineers: St. Joseph, MI, 1983.

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field study, Walter and Altenhofen[11] found a progressive increase in irrigation efficiencies from field (average Ea of 45%), to farm, to efficiency of ditch or sectors (average of 83%). This was due to the reuse of tailwater (10–20% of delivery) and deep percolation (46% of delivery).

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8. Jensen, M.E.; Burman, R.D.; Allen, R.G.; Eds. Evapotranspiration and Irrigation Water Requirements; ASCE Manual No. 70, American Society of Civil Engineers: New York, NY, 1990. 9. Keller, J.; Bliesner, R.D. Sprinkle and Trickle Irrigation; Van Nostrand Reinhold: New York, NY, 1990. 10. U.S. Department of the Interior. Quality of Water— Colorado River Basin: Bureau of Reclamation, Upper Colorado Region; Progress Report no 19; Salt Lake City, Utah, 1999.

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Irrigation: Impact on River Flows

11. Walter, I.A.; Altenhofen, J. Irrigation efficiency studies—northern colorado. Proceedings USCID Water Management Seminar. Sacramento, CA Oct 5–7; USCID: Denver, CO, 1995. 12. Willardson, L.S.; Wagenet, R.J. Basin-Wide Impacts of Irrigation Efficiency; Proceedings of an ASCE Specialty Conference on Advances in Irrigation and Drainage, Jackson, WY, 1983. 13. Willardson, L.S.; Allen, R.G.; Frederiksen, H.D. Elimination of Irrigation Efficiencies; Proceedings 13th Technical Conference, USCID: Denver, CO, 1994.

Irrigation: Metering John A. Replogle Albert J. Clemmens U.S. Water Conservation Laboratory, Agricultural Research Service (USDA-ARS), U.S. Department of Agriculture, Phoenix, Arizona, U.S.A.

The measurement of applied irrigation water is one of the major links in efforts to achieve effective water management worldwide. Measurement of flow for irrigation differs from most municipal and industrial water metering requirements because the water is spread over very large areas. This usually results in the need to measure both very large flows in canals near water supply sources and small flows spread over very large areas, perhaps in small trickle irrigation lines near the points of use. Irrigation was often done with waters that were not needed by other uses, although competition by these other uses is increasing, causing much controversy in water-use planning, policy, and development; therefore enhancing the need for accurate flow metering. Traditionally, flow meters have been classified according to the physical principle or property exploited, such as those related to sound; magnetism; electricity; chemical reactions; mixing; and volume, mass, and energy relations.[1] The device that exploits these properties to interact with the water is called the primary element, and produces an indication that can be detected with a secondary element for the user to observe, or otherwise use. This classification of meters according to exploited properties can be broadly grouped into flow-rate meters or quantity meters, according to the effect that is first observable. For example, a weir is a flow-rate meter, and a bucket is a simple quantity meter. Not all meters are currently practical for use in irrigated agriculture. Major restrictions to irrigation applications are often the lack of electric power at the metering site, capital cost, and poor maintenance support. Thus, practical irrigation metering emphasizes low cost, reasonable accuracy, and simplicity and the ability to meter waters with high sediment and/or trash loads. Meters that meet these criteria for the irrigation setting are discussed later. A wide variety of meters are discussed in more detail in Refs.[2,3]. OPEN-CHANNEL FLOW METERING The most common measurement methods for open channels are: current metering, weirs, and flumes. A

few additional metering methods will be discussed, but they represent a small portion of irrigation meters. Current Metering Current metering is a common method for the measurement of flow in rivers, streams, and large irrigation canals. In this method, a series of velocity measurements are made at many selected points across the channel, usually with a small propeller meter or a cuptype meter similar in concept to the cup-anemometer used for wind velocity. Other types of velocity meters are entering the market that are based on electromagnetic and ultrasonic concepts. The large number of velocity measurements, and their statistical averaging, helps to compensate for odd-shaped channels and the various flow velocities that may exist across the channel. Details of current metering methods are given in Ref.[4]. Current meter measurements are labor intensive and represent only a sample of flow at a particular time. Thus, to be useful for continuous monitoring of flow, a relationship between water depth and flow is used to estimate flow during times between current meter measurements. This adds additional error to the measurement of flow volume over time.[4] For these large flows, measurements with flumes and weirs, discussed later, can be difficult due to the large size of the channel and the need for a drop in water surface as the water passes over the weir or flume. More recently, ultrasonic meters have been used for stream gauging. Transit-time ultrasonic stream gauging is based on detecting stream flow velocity by using the difference in time for sound transmissions sent obliquely across the stream in opposite directions. This difference is translated into average velocity in the sound path that was sampled. Setting these meters so that the average velocity sampled is related to the average flow velocity is a limitation. Improved Doppler ultrasonic meters depend on reflected sound waves from flowing particles in the flow, rather similar to the action of the familiar Police-Radar units, and they are able to sample a series of locations within the flow profile. These meters are becoming more accurate and 655

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Irrigation– Journals

INTRODUCTION

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Irrigation: Metering

Fig. 1 Flume in large canal. Note stilling well and the region of wavy water surface. Sill crest is located about midway between these two features. Canal flow depth is about 2.4 m (8 ft) and sill crest is about 1.4 m (4.5 ft) high.

less expensive. Both types of ultrasonic meters are also applicable to pipelines. Weirs and Flumes Flumes and weirs work on the principal of critical flow, which means that the flow rate is a maximum for a given energy (combination of depth and velocity functions). However, in order to cause critical flow, there must be a drop in water level through the flume or weir. A wide variety of flumes and weirs have been developed since the late 1800s. Most of these are gradually being replaced by a family of flumes and weirs that can be calibrated using computer techniques rather than relying on laboratory calibration. They are called long-throated flumes and broad-crested weirs. For these flumes and weirs, the channel size is reduced by contracting the sides or raising the floor to form what is called a throat section that is long relative to the flow depth, producing nearly parallel flow that can be treated mathematically; hence, these flumes are also called the ‘‘computable flumes.’’ Additionally, the

amount of water surface drop across the flume or weir needed to provide a measurement is not large, typically about 15% of the depth in the throat. Because they can be calibrated by computer, they can be made of essentially any prismatic shape and can be calibrated to asbuilt dimensions (Fig. 1). A number of portable and adjustable flumes have been developed for flow survey work, as opposed to permanent installation, and are available commercially (Fig. 2). Further details on these flumes and weirs can be found in Ref.[5]. Software for design and calibration is also available free-ofcharge on the web: http://www.usbr.gov/wrrl/winflume/ Long-throated flumes and broad-crested weirs are often the simplest and most cost-effective method for measuring flow rates in open channels. They are used worldwide. However, in order to obtain flow volume, the instantaneous flow-rate measurements must be totaled over time. This requires either periodic observation or continuous recording. Devices for accomplishing this often cost more than the flume or weir itself. Also, there are locations where the opportunity for sufficient water surface drop is not available to produce flow measurements over the full range of desired discharges.

PIPELINE FLOW METERING

Irrigation– Journals Fig. 2 Adjustable flume being installed while channel continues to flow. Capacity 56 L/sec (2 cfs).

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While advances in canal flow measurements have significantly aided irrigation water management worldwide, developments in pipe flow measurements have also impacted irrigation flow measurements. A common method for irrigation flow measurement in pipelines, and often the least expensive, has been the propeller meter. Similar to the current meter, the velocity of the turning propeller is related to the average velocity of flow in the pipe. Early meters used mechanical gears to turn a cumulative volume meter, while magnetic indicators of propeller rotation and digital electronics are now common. The biggest problem with

Irrigation: Metering

Fig. 3 Portable ultrasonic flow meter on outlet pipe of an irrigation well. Sensors are usually mounted on the side of pipe to avoid air bubbles that may be in the pipe.

flows in a few locations. These meters are relatively immune to debris in the pipeline and so can be used in culverts or short pipe sections that are supplied by trash-filled open canals. However, they tend to have high cost and/or high maintenance. Doppler ultrasonic meters that measure point velocities are advancing and may prove more accurate and cost effective in the near future. Irrigation applications often require one-time flow surveys rather than permanent installations, and portable Doppler ultrasonic meters, portable transonic meters, and pitot-tube systems are useful for such surveys[7] (Fig. 3). Sometimes the suggested 10 pipe diameters upstream from a meter are not available to assure proper meter functioning. It is sometimes more practical to attempt to modify the flow profile approaching the meter than to increase pipe length. Flow-conditioning devices, such as vanes to keep the flow from swirling and wall obstruction in the form of large-opening orifice to break up wall jetting have proven effective.

REFERENCES 1. Bean, H.S.; Ed. Fluid meters, their theory and application. Report of ASME Research Committee on Fluid Meters, 6th Ed.; The American Society of Mechanical Engineers: New York, 1971; 273 pp. 2. Bos, M.G. Discharge Measurement Structures, 3rd Revised Ed.; International Institute for Land Reclamation and Improvement/ILRI: Wageningen, The Netherlands, 1989. 3. Water Measurement Manual, 3rd Ed.; U.S. Department of Interior, Bureau of Reclamation. Superintendent of Documents, U.S. Government Printing Office: Washington, DC, 1997. 4. Herschy, R.W. Streamflow Measurement; Elsevier Applied Science Publishers: New York, 1985; 453 pp. 5. Clemmens, A.J.; Wahl, T.L.; Bos, M.G.; Replogle, J.A. Water Measurement with Flumes and Weirs, Publication 58; International Institute for Land Reclamation and Improvement/ILRI: Wageningen, The Netherlands, 2001; 382 pp. 6. Miller, R.W. Flow Measurement Engineering Handbook, 3rd Ed.; McGraw-Hill Books: New York, 1996; 553 pp. 7. Replogle, J.A. Some observations on irrigation flow measurements at the end of the millennium. Trans. ASAE 2002, 18 (1), 6–14.

Irrigation– Journals

these meters continues to be bearing wear from sand that is often in pumped wells, as well as errors in flow caused by poor approaching flow conditions. Pipe bends, elbows, valves, etc., can cause most pipeline flow meters to be inaccurate if they are closer than about 10 pipe diameters upstream and 2 pipe diameters downstream.[1,6] Modern electronics have greatly improved the ability of secondary devices to monitor primary devices based on well-known primary elements, for example, the differential pressure across a Venturi meter or orifice meter, or the speed of sonic waves across a pipe. These electronic advances have resulted in lower-cost metering systems, often with improved accuracy. Many metering techniques depend heavily on these advances in electronics, such as vortex-shedding meters, ultrasonic Doppler flow meters, and the ultrasonic transittime flow meters. Many of the older, popular meters are described in a number of references.[6] Vortexshedding meters are becoming more common because of their relatively low cost. However, like propeller meters, they obstruct the flow and are not suitable where debris can enter the pipeline. Multipath ultrasonic meters are being used for large irrigation flows, and single path ultrasonic meters are used for smaller

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Irrigation: Preplant James E. Ayars U.S. Department of Agriculture (USDA), Parlier, California, U.S.A.

INTRODUCTION Irrigation is the process of supplying supplemental water necessary for plant growth and development by several application techniques, e.g., microirrigation, surface irrigation, sprinkler irrigation, subirrigation (by raising a shallow water table), and subsurface irrigation. It is used not only in arid and semiarid areas of the world, but also in humid areas to supplement rainfall during periods of drought. In this context, water is applied after the crop has been planted and growth has begun. The depth and timing of the application are based on the crop water requirement, the irrigation water quality, the crop salt tolerance, and the available soil water storage capacity. These topics are discussed fully in other parts of the encyclopedia. Preplant irrigation is the application of water to a field during a fallow period between crops to accomplish a goal to replenish soil water that is anticipated for future requirements for plant growth and development. Identified uses of preplant irrigation include germination, salinity management, soil water management, fumigation, weed control, and fertilizer placement. Preplant irrigation is used in arid, semiarid, and humid areas throughout the world with the largest applications being in arid and semiarid areas. The depth and timing of the application will depend on the following: the irrigation purpose, the irrigation water quality, the existing stored soil water, the soil salinity, the crop rotation, the crop salt tolerance, and the depth to shallow groundwater.

USES Germination

Irrigation– Journals

Plant establishment and development are critical to achieving yield and production goals in annual cropping systems. Germination of the seed is the first step in this process followed by plant and root development and extension. An adequate supply of soil water is required in the zone of seed placement to assure that these processes occur and are sustained. In humid areas, rainfall is generally adequate for these purposes, but in arid and semiarid areas rainfall is insufficient to 658

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supply the necessary water either due lack of water or poor timing. The San Joaquin Valley of California is typical of this situation. Rainfall occurs primarily during the winter months with the total varying from 150 mm to over 600 mm during this period. The effectiveness of the rainfall as a future water supply is a function of the depth and timing of the occurrences. Most of the rainfall can be lost to evaporation when only small 2–3 mm amounts occur. If the rainfall is in excess of about 12 mm, some of this water will be stored and be available later to meet plant water requirements. If a crop is being planted in the fall after either a period of summer fallow or the harvest of a summer crop, there will be little or no soil water stored and available for germination. As a result, it is necessary to irrigate the field to provide the water necessary for germination and early plant growth. When a crop is planted in the fall immediately following a summer crop, irrigation will probably be withheld until after the crop has been planted. If the crop is planted in the fall following a summer fallow, the field will be irrigated prior to planting, and the crop planted into soil water allowing the producer to schedule operations in a timely fashion. Spring planted crops generally follow a winter fallow period that is used to prepare fields for planting by creating seed beds and preplant irrigating to restore soil water. After irrigation, the seedbed is tilled to provide a mulched surface and prevent further evaporation. At planting, the seed is placed in the soil and allowed to germinate. After germination has been completed, the soil over the seed is removed mechanically, and the seed can sprout and continue to grow. This process is called planting to soil water and is used extensively in cotton production in California. Organic producers often use this method to germinate and sprout direct seeded crops. Usually, care is required in the dry soil layer mechanical removal process and must be applied soon after germination before seedling elongation begins. Weed Control Preplant irrigation plays an important role in weed management in both conventional and organic farming

Irrigation: Preplant

Salinity Management Managing soil salinity is critical to successful agriculture in arid and semiarid areas of the world. Salts naturally occur in the soil, in the water used for irrigation, and in the fertilizers used for production. Crop growth and production will be reduced and eventually eliminated unless salinity in the root zone is controlled. Plants have a wide range of salt tolerance,[1] ranging from sensitive to salt tolerant. In addition to the basic tolerance, plant tolerance varies depending on growth stage with germination being the most sensitive time and maturity being the least sensitive. Salt accumulates in the root zone as crops extract essentially pure water from the stored soil water; thus, leaving salt in the soil. In addition to salt applied by the irrigation water, salt can be transported up to the root zone from a shallow saline water table as the crop uses this water for plant growth. Evaporation from the soil surface also moves salt up into the soil profile and often to the soil surface. This accumulation of salt can have a significant negative impact on germination unless it is removed prior to planting. Leaching is the term used for removal of salt from the soil profile, and it is accomplished by irrigating in excess of the total water needed simply to meet the water requirements of the crop. The leaching requirement is the term used to describe the excess water needed to control the accumulation of salt in the soil profile. This requirement can be met incrementally with each irrigation or once a season. Preplant irrigation is an effective method of managing salt in the profile. Generally, the large application

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made to replenish the stored soil water in the profile is adequate to transport the salt beneath the upper part of the soil profile, critical for germination and early crop development. Data in Fig. 1 show the salt mass in sprinkler irrigated and furrow irrigated plots to a depth of 120 cm for December 1997 and June 1998. In both cases, the salinity was higher at each depth increment in December than in June. The reductions in salinity through the profile was a result of deep percolation and salinity leaching from the preplant irrigation and rainfall. In arid areas, there is generally inadequate rainfall to accomplish the necessary leaching, and preplant irrigation is required. Soil Water Management Crop yield and biomass are directly related to total crop water use. Soil is the reservoir that stores water for plant use, and rainfall and irrigation are the sources of supply of the stored soil water. Soil water holding capacity is a function of soil type with sandy soils being able to store small amounts of water and loams, silty clay loams, and clays storing larger amounts. During the growing season, plants remove water from the stored soil water and irrigation replenishes the depleted water supply. Irrigation scheduling is used to determine when to irrigate and how much to irrigate. It is not a precise science because of the variability in climate, soils, and crop stand and development. The stored soil water acts as a buffer and

Irrigation– Journals

practices. In sugar beet production, preplant irrigation is used to germinate weed seed and carryover crop seeds (barley, wheat, and oats) prior to planting the sugar beet seeds. Sugar beet is not a very competitive crop and significant production can be lost to excessive growth of other plants. Once the weed or volunteer seed has germinated, it can be destroyed by using either chemicals (paraquat or glyphosate) or cultivation. When pre-emergence herbicides are used, preplant irrigation with sprinklers has improved the selectivity and activity of the chemicals because less water is used and less chemical is lost to leaching. Organic producers use preplant irrigation to germinate weed seeds and then eliminate them either through cultivation or by burning. Another technique for weed control uses large amounts of compost applied prior to bed preparation. The compost is preplant irrigated to speed the composting process, which generates large amounts of heat that kills the existing weed seed. This process has to be completed prior to planting to prevent damage to the crop.

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Fig. 1 Salt mass as a function of depth in sprinkler and furrow irrigated field.

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reduces the impacts on plant growth due to water stress from errors in the scheduling process. The total water available increases over the growing season as the root system extends and explores deeper into the soil profile. If water extracted by deep-rooted crops is not replaced by irrigation or rainfall prior to planting the next crop, the following crop might suffer water stress late in the growing season. Preplant irrigation is an effective means to refill the soil profile and to store water for late season plant use in soils with large storage capacity. Fertilizer Placement Microirrigation systems are sometimes used to apply fertilizers in the seedbed prior to planting. This is accomplished by injecting fertilizers into the water and only applying a small amount of water. This is possible with microirrigation and not with other systems. Applying fertilizers with surface irrigation can result in excessive losses due to the operational characteristics of these systems. Fumigation When fumigants are applied with preplant irrigation it has to be done well in advance of planting to ensure no damage is done to the crop. Fumigants are applied by sprinkler and microirrigation systems during preplant irrigation to minimize the losses due to deep percolation. Subsurface drip irrigation is being used to apply fumigants prior to planting strawberries.

MANAGEMENT Depth of Application

Irrigation– Journals

The depth of application will be determined by the intended use. If weed control was the intent, the irrigation has to be small enough to provide water to germinate the weed seed and still permit timely cultivation or create a situation where the weeds subsequently die from lack of water. This might require only 5–10 mm of water. The same would be true with fumigation and fertilization applications. Only enough water would be applied to transport the chemical into the soil and position it in the correct portion of the crop root zone but not enough to transport it out of the root zone. The amount will be determined based on the existing soil water status, the irrigation system, and the size of the plot being irrigated. Leaching and soil water management are often accomplished with a single application of water based on the existing soil water content. In arid areas with

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Irrigation: Preplant

deep-rooted crops, the root zone often extends to a depth of 1m–2 m below the soil surface. The soil water depletion can be in excess of 200 mm in loams to clay soils, and replenishing the water is adequate to transport the salt from the soil surface well into the profile. This occurs in part because of the inefficiency of the irrigation systems. Surface irrigation systems and sprinkler systems have efficiencies of application in the range of 75–85%. This means that for the field to receive at least 200 mm of water, an additional 15–25% more water has to be applied. This inefficiency is generally adequate to provide the necessary leaching fraction when the water quality being used is considered. The depth of application can be determined by measuring the soil water content by soil sampling and a gravimetric determination, by neutron attenuation, by time domain reflectrometry, and by capacitance methods. The method selected will be a function of the crop, soils, and manager preference and experience. The closer the application is made to the time of planting, the more opportunity there is for rainfall to provide part of the water needed to replenish the root zone. Timing of Application Timing for the preplant irrigation will be determined by the intended use. For weed control, water has to be applied such that the weed will be dead prior to emergence of the crop and any irrigation associated with emergence. Fumigation has to occur early enough that the residual effect of the fumigants has dissipated prior to planting. This will be a function of the fumigant, the climate at the time of fumigation, the soil type, and the crop being grown following fumigation, whether it will be direct seeded or transplanted. Some crops, i.e., cotton, require a minimum soil temperature prior to planting to ensure good germination and stand establishment. In this instance, preplant irrigation needs to be done early to allow time for soil heating prior to the optimum planting date for the crop. Managing soil water and leaching has to be done early enough that the soil has time to drain and return to a soil water content that is acceptable for cultivation. In large irrigated areas, this application of preplant irrigation occurs for several months prior to planting. This is possible because of the some of the planting techniques described in a previous section. Water Quality Impacts Improper management of preplant irrigation can have a significant negative impact on shallow groundwater quality and ultimately drainage water quality. This occurs primarily when preplant irrigation is used for

Irrigation: Preplant

soil water management, germination, and salinity control. Large amounts of water are applied in a single application during these operations and if the amount applied is significantly greater than what is required, deep percolation occurs. Deep percolation is the movement of water below the root zone and it is lost to crop production. This water carries along salt, fertilizers, and fumigants that are in the soil profile and mixes with the existing ground water. Depending on the chemical, this can create problems many years into the future.

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and salinity control are the goals. While surface systems are effective, they often have poor irrigation efficiency because the high infiltration rate as result of tillage following the crop. This inefficiency is manifested by excessive deep percolation losses and poor distribution uniformity. Surface systems and sprinklers cover the entire surface area and are thus very effective on large areas. Microirrigation systems are used when fumigation and fertilization are important because of the ability to apply small depths of water and to precisely place the water and chemical.

Method of Application REFERENCE 1. Maas, E.V. Crop salt tolerance. In Agricultural Salinity Assessment and Management; Tanji, K.K., Ed.; ASCE Manuals and Reports on Engineering Practice No. 71; American Society of Civil Engineers: New York, NY, 1990; 262–304.

Irrigation– Journals

Preplant irrigation can be done with any available irrigation system. The system normally used for irrigation during the season is the one that is most often used for preplant. Surface irrigation methods (furrow, flood, and basin) and sprinkler irrigation are the most common application methods when soil water management

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Irrigation: Saline Water B. A. Stewart Dryland Agriculture Institute, West Texas A&M University, Canyon, Texas, U.S.A.

INTRODUCTION As water becomes more limited, there is increasing use of saline waters for irrigation that were previously considered unsuitable. Rhoades, Kandiah, and Marshali[1] classified saline waters as shown in Table 1. Electrical conductivity is a convenient and practical method for classifying saline waters because there is a direct relationship between the salt content of the water and the conductance of an electrical current through water containing salts. Electrical conductivity values are expressed in siemens (S) at a standard temperature of 25 C. Most waters used for irrigation have electrical conductivities less than 2 dS m 1.[1] When water higher than this level is used, there can be serious negative effects on both plants and soils. As salinity in the root zone increases, the osmotic potential of the soil solution decreases and therefore reduces the availability of water to plants. At some point, the concentration of salts in the root zone can become so great that water will actually move from the plant cells to the root zone because of the osmotic effect. Salts containing ions such as boron, chloride, and sodium can also be toxic to plants when accumulated in large quantities in the leaves. The extent that plant growth is affected by saline water is dependent on the crop species. Some plants, such as barley and cotton, are much more resistant to salt than crops like beans. Rhoades, Kandiah, and Marshali[1] list the tolerance levels of a wide range of fiber, grain, and special crops; grasses and forage crops; vegetable and fruit crops; woody crops; and ornamental shrubs, trees, and ground cover. Soils are also negatively impacted by salt, particularly sodium salts. Sodium ions tend to disperse clay particles and this has deleterious effects on infiltration rate, structure, and other soil physical properties.

Irrigation– Journals

IRRIGATING WITH SALINE WATERS Water limitations and the need to increase food and fiber production in many parts of the world have resulted in the use of water for irrigation containing increasing levels of salts. The United States, Israel, Tunisia, India, and Egypt have been particularly active 662

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in irrigating with saline waters.[1] Rhoades, Kandiah, and Marshali[1] published an extensive paper on the use of saline waters for crop production and it is a valuable guide for anyone interested in the subject. They reported that many drainage waters, including shallow ground waters underlying irrigated lands, fall in the range of 2 dS m 1 to 10 dS m 1 in electrical conductivity. Such waters are in ample supply in many developed irrigated lands and have good potential even though they are often discharged to better quality surface waters or to waste outlets. These waters can be successfully used in many cases with proper management. Reuse of second-generation drainage waters with electrical conductivity values of 10 dS m 1 to 25 dS m 1 is also sometimes possible but to a much lesser degree because the crops that can be grown with these waters are atypical and much less experience exists upon which to base management recommendations. Miller and Gardiner[2] suggest that successful irrigation with saline water requires three principles. First, the soil should be maintained near field capacity to keep the salt concentration as low as possible. Second, application techniques should avoid any wetting of the foliage. Third, salts accumulating in the soil should be periodically leached. To accomplish these objectives, Miller and Gardiner[2] recommend the following general rules:  Apply water at or below soil surface. Sprinklers should be used only if they avoid wilting the foliage (such as sprinkling before plant emergence or below-canopy to avoid salt-burn damage).  Keep water additions almost continuous, but at or below field capacity so that most flow is unsaturated. This maintains adequate aeration.  Enough water should be added to keep salts moving downward, thus avoiding salt buildup in the root zone. Miller and Gardiner[2] stress that these rules are difficult to meet and are best satisfied by some form of drip irrigation. They also state that due to the need for high water levels and because of high sodium ratios that sandy soils are more adaptable to the use of saline waters than soils containing high percentages of silt and clay particles.

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Table 1 Classification of saline waters Water class

Electrical conductivity (dS m 1)

Salt concentration (mg L 1)

Non-saline

45,000

Brine

Type of water Drinking and irrigation

Secondary drainage and groundwater Very saline groundwater Seawater

Source: From Ref.[1].

 Selection of crops or crop varieties that will produce satisfactory yields under the existing or predicted conditions of salinity or sodicity.  Special planting procedures that minimize or compensate for salt accumulation in the vicinity of the seed.  Irrigation to maintain a relatively high level of soil moisture and to achieve periodic leaching of the soil.  Use of land preparation to increase the uniformity of water distribution and infiltration, leaching and removal of salinity.  Special treatments (such as tillage and additions of chemical amendments, organic matter and growing green manure crops) to maintain soil permeability and tilth. The crop grown, the quality of water used for irrigation, the rainfall pattern and climate, and the soil properties determine to a large degree the kind and extent of management practices needed.

BLENDING LOW-SALT AND SALTY WATERS Miller and Gardiner[2] reported that countries such as Israel have developed extensive canal and reservoir systems where both low-salt and salty waters are mixed to obtain usable water. Rhoades, Kandiah, and Marshali,[1] however, state that blending or diluting excessively saline waters with good quality water supplies should only be undertaken after consideration is given to how this affects the volumes of consumable water in the combined and separate supplies. They suggest that blending or diluting drainage waters with good quality waters in order to increase water supplies

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or to meet discharge standards may be inappropriate under certain situations. More crop production can usually be achieved from the total water supply by keeping the water components separated. Serious consideration should be given for keeping saline drainage waters separate from the good quality water, especially when the good quality waters are used for irrigation of salt-sensitive crops. The saline waters can be used more effectively by substituting them for good quality water to irrigate certain crops grown in the rotation after seeding establishment.

CONCLUSION There is ample evidence that saline waters once considered unacceptable for irrigation can be used successfully provided that they are properly managed. There is also ample evidence, however, to show that these waters can be highly damaging to the environment and to the soil resource base when improperly managed. Therefore, saline waters should be only used for irrigation after careful study and considering as many factors as possible. Then, when the waters are used for irrigation, a careful monitoring program should be implemented of both the crops produced and of the resulting soil and environmental changes.

REFERENCES 1. Rhoades, J.D.; Kandiah, A.; Marshali, A.M. The use of saline water for crop production. FAO Irrigation and Drainage Paper 48; Food and Agriculture Organization of the United Nations: Rome, 1992. 2. Miller, R.W.; Gardiner, D.T. Soils in Our Environment, 8th Ed.; Prentice Hall: Upper Saddle River, NJ, 1998.

Irrigation– Journals

Rhoades, Kandiah, and Marshali[1] also list specific management practices for producing crops with salty waters. Their list includes the following guidelines:

Irrigation: Sewage Effluent Use B. A. Stewart Dryland Agriculture Institute, West Texas A&M University, Canyon, Texas, U.S.A.

INTRODUCTION One of the primary functions of soil is to buffer environmental change. This is the result of the biological, chemical, and physical processes that occur in soils. The soil matrix serves as an incubation chamber for decomposing organic wastes including pesticides, sewage, solid wastes, and many other wastes. Soils store, decompose, or immobilize nitrates, phosphorus, pesticides, and other substances that can become pollutants in air or water. Consequently, soil has, for centuries, been used for the application of sewage effluents. Sewage effluent provides farmers with a nutrient-enriched water supply and society with a reliable and inexpensive means of wastewater treatment and disposal. It should not, however, be assumed that irrigation is always the best solution for wastewater disposal. Disposal by irrigation should always be compared with alternative options based on environmental, social, and economic costs and benefits. While disposal is the primary objective in many cases, the need of water for irrigation is becoming more often the driver for using sewage effluent on land. This is particularly true in areas like the Middle East where population growth is resulting in severe water shortages. The guidelines for using effluent for irrigation vary considerably among countries and other governing bodies. Cameron[1] conducted a literature review and found wide differences of guidelines for effluent irrigation projects being used throughout the world. In general, however, sustainable and environmentally sound systems can be developed in most situations provided proper management practices are followed.

CONCERNS OF IRRIGATING WITH SEWAGE EFFLUENT Irrigation– Journals

In spite of the documented benefits associated with the use of sewage effluent for irrigation, there are numerous concerns. Many industrial wastewaters have been routinely dumped into municipal sewage lines. While this issue has been addressed in some jurisdictions, it has not in many others. In the United States, the Environmental Protection Agency requires that 664

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wastewaters be treated prior to disposal into municipal treatment plants or back into groundwater. Irrigating with wastewaters partially cleans water by percolation through the soil, but soluble salts and some inorganic and organic chemicals may continue to flow with the water to groundwater or surface supplies. In general, the Environmental Protection Agency allows sewage effluents to be used for irrigation only if it does not cause: 1) extensive groundwater pollution; 2) a direct public health hazard; 3) an accumulation in the soil or water of hazardous substances that can get into the food chain; 4) an accumulation of pollutants such as odors into the atmosphere; and 5) other aesthetic losses, within the limits.[2] Bouwer[3] has also expressed concerns about the use of sewage effluent for irrigation. He is particularly concerned with pathogens and warns that complete removal of viruses, bacteria, and protozoa and other parasites should be required before the effluent can be used to irrigate fruits/vegetables consumed raw or brought into the kitchen, or parks, playgrounds and other areas with free public access. Bouwer also stresses that long-term effects of sewage effluent irrigation on underlying groundwater should be considered in addition to the changes in nitrate and salinity. Ground water in low rainfall regions can be highly affected by percolating sewage effluent because much of the water is used by the growing crops and this greatly concentrates the chemicals in the small amounts of water that actually percolate to the groundwater. These chemicals can include disinfection byproducts, pharmaceutically active chemicals, and compounds derived from humic and fulvic acids formed by the decomposition of plant material. Bouwer claims that many of these chemicals are suspected carcinogens or toxic. Therefore, Bouwer concludes that while sewage irrigation looks good on the surface, a more extensive look reveals a potential for serious contamination of groundwater. He states that municipalities and other entities responsible for irrigation with sewage effluent should do a groundwater impact analysis to develop management protocols and be prepared for liability actions. Those who benefit are local and state institutions in water resources, environmental quality protection, public health, consultants, and operators of effluent irrigation projects.

Irrigation: Sewage Effluent Use

REUSE STANDARDS The standards for using sewage effluent for irrigation of agricultural crops vary widely among different countries of the world. Mexico and many South American countries, e.g., use untreated wastewater for irrigation.[4] Most of these countries do not have the resources or capital to treat sewage effluents. Wastewater is utilized after little or no treatment, and health risks are minimized by crop selection. Mexico does not allow wastewater to be used to irrigate lettuce, cabbage, beets, coriander, radishes, carrots, spinach, and parsley. Acceptable crops include alfalfa, cereals, beans, chili, and green tomatoes. In contrast, Israel has very stringent water reuse requirements. Effluent water requires a high level of treatment (large soil-aquifer recharge systems with dewatering) before the water can be reused for irrigation of vegetables to be consumed raw.[5] Health guidelines for irrigation with treated wastewater developed in California indicate that effluent waters used on food crops must be disinfected, oxidized, coagulated, clarified, and filtered.[6] Total coliform counts cannot exceed a median value of 2.2/100 ml or a single sample value of 25/ 100 ml. Total coliforms must be monitored daily and turbidity cannot exceed 2 nephelometric turbidity units and must be monitored continuously. Less restrictive guidelines developed by Shuval et al.,[7] and adopted by most of the international agencies, suggested that effluent water reuse was relatively safe to use if it contained less than 1 helminth egg L 1, and less than 1000 fecal coliforms/100 ml.

MONITORING GUIDELINES

and textural classes suitable for each land treatment process. Information was provided for designing and monitoring site characteristics for slow rate processes (sprinkler and other typical farm irrigation systems), rapid infiltration basins, and overland flow systems. Monitoring requirements will vary considerably among projects depending on the cropping patterns, soil characteristics, and specific environmental concerns. In most cases, monitoring procedures and criteria will be site specific. In all cases, however, the objectives should be to use the resources effectively, protect the land, protect the groundwater, protect the surface water, and protect the community amenity.

REFERENCES 1. Cameron, D.R. Sustainable Effluent Irrigation Phase 1: Literature Review International Perspective and Standards, Technical Report Prepared for Irrigation Sustainability committee; Canada–Saskatchewan Agriculture Green Plan, 1996. 2. Miller, R.W.; Gardiner, D.T. Soils in Our Environment, 8th Ed.; Prentice-Hall Inc: Upper Saddle River, NJ, 1998. 3. Bouwer, H. Groundwater Problems Caused by Irrigation with Sewage Effluent; Irrigation and Water Quality Laboratory, USDA-ARS: Phoenix, AZ, 2000. 4. Strauss, M.; Blumenthal, U.J. Human Waste Use in Agriculture and Aquaculture: Utilization Practices and Health Perspectives; IRWCD Report No. 09/90; International Reference Centre for Waste Disposal: Deubendorf, 1990. 5. Shelef, G. The role of wastewater reuse in water resources management in Israel. Water Sci. Tech. 1990, 23, 2081–2089, Switzerland. 6. Ongerth, H.J.; Jopling, W.F. Water reuse in California. In Water Renovation and Reuse; Shuval, H.I., Ed.; Academic Press: New York, 1977. 7. Shuval, H.I.; Adin, A.; Fattal, B.; Rawitz, E.; Yekutiel, P. Wastewater Irrigation in Developing Countries. Health Effects and Technical Solutions; World Bank Tech. Pap.; 1986; Vol. 51, 325 pp. 8. U.S. EPA. Process Design Manual: Land Treatment of Municipal Wastewater; EPA 625/1-81-013; U.S. EPA Center for Environmental Research Information: Cincinnati, OH, 1981.

Irrigation– Journals

Site selection is a critical and necessary step in initiating a sewage effluent irrigation system. The U.S. Environmental Protection Agency[8] published detailed information on site characterization and evaluation. Information was provided on the design of systems, site characteristics, expected quality of the effluent water after land treatment, and typical permeabilities

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Irrigation: Site-Specific Dennis C. Kincaid Northwest Irrigation and Soils Research Laboratory, Pacific West Area, Agricultural Research Service (USDA-ARS), U.S. Department of Agriculture, Kimberly, Idaho, U.S.A.

Gerald W. Buchleiter Water Management Research, Agricultural Research Service (USDA-ARS), U.S. Department of Agriculture, Fort Collins, Colorado, U.S.A.

INTRODUCTION Irrigation systems have evolved from flood systems to pressurized sprinkler and trickle systems. In flood irrigation, water is applied to a field in a controlled stream and allowed to flow over the soil surface by gravity, the final distribution being affected by variations in surface slope and water infiltration rates. Well-designed pressurized irrigation systems apply water at sufficiently low rates that it infiltrates with little or no surface movement, thus providing a greater degree of control and improved uniformity of application. The primary objective of irrigation system design is to apply water (and dissolved chemicals) uniformly over a field planted with a uniform crop, the water requirement being determined primarily by the crop and climate. In recent years, sophisticated control systems have been developed that enable water and chemical application to be tailored to smaller areas if and when it is desirable to do so. The term site-specific irrigation (also known as precision-variable irrigation) refers to the practice of intentionally applying different amounts of water to different areas of a field to optimize crop production, minimize chemical and water use, or reduce environmental concerns. Although site-specific irrigation can be applied with any type of pressurized irrigation system, most of the potential application is with continuous-move sprinkler laterals, primarily center pivots.[1–5] DESIGN AND MANAGEMENT OBJECTIVES

Irrigation– Journals

Some of the main reasons for site-specific irrigation are the following:  Avoid watering non-productive areas such as roads, rock outcrops, canals, ditches, and ponds. Center pivots often traverse these areas that lie within a generally circular area.  Apply different amounts of water and nutrients to different zones according to crop production 666

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capability. Soil depth, salinity, or other soil-related factors may limit the potential yield and the total water requirement on some soil types.  Apply reduced amounts of water to steep slopes or zones of low infiltration where runoff is difficult to control. A permanent cover crop may be planted in these areas.  Variable soil types within a field may benefit from different amounts of water during certain time periods. Under water-short scenarios, crops on coarsetextured soils having low water holding capacity need small, frequent water applications to avoid water stress, while the crop on finer-textured soils may be able to withdraw stored soil water.

SCALE CONSIDERATIONS One of the main considerations is determining the minimum size area that must be treated individually.[6] The cost and complexity of the system escalate rapidly as the treatment area decreases. The wetted radius of the individual sprinkler patterns, the start–stop movement of the lateral, and the accuracy with which the lateral position can be determined all affect the minimum practical differential area. Typically, a 300-m2 area is about the smallest desirable unit. Maps defining soil types, unproductive areas, cropping and fertility patterns are used to define management zones (Fig. 1) requiring different water amounts. These zones should be created from the intersecting areas of only the map parameters that affect the water or chemical requirements.

EQUIPMENT FOR SITE-SPECIFIC IRRIGATION Sprinkler Laterals Continuous-move laterals that move in a straight line are called ‘‘linears’’ and those that rotate about a fixed

Irrigation: Site-Specific

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Fig. 1 Schematic of a field irrigated by a 7-span center pivot, with each span subdivided into three segments. Crosshatched areas are special water management zones (photo by Dennis C. Kincaid).

water application patterns overlap, resulting in nearly uniform water distribution along the lateral. The pattern radii of the most popular spray heads are about 5–8 m. The travel speed of the lateral can be varied to change the water application depth in pieshaped differential areas under a pivot or in rectangular differential areas under a linear. However, for all other differential areas, sprinkler flows must be varied along the lateral. There are three main methods of accomplishing this: 1. A variable flow rate sprinkler head uses a fixed nozzle with an insertable pin to produce either a high or low flow rate.[7] The pin can be cycled in or out to produce an effective flow rate anywhere between the high and low flow. 2. Automatic valves can be placed on individual sprinklers or groups of sprinklers on manifolds (Fig. 2). Manifold length is usually a fourth to half the span length. One-directional check

Irrigation– Journals

pivot at one end are called ‘‘center pivots.’’ These laterals consist of several rigid spans, typically 40–50 m in length with a total length of about 400 m, although longer laterals are used. The outermost tower controls the rotation speed. The entire lateral is maintained in a nearly straight line by switches at intermediate towers that start and stop the drive motors according to the flex angle between adjacent spans. Center pivots use a transducer (pivot resolver) to determine the position of the first span with an accuracy of about 1 of rotation and a radial coordinate system to determine the position of any point on the lateral relative to the field map at any time. Recently, differential global positioning system (DGPS) units placed on the outer end of the lateral have been used to improve the positioning accuracy of center pivots. Linear laterals use a guidance system to travel on a predetermined (normally straight) path. A calibrated ground wheel, fixed ground stakes with a trip switch on the lateral, or with a DGPS unit, can determine the lateral position along the travel path. Both end towers control the travel speed and guidance. Additional error is introduced by the guidance system that ‘‘steers’’ the lateral by adjusting the relative speed of the end towers, thus changing the angle of the lateral relative to the travel path. Therefore the positioning accuracy of linears is usually less than that of pivots.

Sprinkler Equipment and Controls Traveling laterals use sprinkler equipment designed to discharge a desired amount of water per unit length of lateral. For pivots, the discharge rate increases with distance from the pivot. Sprinklers or spray heads are placed at fixed or variable spacing such that their

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Fig. 2 An on–off spray manifold on a span of a traveling lateral. Note black automatic valve above manifold (photo by Kincaid).

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Irrigation: Site-Specific

the travel speed, and turns sprinkler control valves on or off according to a predetermined program as the lateral passes over each subarea of the field. Valves are usually electric-solenoid-operated and each requires a separate control wire. Optionally, a code-based control system can send signals to individual valves through a single wire.[9]

CONCLUSION

Fig. 3 Close-up of multiple-manifold spray system (photo by Sadler).

valves are used on the individual heads to prevent the manifold from draining when the manifold valve is off. The manifold valve can be cycled on and off at different time intervals to produce effective application rates between 0% and 100% of the maximum rate. The cycle interval must be much less than the time it takes the full sprinkler pattern to traverse a point on the ground. 3. Two or three complete sets of sprinklers designed with different unit flow rates are mounted on the lateral (Figs. 3 and 4). Any combination of the sprinkler sets can be turned on one at a time, resulting in several distinct rates. Two sets provide four possible rates (e.g., 0, 1/3, 2/3, and 1), and three sets provide eight possible rates. The variable flow sprinkler (method 1) has not yet been commercially developed. At the present time, the on–off manifold (Fig. 2) is likely the most costeffective configuration, as this involves the least additional equipment. The computerized control system is normally located at the pivot or inlet end of the lateral.[8] The computer determines the location of the lateral, adjusts

Irrigation– Journals Fig. 4 A site-specific center-pivot system with selected spray manifolds off (photo by Sadler).

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New technologies have made precision variable water application technically feasible. Many different scenarios of variable soils, different crops, limited water supplies, and environmental concerns may make sitespecific irrigation desirable. Because of the cost and complexity of these systems, economic feasibility will be highly case-dependent.

ARTICLE OF FURTHER INTEREST Irrigation: Sprinklers (Mechanical), p. 670.

REFERENCES 1. Buchleiter, G.W.; Camp, C.R.; Evans, R.G.; King, B.A. Technologies for variable water application with sprinklers. In National Irrigation Symposium, Proceedings of the 4th Decennial Symposium, Phoeniz, AZ, Nov 14–16, 2000; Evans, R.G., Benham, B.L., Trooien, T.P., Eds.; American Society of Agricultural Engineers: St. Joseph, MI, 2000; 316–321. 2. Evans, R.G.; Han, S.; Kroeger, M.W.; Schneider, S.M. Precision center pivot irrigation for efficient use of water and nitrogen. Proc. 3rd International Conf. on Precision Agriculture, Minneapolis, MN, June 23–26, 1996; Robert, P.C., Rust, R.H., Larsen, W.E., Eds.; ASA: Madison, WI, 1996; 75–84. 3. Fraisse, C.W.; Heermann, D.F.; Duke, H.R. Simulation of variable water application with linear-move irrigation systems. Trans. ASAE 1995, 38 (5), 1371–1376. 4. Sadler, E.J.; Camp, C.R.; Evans, D.E.; Usrey, L.J. A sitespecific center pivot irrigation system for highly variable coastal plain soils. Proc. 3rd International Conf. on Precision Agriculture, Minneapolis, MN, June 23–26, 1996; Robert, P.C., Rust, R.H., Larsen, W.E., Eds.; ASA: Madison, WI, 1996; 827–834. 5. Sadler, E.J.; Camp, C.R.; Evans, D.E.; Millen, J.A. Spatial variation of corn response to irrigation. Trans. ASAE 2002, 45 (6), 1869–1881. 6. Sadler, E.J.; Evans, R.G.; Buchleiter, G.W.; King, B.A.; Camp, C.R. Design considerations for site-specific irrigation. In National Irrigation Symposium, Proceedings of the 4th Decennial Symposium, Phoeniz, AZ,

Irrigation: Site-Specific

self-propelled systems. In National Irrigation Symposium, Proceedings of the 4th Decennial Symposium, Phoeniz, AZ, Nov 14–16, 2000; Evans, R.G., Benham, B.L., Trooien, T.P., Eds.; American Society of Agricultural Engineers: St. Joseph, MI, 2000; 322–331. 9. Wall, R.W.; King, B.A.; McCann, I.R. Center pivot irrigation system control and data communications network for real-time variable water application. Proc. 3rd International Conf. on Precision Agriculture, Minneapolis, MN, June 23–26, 1996; Robert, P.C., Rust, R.H., Larsen, W.E., Eds.; ASA: Madison, WI, 1996; 757–766.

Irrigation– Journals

Nov 14–16, 2000; Evans, R.G., Benham, B.L., Trooien, T.P., Eds.; American Society of Agricultural Engineers: St. Joseph, MI, 2000; 304–315. 7. King, B.A.; Wall, R.W.; Kincaid, D.C.; Westermann, D.T. Field scale performance of a variable flow sprinkler for variable water and nutrient application. Paper No. 972216 Presented at the 1997 ASAE Annual International Meeting, Minneapolis, MN, Aug 10–14, 1997; American Society of Agricultural Engineers: St. Joseph, MI, 1997. 8. Evans, R.G.; Buchleiter, G.W.; Sadler, E.J.; King, B.A.; Harting, G.B. Controls for precision irrigation with

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Irrigation: Sprinklers (Mechanical) Dennis C. Kincaid Northwest Irrigation and Soils Research Laboratory, Pacific West Area, Agricultural Research Service (USDA-ARS), U.S. Department of Agriculture, Kimberly, Idaho, U.S.A.

INTRODUCTION Sprinkler irrigation can be defined as the controlled distribution of water as discrete droplets through air. Sprinkler devices were first invented during the late 19th and early 20th centuries, primarily for lawns and gardens.[1] Their widespread use for agricultural crops did not come about until the availability of lightweight aluminum pipe and low cost electricity following World War II. Sprinkler irrigation is particularly well suited to rolling topography, shallow soils, and sandy soils, which are difficult to irrigate efficiently with gravity flow surface irrigation systems. Sprinkler systems are now used on one-half of the 50 million irrigated acres in the United States.[2] Early sprinkler systems used manually moved pipe and fixed or portable pumps and mainlines. Handmove lines are relatively low cost in terms of equipment investment, but are labor intensive. Labor costs provided the primary incentive to develop mechanically moved sprinkler systems. Another factor has been the increase in farm size and the desire to create automated irrigation systems so that one person can irrigate more land. This article describes the mechanics of the various systems. Special sprinklers, mounting devices, and pressure regulators have been developed for these systems. Information on the sprinklers, system design and management, etc. can be found in the listed references.[3–6]

STATIONARY OR PERIODIC-MOVE LATERALS

Irrigation– Journals

A sprinkler lateral is a continuous length of pipe upon which sprinklers are mounted, usually equally spaced. Mechanically moved or hand-moved stationary laterals remain in a fixed position while irrigating, which are then drained and moved to a new predetermined position. They normally irrigate rectangular fields and require several sets to completely irrigate the field.

to move the entire lateral simultaneously when drained (Fig. 1). The pipe is a special high-strength alloy tubing, 100–125 mm in diameter, capable of withstanding considerable torque. The wheel spacing is usually about 12 m, with sprinklers located midway between the wheels. These laterals are typically 400 m in length, but may be as long as 800 m. The wheel radius must be at least as large as the height of the crops to be grown, typically about 0.75–1.0 m. Sideroll laterals are not used to irrigate tall crops such as corn. A powered mover unit located near the center of the lateral provides torque to roll the lateral. Some longer laterals use two movers located approximately onefourth of the distance from each end and connected by a small rotating shaft to coordinate the movement. The movers are powered by a small gas engine, electric motors, or hydraulic motors. The lateral is moved 2–4 complete revolutions between sets (12–15 m). For convenience, some sideroll laterals can be moved by an operator standing at the inlet end of the lateral. A short flexible hose is used to connect the lateral to a water supply outlet. Water is supplied from a fixed mainline with outlets spaced at some unit multiple of the wheel circumference. Trail-Line Lateral The trail-line system, also called a movable solid-set, consists of a lateral mounted on two-wheel support towers (see section ‘‘Continuous-Move Laterals’’), which serves as a movable mainline for a set of trailing sublaterals. The trail-lines are lightweight aluminum sprinkler sublaterals typically spaced about 12–16 m apart and up to 120 m in length. The powered lateral drags the trail-lines between sets. When one irrigation event is complete, the trail-lines are disconnected, and the lateral is moved to the opposite end of the traillines, which are then reconnected to the lateral. The whole system is then moved back across the field dry to the first position, or can irrigate each set in turn as it is moved back to the initial position.

Sideroll Wheeline CONTINUOUS-MOVE LATERALS The sideroll wheeline lateral consists of an aluminum pipe that serves as an axle for a series of rigidly attached wheels, the whole of which is rolled sideways 670

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Continuous-move laterals are those which travel while irrigating, either smoothly or intermittently in small

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Fig. 1 Sideroll lateral with mover unit in foreground. Note the small driveshaft parallel to the lateral pipe on the right, which transfers power to the mover mechanism.

diameter) and is usually supported 3–4 m above ground to provide sufficient clearance for most crops. The lateral is made up of several 30–55 m long spans, where each span pipe is integrated into a rigid truss. The pipe joints between spans are flexible, and one end of each span is supported by a two-wheeled tower, both wheels being powered. The wheels are typically powered by electric motors, but fluid motors (oil or water) are also used. The movement of the wheels on intermediate towers is controlled by switches or valves,

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increments. There are two main types: those called linears that travel in a nearly straight line path, irrigating rectangular areas; and pivoting laterals that rotate about one fixed end, and thus irrigate circular areas. Both types use the same hardware and differ mainly in the way they are controlled and supplied with water. Continuous-move laterals have been built in many variations and styles over the years, but the most common type in use today is shown in Figs. 2 and 3. The lateral pipe is steel or aluminum (100–250 mm

Fig. 2 Center-pivot lateral with swingspan corner system partly extended. Sprinklers are mounted on drop tubes below the lateral.

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Fig. 3 Linear-traveling lateral, hose-drag type, with on-board engine. This lateral can also pivot about the inlet structure. Sprinklers are mounted on drop tubes.

which automatically function to keep the entire lateral in a nearly straight line. Travel speed determines the water application depth. Linear-Move Laterals Linear-move laterals are usually supplied with water through a flexible drag hose (Fig. 3), or alternatively through a suction pipe moving in an open canal. In addition, automatic coupling systems have been built. The hose-drag system can travel twice the length of hose in one set. An on-board diesel powered generator or drag cable provides electricity. The wheels normally follow the same track each pass to minimize crop damage. The outermost towers determine travel speed and travel direction. Special guidance systems must be used to keep the wheels traveling in the same path each pass. One method uses radio antennas to follow a buried cable, while another type follows an aboveground cable or small guide trench. Recently, Global Positioning System receivers have been employed to guide traveling laterals and swingspan pivots (Fig. 2). Traveling laterals up to 800 m in length have been built. Center-Pivot Laterals

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The pivoting lateral or center-pivot system is supplied with water and electrical power through simple swivel couplings at the fixed end. The lateral can rotate continuously about the pivot, so that when the first irrigation is completed the lateral is in position to begin the next irrigation. They require no coupling or uncoupling of hoses or pipes and a pivot without a swingspan requires no guidance system. The low labor requirement of this system has made it very popular, and they are used on about 50% of the sprinkler irrigated land.

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The major disadvantage of the center-pivot is the circular irrigated area, which leaves the corners of a square field unirrigated. In large developments, the circles can be nested to minimize the unirrigated area. Where the economics of the situation dictate that the corners must be irrigated, several options exist. A large sprinkler mounted on the outer end of the lateral and controlled to turn only in the corner areas can irrigate a portion of the corners. Another option is the swingspan corner system (Fig. 2) consisting of an additional span up to 70 m in length that pivots about the outer end of the lateral. As the lateral moves into a corner area, the swingspan pivots outward, effectively extending the length of the lateral. The swingspan wheels are steerable, and a buried cable or GPS guidance system similar to the linear-move controls its movement. Sprinklers on the swingspan are automatically sequenced on or off as the swingspan moves outward or retracts, thus maintaining a nearly equal water application per unit area. A center-pivot equipped with a swingspan can irrigate up to 97% of a square field. Recently, manufacturers have developed traveling laterals that can operate both as linears or pivoting laterals, making it possible to irrigate odd-shaped fields and cover more area with a given length of lateral (Fig. 3). Pivot laterals can also be made towable so that they can irrigate more than one field.

TRAVELING SPRINKLERS Large traveling sprinklers, called big guns, can throw water up to 70 m, and can irrigate large areas. They require relatively high-pressure (up to 1000 kPa) water supplies, so operating costs can be quite high.

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Fig. 4 Hard-hose traveler completing a pass. Note inlet supply hose lower right.

Hard-Hose Travelers

Soft-Hose Travelers A similar traveling sprinkler uses a soft, collapsible hose (Fig. 5). The sprinkler cart is pulled by a cable and winch, usually mounted on the cart itself, and powered by a water turbine or reciprocating cylinder. Initially, the hose is laid out alongside the travel path and the cable is reeled out and attached to a fixed anchor. The sprinkler can travel twice the hose length

Irrigation– Journals

The most popular traveler, called a hard-hose traveler, consists of a big-gun sprinkler mounted on a cart and supplied by a semirigid hose, which retains its round shape when wound upon a reel (Fig. 4). Water is supplied through the center of the reel from a mainline outlet. Initially, the hose is pulled out along the travel path by a tractor. The reel remains in a fixed position while irrigating and slowly rotates, dragging the hose and sprinkler across the field as the hose is reeled in. A water turbine, reciprocating cylinder, or small gas engine provides power to turn the reel. Reel speed is automatically adjusted to account for the change in

diameter due to hose wrap. The hose can be up to 400 m in length and 115 mm diameter. The hose reel is mounted on a trailer for transport by a small tractor.

Fig. 5 Soft-hose traveler beginning a pass. The sprinkler cart is towed by a cable (not visible).

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in one set. The hose must be drained and reeled up between sets.

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because of their inherent advantages, including ease of automation and continuous rotation capability.

Boom Travelers Boom travelers are similar to big-gun travelers except that the gun sprinkler is replaced by a horizontal boom structure extending perpendicular to the travel path, and high enough to clear the crop. This in effect creates a traveling lateral upon which sprinklers or spray heads are mounted. The advantage of this system is that it requires much less water pressure than the large gun sprinklers, and water application uniformity is improved.

CONCLUSION Mechanically moved sprinkler systems will increase in sophistication as computerized controls are developed, particularly for precision variable water and chemical application. Center-pivot systems will likely predominate

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REFERENCES 1. Morgan, R.M. Water and the Land—A History of American Irrigation; The Irrigation Association: Fairfax, VA, 1993. 2. Pair, C.H.; Ed. Farm and ranch irrigation survey. 1997 Census of Agriculture; http://www.nass.usda.gov/ USDA: Washington, DC, 1998. 3. Pair, C.H.; Ed. National Irrigation Symposium, Proceedings of the 4th Decennial Symposium, Phoenix, AZ, Nov 14–16, 2000; American Society of Agricultural Engineers: St. Joseph, MI, 2000. 4. Pair, C.H.; Ed. Design and Operation of Farm Irrigation Systems; ASAE Monograph No. 3; American Society of Agricultural Engineers: St. Joseph, MI, 1980. 5. Keller, J.; Bliesner, R.D. Sprinkle and Trickle Irrigation; Chapman and Hall: New York, 1990. 6. Pair, C.H.; Ed. Irrigation, 5th Ed.; The Irrigation Association: Fairfax, VA, 1983.

Irrigation: Supplemental Phillipe Debaeke Unite d’Agronomie, Institut National de la Recherche Agronomique (INRA), Castanet-Tolosan, France

The main objective of irrigation consists of supplying water to crops when soil-stored water at planting and seasonal rainfall are too erratic or limited to satisfy the plant transpiration demand with enough regularity, at a level defined by the farmer. Supplemental (or supplementary) irrigation (SI) was defined as follows: ‘‘In an area where a crop can be grown by natural rainfall alone but additional water by irrigation stabilizes and improves yield, this irrigation is termed supplemental, the additional water alone being insufficient to produce a crop.’’[1] SI is applied to complete a deficient or uneven precipitation regime, enhancing and securing crop production, both in quantity and quality, in such pedoclimatic conditions where rainfed production is still feasible although less profitable.

AREAS AND CROPPING SYSTEMS CONCERNED When natural contributions by rainfall or groundwater are too scarce to satisfy full crop water requirements only occasionally (amount and distribution within the season), the continuous optimal water regime can be obtained through SI, i.e., by a temporary and discontinuous irrigation regime.[2] Such situations are frequently observed in humid and subhumid regions, generally for spring-sown crops such as soyabean, maize, sugarbeet, potatoes, and for some tree crops. For instance, in southwestern France, most of the maize is grown under SI (up to 250–300 mm). In the regions where both natural and irrigation resources are too limited for ensuring a permanent optimal water regime to crops, SI is mainly supplied at the critical periods of the crop-growth cycle, in order to maintain or improve crop production.[2] This is the case in arid and semiarid regions of the Mediterranean basin where SI is practiced for species generally grown profitably without irrigation but their yields are subjected to great variations over the years because of rainfall variability. These species are: winter cereals (mostly durum and bread wheat), autumn-sown legumes (faba bean, peas), spring-sown crops having a

dense and deep rooting system (sorghum, sunflower, cotton, etc.), and tree crops (olive, almond, peach, vine, etc.). In those regions, spring-sown crops (such as maize or sugarbeet) with high water requirements but restricted rooting system are only grown under intensively-irrigated systems, irrigation amounts (until 800–1000 mm) exceed the contribution of natural resources (rain and stored soil water). Surprisingly, SI is also widely practiced in Northern Europe, such as United Kingdom and Scandinavia.[3] In most years, spring-sown crops (potatoes, sugarbeet, horticultural crops) have their growth restricted and yields reduced by water shortage, the extent of this depending very much on soil type (e.g., shallow soils, low water holding capacity), weather conditions, and the timing and duration of stress periods. Although rainfall is fairly evenly distributed throughout the year, potential evaporation rates exceed rainfall throughout most of the summer months. In Scandinavia, the growing season is much shorter and so early sowing cannot be practiced to moderate the effects of summer drought periods. In Denmark, SI is needed nearly every year on sandy soils to maintain a stable production and 15% of the agricultural land is grown under irrigation.[4] In temperate humid and subhumid environments, where water deficit is occasional, generally terminal and/or of short duration, SI is used by farmers to stabilize yield and quality at higher levels (to improve profits), and to maintain crop uniformity. In drier environments, SI can be considered to be more a dry farming technique since it contributes to optimize the use of limited water resources:[2] its purpose is to prevent complete yield loss (through irrigation at sowing, in exceptionally dry years) or to improve yield in years not excessively dry (by irrigation during shooting). In every case, SI is a means of insuring farmers against climatic risks.

SI SCHEDULING The strategy of applying restricted amounts of water based on the amount and distribution of rainfall in addition to the incremental effect of water on crop yield is the essence of the SI concept.[5] 675

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INTRODUCTION

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Either because irrigation volume (or discharge) is limited (dry winter season, low storage capacity of reservoirs, equipment not available, etc.) or because soil-water deficit is moderate, SI generally results in a limited number of water applications. The goal of these applications is either to save crop life but more generally to improve the efficiency of the other inputs.[5] Positive impacts are expected such as: sowing in due time, assurance of an uneven and minimum plant emergence, more efficient placement and use of fertilizers, thus limiting soil N leaching and residue at harvest, use of high-yielding cultivars and avoidance of moisture stress for plant, particularly during the initial stages of its development in semiarid regions, later (around flowering) in wetter regions. For instance, in the West Asia–North Africa (WANA) region, with a Mediterranean-type climate, wheat production is increasingly declining. Cereal yields are low and variable in response to inadequate and erratic seasonal rainfall (350 mm rainfall and above) and related management factors, such as lack of nitrogen and late sowing. It is clear that small amounts of SI water can make up for the deficits in seasonal rain and produce satisfactory yields.[6] A minimum yield of more than 3.5 Mg/ha is guaranteed for wheat with an amount of irrigation varying from 50 mm to 200 mm depending on the root zone soil water and the amount and distribution of the seasonal rainfall, whereas the average yield is below 1.5 Mg/ha under rainfed management.[1] An addition of only limited irrigation (1/3 full irrigation) may achieve over 60% of the potential increase in yield with full SI. In addition, use efficiency for both soil water and nitrogen is greatly increased by SI. Oweis, Pala, and Ryan[6] observed a wheat yield increase up to a fertilizer input of 100 kg N/ha under SI management in Syria, while optimum response for rainfed conditions was with 50 kg N/ha. In southwestern France, under a temperate subhumid climate, grain yield was increased by 17%, 27%, 37%, and 70% for sunflower, sorghum, soybean, and maize respectively, with an irrigation amount of 120 mm (supplied around flowering) when compared to rainfed management during nine years on a deep silty-clay soil.[7] This shows the differential sensitivity of spring-sown crops to SI as related to ecophysiological traits such as depth and extraction efficacy of rooting system, drought tolerance mechanisms (sunflower and sorghum), indeterminate reproductive period (for soybean) acting as an escaping strategy. With limited available water, the challenge is to satisfy crop water demand at the critical (and most responsive) stages. An extensive review of specific periods for optimizing irrigation was made by FAO.[8] For instance, the most sensitive stages of wheat to water stress are the booting and the early earing stage from

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some research, and the preflowering and ear formation stages according to other research, whereas seed germination and crop emergence periods are only exceptionally considered to be sensitive to water stress.[2] The decision of irrigation at a given growth stage depends on the crop sensitivity to water stress, on the climatic pattern and on the need to exploit natural water resources. Irrigation on cereals in autumn during dry sequences aims at ensuring an optimal plant density and a satisfactory root establishment in order to fully use soil water reserves later but also to cover rapidly the soil surface for controlling soil evaporation and maximizing early radiation interception and biomass accumulation. In semiarid regions, when a single application is available for sunflower, it should be placed either at presowing (soil refillment, crop establishment) or between flower bud appearance and flowering (to increase leaf area index) while in wetter areas, one irrigation is generally recommended after anthesis to enhance the leaf area duration and favor oil production.

SI METHODS The irrigation methods usable for SI must satisfy the following specifications:[2] low equipment cost per ha of irrigated land, high degree of transferability from one field to another, limited labor to set up the irrigation system, high water distribution efficiency, possibility to bring limited water amounts (20–50 mm) timely and accurately at specific growth stages. For these reasons, in rainfed systems, sprinkler irrigation (travelling rainguns, for instance) seem to be the most flexible for field crops while, for vegetables and fruit trees, drip irrigation may be more suitable. The source of water is generally small reservoirs (run-off and rainwater harvesting) but deep groundwater is also used.

NEED FOR MODELS In the last 15 years, on the basis of ecophysiological studies, numerous soil-plant models (either cropspecific or generic) have been developed to simulate the response of major crops to water use. By running on long-term weather records, these mechanistic models, more or less complex, are useful to determine a probabilistic response of grain yield to SI, in interaction with crop management (sowing date, cultivar, crop density, N-fertilization), and to define at field or farm level the optimal irrigation schedules under limited water management (e.g., Refs.[4,9–11]). At farm level, linear programming models have been developed to optimize the crop planning and to

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allocate scarce water between competing fields, according to the response of yield relatively to crop water requirements (resulting from mechanistic models or from simple production functions), to stochastic distribution of rainfall, to input availability (labor, equipment, water), to cost of production (water, other inputs), and to crop value.[12,13] Such models can be used to test if limited amounts of irrigation water can be used more efficiently by applying small amounts to more land than by fully irrigating less land, or to predict the optimal cropping pattern under SI. To conclude, SI cannot be restricted to a simple problem of tactical decision at field level (‘‘when and how much water to apply on this field?’’) but has to be considered also as a major strategical decision at farm level (‘‘which fields and which crops to irrigate using what type of equipment?’’).

REFERENCES

5.

6.

7.

8.

9.

10.

11.

12.

13.

Sept 12–13, 1995; Water Reports 8; ICID-CIID & FAO: Rome, 1996, 177–184. Oweis, T.; Pala, M.; Ryan, J. Management alternatives for improved durum wheat production under supplemental irrigation in Syria. Eur. J. Agron. 1999, 11, 255–266. Oweis, T.; Pala, M.; Ryan, J. Stabilizing rainfed wheat yields with supplemental irrigation and nitrogen in a mediterranean climate. Agron. J. 1998, 90, 672–681. Debaeke, P.; Hilaire, A. Production of rainfed and irrigated crops under different crop rotations and input levels in southwestern France. Can. J. Plant Sci. 1997, 77, 539–548. Doorenbos, J.; Kassam, A.H. Yield Response to Water; Irrigation and Drainage Paper 33, FAO: Rome, Italy, 1979; 193 pp. Perrier, E.R.; Salkini, A.B. Scheduling of supplemental irrigation on spring wheat using water balance methods. In Irrigation: Theory and Practice; Rydzewski, J.R., Ward, C.F., Eds.; Institute of Irrigation Studies, Southampton University: UK, 1989; 447–460. Cabelguenne, M.; Jones, C.A.; Williams, J.R. Strategies for limited irrigations of maize in southwestern France—a modeling approach. Trans. ASAE 1995, 38, 507–511. Debaeke, P. Wheat response to supplementary irrigation in south-western France: II. A frequential approach using a simulation model. Agr. Med. 1995, 125, 64–78. Bryant, K.J.; Mjelde, J.W.; Lacewell, R.D. An intraseasonal dynamic optimization model to allocate irrigation water between crops. Am. J. Agr. Econ. 1993, 75, 1021–1029. Deumier, J.M.; Leroy, P.; Peyremorte, P. Tools for improving management of irrigated agricultural crops. In Irrigation Scheduling: From Theory to Practice, Proceedings of the ICID/FAO Workshop on Irrigation Scheduling, Rome, Italy, Sept 12–13, 1995; Water Reports 8; ICID-CIID & FAO: Rome, 1996, 39–50.

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1. Arar, A. The role of supplementary irrigation in increasing productivity in the near east region. Proceedings of the International Conference on Supplementary Irrigation and Drought Management, Valenzano-Bari, Italy, Sept 27–Oct 2, 1992; CIHEAM; Vol. 1, 2, 1–10. 2. Caliandro, A.; Boari, F. Supplementary irrigation in arid and semiarid regions. Medit 1996, 7, 24–27. 3. Kay, M.G. Supplementary irrigation in northwest Europe. In Consultation on Supplementary Irrigation, Rabat, Morocco, Dec 7–9, 1987; FAO: Rome, 1990; 17–22. 4. Plauborg, F.; Heidmann, T. MARKVAND: An irrigation system for use under limited irrigation capacity in a temperate humid climate. In Irrigation Scheduling: From Theory to Practice, Proceedings of the ICID/FAO Workshop on Irrigation Scheduling, Rome, Italy,

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Irrigation: Surface Wynn R. Walker Biological and Irrigation Engineering Department, Utah State University, Logan, Utah, U.S.A.

INTRODUCTION Surface irrigation, also referred to as ‘‘flood irrigation,’’ is the oldest and most common method of applying water to croplands. There are three broad classifications: 1) basin irrigation; 2) border irrigation; and 3) furrow irrigation. Each classification can be distinguished on the basis of shape, slope, and field boundaries. It is important to understand that references to the ‘‘surface irrigation system’’ may include more than the individually irrigated field. Specifically, the irrigation system may consist of four subsystems, as illustrated in Fig. 1. These are: 1) the water supply subsystem; 2) the water delivery subsystem; 3) the water use subsystem (field); and 4) the water removal (drainage) subsystem.[1] Thus, the terms basin, border, and furrow irrigation are specific configurations of the water use subsystem. Optimizing basin, border, or furrow irrigation practices requires that each component of the irrigation system be designed, constructed, maintained, and operated effectively.

to be squarer in shape while level borders are more rectangular. The most important design parameter for basins is the inflow rate per unit width of the basin (unit discharge). Basins require a high unit discharge. Most soils can be irrigated by basin systems although soils with a moderate to low infiltration rate result in the best efficiency and uniformity. Basin systems typically apply a relatively large depth of water during irrigation and thus deep-rooted, closely spaced crops are best suited for this type of irrigation. Crops, which cannot be inundated for extended periods, should be planted on raised beds or furrows. There are three important advantages of basins: 1) they are effective and efficient methods of leaching salts from the soil profile; 2) they are easily automated with relatively simple flow controls at the basin inlet; and 3) they can achieve efficiencies and uniformities which equal or exceed those of sprinkle systems without the corresponding investment in energy. Border Irrigation

TYPES OF SURFACE IRRIGATION SYSTEMS The advantages and disadvantages of a specific surface irrigation configuration depend on a number of factors. For example, the need and extent of land leveling for furrow irrigation are much less than for basin irrigation. Very small or irregularly shaped fields are more easily irrigated with basins than furrows. The infiltration characteristics of the soil combined with the nature and availability of the water supply in terms of flow rate and duration may favor one method over another. The density and arrangement of the crop as well as the season-to-season cropping pattern will impact the method of applying water; and, the historical traditions of the irrigators may suggest one method over another. Irrigation– Journals

Borders are somewhat like basins though they are rectangular or contoured fields. They typically have a longitudinal but cannot have a lateral slope. They may be free draining or blocked at the lower end. Fig. 3 illustrates three typical border irrigation systems. Borders are suited for most crops and perform well on soils with moderately low to moderately high infiltration characteristics. If the soil crusts easily, borders can be furrowed, so, plants are grown on raised beds. If the border is not diked at the lower end, substantial tailwater losses may occur as illustrated in Fig. 4. Free-draining borders exhibit high application uniformities but generally are less efficient than sprinkle systems. Blocked-end borders enjoy the same high uniformities and efficiencies as basins and have further advantage of better field drainage in cases of excess rainfall or errors in irrigation duration.

Basin Irrigation Two typical examples of basin irrigation are shown in Fig. 2. Basins are level fields with perimeter dikes to prevent runoff. To distinguish them from level borders (discussed in ‘‘Border Irrigation’’), basins tend 678

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Furrow Irrigation By ‘‘furrowing,’’ ‘‘creasing,’’ or ‘‘corrugating’’ a field surface and then regulating a flow to each furrow,

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Fig. 1 Typical elements of a surface irrigation system. Source: From Ref.[1].

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difficulty in setting the proper flow rate into each furrow and thus causing either too much deep percolation or too much tailwater. Salts can accumulate between furrows, which are used for a long period of time. The additional tillage associated with construction of the furrows adds costs to the farm operation. Finally, the danger of erosion from furrow irrigation is higher than with either basins or borders.

IMPROVING THE OPERATION AND MANAGEMENT OF SURFACE SYSTEMS Walker and Skogerboe[2] describe the surface irrigation water management in the following terms. Even though it is the oldest and most common method of irrigation, surface irrigation is the least amenable to consistently high levels of performance. Of all the reasons why this is so, probably none have the significance that is associated with the uncertainty of soil infiltration rates. The rate at which water will be absorbed through the soil surface is a non-linear process which

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a field can be watered with substantially less flow and can have slopes in both the longitudinal and transverse directions. Water flowing in the furrow infiltrates through the wetted perimeter and moves vertically and laterally thereafter to refill the soil reservoir. As noted above, furrows can be used in conjunction with basins and borders, which should be referred to as ‘‘furrowed’’ borders or basins. A typical furrow irrigation system is shown in Fig. 5. Furrows provide somewhat better flexibility in onfarm water management, achieve the same high uniformities as basins and borders, and require less land leveling to implement. Furrow systems require more farm labor and thus tend to be less efficient than basins and borders. Flow rates per unit width can be substantially reduced and topographical conditions can be more severe and variable. Furrows provide operational flexibility important for achieving high efficiencies for each irrigation throughout a season by regulating the flow into each furrow. It is a simple (although labor intensive) matter to adjust the furrow stream size to changing intake characteristics by simply changing the number of simultaneously supplied furrows. Two of the more common ways in which water is introduced to furrows are shown in Fig. 6. In a general situation, furrows are less efficient that either basins or borders, primarily because of the

Fig. 2 Two illustrations of common basin irrigation systems: (A) a basin in Southeast Asia; and (B) A basin in central Utah. (Utah State University Irrigation Photo Archives.)

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Fig. 3 Examples of border irrigation systems: (A) Typical graded border irrigation system; (B) typical level border irrigation system; and (C) typical contour levee or border irrigation system. Source: From Ref.[1].

varies both temporally and spatially. It is affected by year-to-year changes in cropping patterns, cultivation, the weathering due to climate, and many other unknown influences. As a result, neither the irrigator nor the engineer can accurately predict the uniformity and efficiency of an irrigation before it occurs, particularly the first water application following planting.

There are other factors limiting surface irrigation system performance, such as a relative lack of standardized equipment for regulation and automation. These and the intake variability noted above place particular emphasis on the management practices applied to surface irrigation, and the art of surface irrigation management is very important.

Irrigation– Journals Fig. 4 Tailwater runoff under border irrigation. (Bureau of Reclamation Photo: www.yao.lc.usbr.gov/WaterConser/ ConservationDefined.htm.)

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Fig. 5 A typical furrow irrigation system. (Utah State University Irrigation Photo Archives.)

There are four ways in which the operation and management of basins, borders, and furrows can be improved: 1) improving the management of flow and time of cutoff; 2) precision leveling of the field surface; 3) blocking the end of the field; and 4) water recovery and reuse.

time of cutoff. If the inflow can be reduced (cutback) when the water has advanced to the end of the field, then the advantages of both a high flow during advance and a low flow to reduce tailwater can be met. One of the most promising ‘‘cutback’’ practices is surge flow in which the inflow is rapidly cycled on and off to create a ‘‘time-averaged’’ reduction in inflow.

Regulating Inflow and Time of Cutoff Precision Land Leveling There are few practices as important to surface irrigation performance as precision land leveling, grading, or smoothing. The advent of laser guided equipment as shown in Fig. 7 has improved land surface preparation by at least an order of magnitude over historical

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The irrigator of a free-draining surface irrigation system must balance the need for a high inflow to achieve uniform water application against a low inflow to minimize tailwater losses. The intake opportunity time at the end of the field plus the time required for the inflow to reach the end of the field dictate a unique

Fig. 6 A common method of supplying water to furrows using siphon tubes. (Utah State University Irrigation Photo Archives.)

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Fig. 7 Laser guided land leveling equipment. (Utah State University Irrigation Photo Archives.)

practices. The impact on surface irrigation uniformity and efficiency has been very high. It is not unusual to hear an irrigator state that the most important part of surface irrigation is ‘‘lasering.’’

prevented or an emergency drain may be necessary to remove excessive ponding. In any event, the application efficiency of blocked-end systems is usually 15–20% higher than free-draining systems. Blockedend systems should have fairly low field slopes if ponding is a problem.

Blocked-End Systems An alternative to reducing the inflow at the end of the advance period as a way to reduce tailwater losses is simply to block the end of the field and prevent the runoff from occurring. This is a common feature of basins and the reason for their high performance. It is less common in borders and furrows because of the risk of crop damage at the lower end of the field due to lengthy ponding. With proper regulation of inflow and time of cutoff, excessive ponding can be

Wastewater Recovery and Reuse In some areas, the tailwater problem can be resolved by constructing a small reservoir at the end of the field, capturing the runoff, and then reusing it elsewhere on the farm. This is a particularly useful practice where irrigators must control sediments, pesticides, and fertilizer runoff as well. Fig. 8 shows a typical tailwater pond.

Irrigation– Journals Fig. 8 A typical tailwater recovery and reuse facility. (Utah State University Irrigation Photo Archives.)

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CONCLUSION

of the most important water management tasks that arid and semiarid regions will face.

REFERENCES 1. U.S. Department of Agriculture, Soil Conservation Service, Planning Farm Irrigation Systems. National Engineering Handbook; U.S. Government Printing Office: Washington, DC, 1967, Sec. 15, Chap. 3. 2. Walker, W.R.; Skogerboe, G.V. Surface Irrigation: Theory and Practice; Prentice-Hall Inc.: Englewood Cliffs, NJ, 1987; 386 pp.

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Surface irrigation is critically important to the production of the world’s food and fiber. Advances in design, management, and field preparation have made surface irrigation systems as efficient as alternatives such as sprinkle irrigation. Surface irrigation is labor intensive, but requires low energy inputs making this method of watering crops more advantageous in developing countries than in countries like the United States. As the world population increases over the next two or three decades, improving the efficiency of surface irrigation systems to their potential will be one

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Isotopes Michael A. Anderson Department of Environmental Sciences, University of California–Riverside, Riverside, California, U.S.A.

INTRODUCTION The substance we know as water (H2O) is actually comprised of a number of isotopes of O and H. Isotopes are atoms that have the same number of protons and electrons, and therefore the same basic chemical properties, but differ in their mass. Their differences in mass arise due to different numbers of neutrons within their nucleus. Isotopes can be stable or radioactive, and can be both naturally occurring and manmade. Six different isotopes of hydrogen and oxygen are found in nature (Table 1).

BACKGROUND

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The heavy isotopes of oxygen (17O and 18O) were discovered by Giaugue and Johnston.[2] Shortly thereafter, an isotope of hydrogen (2H) was discovered by Urey, Brickwedde, and Murphy[3] for which Urey received the Nobel prize. The isotope was called deuterium and is often written as D. A third isotope of hydrogen (3H, tritium), postulated to exist in 1931, was detected in natural waters in 1950.[4,5] All of the above isotopes are stable, except for tritium (3H), which is radioactive and decays yielding a b
particle to form 3He. The half-life of tritium is 12.43 yr. Tritium is produced naturally in the upper atmosphere by reaction with cosmic radiation to produce 0.25 atoms cm
2 sec
1.[6] Substantially higher amounts of 3H were released to the atmosphere with the above-ground nuclear weapons testing of 1953–1962 that ended with the nuclear test ban treaty ratified in 1963. The peak concentration of 3H in precipitation reached levels shortly before the treaty as high as 10
12% or 1000  higher than concentrations from natural cosmogenically produced 3H.[7] The above isotopes can form a total of 18 different species of water, with the natural abundance of the 4 major species given in Table 2. While the above isotopes of water are chemically essentially the same, there are nevertheless subtle differences in their physical and chemical properties. For example, the density of water is influenced by its isotopic composition. ‘‘Light’’ or normal water (1H16 2 O) has a lower atomic weight, and therefore also 684

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a lower liquid density than ‘‘heavy water’’ (2H16 2 O or, more simply, D2O) (Table 3). The temperature at maximum density, boiling point, vapor pressure, and freezing point are also dependent upon the isotopic composition and mass of water (Table 3). Viscosity is also higher for the heavier isotopes of water (e.g., the viscosity of D2O at 25 C is 1.095 cP, while that for 1H16 2 O is 0.890 cP). Moreover, intramolecular vibrational frequencies, which are influenced by mass of the atoms forming a chemical bond, are also sensitive to the isotopic composition of water. Thus, ‘‘heavy water’’ (2H16 2 O or, more simply, D2O) exhibits lower frequencies of vibration than ‘‘light water’’ (1H16 2 O). The above noted differences in vapor pressure between different isotopes of water have implications for the partitioning of isotopes between the gas and liquid phases. That is, since ‘‘light’’ molecules of water (1H16 2 O) are more volatile than those containing a heavy isotope, molecules evaporating from water are enriched in ‘‘light’’ isotopes and depleted in the heavier isotopes. The relative enrichment or depletion in heavy isotopes is commonly expressed as a d-value (in F or parts per thousand/per mil), defined as:  dðFÞ ¼

Rx
1 Rref

  1000

where R denotes the abundance ratio of the heavy to light isotope (e.g., 2H/1H), and Rx and Rref are the ratios in the sample and the reference standard. The reference for oxygen and hydrogen isotopes in water is taken to be the so-called VSMOW or Vienna Standard Mean Ocean Water. As a result, water evaporated from the ocean is depleted by 12F–15F in 18O (denoted d18O) and by 80F–120F in 2H relative to the source ocean water. Subsequent cooling and condensation of water vapor evaporated from the ocean preferentially removes the less volatile, heavy isotopes of water. Thus, clouds and precipitation are enriched in the heavy isotopes of water, while the remaining vapor is depleted in 2H and 18O. Moreover, the temperature of formation of droplets and precipitation also influences the isotopic composition of rain.[8] Thus, winter precipitation is depleted in heavy isotopes relative to

Isotopes

685

Table 1 Isotopes of hydrogen and oxygen Isotope 1

H

Table 2 Natural abundance of isotopes of water

Weight (amu)

Natural abundance (wt %)

1.0078

99.970

1 1

2

2.0141

0.030

3

3.0160

10
15

16

15.9949

99.732

17

16.9991

0.039

18

17.9992

0.229

H H O O O

Species

Natural abundance (%)

H16 2 O

99.728

H18 2 O 1 17 H2 O 1 2 16

0.200 0.040

HH O

0.032

Source: From Ref.[1].

Source: From Ref.[1].

summer, high latitude precipitation is depleted relative to that formed at lower latitudes, and high altitude precipitation is depleted in heavy water relative to that formed at lower altitudes. These differences have been exploited in a wide range of hydrological, geological, and environmental studies.

ENVIRONMENTAL APPLICATIONS OF ISOTOPE ANALYSES Hydrological Studies The history, age, and pathway of water within the hydrologic cycle can be inferred from the relative abundance of 2H, 3H, and 18O in water. The isotopic composition of groundwater can be used to determine recharge to an aquifer, including the source area and rate of recharge. The source area can be identified by the 2H and 18O concentrations in the groundwater and correlating them with the altitude at which precipitation infiltrated the soil. For example, Friedman and Smith[9] reported an 40F decrease in 2H for every 1000 m increase in altitude on the west slope of the Sierra Nevada range in California. This trend, along with the so-called ‘‘Continental Effect’’ is shown for the western United States in Fig. 1.[10] The mechanism for the ‘‘Continental Effect’’ is the same as that for altitudinal effects, that is, the raining out of heavier isotopes, leaving vapor and subsequent precipitation isotopically lighter as one moves inland. In a somewhat different way, the rate of recharge can be estimated by quantifying the tritium concentrations in the subsurface. Specifically, a peak 3H

concentration associated with nuclear weapons testing would correspond to 1962, and thus its location beneath the land surface would yield a travel distance over approximately 40 yr. For example, Cook et al.[11] have used observed 3H transport to estimate effective unsaturated hydraulic conductivities and recharge rates within arid zone soils in Australia. The rate of movement of nutrients, pesticides, and other chemicals within the subsurface has also been quantified in laboratory and field experiments through co-application of 3H2O. The breakthrough or transport of the chemical relative to that of tritiated water provides a direct measure of chemical transport and the extent of reaction and retardation within the soil. For example, Gupta, Destouni, and Jensen[12] recently compared phosphorus and tritium transport in structured soil and found that 60%–100% of the water flow was associated with 25%–40% available flow paths. Moreover, preferential flow increased phosphorus mass transport by 2–3  than without preferential flow. It should be noted, however, that such studies require careful consideration of safety and other issues. Paleoclimate and Climate Change Studies The relative differences in vapor pressure of heavy and light water and the temperature induced differences in isotopic composition of water referred to above make it possible to infer past climatic conditions. For example, Johnsen et al.[13] used 18O composition of ice cores from Antarctica and Greenland to estimate climatic conditions over the past 100,000 yr. Their study clearly shows colder temperatures from about 70,000 yr to 12,000 yr before present (B.P.), corresponding with the Wisconsin glaciation (Fig. 2).

Species

Densitymax (kg m
3)

Temperature at densitymax ( C)

Boiling point ( C)

Freezing point ( C)

H16 2 O 2 16 H2 O 1 18 H2 O 3 16 H2 O

999.97

3.984

100.0

0.0

1106.00

11.185

101.4

3.8

1112.49

4.211

NA

NA

1215.01

13.403

NA

NA

1

Source: From Ref.[1].

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Table 3 Some properties of different isotopes of water

686

Isotopes

Fig. 1 Isotopic composition of 2H in precipitation in the western United States.[10] (From Elsevier Science.)

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The paleotemperatures of the ancient oceans have been estimated from the oxygen isotope distribution between CaCO3 and water.[14] That is, CaCO3 is enriched in 18O relative to ocean water. Moreover, the relative enrichment, as with other isotopic fractionation reactions, is dependent upon the temperature at which it formed. Thus, the 18O content of CaCO3 provides a record of the ocean temperature at the time it was laid down. Such analyses have demonstrated significant variations in the ocean temperature over the past 700,000 yr.[15] Analyses by Woodruff, Savin, and Douglas[16] show greater variations over the past 20 million yr. Paleoclimatic information on the continents is available from the 18O signature of biogenic apatite[17] and CaCO3 deposits in caves.[18] The d18O values of snail shells have also been used.[19] The isotopic composition of rocks and minerals also provides important information about the conditions at the time of their formation or alteration. For example, the clay minerals in soils over large regions of the United States formed during the Tertiary period under warmer conditions than those of the present.[20] Furthermore, limestone deposited in freshwater environments is depleted in 18O relative to those formed

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Fig. 2 Variation in O composition in ice cores Byrd Station, Antarctica and Camp Century, Greenland.[13] [From Nature (Macmillan Journals, Ltd.).]

in marine settings, reflecting differences in the status of the two types of waters.

18

O

REFERENCES 1. Franks, F. Water; The Royal Society of Chemistry: London, 1984. 2. Giauque, W.F.; Johnston, H.L. An isotope of oxygen, mass 18. J. Am. Chem. Soc. 1929, 51, 1436–1441. 3. Urey, H.C.; Brickwedde, F.G.; Murphy, G.M. A hydrogen isotope of mass 2 and its concentration. Phys. Rev. 1932, 39, 1–15. 4. Faltings, V.; Harteck, P. Der tritium gehalt der atmosphere. Z. Naturforsch 1950, 5a, 438–439.

Isotopes

13. Johnsen, S.J.; Dansgaard, W.; Clausen, H.G.; Langway, C.C. Oxygen isotope profiles through the Antarctic and Greenland ice sheets. Nature 1972, 235, 429–434. 14. Urey, H.C. The thermodynamic properties of isotopic substances. J. Chem. Soc. 1947, 1947, 562–581. 15. Emiliani, C.; Shackleton, N.J. The brunhes epoch: isotopic paleotemperatures and geochronology. Science 1974, 183, 511–514. 16. Woodruff, R.; Savin, S.M.; Douglas, R.E. Miocene stable isotope record: a detailed deep pacific ocean study and its paleoclimatic implications. Science 1981, 212, 665–668. 17. Kolodny, Y.; Luz, B.; Navon, O. oxygen isotope variations in phosphate of biogenic apatites. I. Fish bone apatite—rechecking the rules of the game. Earth Planet. Sci. Lett. 1983, 64, 398–404. 18. Schwarz, H.P.; Harmon, R.S.; Thompson, P.; Ford, D.C. Stable isotope studies of fluid inclusions in speleothems and their paleoclimatic significance. Geochim. Cosmochim. Acta 1976, 40, 657–665. 19. Magaritz, M.; Heller, J. A desert migration indicator— oxygen isotopic composition of land snail shells. Paleo. Paleo. Paleo. 1980, 32, 153–162. 20. Faure, G. Principles of Isotope Geology, 2nd Ed.; John Wiley and Sons: New York, 1986.

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5. Grosse, A.V.; Johnston, W.M.; Wolfgang, R.L.; Libby, W.F. Tritium in nature. Science 1951, 113, 1–2. 6. Lal, D.; Seuss, H.E. The radioactivity of the atmosphere and hydrosphere. Ann. Rev. Nucl. Sci. 1968, 18, 407–435. 7. IAEA. World Survey of Isotope Concentrations in Precipitation. Tech. Report Series No. 96, 1969. 8. Dansgaard, W. Stable isotopes in precipitation. Tellus 1964, 16, 436–463. 9. Friedman, I.; Smith, G.I. Deuterium content of snow cores from Sierra Nevada area. Science 1970, 169, 467–470. 10. Ingraham, N.L. Isotopic variations in precipitation. In Isotope Tracers in Catchment Hydrology; Kendall, C., McDonnell, J.J., Eds.; Elsevier Science: Amsterdam, NL, 1998; 87–118. 11. Cook, P.G.; Jolly, I.D.; Leaney, F.W.; Walker, G.R.; Allan, G.L.; Fifield, L.K.; Allison, G.B. Unsaturated zone tritium and chlorine-36 profiles from southern Australia—their use as tracers of soil water movement. Water Resour. Res. 1994, 30, 1709–1719. 12. Gupta, A.; Destouni, G.; Jensen, M.B. Modelling tritium and phosphorus transport by preferential flow in structured soil. J. Contam. Hydrol. 1999, 35, 389–407.

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Journals Joseph R. Makuch Water Quality Information Center, National Agricultural Library, Beltsville, Maryland, U.S.A.

Stuart R. Gagnon University of Maryland, College Park, Maryland, U.S.A.

INTRODUCTION To understand a field of study, you must read its literature, including the bulwark of any discipline, its scholarly journals. Capturing the breadth of hydrologic sciences literature that addresses agricultural issues requires exploring a variety of journals. While there may be overlap in topical coverage, each journal fills a particular subject matter and readership niche. This article focuses on key journals in the hydrologic sciences, especially those covering agricultural issues.

OVERVIEW OF JOURNALS

Irrigation– Journals

Scholarly journals are a means of sharing information in a given field. Journals serve to expand the knowledge base of a subject. Journal articles in hydrologic science—a multidisciplinary field that focuses upon the occurrence, movement, and properties of water— describe experimental studies and their implications, discuss theoretical approaches or analyze the ramifications of public policy. A special type of journal article—the literature review—synthesizes the findings of seminal publications on a given topic. A literature review, like other journal articles, includes a list of publications referenced in the article. Journals may also publish commentaries or letters to the editor on issues relevant to the field or on previously published articles. Journal articles are written by researchers from academia, government, and the private sector. Published articles are often viewed as a measure of professional productivity. Institutions gain prestige when their employees’ articles are published in respected scholarly journals. People who need to stay abreast of developments in their fields read or review scholarly journals and articles of interest. Researchers learn about advancements in experimental techniques or theories. Reported findings and methodologies influence how researchers approach their own scientific inquiries. Educators 688

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acquire information to keep their courses current. Decision makers obtain unbiased scientific bases for recommending or implementing actions. Undergraduate and graduate students gain knowledge in their fields by reading and studying scholarly articles. Most journals in the hydrologic sciences are published by either commercial scientific publishers or professional societies. Publication schedules vary, as do subscription prices. Online versions of journals are increasingly available. Publishers often allow nonsubscribers online access to tables of contents, abstracts, or sample issues. Libraries at institutions with departments involved with hydrologic sciences have print and/or online subscriptions to relevant journals.

SPECIFIC HYDROLOGIC SCIENCE JOURNALS Table 1 lists hydrologic science journals that are important for the agricultural sector. These journals cover hydrologic science issues beyond agriculture, but they publish many agriculturally related articles. There are journals for hydrologic sciences in addition to those listed in Table 1, but they have less relevance to agriculture. Relevant articles concerning water resources and agriculture can also be found in broader environmental publications and in the agricultural literature. See Tables 2 and 3 for a sampling of germane journals in these fields. In addition, journals in fields such as meteorology, climatology, limnology, aquatic biology, and forestry are sources of related information. Tables 1–3 also provide the World-Wide-Web address of each journal’s publisher. Visit the Web sites of the respective journals to obtain detailed information about each journal, including frequency of publication, subscription cost, and scope of coverage. Most journals also provide this information in each issue. Indexes to journal articles are usually published in the last issue of a volume. Articles can also be located by using abstracting and indexing services such as the Water Resources Abstracts and AGRICOLA

Journals

689

Table 1 Journals for hydrologic sciences (important for agriculture) with web addresses of publishers Publisher Web address

Advances in Water Resources

www.elsevier.com/

Agricultural Water Management

www.elsevier.com/

Groundwater

www.ngwa.org/

Groundwater Monitoring and Remediation

www.ngwa.org/

Hydrological Processes

www.interscience.wiley.com/

Journal of the American Water Resources Association

www.awra.org/

Journal of Contaminant Hydrology

www.elsevier.com/

Journal of Geophysical Research-Atmosphere

www.agu.org/

Journal of Great Lakes Research

www.iaglr.org/

Journal of Hydraulic Research

www.iahr.org/

Journal of Hydrologic Engineering

www.pubs.asce.org/

Journal of Hydrology

www.elsevier.com/

Journal of Soil and Water Conservation

www.swcs.org/

Journal of Water Resources Planning and Management

www.pubs.asce.org/

Water Environment Research

www.wef.org/

Water Research

www.elsevier.com/

Water Resources Research

www.agu.org/

Water Science and Technology

www.iwap.co.uk/

Wetlands

www.sws.org/

databases. There is a trend for online indexes to hyperlink directly to journal articles when those articles are available electronically. Reference directories, such as Ulrich’s International Periodicals Directory and the Serials Directory: An International Reference Book, available in paper or online at many libraries, are other good sources of information on particular journals. These directories are also useful for finding information about journal name or publisher changes. Additional information about several journals representative of the literature is provided below. These journals were recommended by knowledgeable professionals working in various areas of water resources and agriculture. The brief descriptions are based on reviews of recent issues and information published in the journals and their respective Web sites. See these sources for additional information. Examples of Journals for Hydrologic Sciences Covering Agricultural Issues Agricultural Water Management: an international journal. 1977–present. 15/yr. Elsevier Science BV, P.O. Box 211, 1000 AE Amsterdam, Netherlands. http://www.elsevier.com/. ISSN: 0378-3774. Readers of this international journal include agricultural engineers, agricultural hydrologists, and agronomists. Research articles on various aspects of

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irrigation are a major theme of the journal. Other areas covered include drainage, erosion, and water quality. Theme issues, such as ‘‘The Use of Water in Sustainable Agriculture,’’ are occasionally published. Journal of the American Water Resources Association. 1965–present. Bi-monthly. American Water Resources Association, P. O. Box 1626, Middleburg, VA 201181626. http://www.awra.org/. ISSN: 0043-1370. Formerly known as the Water Resources Bulletin, the main focus of this journal is on water resources management issues of broad interest. The journal organizes articles under one of three categories: ‘‘Dialogue on Water Issues,’’ ‘‘Technical Papers,’’ and ‘‘Discussion Papers’’ (and replies). Articles in the ‘‘Dialogue’’ section cover policy and critical management issues. The ‘‘Technical’’ section contains articles on subjects such as phosphorus and wetlands, stream channel instability, and artificial neural networks for subirrigation systems. Comments on earlier articles are published in the ‘‘Discussion’’ section, along with replies from the original author(s). Special issues, such as ‘‘Water Resources and Climate Change,’’ are sometimes published. Impact is a sister publication of the journal that focuses on a single theme for each issue. Articles address timely issues for water resource professionals. Journal of Hydrology. 1963–present. 56/yr. Elsevier Science BV, P.O. Box 211, 1000 AE Amsterdam, Netherlands. http://www.elsevier.com/. ISSN: 0022-1694.

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Journal title

690

Journals

Table 2 Environmental journals covering water-related topics (important to agriculture) with web addresses of publishers Journal title

Publisher Web address

Agriculture, Ecosystems and Environment

www.elsevier.com/

AMBIO

ambio.allenpress.com/

Aquatic Toxicology

www.elsevier.com/

Archives of Environmental Contamination and Toxicology

www.springer-ny.com/

Bulletin of Environmental Contamination and Toxicology

www.springer-ny.com/

Chemosphere

www.elsevier.com/

Critical Reviews in Environmental Science and Technology

www.crcpress.com/

Ecotoxicology

www.wkap.nl/

Ecotoxicology and Environmental Safety

www.academicpress.com/

Environmental Management

www.springer-ny.com/

Environmental Monitoring and Assessment

www.wkap.nl/

Environmental Pollution

www.elsevier.com/

Environmental Science and Technology

www.pubs.acs.org/

Environmental Toxicology

www.interscience.wiley.com/

Journal of Environmental Economics and Management

www.academicpress.com/

Journal of Environmental Engineering

www.pubs.asce.org/

Journal of Environmental Management

www.academicpress.com/

Journal of Environmental Quality

www.agronomy.org/

Journal of Environmental Science and Health, Part A-Toxic/Hazardous Substances and Environmental Engineering

www.dekker.com/

Journal of Environmental Science and Health, Part B-Pesticides, Food Contaminants, and Agricultural Wastes

www.dekker.com/

Journal of Range Management

www.srm.org/

Journal of Toxicology and Environmental Health—Part A

www.tandf.co.uk/

Journal of Toxicology and Environmental Health—Part B: Critical Reviews

www.tandf.co.uk/

Nature

www.nature.com/

Science of the Total Environment

www.elsevier.com/

Water, Air and Soil Pollution

www.wkap.nl/

Irrigation– Journals

This journal, with U.S. and international editors, publishes highly technical research papers, and comprehensive reviews in the hydrologic sciences. Topics include the ‘‘physical, chemical, biogeochemical, stochastic and system aspects of surface and groundwater hydrology, hydrometeorology, and hydrogeology.’’ Articles may be of an empirical, theoretical, or applied focus. Theme issues are sometimes published. Journal of Soil and Water Conservation. 1946– present. Quarterly. Soil and Water Conservation Society, 7515 N. E. Ankeny Rd., Ankeny, IA 50021. http://www.swcs.org/. ISSN: 0022-4561. Published by a professional society whose members include soil and water conservation professionals, researchers, planners, educators, administrators, and others, this journal focuses on the conservation, improvement and sustainable use of soil, water and related natural resources worldwide. Journal issues contain primarily ‘‘Features’’ and ‘‘Research’’ articles. The former are overview and

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synthesis articles, including literature reviews, while the latter report on specific research studies. Articles in both sections address a broad range of soil and water conservation topics and include articles focussing on social sciences and economics. The general emphasis of the journal is on agricultural and other rural lands. The journal also publishes commentaries from readers, a listing of future conferences and some advertisements, including employment opportunities. A less technical, magazine-style sister publication, Conservation Voices, also covers soil and water conservation topics. In January 2002, Conservation Voices and the Journal of Soil and Water Conservation will be merged into a single journal published six times a year. Water Resources Research. 1965–present. Monthly. American Geophysical Union, 2000 Florida Ave. N. W., Washington, DC 20009. http://www.agu.org/. ISSN: 0043-1397. This is an interdisciplinary journal that covers research in the social and natural sciences related to

Journals

691

Table 3 Agricultural journals that often cover water-related topics with web addresses of publishers Journal title

Publisher Web address

Advances in Agronomy

www.academicpress.com/

Agronomy Journal

www.agronomy.org/

American Journal of Agricultural Economics

www.aaea.org/

American Journal of Alternative Agriculture

www.winrock.org/

Applied Engineering in Agriculture

www.asae.org/

Irrigation and Drainage

www.interscience.wiley.com/

Irrigation Science

www.springer-ny.com/

The Journal of Agricultural Science

www.journals.cambridge.org/

Journal of Irrigation and Drainage Engineering

www.pubs.asce.org/

Nutrient Cycling in Agroecosystems

www.wkap.nl/

Poultry Science

www.poultryscience.org/

Soil Science Society of America Journal

www.soils.org/ www.elsevier.com/

Soil and Tillage Research Swedish Journal of Agricultural Research

a

Transactions of the ASAE

www.asae.org/

Note: Ceased publication in 1998.

water. The journal publishes articles in scientific hydrology covering the biological, chemical, and physical sciences as well as economics, sociology, and law. There are deputy editors for erosion, sedimentation, and geomorphology; geochemistry and geobiology; groundwater; surface water; vadose zone; and water policy, economics and systems analysis. Many articles are theoretical. Examples of Environmental Journals that Cover Water and Agriculture Agriculture, Ecosystems and Environment. 1974– present. 15/yr. Elsevier Science BV, P.O. Box 211, 1000 AE Amsterdam, Netherlands. http://www. elsevier.com/. ISSN: 0167-8809. Covering the interface between agriculture and the environment, Agriculture, Ecosystems and Environment promotes interdisciplinary approaches to research. The journal is aimed at scientists studying many aspects of agricultural ecosystems. Papers in this journal have covered topics such as water availability and use, non-point-source pollution, and seasonal flooding. Special issues are occasionally published, including an issue on sustainable land management. Agriculture, Ecosystems and Environment is an amalgamation of two earlier journals, Agro-Ecosystems and Agriculture and Environment. A section of Agriculture, Ecosystems and Environment is currently published as Applied Soil Ecology. Both journals are included in the same subscription.

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Journal of Environmental Quality. 1972–present. Bi-monthly. American Society of Agronomy, 677 S. Segoe Rd., Madison, WI 53711. http://www.agronomy. org/. ISSN: 0047-2425. The professional societies, American Society of Agronomy, Crop Science Society of America and Soil Science Society of America, cooperatively publish the Journal of Environmental Quality. The journal contains research and review articles on topics covering agricultural and natural ecosystems. A section of the journal, headed ‘‘Environmental Issues,’’ contains articles from ‘‘a combination of scientific, political, legislative, and regulatory perspectives.’’ Water, Air, and Soil Pollution: An International Journal of Environmental Pollution. 1971–present. 32/yr. Kluwer Academic Publishers, P.O. Box 17, Dordrecht, 3300 AA, Netherlands. http://www.wkap.nl/. ISSN: 0049-6979. All aspects of the biological, chemical, and physical processes of environmental pollution are covered by this interdisciplinary journal. Water, Air, and Soil Pollution includes articles on wastewater irrigation, forest management, and nitrate depletion. Other papers published in the journal include those describing methods used to study and measure environmental pollutants. Examples of Agricultural Journals that Cover Hydrologic Sciences Journal of Irrigation and Drainage Engineering. 1956–present. Bi-monthly. American Society of Civil

Irrigation– Journals

a

N/A

692

Engineers, 1801 Alexander Graham Bell Dr., Reston, VA 20191. http://www.pubs.asce.org/. ISSN: 0733-9437. This journal covers research on ‘‘engineering hydrology, irrigation, drainage, and related water management subjects, such as watershed management, weather modification, water quality, groundwater, and surface water.’’ Articles appear in three categories: ‘‘Technical Papers’’ discuss experimental results and conclusions or analytical approaches to water management problems. Shorter articles, or reports of preliminary research results, are published in ‘‘Technical Notes.’’ The ‘‘Discussion’’ section contains short pieces that offer substantive comments on previously published papers and includes a closure response from the original author(s). The journal was formerly known as the Journal of the Irrigation and Drainage Division, Proceedings of the American Society of Civil Engineers. Soil Science Society of America Journal. 1936– present. Bi-monthly. Soil Science Society of America, 677 S. Segoe Rd., Madison, WI 53711. http:// www.soils.org/. ISSN: 0361-5995. The table of contents for this journal, formerly Soil Science Society of America Proceedings, divides the articles into several different subject categories. While the section on ‘‘Soil and Water Management and Conservation’’ contains articles most relevant to hydrologic science, other sections may also contain water-related articles. Articles primarily describe and discuss different aspects of soil science research. Comments on specific articles or other soil science topics are also published, as are the occasional invited essay or review. A list of new soil science books is sometimes published.

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Journals

Transactions of the ASAE. 1958. Bi-monthly. American Society of Agricultural Engineers, 2950 Niles Rd., St. Joseph, MI 49085-9659. http://www. asae.org/. ISSN: 0001-2351. Each issue in this journal, from ASAE—the professional society for engineering in agricultural, food, and biological systems—contains a soil and water section. Articles cover a range of water-related topics such as water flow in riparian areas, irrigation efficiency, and nitrate leaching. Topics are approached from various perspectives including computer modeling, engineering design, and scientific investigation. Resource is a magazine by ASAE that occasionally publishes articles or short news pieces covering water and agriculture issues.

BIBLIOGRAPHY 1. Delphino, J.J. Water chemistry. Seeking information. Environ. Sci. Technol. 1977, 11 (7), 669–672. 2. Haas, J.; Clark, M. Research journals and databases covering the field of agrochemicals and water pollution. Sci. Technol. Libr. 1992, 13 (2), 57–63. 3. The Serials Directory. An International Reference Book, 13th Ed.; EBSCO Publishers: Birmingham, AL, 1999. 4. Ulrich’s International Periodicals Directory, 2000, 38th Ed.; R.R. Bowker: New Providence, NJ, 1999. 5. Walker, R.D.; Ahn, M.L. Literature cited by water resources researchers. In Changing Gateways. The Impact of Technology on Geoscience Information Exchange, Proceedings of the 29th Meeting of the Geoscience Information Society, Seattle, WA, Oct, 24– 27, 1994; Haner, B.E., O’Donnell, J., Eds.; Geoscience Information Society: Alexandria, VA, 1995; 67–77.

Karst–Neuse

Karst Aquifers John Van Brahana Division of Geology, Department of Geosciences, University of Arkansas, Fayetteville, Arkansas, U.S.A.

INTRODUCTION Karst aquifers are water-bearing, soluble rock layers at or near the earth’s surface in which groundwater flow is concentrated along secondarily enlarged fractures, fissures, conduits, and other interconnected openings. They are formed by the chemical dissolving action of slightly acidic water on highly soluble rocks, most notably limestone and dolomite, and to a lesser degree, gypsum, anhydrite, and halite. For the processes of karst to be active, water must dynamically circulate through these soluble rocks—exposing the rock to interaction with water and enabling transport of solutes, and the water must be undersaturated with respect to the chemical constituents of the rock— enabling dissolution to occur. This interplay of flow (hydrology) and dissolution (geochemistry) removes rock, creating increasingly larger voids along the pathways the water follows through time. Karst aquifer development commonly results in distinctive landforms, but visible surface features are not an essential attribute of karst, because in many instances, the surface features may be covered by soil or regolith. Although karst and karst aquifers most commonly are recognized as having distinctive landforms and topography (e.g., closed depressions, sinkholes, sinking streams, dry valleys, caves, dissolutionally enlarged joints or bedding planes, grikes, karren, and springs), these are indicators of karst rather than definitive elements. Whereas most karst areas express part or all of these features, the key essential element of the karst definition must include ‘‘distinctive subsurface hydrology,’’[1] characterized by secondarily enlarged flow pathways. Other distinctive hydrology components include a high degree of interconnectivity between surface and groundwater, relatively rapid groundwater flow, great areal and temporal variability of aquifer properties, numerous springs, great susceptibility for contamination from human and natural activities at the land surface, and lack of filtration and attenuation of contamination. Water-bearing rocks with these attributes are likely karst aquifers if they are highly soluble, even if no karst landforms are present.[1] Karst aquifers are widespread and intensively utilized. Their worldwide occurrence ranges from 12%

to 25% of the earth’s surface.[1–4] About 25% of the world’s population is estimated to rely on freshwater supplies from these aquifers.[3] Fig. 1 shows the dominant regional karst aquifers in the United States in terms of water use, well yields, and spring discharge.[4] Globally, karst aquifers comprise important water resources in, e.g., southern China, southeast Asia, western Europe and the Mediterranean basin, and the Caribbean islands.

MAJOR SOURCES OF KARST INFORMATION Karst-forming processes can be some of the most dynamic, erosive forces that counterbalance the uplifting forces of tectonics.[5] Karst can be responsible for the most active surface water/groundwater interactions of all aquifer types.[6] Coupled with more than 60 controlling influences (e.g., lithologic, structural, hydrologic, geochemical, and geomorphic), these processes result in the most variable hydrogeology within a single aquifer type of all the earth’s rocks.[7] These facts notwithstanding, the hydrology of these aquifers is not unpredictable. Refs.[2,3,8–20] synthesize the current state of understanding of the hydrology of karst aquifers as well as numerical simulation as a hydrogeologic tool. Websites include the National Speleological Society,[21] the Karst Waters Institute,[22] and the Karst Commission of the International Association of Hydrogeologists.[23,24]

UNIQUE ATTRIBUTES OF KARST AQUIFERS Notable differences between karst and porous media aquifers are: groundwater flow is commonly turbulent in karst, and Darcy’s law is not appropriate for quantification in a karst aquifer, except at large scales exceeding several kilometers; groundwater in karst aquifers contains multiple components of flow types, a mix of fastflow and slowflow conditions from the epikarst, the vadose zone, and the phreatic zone; fastflow in karst aquifers dominates advective transport; fastflow paths in karst aquifers are difficult to predict without tracing studies; the interaction between surface water and groundwater in karst lands is more pronounced 693

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solute retardation; karst aquifers are generally more prone to contamination; karst aquifers tend to concentrate and accumulate flow, whereas porous-media aquifers tend to retain diffusive conditions; springs integrate flow from all parts of a karst aquifer; springs and cave streams are excellent locations to sample characteristic water quality from a karst aquifer; and wells are not a representative way to sample karst aquifers, unless it has been documented that the wells intersect the zones of fastflow.

Fig. 1 Distribution of karst aquifers in the U.S. Source: From Ref.[4].

than in non-karst terrane; allogenic recharge to karst aquifers by subsurface capture of streamflow derived outside of the groundwater basin is common in karst; spring recharge boundaries in karst commonly do not coincide with surface water boundaries; karst aquifers generally exhibit a much wider range of variability of hydraulic characteristics than porous media aquifers (Table 1); hydraulic characteristics can vary areally, in orientation, and temporally, increasing as dissolution progresses, and decreasing as deposition of insoluble sediments occur or chemical precipitation occludes zones of enhanced permeability; karst aquifers commonly have greater hydraulic conductivity but lower storativity than comparably sized non-karst aquifers; water quality in karst aquifers varies temporally, a result of mixing from the different flow components; karst aquifers typically have more permeable, open flow systems, and less chance for filtration and

FACTORS CONTROLLING THE FLOW OF GROUNDWATER IN KARST AQUIFERS The laws of physics that govern the flow of groundwater in other aquifers apply equally to karst (water flows from areas of high energy to areas of low energy). Groundwater obeys the laws of thermodynamics, always following the path of least resistance. While all factors responsible for the creation of a karst aquifer are important (Table 2), there is a general hierarchy of relative importance of geologic factors as they affect karst aquifer formation. From most important to less important, one version of this ranking follows.[7] Lithology of the hydrogeologic framework of aquifers and confining beds, including bulk chemical purity, grain size, original porosity and permeability inherited from early diagenesis, layer thickness, sequence thickness, caprock integrity and strength, and vertical variability in permeability, plays an essential role in karst aquifer formation. Simply stated, if no soluble rocks exist, there is no karst. As well as defining the aquifer potential of the rock matrix to be dissolved,

Table 1 Comparison of typical flow and hydraulic characteristics of karst aquifers Rock type

Primary porosity and permeability

Secondary porosity and permeability

Flow regime

Discharge from large springs

Low porosity anisotropic; Cavernous Low porosity low huge permeability in limestone average but highly variable permeability conduits, very low permeability in rock mass

Laminar to turbulent, if deeply buried, may be stagnant

Regional aquifers > 103 L sec at major springs

Chalk

High porosity, low permeability, intergranular

Usually not significant, usually undeveloped regionally, jointing can be important

Usually stagnant, very sluggish, commonly only local if at all

Commonly confining layers— >103 L sec 1 or less; 0 L sec 1 is common, may be local aquifer if fractured

Dolomite

Low porosity, low to moderate permeability

Variable from diagenetic pin-point porosity to conduit flow. Joints and faults can be important

Variable usually laminar, can be turbulent to sluggish

Vary from confining layers to regional aquifers: 101 to 102 L sec 1

Marble

Very low, essentially Significant if jointed or none fractured, solution conduits can develop if flow system complete

Source: Adapted from Ref.[7].

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1

Usually laminar where Typically local aquifers yield secondary permeability 102 to 101 L sec 1, 0 is common developed—can be variable, turbulent to sluggish

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Table 2 Geologic factors, processes and controls that affect porosity and permeability of karst aquifers Factor

Processes

Controls

General influence

Diagenetic

Compaction Cementation Pressure solution Solution (includes recrystallization, inversion, micritization)

Original porosity and permeability; original mineralogy; grain size/ surface area; proximity to sea level (uplift or burial); volume and rate of water movement; fluid chemistry: pH, pCO2 salts in solution; temperature, pressure

Influences initial distribution of porosity and permeability of indurated rock mass. Many of these are geochemical in nature; they occur very early in the history of the rock

Geochemical

Solution (dissolution) Dolomitization Dedolomitization Precipitation Sulfate reduction Redox

Groundwater flux; original porosity and permeability; mineralogy; fluid chemistry: pH, pCO2, salts in solution, temperature, pressure, mineral-water saturation

Influences later development of porosity and permeability; influences water chemistry

Layer thickness; sequence thickness; variability in texture (vertical); variability in permeability (vertical); original porosity and permeability inherited from diagensis; bulk chemical purity; grain size

Influences anisotropy of rock mass, thereby resulting in zones potentially more permeable if other geologic factors are favorable

Fracture density; openness of fractures; layer (permeability) orientation

Influences orientation of permeability zones.

Lithologicstratigraphic

Structuraltectonic

Uplift Tilting Folding Jointing Faulting Metamorphism

Influences integrity of confining layers. In extreme instances (metamorphism), influences existence of permeability zones

Hydrologic

Dynamic groundwater

Climatic—temperature; climatic— precipitation; depth of circulation; location of boundaries; existence of complete flow systems; flux; initial anisotropy–vertical variation; springs; surface water/groundwater relation; recharge; hydraulic gradient; size of groundwater basin

Influences existence of flow systems. Influences rate of flow system evolution

Weathering geomorphic

Infilling (fluvial and glacial) Unloading

Topography; relief; soil development by sedimentation; cap rock; degree of karstification; base level; surface slope

Influences development of flow systems. Influences destruction of permeability. Influences shallow porosity– permeability development

Sequence of events; duration of events

Influences stage of development of specific permeability zones

Historical geologic– chronologic Source: From Ref.[7].

lithologic factors influence the anisotropy and heterogeneity of the rock mass, and define zones potentially more permeable and therefore more favorable for flow. For given groundwater conditions, there is a preferential pathway of flow, one that successfully captures increasingly large volumes of flow in the karst system at the expense of other competing pathways. Structural and tectonic factors, including brittle fracture and folding processes, uplift, tilting, and metamorphism, can be very important in karst aquifer development.[2,3,7,9]

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Structural processes not only influence the orientation of preexisting permeability zones through folding, tilting, and uplift, they also create new pathways in the form of secondary rock fractures (such as joints and faults), both in confining beds and aquifers. Under the extreme conditions of metamorphism, structural processes can obliterate previous permeability by recrystallizing carbonate aquifers into marble. The role of structure in creating vertical short circuits along subsurface flowpaths is almost as important as

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lithology, but the dimensions are typically smaller by several orders of magnitude, and structure is ranked lower only for this reason. Other factors (Table 2), including hydrologic, geochemical, diagenetic, weatheringgeomorphic, and historical geologic–chronologic influences,[7] can be important at a local scale, but owing to the extensive variability of karst aquifers, their role and influence is not ubiquitous regionally.

SINKHOLE FLOODING Sinkhole flooding is a common problem around karst aquifers, especially in areas where there is rapid development by humans. Construction in a watershed locally modifies the hydrologic budget, inhibiting infiltration in some areas, diverting infiltration to overland flow which is in turn pirated underground at focused recharge points elsewhere. When storms generate more discharge than the karst aquifer can transmit, water levels in the aquifer rise rapidly. Unlike surface streams that have wide flood plains, karst aquifers have relatively narrow, confined boundaries within the rock. The water level of these aquifers reflects the leastconstrained dimension, and when more flow is introduced than can pass, water levels rise precipitously (>100 ft), and flood low-lying areas of the karst flow system. Some flood-prone sinkholes are miles from the nearest surface stream or flood plain, and property owners may not realize they are at risk until a flood occurs.[20]

CAVITY COLLAPSE, SUBSIDENCE, AND OTHER ENGINEERING AND CONSTRUCTION PROBLEMS Collapse of cave passages, underground voids, and more commonly, collapse of the soils and sediments overlying karst bedrock is a natural process that occurs in response to the continual evolution of the enlarging karst ground water-flow system. This process is exacerbated by human activity, including rapid lowering or raising of the water level, changes in runoff chemistry (pH), increased vibrations, and loading the land above the aquifer with buildings and other massive structures. Highways, bridges, dams, buildings, cars, and homes have been lost as karst rocks equilibrate to the loads they support. Millions of dollars in construction have been spent repairing cracks, filling openings, grouting leaks, and otherwise restoring the structures. The most catastrophic sinkhole event in recorded history occurred in December 1962 in West Driefontein, South Africa. Twenty-nine lives were lost by the sudden disappearance of a building into a huge sinkhole that measured more than 180 ft across.[4] Loss of life

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is not common, but this incident reflects the dynamic, rapid changes that are associated with water resources and hydrogeologic framework in areas underlain by soluble rocks.

CONCLUSION Karst aquifers occur beneath a significant portion of the earth’s surface (from 12% to 25%, depending on one’s definition of karst). Karst aquifers are more highly soluble than other rock types, and most commonly include limestones, dolomites, marbles, and evaporites. In fact, given enough time, karst-like dissolution features have been observed in most lithologies. Karst aquifers are unique compared to porous media aquifers in that they are highly variable in water transmission properties, and if flow is concentrated along focused pathways, and the water in the aquifer is aggressive, these aquifers evolve, flowpaths become enlarged, and rapid movement of groundwater becomes the norm. Karst aquifers are more easily contaminated than porous media aquifers; attenuation of contamination in these aquifers typically is minimal, owing to significantly reduced surface area of solid aquifer/water contact Collapse of the cave passages, underground voids, and more commonly, collapse of the soils and sediments overlying the soluble bedrock is a natural process, and the resulting topography that overlies these aquifers typically is characterized by internally drained depressions called sinkholes (dolines).

REFERENCES 1. Quinlan, J.F.; Smart, P.L.; Schindel, G.M.; Alexander, E.C., Jr.; Edwards, A.J.; Smith, A.R. Recommended Administrative/Regulatory Definition of Karst Aquifer, Principles for Classification of Carbonate Aquifers, Practical Evaluation of Vulnerability of Karst Aquifers, and Determination of Optimum Sampling Frequency at Springs, Proceedings Volume of the Third Conference on Hydrogeology, Ecology, Monitoring and Management of Ground Water in Karst Terranes; Nashville Tennessee Association of Ground Water Scientists and Engineers, 1991, 573–635. 2. Ford, D.C.; Williams, P.C. Karst Geomorphology and Hydrology; Unwin Hyman: London, 1988; 601 pp. 3. White, W.B. Geomorphology and Hydrology of Karst Terrains; Oxford University Press: New York, 1988; 464 pp. 4. Veni, G.; DuChene, H.; Crawford, N.C.; Groves, C.G.; Huppert, G.N.; Kastning, E.H.; Olson, R.; Wheeler, B.J. Living with Karst—A Fragile Foundation; American Geological Institute Environmental Awareness Series 4: Alexandria, VA, 2001; 64 pp.

5. White, W.B.; Culver, D.C.; Herman, J.S.; Kane, T.C.; Mylroie, J.E. Karst lands. Am. Sci. 1995, 83, 450–459. 6. Winter, T.C.; Harvey, J.W.; Franke, O.L.; Alley, W.M. Ground Water and Surface Water—A Single Resource; U.S. Geological Survey Circular 1139: Denver, CO, 1999; 1–79. 7. Brahana, J.V.; Thrailkill, J.; Freeman, T.; Ward, W.C. Carbonate rocks. In Hydrogeology; Back, W., Rosenshein, J.S., Seaber, P.R., Eds.; Geological Society of America: Boulder, CO, 1988; Vol. O-2, 333–352. 8. Dreybrodt, W. Processes in Karst Systems—Physics, Chemistry, and Geology; Springer-Verlag: Berlin, 1988; 288 pp. 9. Palmer, A.N.; Palmer, M.V.; Sasowsky, I.D.; Eds. Speleogenesis—Evolution of Karst Aquifers; National Speleological Society, Inc.: Huntsville, AL, 2000; 527 pp. 10. Kresic, N. Karst Water Resources; Lewis Publishers: Boca Raton, FL, 1998; 384 pp. 11. Worthington, S.R.H. Karst Hydrogeology of the Canadian Rocky Mountains. Unpublished Ph.D. dissertation, McMaster University: Hamilton, Ontario, Canada, 1994; 227 pp. 12. Palmer, A.N. Groundwater processes in karst teranes. In Groundwater Geomorphology; Special Paper 252; Higgins, C.G., Cates, D.R., Eds.; Geological Society of America: Boulder, CO, 1990; 177–209. 13. Ford, D.C.; Palmer, A.N.; White, W.B. Landform development—karst. In Hydrogeology; Back, W., Rosenshein, J.S., Seaber, P.R., Eds.; Geological Society of America: Boulder, CO, 1988; Vol. O-2, 401–412. 14. Palmer, A.N.; Palmer, M.V.; Sasowsky, I.D.; Eds. Groundwater Flow and Contaminant Transport in Carbonate Aquifers; A. A. Balkema: Rotterdam, Netherlands, 2000; 288 pp.

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15. Palmer, A.N.; Palmer, M.V.; Sasowsky, I.D.; Eds. Karst Modeling; Special Publication 5; Karst Waters Institute: Charles Town, 1999; 1–294. 16. Quinlan, J.F.; Davies, G.J.; Jones, S.W.; Huntoon, P.W. The applicability of numerical models to adequately characterize ground-water flow in karstic and other triple-porosity aquifers. In Subsurface Fluid-Flow Modeling; Special Technical Publication 1288; Ritchy, J.D., Rumbaugh, J.O., Eds.; American Society for Testing and Materials: West Conshohocken, PA, 1996; 114–133. 17. Labat, A.; Ababou, R.; Mangin, A. Introduction of wavelet analyses to rainfall/runoffs relationship for a karstic basin: the case of licq-atherey karstic system (France). Ground Water 2001, 39 (4), 605–615. 18. Dreiss, S. Regional scale transport in a karst aquifer. 1. Component separation of spring flow hydrographs. Water Resour. Res. 1989, 25 (1), 117–125. 19. Dreiss, S. Regional scale transport in a karst aquifer. 2. Linear systems and time moment analysis. Water Resour. Res. 1989, 25 (1), 126–134. 20. Pinault, J.-L.; Plagnes, V.; Aquilina, L.; Bakalowicz, M. Inverse modeling of the hydrological and hydrochemical behavior of hydrosystems: characterization of karst system functioning. Water Resour. Res. 2001, 37 (8), 2191–2204. 21. http://www.caves.org (accessed February 2002). 22. http://www.karstwaters.org (accessed February 2002). 23. Drew, D.; Hotzl, H. Karst Hydrogeology and Human Activities: Impacts, Consequences, and Implications; International Association of Hydrogeologists, A. A. Balkema: Rotterdam, Holland, 1999; 322 pp. 24. http://www.iah.org (accessed February 2002).

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Karst Aquifers: Water Quality and Water Resource Problems John Van Brahana Division of Geology, Department of Geosciences, University of Arkansas, Fayetteville, Arkansas, U.S.A.

INTRODUCTION Owing to the rapid flow, the open nature of the flow paths, and the general lack of aquifer surface area for water/rock interaction, karst aquifers are more at risk to contamination than porous-media aquifers. Problems typically include lack of attenuation, little or no response time, and unexpected flow directions and plume transport. In addition to water quality problems, catastrophic subsidence and flooding are common problems where soluble rocks occur near land surface.

FACTORS CONTROLLING GROUNDWATER QUALITY IN KARST AQUIFERS Dissolution is the dominant chemical process in karst aquifers, and water type is controlled by the inorganic reactions of the recharge water and the aquifer matrix.[1–5] In carbonate aquifers, bicarbonate (HCO
3) is the dominant anion. Where limestone is the dominant lithology, calcium (Caþ2) is the prevailing cation. Where dolomite aquifers predominate, magnesium (Mgþ2) and Caþ2 are the dominant cations, typically in a ratio of about 1 : 3, respectively. Carbonate aquifers yield hard water, hardness being a measure of the soap-consuming properties of water, a function of the concentration of Mgþ2 and Caþ2 ions in solution. The chemistry of meteoric recharge water to the aquifer is predominantly a weak (pH 5–7) carbonic acid (H2CO3), formed from H2O mixing with carbon dioxide (CO2) in the atmosphere and from soil gas picked up in solution during recharge through organic-rich soil and regolith. At some locations, however, such as the area of the Pecos River in west Texas and eastern New Mexico (see Fig. 1) in the vicinity of Carlsbad Caverns, meteoric H2O has mixed with hydrogen sulfide gas (H2S) generated by organic matter degradation and oil formation from basins to the east. This reaction formed sulfuric acid (H2SO4), resulting in groundwater of a mixed sulfate/bicarbonate type. Karst aquifers formed in evaporite rocks are much less common than karst aquifers in carbonate rocks, primarily because their solubility is much greater than carbonates, and in humid climates the chemical rates 698

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of erosion are extremely rapid. Evaporite karst is preserved in arid and semiarid climates of the western United States (see Fig. 1) and other areas of the world, but these aquifers typically yield highly-mineralized, non-potable water and are seldom used for water supply. Gypsum and anhydrite yield calcium sulfate type waters (Caþ2, SO
2 4 ), and halite produces brines of sodium chloride (Naþ, Cl
) character.[1] The overall water quality of karst aquifers is a reflection of the geochemistry of all the sources of recharge, weighted by the proportion of the percentage contribution of that source to the total hydrologic budget, coupled with water/rock reactions within soils and rocks upgradient from the aquifer.[1–5] Mixing of karst waters is a major chemistry-controlling process. If land use in the recharge area of the aquifer allows undesirable constituents, water quality in the aquifer will be degraded, as elaborated under the pollution section that follows.[6–11] Sediment is an important part of water quality of karst aquifers. Turbulent flow, coupled with open flow systems allows clay-sized particles of soil to be transported through these groundwater systems.[12] The importance of sediment to water quality is that these particles have huge surface areas, and they typically have electrical charges that attract and hold potentially hazardous substances, such as trace metals and organic chemicals. Sediments also provide a substrate for the preservation and transport of bacteria, virus, and other pathogens. Therefore, contaminants that otherwise would not be present within the aquifer may be mobilized and attached to sediment particles. Contaminants here are sorbed to particles in suspension, and are not in solution; they are in transit through the aquifer just the same, and are available for reaction. If these are ingested in any form by organisms (such as mercury attached to clay may be ingested by bottom-feeding fish) the contaminant may become biologically active (methylated) and bioaccumulated.

WATER-RESOURCE PROBLEMS IN KARST AQUIFERS The most recurring problems humans experience when dealing with karst aquifers are those related to

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for movement into the subsurface is great.[13] These risks, combined with the susceptibility of this type of aquifer to contamination, call for vigilance in conducting most human activities in all karst areas, and longterm planning and zoning for those areas that are particularly susceptible. Websites include the the Karst Waters Institute,[14] and the Karst Commisssion of the International Association of Hydrogeologists.[15]

REFERENCES Fig. 1 Distribution of Karst aquifers in the U.S. Source: Modified from Ref.[6].

pollution,[6–11] flooding,[3,4,6] and subsidence.[3,4,6] This is because human activity creates disequilibrium. Changes within the aquifer occur rapidly in response to changes at the surface, flow velocities can typically transport sediment and other potentially large conduit-plugging debris into the aquifer, and there is less chance for filtration or sorption of unwanted solutes and constituents in karst. Thus, land use in the area contributing recharge to a karst aquifer needs to be closely monitored to minimize negative impacts. Pollution Pollution is common in karst aquifers near areas with high human or animal population densities. Unwanted constituents can move into an aquifer and mix with the native ground water, polluting the aquifer, rendering it unsuitable for further use. Many pollutants move through fast-flow karst aquifers rapidly, and the duration of the impact is short, a matter of days, weeks, or months. Some contaminants, however, such as dense, non-aqueous phase liquids (DNAPLs), sink to the bottom of flow zones in karst aquifers and remain until decomposed by natural processes; the duration of pollution of this sort may be hundreds of years or more. Manufacturing spills, illegal dumping in sinkholes, spills from transporting hazardous wastes, leakage from underground storage tanks, leachate plumes from landfills, excess nutrients from animal manure and inorganic fertilizers, pharmaceuticals and endocrine disruptors from water-treatment facilities, and microbial pathogens from leaky sewers, improperly functioning septic systems, and confined animal feeding operations (CAFOs) are but a few of the well-documented pollution problems found in karst aquifers.[4,6–11] Wherever concentrations of constituents exist at the surface in karst terrane, the chance

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1. Hanshaw, B.B.; Back, W. Major geochemical processes in the evolution of carbonate aquifer systems. J. Hydrol. 1979, 43, 287–312. 2. Thrailkill, J.; Robl, T.R. Carbonate geochemistry of vadose water recharging limestone aquifers. J. Hydrol. 1981, 54, 195–208. 3. Ford, D.C.; Williams, P.C. Karst Geomorphology and Hydrology; Unwin Hyman: London, 1989; 601 pp. 4. White, W.B. Geomorphology and Hydrology of Karst Terrains; Oxford University Press: New York, 1988; 464 pp. 5. Worthington, S.R.H. Karst Hydrogeology of the Canadian Rocky Mountains. Unpublished Ph.D. dissertation, McMaster University, Hamilton, Ontario, Canada, 1994; 227 pp. 6. Veni, G.; DuChene, H.; Crawford, N.C.; Groves, C.G.; Huppert, G.N.; Kastning, E.H.; Olson, R.; Wheeler, B.J. Living with Karst—A Fragile Foundation; American Geological Institute Environmental Awareness Series 4: Alexandria, Virginia, 2001; 64 pp. 7. White, W.B.; Culver, D.C.; Herman, J.S.; Kane, T.C.; Mylroie, J.E. Karst lands. Am. Sci. 1995, 83, 450–459. 8. Winter, T.C.; Harvey, J.W.; Franke, O.L.; Alley, W.M. Ground water and surface water—a single resource. U.S. Geological Survey Circular 1999, 1139, 1–79. 9. Kresic, N. Karst Water Resources; Lewis Publishers, 1998; 384 pp. 10. Sasowsky, I.D.; Wicks, C.M.; Eds. Groundwater Flow and Contaminant Transport in Carbonate Aquifers; A.A. Balkema: Rotterdam, Netherlands, 2000; 288 pp. 11. Drew, D.; Hotzl, H. Karst Hydrogeology and Human Activities: Impacts, Consequences, and Implications; International Association of Hydrogeologists: A.A. Balkema: Rotterdam, Holland, 1999; 322 pp. 12. Mahler, B.J.; Bennett, P.C.; Zimmerman, M. Lanthanidelabeled clay: a new method for tracing sediment transport in karst. Ground Water 1997, 35, 835–843. 13. Katz, B.G.; Coplen, T.B.; Bullen, T.D.; Davis, J.H. Use of chemical and isotopic tracers to characterize the interactions between ground water and surface water in mantled karst. Ground Water 1997, 35, 1014–1028. 14. http://www.karstwaters.org (accessed February 2002). 15. http://www.iah.org (accessed February 2002).

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La Nin˜a David E. Stooksbury Biological and Agricultural Engineering Department, University of Georgia, Athens, Georgia, U.S.A.

INTRODUCTION La Nin˜a is a change in global weather patterns associated with colder than normal sea surface water temperatures in the equatorial Pacific Ocean (see Fig. 1). La Nin˜a is the cool phase of the best known example of interannual variation in the earth’s weather and climate patterns, El Nin˜o-Southern Oscillation (ENSO). Since El Nin˜o, which means boy in Spanish, is the warm phase of ENSO, the cool phase of ENSO is called La Nin˜a, which means girl in Spanish. Another term for La Nin˜a is El Viejo, which means old man in Spanish. The La Nin˜a pattern leads to changes in positions of jet streams, the steering currents for weather systems. Changes in the polar jet streams cause changes in weather patterns across North America during the winter. While the change in weather patterns associated with La Nin˜a can be dramatic, most regions experience minimal to no direct impact from La Nin˜a. La Nin˜a events occur every three to seven years lasting a few months to a year or more.

IMPACTS In the United States, La Nin˜a weather patterns usually mean a dry to very dry winter for the coastal plain of Georgia and the Carolinas as well as Florida (see Fig. 2). While the Southeast is experiencing a dry winter, the lower Mississippi River and the Ohio River Valleys normally experience a wet winter. Much of the remainder of the United States experience very little winter time precipitation impacts associated with La Nin˜a. During La Nin˜a winters, the polar jet stream is positioned such that most storms move from the lower Mississippi River to the Ohio River Valleys exiting the Northeast coast. Compared to a normal winter, the southeastern United States experiences fewer storms during a La Nin˜a winter. During the summer and fall, La Nin˜a events are associated with increased tropical weather activity in the Gulf of Mexico and Atlantic Ocean. This increase in tropical weather makes the east coast more 700

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vulnerable to tropical storms and hurricanes during La Nin˜a years. The increase in tropical weather activity associated with La Nin˜a events, usually causes the Southeast to experience wetter than normal falls during La Nin˜a years. Not all regions impacted by La Nin˜a have decrease in precipitation. The La Nin˜a weather pattern usually brings wetter than normal conditions to northern Australia, Indonesia, and the Philippines. The impacts of La Nin˜a weather patterns vary from one event to another. The impacts depend on the coolness of the surface water, the exact location of the cool surface water, the areal extent of the cool surface water, and other regional and global weather patterns.

Mechanism The oceans and the atmosphere are linked. The discovery of the ocean–atmosphere linkage is one of the most important breakthroughs in modern environmental research. The linkage between the equatorial Pacific Ocean and the atmosphere helps to determine weather patterns across much of the earth. The variation in atmospheric pressure patterns over the Pacific Ocean that are linked to the equatorial Pacific Ocean temperature patterns is called the Southern Oscillation, SO. The strength of SO is calculated by the surface atmospheric pressure anomaly differences between Tahiti and Darwin, Australia (Tahiti anomaly minus Darwin anomaly). This measure of SO strength is called the Southern Oscillation Index, SOI. A surface atmospheric pressure anomaly is calculated by subtracting the mean atmospheric surface pressure from the observed atmospheric surface pressure. Thus, if the observed atmospheric surface pressure is more than the mean, the anomaly has a positive value. When the SOI has a positive value, it means that the surface atmospheric pressure difference between Tahiti and Darwin is greater than normal. A positive SOI is correlated with a cooler than normal surface water in the eastern and central equatorial Pacific Ocean. The linkage between the SO and La Nin˜a is complex. At the most basic level, sea surface temperature patterns influence atmospheric pressure patterns, and

˜a La Nin

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Fig. 1 Atmospheric and oceanic patterns during a La Nin˜a. (From the National Weather Service Climate Prediction Center, Camp Springs, MD.)

atmospheric pressure patterns influence wind speed and direction and thus the sea surface temperature patterns. Under neutral conditions (climatologists prefer the term ‘‘neutral’’ instead of ‘‘normal’’) the normal cold surface water of the eastern Pacific Ocean is associated with relatively high surface atmospheric pressure over the region. Air moves (wind) from areas of high atmospheric pressure to areas of low atmospheric pressure. The greater the pressure gradient (pressure difference between two locations divided by the distance between the two locations), the greater the wind speed. The moving air in contact with the ocean surface causes ocean surface currents, which redistribute the ocean surface temperature pattern. The stronger the wind, the more the redistribution of surface water temperatures. With ENSO (either the cool phase, La Nin˜a or the warm phase El Nin˜o), the linkage between the ocean and the atmosphere results in decreasing or increasing easterly trade-wind (wind from the east to the west) speeds over the equatorial Pacific Ocean. When the SOI is positive (La Nin˜a phase), the pressure gradient across the eastern and western Pacific Ocean is increased. With an increased pressure gradient, the speed of the easterly trade-winds increases, and allows cold upwelled water from the eastern equatorial Pacific Ocean to cool the central equatorial Pacific Ocean thus producing a La Nin˜a event. Since La Nin˜a and neutral phases of ENSO have the same atmospheric pressure and wind patterns, many climatologists consider a La Nin˜a event ‘‘an extreme case of normal.’’ While the atmospheric pressure and wind patterns are the same for La Nin˜a and neutral phases, the pressure gradient and thus the easterly winds are much more pronounced during La Nin˜a events. The stronger easterly winds cause a major

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Fig. 2 Typical jet stream and climate patterns during an El Nin˜o (top) and a La Nin˜a (bottom). (From the National Weather Service Climate Prediction Center, Camp Springs, MD.)

expansion of cold sea surface temperatures across the central equatorial Pacific Ocean and the associated changes in global weather patterns. Since the 1990s scientists have used Pacific Ocean surface temperature data and computer models to predict the occurrence of a La Nin˜a event months in advance. While these predictions are not perfect, they allow for planning to mitigate or take advantage of a shift in weather patterns. Thus, regions that normally experience drought during a La Nin˜a event can plan to mitigate the impacts. For regions like the southeastern United States, drought mitigation plans can be activated months in advance. For more detailed information about La Nin˜a, see Ref.[1]. REFERENCE 1. National Oceanic and Atmospheric Administration Climate Prediction Center. El Nin˜o/La Nin˜a website. http://www.cpc.ncep.noaa.gov/products/analysis_ monitoring/lanina/ (accessed August 2002).

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Land Drainage: Subsurface Stephen R. Workman University of Kentucky, Lexington, Kentucky, U.S.A.

INTRODUCTION

SUBSURFACE LAND DRAINAGE DESIGN

Subsurface land drainage is the removal of excess soil water through the use of ditches or buried perforated pipe. Ideally, the excess water is removed quickly enough to allow timely field operations to occur or to limit crop damage if a crop is present. This chapter presents the design of subsurface drainage systems using knowledge of soil physical properties, crop susceptibility to excess water, and the flow of water in pipe systems. The water table (saturated soil conditions) is present near the soil surface in many places. Soils located near streams, lakes, and oceans are familiar examples. Other cases might include areas where an impermeable or slowly permeable layer exists in the soil profile that causes a perched water table to exist. The primary limitation for effectively using these soils for recreation, construction, or agriculture is the high water table.

A combination of factors influences the depth and spacing of laterals within a drainage system. These factors include hydraulic conductivity, drainable porosity, thickness of soil layers, depth to a restrictive layer, and quality of surface drainage. The single most important soil property affecting the design of drainage systems is hydraulic conductivity.[2] Hydraulic conductivity (K) is spatially variable and should be determined at numerous positions across the site. In the design of large systems where large differences in conductivity may occur, the spacing may be wider or narrower in some regions of the field. From a construction standpoint, however, it is best to maintain uniform drain spacing for as large an area as possible. Whereas the hydraulic conductivity indicates the rate that water can move through the soil, another parameter called the drainage coefficient (DC) has been defined to represent the rate of water to be removed by the drainage system. The DC is the depth of water to be removed in 24 hr by the drainage system and has the same units as hydraulic conductivity (L/t). In humid areas where uniform, low-intensity rainfall events are common, the DC depends largely on a design rainfall event. The DC should be selected to remove excess water rapidly enough to prevent serious damage to the crop. Loss of crop will generally occur if the soil profile is allowed to remain saturated for more than 24 hr.[3,4] Typical values of DC are 10–20 mm/ day.[5] The DC should be increased for high value crops or special soil conditions.[5]

SYSTEM LAYOUT The drains in a subsurface land drainage system can either be placed in a systematic pattern if a large area is to be drained or randomly placed near hard-to-drain areas. A random layout of drains is used to remove excess water from localized depressions in a field. The primary design considerations are to maintain adequate slope to the outlet and to ensure that adequate cover over the drain is maintained. A random layout is typically used as the drainage system under rolling topography, constructed grass waterways, and the greens of golf courses. For most cases where the entire field is poorly drained, a system of parallel drains or laterals is installed throughout the entire field. The laterals are connected to a series of sub-mains and mains that transport the water to the outlet. The spacing between the parallel drains is important to the cost of the system and can result in considerable savings if an optimal design is chosen. Besides random or parallel layouts, an interceptor drain may be used near seepage or overland flow areas. These drains are typically installed at locations of water seeps and excess runoff such as at the bottom of a hill.[1] 702

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Steady-State Design One method of designing a subsurface drainage system is to assume that a uniform rainfall occurs on the soil surface over a long period of time. At steady state, all inflows (rate of groundwater recharge) must equal outflows (discharges through the drainage system). These steady-state conditions can be computed by the Hooghoudt equation where S is drain spacing (L), R is the drainage coefficient or rainfall rate per unit area (L/t), K is the hydraulic conductivity (L/t), d is the depth of the drain to the restrictive layer (L), and b is the vertical distance from the drain to

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the water table position at the midpoint between drains (L).[3]
S ¼

8Kdb þ 4Kb2 R

1=2 ð1Þ

The primary limitation of the earlier equation is that flow is assumed to move horizontally towards a ditch.[4] For the case of flow to drain pipes, convergence losses near the drain must be included. The effect of the convergence is to reduce the amount of flow because there are vertical and horizontal components of flow, conceptually similar to reducing the thickness of the soil profile. An effective depth (de) can be computed by the equation, de ¼

pS     S 8 ln pre þ FðxÞ

FðxÞ ¼

1 X

for

2 ln coth ðnxÞ ¼

n¼1

x ¼

2pd S

ð2Þ

1 X

4e
2ix ; ið1
e
2ix Þ i¼1

ð3Þ

ðn ¼ 1; 2; 3; . . .Þ; ði ¼ 1; 3; 5; . . .Þ

x p2 þ ln 4x 2p

ð4Þ

where re is the effective radius for the drain that accounts for the fact the drain has a limited amount of openings for water entry. In the design process for steady-state analysis, the drainage coefficient is considered to be the design rainfall amount. Transient Methods In areas where frequent, high-intensity storms occur during the growing season, a transient analysis should be conducted to assure that the water table falls quickly enough to maintain a well-aerated root zone. Mathematical analysis of the falling water table case is far more difficult than for the steady-state case. If the assumptions of horizontal flow, vertical equipotentials in the saturated zone, and instantaneous release/ addition of water at the water table can be imposed, then the unsaturated zone can be neglected. In the transient case, the drainage coefficient is related to the water table drop over a period of a day. The drainage rate is then the amount of water released as the water table drops DC ¼ ðb0
bÞ24 hr  f

ð5Þ

where (b0
b)24 hr is the prescribed drop in the water table in 24 hr and f is the drainable porosity (L3/L3).

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2

31=2 9Ktd e  5 S ¼ 4 o ð2de þ bÞ f ln bbð2d e þ bo Þ

ð6Þ

where the effective depth to the impermeable layer (de) has been included to account for convergence losses to the drain, t is the time period for the water table to drop from b0 to b, and all other parameters are as defined earlier. Both the steady state and the transient methods involve the determination of S and de through the process of iteration.

SYSTEM DESIGN Layout

for x < 0.5, F(x) may be closely approximated by, FðxÞ ¼

Note that the drainable porosity is much smaller than the porosity of the soil because only the pores that drain as the water table falls from b0 to b are included. For the falling water table case the drain spacing can be computed as,[2]

The subsurface drainage system is designed to transport excess water to the outlet. The outlet has to have the capacity of carrying the excess water away from the site. The outlet may be a drainage channel, an existing main, or a stream channel. After the outlet location is chosen and checked for capacity, the layout of the system in the field is determined. Some key characteristics of an efficiently designed system include making the laterals as long as possible, placing the laterals parallel to the topographic contour of the field, and minimizing lengths of sub-mains, mains, connections, and fittings.[6] Design Flow The design flow for the system is related to the drainage coefficient used to compute the drain spacing and can be computed from the equation, Q ¼ DC  Ad

ð7Þ

where Q is the flow rate (L3/t), DC is the drainage coefficient (L/t), and Ad is the area-drained (L2). Grades Maximum grades are limiting only where pipes are designed for near-maximum capacity or where pipes are placed in unstable soil. For nearly level areas, the drain should be as steep as possible while maintaining adequate depth at all locations to reduce the size of the

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mains and sub-mains. Drains should not be placed at a grade less than 0.2% under most conditions. If fine sand or silt is present, then larger grades may be necessary to keep the pipe clean. Pipe Size The following equation can be used to size the pipe needed to carry the drainage water. d ¼ 51:7ðDC  A  nÞ3=8 s
3=16

ð8Þ

where A is the drainage area entering the drain (ha), d is the pipe diameter for a pipe flowing full (mm), n is Manning’s roughness coefficient, s is the drain slope (m/m), and DC is the drainage coefficient (mm/day). Typical Manning’s roughness values for corrugated plastic pipe are 0.015 for 75–200 mm diameter pipe, 0.017 for 250–300 mm diameter pipe, and 0.2 for pipes greater than 300 mm.

a T- or a Y-manufactured junction. Manufactured connections should be used when changing pipe size, and end caps or plugs are required for preventing soil from entering the end of the lateral. Finally, the necessary documentation includes a map of the drainage system filed with the deed to the property. The use of GPS and GIS technologies is encouraged for correct identification of the drainage system.[8]

CONDUIT LOADS Loads on underground conduits include those caused by the weight of the soil and by concentrated loads resulting from the passage of equipment or vehicles. At shallow depths, concentrated loads from field machinery largely determine the strength requirements of conduits; at greater depths, the load from the soil is the most significant factor.[6,9]

REFERENCES ACCESSORIES Accessories for subsurface drainage systems include surface inlets, blind inlets, sedimentation basins, and control structures.[2] A surface inlet, sometimes called an open inlet structure, is used to remove surface water from potholes, road ditches, or other depressions. Blind inlets may be used where the amount of surface water to remove is small or the amount of sediment is too great to permit surface inlets. Sedimentation basins are any type of structure that provides for sediment accumulation, reducing deposition in the drain. Finally, control structures are placed in the drainage system to maintain the water table at a specified level.

INSTALLATION CONSIDERATIONS During installation, several factors such as machinery, grade control, corrections, and documentation must be considered. Examples of machinery used in the installation process include a trencher or backhoe to install the mains, a backhoe to clear enough soil away at junctions to make a connection and insert a plow or trencher boot, a drain plow or trencher to install the laterals, and pipe feeders to reduce stretch in pipe. For grade control, the bottom of the trench should be shaped with a supporting groove to provide good alignment and bottom support. Laser or manual methods may be used to establish grade.[6,7] Connections, an essential part of the drain system, should be made with

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1. Carter, C.E. Surface drainage. In Agricultural Drainage; Skaggs, R.W., van Schilfgaarde, J., Eds.; ASA, CSSA, SSSA: Madison, WI, 1999; 1023–1050. 2. Madramootoo, C.A. Planning and design of drainage systems. In Agricultural Drainage; Agron. Monogr. 38; Skaggs, R.W., van Schilfgaarde, J., Eds.; ASA, CSSA, SSSA: Madison, WI, 1999; 871–892. 3. Ritzema, H.P. Subsurface flow to drains. In Drainage Principles and Applications; Ritzema, H.P., Ed.; ILRI: Wageningen, The Netherlands, 1994; 263–304. 4. van der Ploeg, R.R.; Horton, R.; Kirkman, D. Steady flow to drains and wells. In Agricultural Drainage, Agron. Monogr. 38; Skaggs, R.W., van Schilfgaarde, J., Eds.; ASA, CSSA, SSSA: Madison, WI, 1999; 213–264. 5. ASAE Standards; EP480 Design of Subsurface Drains in Humid Areas, 47th Ed.; ASAE: St. Joseph, Mich., 2000. 6. Cavelaars, J.C.; Vlotman, W.F.; Spoor, G. Subsurface drainage systems. In Drainage Principles and Applications; Ritzema, H.P., Ed.; ILRI: Wageningen, The Netherlands, 1994; 827–930. 7. ASAE Standards, EP481 Construction of Subsurface Drains in Humid Areas, 47th Ed.; ASAE: St. Joseph, Mich., 2000. 8. Chieng, S. Use of geographic information systems and computer-aided design and drafting techniques for drainage planning and system design. In Agricultural Drainage, Agron. Monogr. 38; Skaggs, R.W., van Schilfgaarde, J., Eds.; ASA, CSSA, SSSA: Madison, WI, 1999; 893–910. 9. Schwab, G.O.; Fouss, J.L. Drainage materials. In Agricultural Drainage, Agron. Monogr. 38; Skaggs, R.W., van Schilfgaarde, J., Eds.; ASA, CSSA, SSSA: Madison, WI, 1999; 911–926.

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Land Drainage: Wells Rameshwar Kanwar Agricultural and Biosystems Engineering Department, Iowa State University, Ames, Iowa, U.S.A.

Allah Bakhsh Department of Irrigation and Drainage, University of Agriculture, Faisalabad, Pakistan

INTRODUCTION Land drainage is the removal of excess surface and subsurface water from the land to enhance crop growth, including the removal of soluble salts from the soil.[1] In a more scientific sense, drainage can also be defined as the techniques or methods employed to lower and control the elevation of shallow water table depths at such a depth within the root zone where maximum crop productivity can be achieved.[2] Drainage increases the depth of soil free of water for plants to develop their rooting system, absorb more nutrients and, therefore, sustain yields over a longer period of time.[3] Drainage can be either natural or artificial. Artificial drainage usually supplements and enhances the natural drainage capability of the soils to keep a favorable balance of water, salts, and air in the root zone for optimum plant growth. The man-made landscape changes, however, not only increase crop production by bringing more area under cultivation but also alter the natural drainage ways. Moreover, the construction of roads, highways, and installation of irrigation sources/practices and land-forming operations also affect the natural drainage capability of the soils. Under these conditions, the long-term benefits and productivity of the poorly drained soils and mitigation of the irrigation effects can only be sustained by providing and maintaining the adequate surface and subsurface drainage systems for poorly drained soils. Benefits The main objective of land drainage is to provide and maintain a healthy root zone environment that is suitable for maximum crop productivity with the following benefits: 1. Drainage helps maintain a balance of air, water, and salts in the root zone and also increases root growth and crop yields by increasing uptake of plant nutrients.

2. Drainage increases biological activities in the root zone and, therefore, improves the nitrogen availability to plants. 3. Drainage removes excess water from the soil profile and improves the soil structure, workability, and other related field operations such as cultivation, planting, and harvesting practices. 4. Drainage improves soil temperature that is important for seed germination and plant growth processes. 5. Drainage helps reclaim the salt-affected soils. 6. The other associated benefits include mosquito control, improvement, and maintenance of roads, easy movement of farm machinery, wider selection of crops, longer growing season, better crop yields, and overall lower operational costs. Primarily, there are two types of drainage systems. One is horizontal drainage by means of surface ditches or subsurface buried field drains and the other is vertical drainage that is performed using wells. This article is mainly concerned with the vertical drainage and the subsequent discussion is limited to drainage with wells.

OPEN WELLS The open wells consist of a pit, dug to the groundwater level so that enough water can be collected. The depth of the well also depends on the soil formation and aquifer yield. The dug wells are mostly shallow because of the difficulty in their digging deeper through high water table soils and, therefore, have relatively lesser water yields. The dug wells are sometimes lined with masonry, gravel, rocks, or stone to avoid caving problems. After construction of these wells, water is lifted from wells at a faster rate to remove the fine soil particles in the adjacent area of the wells. This practice is called ‘‘well development,’’ which opens the pores of the soil formation and increases flow of water to the wells. Larger diameter open wells are also 705

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constructed to increase the flow area to the wells and also to increase the storage volume of water in the wells. At some places, tile drains discharge into shallow wells called ‘‘sumps’’ and water is lifted from these wells into nearby canals, reservoirs, or natural streams for its use in irrigation or disposal purposes. This method is well illustrated in a number of places in California, e.g., Sutter Basin.[4] These conditions, however, may vary from site to site. Higher well yields are not possible as sand can start slipping into the well when water depth is lowered to a certain level in the well. Adding stones or gravel at the bed of the well can help stop slipping of sand and increase water yield at some places. Water yields of open wells, however, can be increased by installing strainers at the bottom of the wells to tap groundwater from the deep layers, depending on the local hydrogeological conditions. Persian wheels are also used to lift water from such wells.

PERSIAN WHEELS Persian wheels lift water from open wells by means of small buckets tied with flexible rope or chains on a rotating drum or wheel. Power to this rotating wheel is provided through the shaft coming from another rotating drum driven by a pair of bullocks. The rotation of rope or chain brings the buckets up, full of water, from the well and empties them automatically as the buckets descend over the rotating drum. This mechanism provides continuous lifting of water from the well. Water lifted from open wells was mostly used for irrigation purposes, but at the same time it was providing drainage to the surrounding area because of its wide diameter and sizable capacity to store the groundwater seeping from the water bearing formations. A Persian wheel can yield water between 0.1 and 0.2 ft3/s, and can command an area of about 5–15 acres.[5] The use of the Persian wheel is feasible where groundwater is shallow and of good quality for usable purposes. These wells, in addition to their use for supplying water, have also been used as a source to drain the excess water from the soil surface as well as from the root zone called ‘‘drainage wells.’’

DRAINAGE WELLS The drainage of low lands or field pockets, which have no gravity outlet, was usually made possible by drilling sinkholes or wells into the water absorbing formations at a certain depth below the ground surface. The excess water was used to drain into these wells where it can flow into the coarse formations below the ground

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Land Drainage: Wells

surface. Such vertical drainage through holes or wells can fall in two categories, depending on the depth of the wells, i.e., shallow and deep. Both these conditions are site-specific and depend on the local hydrogeology of the area and the amount of drainage water to be disposed of. When the water bearing formation of sufficient thickness is available at a shallow depth of about 6 –10 m below the ground surface, it is termed as the shallow vertical drainage. When a deep well is drilled, it transports the surface or subsurface drainage water into the deep layers called deep vertical drains. The functions of such drainage wells, however, depend on the transmissivity characteristics of the geologic formations. Both shallow and deep wells serve the same purpose of removing excess water from the root zone by flowing it into the main groundwater body of the area. These wells have also been called ‘‘agricultural drainage wells.’’

AGRICULTURAL DRAINAGE WELLS (ADWs) When the state of Iowa in the United States was first inhabited by Europeans, the North Central part of the state was covered with wetlands and depressions were scattered. The lands were waterlogged with standing water, especially in the low-lying areas. Subsurface tile lines were laid and a drainage network was established to drain the excessive water from the surface as well as from the root zone. Meanwhile, agricultural drainage wells were also constructed to drain the surface as well as subsurface water from those depressions that otherwise were not drained. The collected water was guided downward to the bedrock aquifer through ADWs. Of the estimated 400–500 ADWs in Iowa, 359 were registered in four counties.[6] These ADWs have been used to provide outlets for removal of excessive soil moisture from the areas where other drainage systems were not feasible. These wells allow injection of surface or subsurface water using surface outlets down to the aquifer. Agricultural drainage wells help manage soil moisture so that crops can grow better. Depending on the local conditions, these wells are also used to recharge the aquifers. These wells include buried collection basins, one or more tile drainage lines to collect water, and a drilled or dug well in the low-lying areas of the fields (Fig. 1). The design of this system should consider the aquifer capabilities to handle such large quantities of water generated through thunderstorms. The United States Environmental Protection Agency has reported that there are more than 2842 agricultural drainage wells in the nation, including 359 in Iowa.[6] These ADWs, however, have been reported to be a concern of aquifer pollution with

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Land Drainage: Wells

Fig. 1 A schematic diagram of ADW installation. Source: From Ref.[6].

agricultural chemicals such as fertilizers, pesticides, or bacteria and other microorganisms. These wells short-circuit the drainage water from the surface or root zone directly into the aquifer without passing through the soil matrix. The quality of drainage water improves when it goes through the soil filtering process and chemical degradation before joining the main groundwater body.

TUBEWELL DRAINAGE This method of drainage uses wells to control the water table levels in the root zone. These wells can be shallow or deep depending on the hydrogeologic conditions of the area. The suitability of this system, however, is governed by the hydraulic properties of the soil horizons or aquifer. The shallow wells will directly lower the water table in the root zone whereas deep wells may tap the confined aquifers. Proper design and installation of wells can quickly lower the water levels depending on the local conditions. Vertical drainage or tubewell drainage can be performed using different types and methods related to pumping groundwater from the aquifer under different hydrogeological conditions, such as confined and unconfined aquifers. Tubewell drainage is a technique of removing excess water from the root zone by lowering the groundwater level of the waterlogged area using a single well or a series of wells.[7] To maintain the water table at a certain depth below the ground surface, tubewell discharge is usually equal to the drainable surplus of the area being drained. The success of tubewell drainage lies in pumping enough water from the aquifer so that the root zone can provide a favorable environment for crop productivity on a sustainable basis. The rate of

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lowering the water table depends on many factors such as hydrogeological conditions of the area, physical properties of the aquifer, and the physical properties of the overlying soil layers of the earth profile. The earth profile can basically be divided into two zones: fractured rock zone and the rock flowage zone. From a groundwater point of view, the fractured rock zone is of interest and is further composed of the saturated and unsaturated zones. The unsaturated zone has no free drainage of water but may contain perched water table depending on the soil horizon properties of the area. The saturated zone, however, contains the main groundwater body in the unconfined or confined soil layers depending on the geologic formation of the area and the sources of recharge of the aquifers. The wells, based on the availability of water from these soil layers, can be divided into gravity, artesian, or both types, depending on the soil conditions and the aquifer characteristics. Tubes or pipes in combination with the pumping set equipment can be used to pump groundwater for irrigation or drainage purposes. The pumping set increases the flow energy of groundwater by creating an artificial sink within the saturated zone called ‘‘drawdown.’’ This drawdown extends to the crop root zone and controls the water table at the desired depth within a desired area, depending on the well spacing and aquifer conditions. The tubewell or pumped well drainage system is also characterized by its higher initial and operating costs. This high cost, however, is balanced with the benefits obtained from tubewell drainage in comparison with other drainage systems that are described as follows: 1. Tubewell drainage is a quick and effective method to lower the water table at a faster rate than any other method.

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2. Water table can be lowered to a greater depth and can also be maintained at a desired level within the root zone because of the flexible control of its operational schedule. 3. The deeper installation of tubewell perforated pipes also offers an option to select the more permeable sandy layers from the soil horizon data ‘‘well log.’’ 4. The productive land is saved from construction of open ditches and farm machinery movements become easy. 5. Depending on the undulating topography and landscape features, sometimes tubewell drainage is the only option left to avoid digging long channels through hilly ridges. 6. The maintenance cost is less when compared with other drainage systems and its easy operation offers advantages over the others. The water-borne diseases are also preventable. 7. The pumped drainage water can also be used to supplement the irrigation water depending on its quality and would be available at the time when crops need it. The feasibility of the well drainage method, however, depends on the hydrogeological characteristics of the area, sources of excess water, aquifer properties, well spacing, energy charges, groundwater quality, and also the feasibility to dispose the drainage effluents.

DRAINAGE EFFLUENTS’ DISPOSAL PROBLEMS The cost-effective and environmentally safe disposal of drainage effluents is a real problem both in the humid and arid regions, although characteristics of the drainage effluents can vary for both the regions. In humid areas, soil salinity is not the problem because of the availability of fresh water from rainfall, but at the same time, studies have shown that drainage effluents are a potential carrier of nutrients, pesticides, and herbicides from the agricultural fields to surface and groundwater bodies. Although application rates of agrochemicals are high in some humid zones of the world because of high cropping intensity to obtain higher crop yields, drainage of these agricultural lands has played a significant role in transporting these chemicals to lakes, streams, reservoirs, and oceans. Therefore, disposal of drainage effluents from humid areas needs treatments to reduce nutrient loads to minimize eutrophication problems causing low oxygen levels that are preventable. Similarly, the drainage water from an arid zone carries significant amounts

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of salts and can neither be used for irrigation nor be found suitable for wild habitat and aquatic life. The disposal of salty drainage water is a serious issue, particularly in the arid zones where enough water is not available for its dilution.

CONCLUSIONS In arid regions, irrigated agriculture has increased agricultural production, but at the same time, it has disturbed the ecosystem from the construction of different size reservoirs, canals, and irrigation and drainage ditches. The irrigation network usually increases seepage, percolation, and ultimately contributes to the rise of groundwater levels as well as salt accumulation in the root zone. To mitigate the effects of irrigation and sustain the productivity of irrigated fields, drainage of irrigated lands becomes essential to keep a favorable balance of salts, air, and water in the root zone. Besides overcoming the irrigation effects, drainage also helps in bringing more areas under cultivation, which were earlier waterlogged, under depressions and potholes, or in swamps. Drainage of agricultural land not only maintains productivity of the poorly drained soils but also induces transport of nutrients and chemicals from the agricultural fields to as near as lakes and as far as oceans.[8] According to Newton’s third law of motion, every action has a reaction. The same is true in the sense of our ecosystem. The man-made structures and human activities exploited the natural resources to grow more and more food by increasing the crop yields, from increased use of chemical inputs, and by bringing more area under cultivation at the cost of losing forest lands, wetlands, or other places of wildlife habitat, which had devastating effects on the environment. The development of society in the early 1900s did not recognize the long-term effects of development but now conditions are reaching to such a level where global warming, glacier melting, sea level rising, more floods, hurricanes, and El-Nino effects could be the reaction of disturbing the nature in response to the human actions. Depending on the climate, hydrogeology, soil characteristics, topography, and farming practices, drainage effluents and their chemical characteristics and their effects on agroecology during transport of drainage effluents need to be managed. In humid regions, reducing drainage effluents by using wetlands as ecological filters can help in reducing the adverse effects of the drainage waters on the soil and water quality. Similarly in arid regions, improving irrigation efficiency, identifying, and checking the sources of excess water and salts need to be identified and managed accordingly. The disposal and management

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of drainage effluents require effective practices to minimize their offsite effects on the environment. 4.

REFERENCES

5.

1. ICID. Amendments to the constitution, agenda of the International Council meeting at Rabat. International Commission on Irrigation and Drainage, Morocco; ICID. New Delhi, 1979; 156–163. 2. Kanwar, R.S.; Bjorneberg, D.; Baker, D. An automated system for monitoring the quality and quantity of subsurface drain flow. J. Agric Engng. Res. 1999, 73 (2), 123–129. 3. Skaggs, R.W.; Schilfgaarde, J.V. Introduction. In Agricultural Drainage; Skaggs, R.W., van Schilfgaarde, J., Eds.;

6.

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7.

8.

Agron. Monogr. 38; ASA, CSSA, SSSA: Madison, WI, 1999; 3–10. Roe, H.B.; Ayres, Q.C. Engineering for Agricultural Drainage; McGraw Hill Book Company, Inc.: New York, 1954. Ahmad, N. Tubewells construction and maintenance. Scientific Research Stores 4, Akbari Road, Lahore, 1969. IDALS, Agricultural drainage well. Annual Report and Project Summary, Iowa Department of Agriculture and Land Stewardship, in cooperation with, Iowa State University, Ames, 1994. ILRI. Drainage principles and applications. International Institute for Land Reclamation and Improvement: Wageningen, The Netherlands, 1994. Bakhsh, A.; Kanwar, R.S. Simulating tillage effects on nonpoint source pollution from agricultural lands using GLEAMS. Trans. ASAE 2001, 44 (4), 891–898.

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Leaf Water Potential Andreas J. Karamanos Laboratory of Crop Production, Agricultural University of Athens, Athens, Greece

INTRODUCTION Leaf water potential is the thermodynamic expression of the water status of leaves, which is widely used in plant science research in the last 40 years. In this article, a description of the physical principles leading to the definition of the leaf water potential and its component potentials is initially given. Then, the way of development of water deficits in leaves and the compartmentation of the water and its component potentials in leaf cells and tissues subjected to different degrees of water shortage are presented. Some useful implications for the development of adaptive mechanisms to drought result from this analysis. Finally, the existing methods for the determination of leaf water potential are presented and classified in groups according to their basic principles.

DEFINITIONS According to Slatyer and Taylor,[1] the state of water in any system is expressed thermodynamically in terms of its chemical potential (mw) or the partial molal Gibbs  w Þ as follows: free energy ðG w ¼ mw ¼ G




@G @nw

 ð1Þ T;P;ns

where G is the Gibbs free energy of the system (a function of its internal energy) and nw the number of water  w Þ are defined more simply as the moles. Thus, mw or ðG change that occurs in the energy content of a system for a given change in the number of moles of water in the system at constant temperature (T), pressure (P), and solute content (ns). The water potential of a system (W) is defined as: W ¼

mw
m0w V w

ð2Þ

where m0w is the chemical potential of pure free water and V w the partial molal volume of water (18 cm3 mole
1). W is expressed as energy per unit volume, which is dimensionally equivalent to pressure. 710

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Thus, traditional pressure units (e.g., bar, MPa, etc.) are used for the expression of W. The concept of water potential can be realized as a measure of the capacity of water at a point in the system to do work in comparison with the work capacity of pure free water ðm0w Þ which is taken arbitrarily as zero. Accordingly, W provides a unified measure for the energy status of water at any place within the soil–plant–atmosphere continuum. Since the energy status of water results from the interaction of forces of different origin exerted on water molecules, W is thought to consist of several components each one representing a different kind of the forces involved. The effect of solutes, which are very common in any aqueous system, is expressed as the osmotic component of W, the osmotic or solute potential (Cs). The solutes lower the vapor pressure, and, hence, the potential energy of water in the system. Thus, Cs takes values below zero, which are proportional to the concentration of osmotic substances (cs ¼ n/V, where n ¼ number of moles and V ¼ volume): Cs ¼
RTcs ¼
p

ð3Þ

where R is the gas constant, T the absolute temperature, and p the osmotic pressure. External pressure, either above or below the value of the local atmosphere, increases or decreases, respectively, the water potential. Its effect is described in terms of the pressure potential (cp), which usually takes values above zero. Forces arising both at the liquid–air and at the solid–liquid interfaces are responsible for another component of W, the matric potential (cm). They are considered to arise in systems rich in matrix, i.e., with large surface to volume ratios (e.g., soils, plant cell walls, gels, and colloids). Water is bound by matrix with forces related to its water content. Thus, the capacity of water to do work is reduced, and, hence, cm takes negative values. It follows that, at any time, W is a function of cs, cp, and cm: W ¼ fðcs ; cp ; cm Þ

ð4Þ

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However, Eq. (4) is usually described, in a simplified way, as an algebraic sum: W ¼ cp þ cs þ cm

ð5Þ

A further component due to gravity, called gravitational potential (cg) might also be added to Eqs. (4) and (5) only in specific cases (e.g., in tall trees or when reverse water flow is considered).

WATER IN THE LEAVES Water enters the leaves via the vascular system of the petiole and it flows through the veins of the lamina down to the vein endings which are embedded in the mesophyll. Lateral water movement towards neighboring mesophyll tissue along the xylem tissue of the veins also occurs to a substantial extent. The specific important characteristic of the vascular system of the leaf is its close spatial relation to the mesophyll.[2] According to Kramer,[3] the actual distribution of water in the mesophyll occurs chiefly from the smaller veins, which are so numerous that most cells of a leaf are only a few cells away from a vein or vein ending. Fig. 1 shows diagrammatically the pathway of water from a xylem vessel to the evaporating surfaces in a substomatal cavity through a mesophyll cell. Water moves mainly through the apoplast (cell walls and intercellular spaces) where the resistance (rw) is minimal. Water exchanges (f) from and to the vacuole (symplastic pathway) are more difficult because of

Fig. 1 (A) Schematic representation of the water pathway from a xylem vessel (X) to the evaporating surfaces at the substomatal cavity (SC) through a mesophyll cell. (B) A simplified electric analog of the water movement in (a). F: main flow through the cell wall (W), f: secondary flow from the vacuole (V), rw: apoplast resistance, rv: symplast resistance, cv: symplast capacitance.

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the resistance of the two cell membranes (plasmalemma and tonoplast) to be crossed (rv). The water flux can be represented by an electric analog [Fig. 1(B)] where the apoplast exhibits mainly conductive properties (appearing as the resistance rw), whereas the symplast both conductive (rv) and storage properties represented as the capacitance cv. At any time, leaf water balance results from the relative magnitudes of the water supply through the roots and stem and the water loss through transpiration. To understand the mode of development of leaf water deficits we can consider the simple physical model proposed by Dixon in 1938 as quoted by Weatherley[4] (Fig. 2). In this model, the plant is regarded as consisting of two porous pots filled with water and connected by a tube through a manometer. The lower pot representing the root is semipermeable and buried into the soil. The upper pot indicates the transpiring leaf surface; the manometer represents the vacuoles of the mesophyll cells, which are off the main pathway since water moves mainly through the apoplast. Once transpiration starts, water evaporates quickly from the upper pot and a negative tension is transmitted through the tube to the lower pot resulting in water absorption from the soil. The existence of a considerable root resistance induces a lag of absorption behind transpiration, and water absorption cannot meet instantaneously transpirational fluctuations. Thus,

Fig. 2 Dixon’s simplified model of a transpiring plant. The upper and lower porous pots represent the leaves and roots, respectively, while the manometer (m) represents the mesophyll cells. The negative pressure registered by the manometer is equivalent to the leaf water potential (Wl) which fluctuates in response to lateral water movement (fc) from and to the cells. F denotes the main transpiration stream. Source: From Ref.[4].

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water will be drawn from the vacuoles of the mesophyll cells and the manometer will register the tension in the system. The leaf water potential (Wl) is equivalent to the tension registered by the manometer and proportional to the amount of water drawn from the cells. Accordingly, the fluctuations in Wl arise from mesophyll-water exchanges with the main transpiration stream.

WATER POTENTIAL AND ITS COMPONENTS IN LEAVES The Osmometer Concept In order to study leaf water relations, it is necessary to consider a leaf as consisting of mature parenchyma cells. Mature cells consist of three distinct phases: an elastic cell wall, the parietal cytoplasm with the nucleus and the organelles, and a central vacuole containing a dilute aqueous solution of sugars, ions, organic acids etc. [Fig. 3(B)]. The vacuole occupies about 80–90% of the cell volume and is surrounded by a semipermeable membrane, the tonoplast. It is, therefore,

Fig. 3 Diagrammatic representation of (A) a meristematic, and (B) a mature parenchyma cell from higher plants. The large vacuole of the mature cell controls its water exchanges in a manner close to that of an ideal osmometer. The same may not apply to the meristematic cell where vacuolation is small. Source: From Ref.[5], p. 129.

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reasonable to consider, as a first approximation, that cell water exchanges are controlled by the vacuole, which behaves as an osmometer. The adoption of the osmometer-concept for mature leaf parenchyma cells presupposes that the contribution of matrix to Wl is negligible (i.e., cm ¼ 0). Thus, Eq. (5) becomes: W1 ¼ cp þ cs

ð6Þ

The solute potential (cs) is determined by the concentration of the osmotically active substances in the vacuole. In leaf cells, cs always takes negative values which vary with cell volume: they are least negative in fully hydrated cells (maximum volume) and more negative in dehydrated ones (smaller volume) and they are supposed to follow the Boyle–van’t Hoff relationship: cs V ¼ constant

ð7Þ

It follows that a linear relationship is expected either between cs and 1/V or between 1/cs and V. The changes in cell volume are also responsible for the development of the pressure potential (cp). When water enters the cell, the vacuolar volume increases, and a pressure, called turgor pressure, is exerted on cell wall. At the same time, a pressure equal to turgor pressure is developed at the opposite direction, namely, from the wall to cell interior. This latter pressure, called wall pressure, acts like a hydrostatic pressure, increases the energy status of water in the cell by raising the pressure exerted on it above the local atmosphere, and represents the cell pressure potential (cp). Obviously, cp takes positive values as long as the vacuole exerts a pressure on the surrounding wall. When cells lose water, the vacuole shrinks progressively with a concomitant fall in cell turgor and cp. The relationship between cell volume and cp is curvilinear and depends on the elastic properties of the cell wall. Fig. 4 shows the relationship between W and its component potentials and the changes in cell volume (Hoefler diagram). When the cell achieves its maximum hydration (full turgor) W bears its maximum value (zero) because jcpj ¼ jcsj. As cell hydration falls, W drops curvilinearly to more negative values, and at incipient plasmolysis cp ¼ 0 and W ¼ cs. From this point onwards W is exclusively determined from the changes in cs: it falls linearly with any further dehydration according to Eq. (7). Fig. 5 shows the relations between 1/Wl against water deficit, known as pressure–volume curves[7] for leaves from different plant species. The curvilinear (turgor component) and the straight line (solute component) portions of the relationship are evident in all cases.

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high levels of leaf dehydration should be beneficial for plants growing in arid regions. This can be achieved by means of a more elastic cell wall, which makes the fall of cp with increasing dehydration less abrupt. Secondly, it is possible that an accumulation of osmotically active substances takes place in the vacuole. This leads to a drop of cs to values more negative than those expected by a simple volume reduction caused by cell dehydration. This solute accumulation in cells subjected to water stress constitutes an adaptive mechanism known as osmotic adjustment or osmoregulation. Osmotic adjustment has been detected in many plant species[9] and acts in two ways: 1) it enables cells to lose more water before their turgor drops to zero; 2) it increases the ability of cells to absorb water under dry conditions by lowering the cell water potential and thus maintaining a potential gradient between plant cells and their medium, necessary for water absorption.

The Effects of Cell Matrix Fig. 4 The relationship between the volume of water lost (DV) and the water (W) and its component potentials, solute (cs), and pressure potential (cp) for a cell or tissue showing ideal osmotic behavior. The dashed line indicates the point of zero turgor (incipient plasmolysis). Source: From Ref.[6].

The diagram of Fig. 4 leads to two ecologically important conclusions. First, in view of the great significance of cell turgor to many physiological processes,[8] the maintenance of cp above zero at relatively

As stated before, matric effects arise in systems rich in substances with large surface to volume ratios. At the cellular level, matrix is present in the cell walls in the form of interwoven cellulose microfibrils, and in the cytoplasm as the various gels and colloids. The real nature of cm has been the subject of many discussions among specialists. Initially, cm was thought to be the result of forces retaining water molecules by capillarity, adsorption, and hydration.[5,10] Tyree and

Fig. 5 The relationship between the inverse of the leaf water potential (1/Wl) and the leaf water saturation deficit (WSD) (pressure–volume curves) for leaves of wheat and cotton plants. No obvious deviations in the straight-line portions of the curves at high levels of dehydration are detectable.

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Karamanos,[11] based on a physicochemical study of the forces arising near solid phases with substantial surface charge densities, identified cm as the energy of interaction of the water dipoles with the electric field in the double layer,[12] and separated the effects of surface tension within the micropores as belonging to forces of a different nature (i.e., negative pressure). On this account, cm exerts its major impact in cell walls within a very short distance from the charged surfaces and its contribution is minor in comparison to both cp and cs. The osmometer approach assumes that the effects of cell matrix on tissue water exchanges are negligible and, thus, cm equals zero. This assumption might be almost true in fully vacuolated parenchyma cells [Fig. 3(B)], because the vacuole contains no matrix. In meristematic cells, however, the situation could not be so clear because of the poor vacuolation and the large volume fraction of cytoplasm [Fig. 3(A)]. Within the cytoplasm cm is important only in the matrix double layers which are expected to be less abundant in comparison with the cell wall. Since we have already concluded that cm influences the state of only a small fraction of the total cell wall water, the overall effect in the cytoplasm is probably even smaller.[11] Nevertheless, no data on water exchanges of growing leaves are available to test this hypothesis. A further complication to the osmometer approach is the question concerning the role of the apoplast as a water reservoir of cells and tissues. There is evidence that a high apoplastic water content is a common feature of xerophytes.[13,14] On this account, apoplastic

Leaf Water Potential

water could compensate for any water loss from the symplasm. Such an assumption does not seem to be valid for several reasons. First, no systematic deviations along the pressure–volume curves were detected, even in drought adapted species[15] and at high levels of dehydration (Fig. 5), an indication that the osmometer model holds satisfactorily. Secondly, the values of leaf relative water content seldom fall below 50% in the most severe cases of natural dehydration. Accordingly, significant amounts of not easily extractable water are still retained in leaves suffering from intense water stress. Thirdly, the rate of net water loss from leaves accounts for less than 5% of the evapotranspiration rate of plants during daylight hours.[16] Thus, no part of the leaf water content could ever act as an effective reservoir for water, especially cell apoplast which functions as the pathway for free water movement from the xylem to the evaporating surfaces when leaf is transpiring (Fig. 1). In conclusion, cell matrix does not seem to play a detectable role in leaf water relations. The osmometer concept holds satisfactorily, an indication that the cell vacuole is the key-site which regulates the water status of mature leaves.

METHODS OF MEASURING LEAF WATER POTENTIAL A relatively large number of methods for determining leaf water potential have been used. The existing methods can be classified into three basic groups:

Fig. 6 Determination of the leaf water potential of fava bean leaves by the change in length of leaf strips floated on a series of mannitol solutions of different osmotic potentials (Cs). The leaf water potential (W) coincides with the Cs of the solution where the initial length of the strip (214 grid units) remained unchanged. The osmotic potential at zero turgor (Cs0) can also be traced on the axis of mannitol osmotic potentials from the point E of intersection of lines AB and CD. Source: From Ref.[20].

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1) compensation methods; 2) psychrometric methods; and 3) pressure chamber method. A thorough description of these techniques is given in the works of Slavı`k[17] and Wiebe et al.[18] Compensation Methods In the compensation methods, we search for the solution of a known osmotic potential which is isotonic to the leaf water potential of the sample. A set of uniform parallel leaf tissue samples (discs, strips, etc.) are floated in a series of graded test solutions of known osmotic potential and left for equilibration.[19] The net water transfer between the tissue and the solution depends on the relative magnitudes of Wl and the cs of the solution and is manifested as a change either in the weight or the size (length, thickness, or area) of the sample. The isotonic solution is usually detected by interpolation. Fig. 6 shows the determination of Wl

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by the change in length of leaf strips. This technique can also give the cs of the tissue at zero turgor. The test solutions to be used must not: 1) be harmful to the tissues; 2) penetrate through cell membranes; and 3) be metabolized by plants. Mannitol, polyethylene glycol, and sucrose are the most common osmotica, but none of them completely fulfils all the requirements set above. An alternative method is the equilibration of the leaf samples in the gaseous phase. A parallel set of samples is left to equilibrate in closed vessels over a series of graded test solutions.[21] The direction and relative rate of the water vapor transfer between the sample and the solution is then determined by measuring either the sample weight or the volume of the osmotic solution. All compensation methods are time-consuming, temperature-dependent, laborious, and of relatively low accuracy (from 1 MPa to 0.3 MPa).

Fig. 7 Different types of thermocouple psychrometers. (A) A type. 1: wires, 2: copper rods, 3: rubber stopper, 4: measuring chamber, 5 and 6: chromel–constantan wires, 7: copper wires. (B) B type. 1: wire, 2: single wire, 3: brass flange, 4 and 5: chromel–constantan wires, 6: silver ring, 7: brass tube, 8: stainless steel cap, 9: brass rod closing the measuring chamber during equilibration, 10: brass piston with O-rings. (C) C type. 1: glass vessel, 2: rubber cap, 3: copper net cylinders with insulation, 4: rubber stopper, 5: glass tube, 6: thermistor, 7: plastic ‘‘spoon’’ filled with water, 8: holder of the plastic ‘‘spoon,’’ 9: wetting thread, Pb: copper plate Pb—lead weight. Source: From Ref.[17], Figs. 1.14, 1.18, and 1.21.

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Psychrometric Methods These methods measure water potential by determining the wet bulb depression in a closed gaseous system which is in equilibrium with the leaf sample. The wet bulb depression depends on the relative humidity of the air in the system, which in turn is related to W as follows:
W ¼

 RT e ln eo V w

ð8Þ

where e/eo is the relative humidity. There are three kinds of psychrometers for measuring the wet bulb depression. In the first (type A, [Fig. 7(A)], a thermocouple junction (chromel and constantan wires) is used alternatively as wet and dry: the output of the thermocouple is read first when the junction is dry, then condensation of a fine water droplet is achieved by Peltier cooling.[22] The thermocouple functions as a wet junction as long as water remains on its surface, and the difference between the two readings is equivalent to the wet bulb depression. In the second type (type B, Fig. 7(B)), the output of a thermocouple with the thermojunction permanently wetted by a small drop of pure water is measured.[23] The droplet of water is held on a small silver ring supported by thin chromel and constantan wires. The diffusion flux of water from the junction to the leaf-tissue sample serving as a vapor sink is measured. In another version of this type, an additional similar dry thermocouple is included in the sample chamber and measured as reference.[24] In the third type (type C, Fig. 7(C)), temperature sensitive resistance units (thermistors) are used instead

of thermocouples.[25] The dry bulb temperature is measured with a thermistor to which a miniature chamber filled with water or wetted filter paper can be tightly attached. The wet bulb temperature is then measured by the same thermistor after temporarily lowering the wet chamber: water starts evaporating from the thermistor at a rate depending on the vapor pressure in the sample chamber, which is in equilibrium with the water potential of the plant material. The readings from all types of psychrometers are calibrated against salt or sucrose solutions of known cs. Leaf tissue segments (punched discs or strips) are used as samples. However, specially designed psychrometers or hygrometers are also used for the in situ measurement of the water potential of attached leaves (see Ref.[18] for a review). Psychrometric techniques are extremely temperaturesensitive. Nevertheless, their mean accuracy is very high (up to  0.01 MPa).

The Pressure Chamber Method The pressure chamber (or pressure bomb) was first used extensively by Schollander et al.[26] A cut leaf is inserted within a cylinder with its petiole protruding from the lid, so that it can be observed for sap exudation (Fig. 8). The chamber is then hermetically sealed and the pressure inside is gradually increased by compressed air or nitrogen until sap appears at the xylem vessels on the cut surface. According to the theoretical analysis of the technique,[7,27,28] the water potential in the xylem vessel (Wx) is dominated by the negative hydrostatic pressure (tension) cp,x caused by the transpiration stream. Both solute (cs,x) and

Fig. 8 Diagrammatic representation of a pressure chamber apparatus. 1: cylinder, 2: lower cover, 3: upper cover, 4: O-rings, 5: insertion held with four screws (6) used to seal the stem by means of an O-ring (7), 8: rubber stopper, 9: binocular microscope, 10: pressure gauge, 11: inlet valve, 12: outlet valve. Source: From Ref.[17], Fig. 1.33.

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Fig. 9 The time course of the leaf water potential (Wl) for the wheat cultivar Yecora grown in Athens under rainfed conditions. The shaded area indicates the WPD and the vertical bars, the standard errors of the means. Source: From Ref.[29].

matric components (cm,x) in the xylem are negligible.[26] It follows that: Wx ffi cp;x

The integral of the course of Wl over time (i.e., the shaded area in Fig. 9) describes the ‘‘duration’’ of Wl [water potential duration (WPD), in MPa days]:

ð9Þ WPD ¼

The pressure required to force the sap to the cut petiole (P) compensates the original negative pressure in the intact xylem vessels (cp,x), so that:
P ¼ cp;x ¼ Wx

ð10Þ

Wx then equals Wl when single leaves are used. The technique is quick, easy, and quite accurate (0.02 MPa) provided that some precautions are taken: 1) an appropriate rate of pressure increase is applied; 2) the water loss from the leaf during determination is minimal; 3) the pressure gauge is as accurate as possible. In addition, this technique is very useful for producing pressure–volume curves[7] which offer useful and reliable information on the mechanisms involved in the regulation of leaf water status.

Z

v

Wl;i dt

ð11Þ

i¼1

where Wl,i is the leaf water potential at day t within the observation period, i.e., from days 1 to v. In order to make the values of WPD comparable among observation periods of different duration, the Water Potential Index (WPI) is derived by dividing WPD by the length of the period of study: WPI ¼ WPD=n

ð12Þ

where n is the length of a period in days. The use of the WPI as an objective indicator of the total water stress experienced by plants in a given environment looks promising in genotype evaluation studies for drought stress resistance.[29]

REFERENCES THE WATER POTENTIAL INDEX At any time, leaf water potential is the combined result of interactions of soil water availability, evaporative demand, and plant responses. Accordingly, it can be considered as a reliable indicator of leaf water balance. Karamanos and Papatheohari[29] suggested a method to assess the water stress history experienced by a plant or crop by using serial values of Wl over an observation period (Fig. 9).

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1. Slatyer, R.O.; Taylor, S.A. Terminology in plant- and soil-water relations. Nature 1960, 187, 922–924. 2. Esau, K. Plant Anatomy; John Wiley and Sons, Inc: New York, 1965; 767 pp. 3. Kramer, P.J. Plant and Soil Water Relationships. A Modern Synthesis; McGraw-Hill: New York, 1969; 482 pp. 4. Weatherley, P.E. Water movement through plants. Philos. Trans. R. Soc. Lond. 1976, B273, 435–444. 5. Slatyer, R.O. Plant–Water Relationships; Academic Press: London, 1967; 366 pp.

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6. Weatherley, P.E. Water in the leaf. In The State and Movement of Water in Living Organisms; Cambridge University Press: Cambridge, 1965; 157–184. 7. Tyree, M.T.; Hammel, H.T. The measurement of the turgor pressure and the water relations of plants by the pressure bomb technique. J. Exp. Bot. 1972, 23, 267–282. 8. Hsiao, T.C. Plant responses to water stress. Annu. Rev. Plant Physiol. 1973, 24, 519–570. 9. Morgan, J.M. Osmoregulation and water stress in higher plants. Annu. Rev. Plant Physiol. 1984, 35, 299–319. 10. Dainty, J. Water relations of plant cells. Adv. Bot. Res. 1963, 1, 279–326. 11. Tyree, M.T.; Karamanos, A.J. Water stress as an ecological factor. In Plants and Their Atmospheric Environment; Grace, J., Ford, E.D., Jarvis, P.G., Eds.; Blackwell Scientific Publications: Oxford, 1981; 237–261. 12. Bolt, G.H.; Miller, R.D. Calculation of total and component potentials of water in soil. Trans. Am. Geophys. Union 1958, 39, 917–928. 13. Gaff, D.F.; Carr, D.J. The quantity of water in the cell walls and its significance. Aust. J. Biol. Sci. 1961, 14, 299–311. 14. Cutler, J.M.; Rains, D.W.; Loomis, R.S. Role of changes in solute concentration in maintaining favourable water balance in field-grown cotton. Agron. J. 1977, 69, 773–779. 15. Richter, H.; Duhme, F.; Glatzel, G.; Hinckley, T.M.; Karlic, H. Some limitations and applications of the pressure–volume curve technique in ecophysiological research. In Plants and Their Atmospheric Environment; Grace, J., Ford, E.D., Jarvis, P.G., Eds.; Blackwell Scientific Publications: Oxford, 1981; 263–272. 16. Weatherley, P.E. Some aspects of water relations. Adv. Bot. Res. 1970, 3, 171–206. 17. Slavı`k, B. Methods of Studying Plant Water Relations; Chapman and Hall Ltd: London, 1974; 449 pp.

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18. Wiebe, H.H.; Campbell, G.S.; Gardner, W.H.; Rawlins, S.L.; Cary, J.W.; Brown, R.W. Measurement of Plant and Soil Water Status, Bull, 484; Utah Agricultural Experiment Station: Logan, Utah, 1971; 71 pp. 19. Ursprung, A.; Blum, G. Eine neue methode zur messung der saugkraft von hartlaub. Jahrb. Wiss. Bot. 1927, 67, 334–348. 20. Kassam, A.H. Determination of water potential and tissue characteristics of leaves of Vicia faba L. Hort. Res. 1972, 12, 13–23. 21. Slatyer, R.O. The measurement of diffusion pressure deficit in plants by a method of vapour equilibration. Aust. J. Biol. Sci. 1958, 11, 349–365. 22. Spanner, D.C. The peltier effect and its use in the measurement of suction pressure. J. Exp. Bot. 1951, 2, 145–168. 23. Richards, L.A.; Ogata, G. Thermocouple for vapour pressure measurement in biological and soil systems at high humidity. Science 1958, 128, 1089–1090. 24. Kramer, P.J. Measurement of Water Potential and Osmotic of Leaf Tissue with a Thermocouple Psychrometer; Department of Botany, Duke University: Durham, N.C., 1967. 25. Kreeb, K. Untersuchungen zu den osmotischen zustangroessen. I. ein tragbares elektronisches mikrokryoskop fur oekophysiologische arbeiten. Planta 1965, 65, 269–279. 26. Schollander, P.F.; Hammel, H.T.; Hemingsen, E.A.; Bradstreet, E.D. Hydrostatic pressure and osmotic potential in leaves of mangroves and some other plants. Proc. Natl Acad. Sci. U.S.A. 1964, 52, 119–125. 27. Waring, R.H.; Cleary, B.D. Plant moisture stress: evaluation by pressure bomb. Science 1966, 155, 1248–1254. 28. Ritchie, G.A.; Hinckley, T.M. The pressure chamber as an instrument for ecological research. Adv. Ecol. Res. 1975, 9, 165–254. 29. Karamanos, A.J.; Papatheohari, A.Y. Assessment of drought resistance of crop genotypes by means of the water potential index. Crop Sci. 1999, 39, 1792–1797.

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Library Resources Robert Teeter Library, Santa Clara Valley Water District, San Jose, California, U.S.A.

INTRODUCTION This article describes some representative water libraries in different kinds of institutions, both in the United States and internationally. It also describes the major databases available for finding journal articles and other literatures about water and related fields.

LIBRARIES: UNITED STATES The U.S. Geological Survey’s library system claims to be ‘‘the largest earth-science library in the world.’’ Just as water is a major part of the Survey’s mission, so too is it a major subject in its libraries. The USGS libraries collect materials in water resources, water quality, hydrology, surface water, groundwater, meteorology, glaciology, aquatic ecology, and aquaculture. The libraries hold more than 450,000 maps and 350,000 photographs, as well as more than 1.2 million scientific books and a comprehensive collection of the USGS’s own publications. The USGS has its main library in Reston, Va., with regional libraries in Denver, Colorado; Flagstaff, Arizona; and Menlo Park, California. These libraries are open to the public as well as USGS staff. There are also smaller ‘‘science center libraries’’ around the country, such as the Water Resources Division library in New Cumberland, Pennsylvania, and the National Wetlands Research Center library in Lafayette, Louisiana.[1] As part of the National Agricultural Library in Beltsville, Maryland, the Water Quality Information Center (WQIC) focuses on water management issues as they relate to farming and animal husbandry. In order to serve users nationwide, many of WQIC’s products are provided through its Website. These include a database of electronic publications on water and agriculture and bibliographies on water-related subjects. Some of the bibliographies are automatically updated as the National Agricultural Library’s catalog is updated.[2] Agencies like the Illinois State Water Survey perform research and disseminate information on their state resources, just as the USGS does at the national level. The library of the ISWS, in Champaign, Ilinois, collects materials on groundwater, hydrology, water

supply and treatment, water-use planning, and climate change. In addition to its own survey publications, the library has significant collections of reports from the University of Illinois Water Resources Center, U.S. Environmental Protection Agency, and other state and federal agencies.[3] The University of Wisconsin Water Resources Library, in Madison, has a collection of about 30,000 volumes focusing on the water resources of Wisconsin and the other Great Lakes states. Unlike many academic libraries, this one lends materials to the general public (as long as they are Wisconsin residents) and even has services for children. Its Website features research guides, advice on finding a water-related job, a directory of water experts, and links to water Websites.[4] The library is affiliated with the university’s Water Resources Institute, one of 54 such state institutes, many of which have libraries.[5] Many other universities have extensive archives from local water agencies and officials. A product of 12 universities, the Western Waters Digital Library provides online access to records concerning the major watersheds of the Western United States.[6] The Everglades Digital Library, a service of Florida International University and other organizations, presents scientific reports, maps, data sets, photos, and other materials related to the wetlands of south Florida.[7] The Water Resources Center Archives at the University of California, Berkeley, has an important collection of documents and archives relevant to its state.[8] The StreamNet Library, in Portland, Oregon, serves scientists and the general public interested in the fisheries (especially salmon) and ecosystems of the Columbia River Basin in the Pacific Northwest. The library has approximately 20,000 items—books, technical reports, journals, computer files, and other formats.[9] The Rivers Institute at Hanover College in Hanover, Indiana, maintains a physical collection in the college’s library and an online learning center on the Web. Among the online resources are an extensive directory of Web links, resources for K-12 teachers, a list of water dates and events, and lists of publications and presentations by institute staff.[10] The Santa Clara Valley Water District manages the wholesale water supplies of Santa Clara County, California (Silicon Valley), while providing flood protection to its 1.7 million residents and serving as 719

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stewards of the county’s more than 800 miles of streams. The district’s library, in San Jose, consists of approximately 18,000 books, reports, government documents, electronic databases, and audio-visual materials. The collection includes the areas of engineering, environmental and earth sciences, and management. The library also serves the general public by making available district reports and by interlibrary lending.[11] St. Johns River Water Management District, based in Palatka, Florida, provides regional water supply, surface water protection, and flood protection to the residents of 18 counties in northwestern Florida. The library covers water resources (surface and groundwater), civil engineering, environmental and life sciences, earth sciences, and Florida local and natural history. The library contains about 20,000 books, government documents, and technical reports. Checkout privileges are for staff only, but members of the public are welcome to visit.[12]

LIBRARIES: INTERNATIONAL The Centre for Ecology and Hydrology, a research institute in the United Kingdom, maintains libraries at its eight locations. The libraries hold 50,000 books and reports on hydrology, aquatic ecology, climate change, and related subjects. Access is restricted to staff and approved external researchers.[13] Natural Resources Canada’s library covers all aspects of earth sciences, including water resources. Its main library is in Ottawa, but a branch in Quebec City specializes in groundwater; there are branches elsewhere in the country. The library serves Canadian government staff, scientists, and the general public.[14] The Bundesanstalt fu¨r Gewa¨sserkunde (German Federal Institute of Hydrology), a government agency in Koblenz, Germany, holds 85,000 publications in hydrology, water conservation, water resources management, aquatic biology, and related topics. It directly circulates materials to residents of the region and to others via interlibrary loan.[15] The Water Reference Library, University of New South Wales, Australia, is part of the university’s library, but also serves the Water Resource Laboratory. Its collection numbers 21,000 items in the fields of hydraulics, hydrology, coastal engineering, groundwater, irrigation, water quality, and oceanography.[16] The library serving WaterCare Services, Ltd, in Auckland, New Zealand, has a collection in the areas of water supply, water resources planning, water quality, sewage treatment, engineering, and financial and asset management. Librarian Sarah Knight provides research and current awareness for staff in these subjects.[17]

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Library Resources

Sydney Water, a utility serving Australia’s largest city, has a library serving staff and the general public (by appointment). Its collection focuses on water, wastewater, environment, and engineering. Library staff provide books, reports, internal documents, audiovisual material, and online and CD-ROM databases. Library staff members also offer current awareness, journal alert, and interlibrary loan services.[18] The Winnipeg (Manitoba) Water and Waste Department’s library contains civil engineering references and standards, department reports, manufacturers’ catalogs, and archival material. The library primarily serves department staff, but other city staff and engineering students also use it.[19]

DATABASES: SUBSCRIPTION The following databases are available by subscription only. Many academic libraries, as well as special water libraries, have access to one or more of them. Except for the Water Library, all of these databases contain citations and abstracts rather than full text. Water Resources Abstracts, published by Proquest CSA, indexes journals, conference proceedings, books, and reports in the areas of groundwater, surface water, water supply, water quality, watersheds, water engineering, and desalination. Coverage dates from 1967. A complementary database from Proquest CSA, Aquatic Sciences and Fisheries Abstracts (ASFA) focuses on aquatic biology.[20] Aqualine covers a similar range of subjects as Water Resources Abstracts, but with a British emphasis. Its index goes back to 1960. It is jointly produced by Proquest CSA and the Water Research Centre (WRc).[21] The American Water Works Association’s database Waternet indexes journals, books, reports, and conference proceedings from the AWWA and other publishers. It covers water infrastructure, water treatment, water utility management, water conservation, water law, and water security from 1971 on. It is sold on CD-ROM from AWWA and is also available through the Dialog database service. The Water Library, an online service of AWWA, provides the full text of articles (for a fee) from the association’s journal back to 1971.[22]

DATABASES: FREE The American Society of Civil Engineers offers a database of its publications (many water related)— the CE Database. Citations and abstracts are free; full articles may be purchased for a fee.[23]

Similarly, the National Ground Water Association permits free searching of its Ground Water Online database. Abstracts and articles are free to NGWA members; nonmembers must pay a fee for articles.[24] Researchers worldwide post abstracts of their current and recent work at the Water Research Network, based in Norway.[25] The University of Florida Center for Aquatic and Invasive Plants makes available a database known as APIRS, which indexes the literature of aquatic, wetland, and invasive plants.[26] CONCLUSION Water libraries exist wherever scientists and engineers need the support a good library (and librarians) can provide to do their research. However, if there is not one near you, check to see what your nearest university or research institution can offer. REFERENCES 1. http://library.usgs.gov/, http://library.usgs.gov/ coldevpol.pdf, and http://library.usgs.gov/scicenterlibs. html (accessed January 24, 2007). 2. http://www.nal.usda.gov/wqic/ (accessed January 21, 2007). 3. http://www.sws.uiuc.edu/chief/library/gen_info.htm (accessed January 25, 2007). 4. http://www.wri.wisc.edu/library/, http://www.aqua. wisc.edu/waterlibrary/, and http://www.aqua.wisc.edu/ waterlibrary/kids/ (accessed January 26, 2007). 5. See list at; http://niwr.montana.edu/institutes/ (accessed January 26, 2007). 6. http://westernwaters.org/, and http://westernwaters. org/participants.html (accessed January 28, 2007). 7. http://everglades.fiu.edu/ (accessed January 28, 2007). 8. http://www.lib.berkeley.edu/WRCA/ (accessed January 28, 2007).

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9. http://www.fishlib.org/ (accessed January 21, 2007). 10. http://www.riversinstitute.org/education/learningcenter/ (accessed January 25, 2007). 11. http://www.valleywater.org/News_and_events/ Publications/District_library.shtm (accessed January 21, 2007). 12. Susie Hallowell, personal communication, January 12, 2007. 13. http://www.ceh-nerc.ac.uk/library/, and http://www. ceh-nerc.ac.uk/library/collection.html (accessed January 28, 2007). 14. Diane Thompson, personal communication, December 15, 2006; http://www.nrcan.gc.ca/libraries/03_e.html and http://www.nrcan.gc.ca/libraries/01a_e.html; (accessed January 28, 2007). 15. http://www.bafg.de/servlet/is/9427/ and http://www. bafg.de/servlet/is/7241/ (accessed January 28, 2007). (accessed January 28, 2007). 16. http://www.wrl.unsw.edu.au/resources/library.php3 (accessed January 23, 2007). 17. Sarah Knight, personal communication, December 18, 2006. 18. Connie Messina, personal communication, January 10, 2007; http://www.sydneywater.com.au/ContactUs/ Library.cfm (accessed January 28, 2007). 19. JoAnn Da Silva, personal communication, January 16, 2007. 20. http://www.csa.com/factsheets/water-resources-set-c.php (accessed January 29, 2007). 21. http://www.csa.com/factsheets/aqualine-set-c.php (accessed January 29, 2007). 22. http://www.awwa.org/bookstore/product.cfm?id= WWATCD, http://library.dialog.com/bluesheets/ html/bl0245.html, and http://www.awwa.org/AWWA/ waterlibrary/wlfaq.aspx (accessed January 29, 2007). 23. http://www.pubs.asce.org/cedbsrch.html (accessed January 29, 2007). 24. http://www.ngwa.org/gwonline/gwol.cfm (accessed January 29, 2007). 25. http://www.waternet.org/ (accessed January 29, 2007). 26. http://plants.ifas.ufl.edu/search80/NetAns2/ (accessed January 29, 2007).

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Livestock and Poultry Production: Water Consumption David B. Parker Michael S. Brown Division of Agriculture, West Texas A&M University, Canyon, Texas, U.S.A.

INTRODUCTION

WATER CONSUMPTION FOR LIVESTOCK

Competition for drinking water has increased in many locations across the world. Groundwater continues to decline in many areas. Water once used for agriculture is now being directed to municipal and industrial uses. There is a trend away from the family-sized farm or ranch to fewer large-sized animal production operations called concentrated animal feeding operations (CAFOs). Water consumption data are a necessary component in the design of a drinking water supply system for new livestock and poultry production operations. Water is the primary constituent in the body of livestock and poultry, constituting 50–80% of the live weight of the animal. Water serves as an essential solvent and plays a vital role in regulation of body temperature, lactation, digestion, elimination of waste products of digestion and metabolism, regulation of osmotic pressure, reproduction, transportation of sound, and vision.[1,2] Livestock and poultry fulfill their water needs by drinking or eating snow and/or ice, ingesting water contained in feed, and in minor amounts through metabolism (water produced by the oxidation of carbohydrate, fat, and protein). The primary avenues of water loss are urinary excretion, fecal excretion, perspiration, and respiration.[1] When evaluating water requirements of livestock and poultry, it is important to distinguish between free water intake (FWI), which is water consumed by the animal by drinking, and total water intake (TWI), which is the sum of FWI and any water ingested in the feed. The amount of water ingested in processed feed and grazed forage can be substantial. For example, many ‘‘dry’’ feedstuffs contain 10–14% water, while grazed forages and silages contain 60– 80% water. Free water intake is usually measured as the amount of water disappearance from a water trough. Because of spillage by ‘‘leisure’’ activities, water disappearance does not always equal the amount of water ingested or actual water requirement. However, water disappearance and FWI are assumed to be equal in most production situations.

Cattle

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Cattle can be divided into three groups: feedlot cattle, range cattle, and dairy cattle. Within these groups, a variety of factors may affect water consumption, including species or breed, type of diet, feed intake, rate and composition of gain, pregnancy, lactation, activity, and environmental conditions. One of the first references for daily water intake by dairy and beef cattle was published by Winchester and Morris.[3] Although the water intake values presented in this publication are often referenced, there are many things that have changed to alter water intake in cattle today, most notably breed characteristics, feeding management, and dietary ingredients. In this chapter, we have attempted to provide a concise summary of the latest in water consumption data available for current feeding and management conditions. Feedlot cattle Most commercially fed cattle in the United States are housed in open, earthen-surfaced lots with 50–200 animals per pen. Until recently, few data were available from which to establish water requirements for feedlot cattle.[1] Parker et al.[4] monitored total water usage over a two-year period at a 50,000 animal beef cattle feedlot. Average daily water usage over the two year period was 40.9 L per animal, which included all water used for drinking, in the feedmill for steam flaking grain, and water used in overflow water troughs in the winter to prevent ice formation. In the winter, 66% of total usage was for drinking and in the summer 89% was used for drinking. A regression equation was developed to predict daily water usage for the entire feedlot: DFWU ¼ 39:2
0:648MaxT þ 0:0421 MaxT2
0:0717 MinRH

ð1Þ

2

R ¼ 0:60 where DFWU is the daily feedlot water use (L per animal), MaxT, the maximum daily temperature ( C), and

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MinRH, the minimum daily relative humidity (%). Jeter[5] measured FWI by feedlot steers and developed the following equation:

Table 1 Total water intake by beef cattle with an ambient temperature of 4.4–32.2 C

DFWI ¼ 40:61 þ 0:46 MinT
0:45 MinRH

Class of cattle

R2 ¼ 0:93

ð2Þ

where DFWI is the daily free water intake (L/day), MinT, the minimum daily temperature ( C), and MinRH, the minimum daily relative humidity (%). Range cattle The livestock category of range cattle describes both diverse classes of cattle (non-lactating and lactating cows, growing steers and heifers, and mature bulls) and diverse types of grazing conditions (e.g., introduced forages and winter or spring cereal grains, native range) in which forage quality varies. The potential relationships between climatologic variables and the distance animals must travel to obtain forage and drinking water are poorly understood for grazing cattle. Although the majority of data available do not consider the water consumed in forage, it is well recognized that forage dry matter concentration is dynamic and dependent on environmental influences. Winchester and Morris[3] summarized total water intake data from six studies generally involving various classes of cattle that were individually fed and housed in environmental chambers for periods of 7–14 day (Table 1). Although these data were primarily derived under ‘‘laboratory’’ conditions, few data from production studies are available. Kattnig et al.[6] reported that FWI by 234 kg Holstein steers fed hay in individual pens was 91 mL/kg of body weight; average daily maximum temperature was 20.7 C. Ojowi et al.[7] conducted an 84 day grazing study by using growing steers (308 kg) gaining 0.9 kg/day and indicated that FWI averaged 94 mL/kg of body weight. Ali, Goonewardene, and Basarab[8] monitored ambient temperature, relative humidity, and FWI of grazing cattle (cows with calves, heifers, and a mature bull) during an 84 day grazing period. The average daily maximum temperature was 22.2 C, and FWI averaged 108 mL/kg of body weight.

Daily total water intake L/day

mL/kg of body weight

28–66 33–78

51–121 40–96

25–37 23–33

61–91 46–66

Bulls 544 kg 816 kg Non-lactating, pregnant cowsa 408 kg 500 kg

Lactation adjustment [added for each 1 kg of milk produced (4% milk fat)]b 2.1–2.7 Growing heifers and steers (average daily body weight gain of 0.4 –0.7 kg/day) 180 kg 360 kg

15–22 24–35

83–122 67–97

Total water intake was determined to be constant below 4.4 C. a Data were not determined above an ambient temperature of 21.1 C. b Data were derived from lactating dairy cows with an ambient temperature of 4.4–32.2 C. Source: From Ref.[3].

Akerlind, and Gustafson[11] developed the following equation: DFWI ¼ 14:3 þ 1:28 MP þ 0:32 DM

ð3Þ

where DFWI is the daily free water intake (drinking only), MP, the milk production (kg/day), and DM, the dry matter (% of diet). A summary of estimated water requirements for various classes of dairy cattle is presented in Table 2. Swine Weanling pigs age (3–6 weeks) will drink about 0.5 L/ day in the first week after weaning and 1.5 L at age 6 weeks.[13,14] Growing swine will consume about 2.5– 3.0 L of water for each kg of feed consumed.[14] Pigs drink about the same at temperatures of 7–22 C, but the amount of water consumed increases considerably at 30 C.[15] A summary of estimated water requirements for various classes of swine is presented in Table 3.

Dairy cattle Horses Water is the most important dietary component for dairy cattle, as insufficient water can limit milk production.[3] Lactating dairy cows require about 4 L of water for each kg of milk produced.[9] Many regression equations have been developed to predict the amount of water consumed by dairy cows.[10] Dahlborn,

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Horses consume about 8.4 L per 100 kg of body weight. Horses need 2–3 L of water per kg of dry matter intake.[18] Fonnesbeck[19] determined that horses fed an all-hay diet resulted in a water-to-feed ratio of 3.6:1, while horses fed a hay–grain diet had a

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Table 2 Water requirements for various classes of dairy cattle Class

Production Estimated water (kg milk/day) consumption (L/day)

Holstein calves (1 mo) (2 mo) (3 mo) (4 mo)

— — — — —

5–8 6–9 8–11 11–13

Table 4 Water requirements for various classes of horses

Class Maintenance, 500 kg, thermoneutral environment

23–30

Maintenance, 500 kg, warm environment

30–57

Lactating mare, 500 kg

38–57

Working horse, 500 kg, moderate work

38–45

Working horse, 500 kg, moderate work, warm environment

45–68 23–30

Holstein heifers (5 mo) (15–18 mo) (18–24 mo)

— — — —

14–17 22–27 28–36

Dry cows (pregnant)

—

26–49

Weanling, 300, thermoneutral environment

Jersey cows

13.6

49–59

Source: From Ref.[16].

Guernsey cows

13.6

52–61

Ayrshire, Brown Swiss, and Holstein cows

13.6 22.7 36.3 45.4

55–64 91–102 144–159 182–197

Source: From Refs.[10,12].

water-to-feed ratio of 2.9 : 1. Working horses may drink 20–300% more water than horses at rest.[18] A summary of estimated water requirements for various classes of horses is presented in Table 4.

met. Goats are one of the most efficient animals in the use of water, having one of the lowest rates of water turnover per unit of body weight.[21] Data for water consumption by dairy and meat goats is limited. Dairy goats require about 1.43–3.5 L of water per kg of milk, significantly less than dairy cattle.[21] Penned meat goats drink about 0.7 L of water per day.

WATER CONSUMPTION FOR POULTRY Chickens

Sheep The water required for sheep depends on the same factors as other livestock. In addition, the water requirements may vary with wool covering.[20] A summary of estimated water requirements for various classes of sheep is presented in Table 5. Goats A high proportion of the world’s goat population lives in arid areas where water requirements are not easily Table 3 Water requirements for various classes of swine Class

Estimated water consumption (L/day)

Estimated water consumption (L/day)

Weanling pig

0.5–1.5

Xin[22] developed the following equation for broilers between 1 day and 56 day of age: DFWI ¼ 2:78 þ 4:70D þ 0:128D2
0:00217D3 R2 ¼ 0:999

ð4Þ

where DFWI is the daily free water intake (L per 1000 birds) and D, the age in days. A summary of estimated water requirements for various classes of chickens is presented in Table 6. Turkeys Parker, Boone, and Knechtges[24] monitored water intake in tom turkeys at temperatures ranging from

11 kg pig

1.9

27 kg pig

5.7

Table 5 Water consumption of various classes of sheep

45 kg pig

6.6

Class

Water requirement (L/day)

90 kg pig

9.5

Rams

7.6

Gestating sow

17.0

Dry ewes

7.6

Pregnant gilt

20.8

Ewes with lambs

Sow plus litter

22.7

5–20 lb lambs

0.4–1.1

10–15

Feeder lambs

5.7

Boar Source: From Refs.[16,17].

© 2008 by Taylor & Francis Group, LLC

Source: From Ref.[16].

11.3

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Table 6 Water requirements for various classes of chickens and turkeys Water consumption (L per bird per week) Large white turkeys Age (weeks)

Broiler chickens

White leghorn hens

Brown egg-laying hens

Males

Females

1

0.22

0.20

0.20

0.38

0.38

2

0.48

0.30

0.40

0.75

0.69

4

0.10

0.50

0.70

1.65

1.27

6

0.15

0.70

0.80

2.87

2.15

8

0.20

0.80

0.90

4.02

3.18

10

—

0.90

1.00

5.34

4.40

12

—

1.00

1.10

6.22

4.66

14

—

1.10

1.10

6.68

4.70

16

—

1.20

1.20

6.92

4.74

18

—

1.30

1.30

7.00

—

20

—

1.60

1.50

7.04

—

Source: From Ref.[23].

10.0 to 37.8 C, with water consumption of 0.3 L/day and 1.3 L/day, respectively. A summary of estimated water requirements for various classes of turkeys as reported at commercial turkey production companies is presented in Table 6.

Ducks Veltman and Sharlin[25] evaluated water consumption in White Pekin ducks between 14 day and 42 day of age. Ducks that were allowed access to water 24 hr per day consumed 0.8 L/day, while ducks provided access to water only 4 hr/day consumed 0.6 L/day.

REFERENCES 1. NRC. Nutrient Requirements of Beef Cattle, Update 2000, 7th Ed.; National Research Council, National Academy Press: Washington, DC, 2000; 80–84. 2. Okine, E. Water Requirements for Livestock. Alberta Agriculture, Food and Rural Development: Lacombe, Alberta, Canada. http://www.agric.gov.ab.ca/agdex/ 400/00716001.html (accessed August 2001). 3. Winchester, C.F.; Morris, M.J. Water intake rates of cattle. J. Anim. Sci. 1956, 15, 722–740. 4. Parker, D.B.; Perino, L.J.; Auvermann, B.W.; Sweeten, J.M. Water use and conservation at Texas high plains beef cattle feedyards. Appl. Eng. Agric. 1998, 16 (1), 77–82. 5. Jeter, M.B. Drinking water intake by finishing yearling beef steers M.S. Thesis, West Texas A&M University: Canyon, TX, 2001.

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6. Kattnig, R.M.; Pordomingo, A.J.; Schneberger, A.G.; Duff, G.C.; Wallace, J.D. Influence of saline water on intake, digesta kinetics, and serum profiles of steers. J. Range Manag. 1992, 45 (6), 514–518. 7. Ojowi, M.O.; Christensen, D.A.; McKinnon, J.J.; Mustafa, A.F. Thin stillage from wheat-based ethanol production as a nutrient supplement for cattle grazing crested wheatgrass pastures. Can. J. Anim. Sci. 1996, 76 (4), 547–553. 8. Ali, S.; Goonewardene, L.A.; Basarab, J.A. Estimating water consumption and factors affecting intake in grazing cattle. Can. J Anim. Sci. 1994, 74 (3), 551–554. 9. Grant, R. Water Quality and Requirements for Dairy Cattle; Publication No. G93-1138-A; University of Nebraska Cooperative Extension: Lincoln, NE, 1993. 10. NRC. Water. Nutrient Requirements of Dairy Cattle, 7th Ed.; National Research Council. National Academy Press: Washington, DC, 2001; Chap. 8, 178–183. 11. Dahlborn, K.; Akerlind, M.; Gustafson, G. Water intake by dairy cows selected for high or low milkfat percentage when fed two forage to concentrate ratios with hay or silage. Swed. J Agric. Res. 1998, 28, 167–176. 12. NRAES. Dairy Reference Manual; NRAES-63, Northeast Regional Agricultural Engineering Service: Ithaca, NY, 1995; 144–145. 13. Almond, G.W. How Much Water Do Pigs Need? In Proceedings of the North Carolina Healthy Hogs Seminar, North Carolina State University, Greenville, NC, November 1, 1995; http://mark.asci.ncsu.edu/ HealthyHogs/book1995/almond.htm (accessed August 2001). 14. NRC. Nutrient Requirements of Swine, 10th Revised Ed.; National Research Council, National Academy Press: Washington, DC, 1998; 90–93. 15. Mount, L.E.; Holmes, C.W.; Close, W.H.; Morrison, S.R.; Start, I.B. A note on the consumption of water

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16.

17.

18.

19.

20.

Livestock and Poultry Production: Water Consumption

by the growing pig at several environmental temperatures and levels of feeding. Anim. Prod. 1971, 13 (3), 561–563. Lardy, G.; Stoltenow, C. Livestock and Water; Publication No. AS-954; North Dakota State University Extension Service: Fargo, ND, 1999. Straub, G.J.; Weniger, J.H.; Tawfik, E.S.; Steinhauf, D. The effects of high environmental temperatures on fattening performance and growth of boars. Livest. Prod. Sci. 1976, 3 (1), 65–74. NRC. Nutrient Requirements of Horses; National Research Council, National Academy Press: Washington, DC, 1989; 30–31. Fonnesbeck, P.V. Consumption and excretion of water by horses receiving all hay and hay–grain diets. J. Anim. Sci. 1968, 27, 1350–1356. NRC. Nutrient Requirements of Sheep; National Research Council, National Academy Press: Washington, DC, 1985; 26–27.

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21. NRC. Nutrient Requirements of Domestic Animals, Number 15, Nutrient Requirements of Goats: Angora, Dairy, and Meat Goats in Temperate and Tropical Countries; National Academy Press: Washington, DC, 1981; 9–10. 22. Xin, H. Feed and water consumption, growth, and mortality of male broilers. Poult. Sci. 1994, 73 (5), 610–616. 23. NRC. Nutrient Requirements of Poultry, 9th Ed.; National Research Council, National Academy Press: Washington, DC, 1994; 15–17. 24. Parker, J.T.; Boone, M.A.; Knechtges, J.F. The effect of ambient temperature upon body temperature, feed consumption, and water consumption, using two varieties of turkeys. Poult. Sci. 1972, 51 (2), 659–664. 25. Veltman, J.R.; Sharlin, J.S. Influence of water deprivation on water consumption, growth, and carcass characteristics of ducks. Poult. Sci. 1981, 60 (3), 637–642.

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Livestock Water Quality Standards Floron C. Faries Jr. College of Veterinary Medicine, Texas A&M University, College Station, Texas, U.S.A.

Guy H. Loneragan Feedlot Research Group, Division of Agriculture, West Texas A&M University, Canyon, Texas, U.S.A.

John C. Reagor Veterinary Diagnostic Laboratory, Texas A&M University, College Station, Texas, U.S.A.

John M. Sweeten Agricultural Research and Extension Center, Texas Agricultural Experiment Station (TAES), Amarillo, Texas, U.S.A.

INTRODUCTION A plentiful and consistent supply of high-quality water is essential for optimal production and health of feedlot cattle. Water of inadequate quality can result in decreased gains, poor feed conversion, and adverse affects on animal health. The greatest losses to livestock producers from low-water quality are often through undetected production inefficiencies and hidden but considerable influences on profitability.

WATER REQUIREMENTS Water constitutes 60–70% of the body of livestock. Water consumption is critical for animal maintenance. Animals that do not drink sufficient water may suffer stress or even dehydration. The amount consumed depends on the species, weather, and characteristics of feedstuffs consumed. For instance, dry beef cows need about 8–10 gal of water daily, whereas beef cows in their last 3 mo of pregnancy may drink up to 15 gal a day. Those in milk need about five times as much water as the volume of milk produced. Also, calves require much more water after weaning than before. Ignoring this fact may irreversibly retard growth of calves from which they may never fully recover.

WATER QUALITY Safe supplies of water are absolutely essential for livestock. Livestock may suffer health problems or belownormal consumptions resulting from substandard quality water. Ingestion of mineral or organic contaminants can cause poor performance or non-specific

disease conditions. Major livestock health problems associated with water quality are seldom reported except in site-specific instances. When evaluating the quality of water for livestock, one has to consider whether livestock performance will be affected; whether water could serve as a carrier to spread disease; and whether the acceptability or safety of animal products for human consumption will be affected. The most common water quality problems affecting livestock production are the following.
Excess salinity—high concentration of minerals, measured as total dissolved solids (TDS).
High nitrates or nitrites.
Bacterial contamination.
Blue-green algae.
Accidental spills of petroleum, pesticides, or fertilizers into water supply. The importance of nitrate, nitrite, sulfate, and TDS as factors influencing water quality for livestock has been recognized. Concentrations generally considered safe for consumption by cattle have been established (Table 1). However, these values may vary slightly depending on type and formulation of rations fed to cattle. In 1999, the USDA’s National Animal Health Monitoring System[1] conducted a water sampling study on beef feedlots with 1000 head or more capacity in the 12 leading cattle feeding states. These feedlots accounted for 96.1% of U.S. cattle feedlot inventory (January 1, 2000) and 84.9% of feedlots with 1000 head or more capacity.[1] One representative water sample per feedlot was analyzed for nitrate, nitrite, sulfate, and TDS. A total of 263 feedlots from 10 states (all west of the 727

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Table 1 Concentrations of nitrate, nitrite, sulfate, and total dissolved solids in water typically considered safe for livestock usage Measurement

Concentration considered safea (mg L1)

Nitrate, NO3

Less than 440

Nitrate, NO3-N

Less than 100

Nitrite, NO2

Less than 33

Nitrite, NO2-N

Less than 10

Sulfate, SO4

Less than 300

Total dissolved solids, TDS

Less than 3000

a

mg/L is equivalent to parts per million (ppm). Source: From Ref.[1] and National Research Council, National Academy of Sciences, Washington, DC.

Mississippi River) supplied a water sample for analysis. (No water samples were submitted from Arizona or Oklahoma.) The majority of samples (89.7%) were drawn from a well. Other sources included municipal/city (4.6% of samples), spring/river (2.3%), and pond/lake (2.3%). Only 1.7% of samples came from shallow wells (less than 30 ft deep), while 45.3% of samples were from wells that were 101 ft–300 ft deep, and 22.5% were from wells deeper than 300 ft. The mean nitrate concentration was 33.6 mg L1  3.5 mg L1 NO3 (or 7.6 mg L1  0.8 mg L1 NO3-N) while sulfate averaged 205 mg L1  24 mg L1 SO4. Both these values are considered safe levels (See Table 1). Nitrite was detectable in only 0.4% of the samples. No water samples exceeded the recommended nitrate limit and only 23% exceeded the sulfate limit.

MINERALS AND SALINITY Livestock tolerance of minerals in water depends on many factors: kind, age, diet, and physiological condition of the animal; season; climate; and kind of salt ions in the water. Livestock may drink less if the water tastes bad. Livestock restricted to waters with high salt content may suffer physiological upset or death. Several mineral elements found in water seldom offer problems to livestock because they do not occur at high levels in soluble form, or because they are toxic only in excessive concentrations. Examples are iron, copper, cobalt, zinc, iodide, and manganese. These elements do not seem to accumulate in meat or milk to the extent that they would cause problems. Common compounds found in waters with excess salinity include sodium, chloride, calcium, magnesium, sulfate, and bicarbonate. Bicarbonates and carbonates may contribute to alkalinity (pH) levels. When feed also is high in salt, lower water salinity would be desirable. Moreover, animals consuming high-moisture

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forage can tolerate more saline waters than those grazing dry grain rations, dry brush, or scrub. Hard water without high salinity does not harm animals.

NITRATE AND NITRITE Sources of nitrates and nitrites include decaying animal or plant protein, animal metabolic waste, nitrogen fertilizers, silage leachate, and soil high in nitrogen-fixing bacteria. Nitrates and nitrites are water soluble and may be leached away to the water table or into ponded water. Nitrate is important in livestock health. Although nitrate is not a particularly potent toxin, it is readily reduced to highly toxic nitrite within the rumen. Nitrite is about 10 times more toxic than nitrate. Nitrite is absorbed where it interacts with red blood cells by inhibiting their ability to effectively transport oxygen. Moderate nitrate intake may not cause any noticeable effect on animal health but may result in decreased animal gains and poorer feed conversion. Intake of large amounts of nitrate may result in death.

SULFATE Sulfur is required by all animals. The recommended sulfur intake for beef cattle is 0.15% of the ration and the maximum tolerable limit is 0.4% of the ration on a dry matter basis. Water can contribute significant quantities of sulfur, as sulfate, towards total sulfur consumption. Sulfur and sulfate are relatively non-toxic in these forms, but sulfate/sulfur is readily reduced in the rumen to highly toxic sulfide products. Excessive total sulfur consumption through feed and water can result in decreased water consumption, feed intake, and reduced average daily gains feed conversion.[2] Cattle on pasture can tolerate up to a maximum of 2000 mg L1 sulfate (SO4). However, cattle on full feed with a heavy proportion of concentrate in the diet should have water containing 300 mg L1 sulfate or less. If native cattle are moved onto water with a high sulfate level during a hot, dry period, there may be problems with adequate consumption. The same water may be well tolerated and consumption increased as needed in animals that were introduced to the water source in a cooler season, when demand was initially low (due to sufficient time for adaptation/acclimatization).

BIOLOGICAL ORGANISMS All surface waters must be assumed to carry bacteria. Livestock should be kept away from contaminated

water that has not been adequately aerated (oxygenated) because of the likelihood of excessive levels of bacterial pathogens that may be present. Surface water sources may have problems with algae growth as a result of high-nutrient loading in runoff water. Avoid using waters bearing heavy growths of blue-green algae, as several species can produce animal toxins (poisons). To control algae in storage tanks, reduce the introduced organic pollution and exclude light. Water storage tanks can be disinfected by adding 1 oz of chlorine bleach per 30 gal of water, holding for 12 hr before draining, and then refilling with clean water. Chlorination can also control certain bacteria.

WATER QUALITY CRITERIA There are no regulations regarding livestock water quality. Suggested limits of concentrations of specific substances in water for livestock, where these have been established, are shown in Table 2, which shows that suggested upper limits for livestock are generally higher than for humans, with the exception of copper and fluoride. Generally, salinity is more restrictive for young animals, pregnant, or lactating animals. Also, monogastic animals (poultry and swine) are less tolerant to salinity than ruminant animals (cattle or sheep).

WATER QUALITY EVALUATION To evaluate water quality in relation to livestock health problems, it is imperative to obtain a thorough history, make accurate observations, ask intelligent questions, and submit suspected water and properly prepared tissue specimens without delay to a qualified laboratory.[7] Obtain assistance from a local veterinarian, county Extension agent, or state veterinary medical diagnostic laboratories, usually affiliated with the land grant university in each state.

SOURCES OF WATER QUALITY CONTAMINATION Contaminant levels may be affected by runoff from surrounding lands and by concentration caused by water evaporation from a pond or storage tank.[8] Salinity is of special concern in the western half of the United States where naturally occurring salinity in watersheds or geological formations can restrict livestock water uses (Table 3). Livestock grazing operations may influence stream water quality where cattle are watered in or along the streams or drainage features. Potential sources of localized groundwater contamination include: livestock manure accumulations

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Table 2 Recommended limits of concentration of some potentially toxic substances in drinking water for livestock vs. comparable values for humans

Selected inorganic constituents

Comparable U.S. EPAa criteria (for humans)

Safe concentration (upper limit) for livestock, (mg L1)[3] NAS[5]

CAST[6]

Primary Inorganic chemicals

(MCL), mg L1

Antimony

0.006

Arsenic

0.05

Asbestos

7 MFL

Barium

2.0

Beryllium

0.004

Boron

N.A.

0.2

0.5

N.E. 5.0

Cadmium

0.005

0.05

0.5

Chromium

0.1

1.0

5.0

Chloride

N.A.

Cobalt

N.A.

1.0

1.0

Copper

1.3

0.5

0.5

Cyanide, free

0.2

Fluoride

4.0

2.0

3.0

Iron

N.A.

N.E.

No limitb

Lead

0.015

0.1

0.1

Manganese

N.A.

N.E.

No limit

Mercury

0.002

0.01

0.01

Nickel

N.A.

1.0

Nitrate-N

10.0

100

300

Nitrite-N

1.0

10

10

Salinity

N.A.

Selenium

0.05

Sulfate

N.A.

Thallium

0.002

Total dissolved solids

N.A.

See Table 3

Vanadium

N.A.

1.0

Zinc

N.A.

25.0

MCL ¼ Maximum contaminant level, highest level allowed in drinking water, an enforceable standard for public drinking water supply; MFL ¼ million fibers per liter; N.A. ¼ Not applicable; N.E. ¼ Not established. a Primary standards only, not including current human drinking water quality standards for micro-organisms; disinfectants or disinfection byproducts; organic chemicals; or radionuclides.[4] b Available data are not sufficient to warrant definite recommendations.

around water wells, ponds and stock pens, and agricultural chemicals or containers at spray pens, dipping vats, and disposal sites. Other potential non-point pollution sources that require careful site selection

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Table 3 Guide to using saline waters for livestock Total soluble salts content of waters (mg L1)

Comments—livestock water use

Less than 1,000

Relatively low level of salinity, no serious problem expected

1,000–2,999

Considered satisfactory; may cause temporary mild diarrhea in livestock unaccustomed to them, but should not affect animal health or performance

3,000–4,999

Should be satisfactory; may cause temporary diarrhea or be refused at first by animal unaccustomed to them

5,000–6,999

Can be used with reasonable safety; avoid using those approaching the higher limits for pregnant or lactating animal

7,000–10,000

Use should be avoided; considerable risk for pregnant or lactating livestock, young animals, or for any animals subjected to heavy heat stress or water loss; older livestock may subsist under conditions of low stress

More than 10,000

Excessive risks; cannot be recommended for use under any conditions

[5]

Source: From Ref. .

and management include:[9] concentrated animal feeding operations; wastewater holding ponds; lagoons; manure stockpiles; silos; dead animal disposal sites; and onsite sewage treatment systems. Fertilizers, including manure and wastewater, should be carefully selected and applied to land in accordance with soil and crop requirements or nutrient management plans. This will help prevent contaminating underlying aquifers and with nutrients or salts. Always handle and apply pesticides in strict accordance with the recommendations on the label. Do not apply pesticides around a water supply or other vulnerable sites. Wellhead protection measures are specified in water well drillers’ guidelines in most states. Locate wells at least 150–300 ft from livestock corrals, septic tanks, manure treatment lagoons, and runoff holding ponds.[8] To prevent infiltration, case, and grout wells down to a restrictive layer or to the water table, and seal around the wellhead with a concrete pad.

CONCLUSION Livestock producers should provide sufficient safe water for animals by preventing contamination and providing adequate sources of year-round, highquality drinking water supply. Livestock should be protected from unsafe drinking water by providing alternative sources of acceptable quality water. Water-related health problems in livestock are usually caused by stress conditions that may include

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inadequate water supply or unpalatable water with a high level of dissolved substances.

REFERENCES 1. National Animal Health Monitoring System (NAHMS), Water Quality in U.S. Feedlots, Information Sheet #N341. 1200; USDA Animal and Plan Health Inspection Service: Veterinary Services: Centers for Epidemiology and Animal Health: Ft. Collins, CO, 2000. 2. Loneragan, G.H.; Wagner, J.J.; Gould, D.H.; Garry, F.B.; Thoren, M.A. Effects of water sulfate concentration on performance, water intake, and carcass characteristics of feedlot steers. J. Anim. Sci. 2001, 79, 2941–2948. 3. Herrick, J.B. Water quality for animals. Symposium Proceedings, Ames, Iowa, , 1976. 4. U.S. Environmental Protection Agency. National Primary Drinking Water Proposed Interim Standards. EPA 816-F-01-07, Office of Water: Washington, DC, 2001; 4, www.epa.gov/safewater. 5. National Academy of Sciences Nutrients and Toxic Substances in Water for Livestock and Poultry. Washington, DC, 1974. 6. Council for Agricultural Science and Technology. Quality of Water for Livestock, Vol. I. Report No. 26; 1974. 7. Faries, F.C.; Sweeten, J.M.; Reagor, J.C. Water Quality: Its Relationship to Livestock. L-2374; Texas Agricultural Extension Service, Texas A&M University: College Station, TX, 1991. 8. Midwest Plan Service, Private Water Systems Handbook; MWPS-14, Iowa State University: Ames, IA, 1979. 9. Sweeten, J.M. Groundwater Quality Protection for Livestock Feeding Operations. Texas Agricultural Extension Service, L-2348; 1991.

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Livestock: Water Harvesting Methods Gary W. Frasier U.S. Department of Agriculture (USDA), Fort Collins, Colorado, U.S.A.

INTRODUCTION Approximately, 40% of the world’s land area is classified as rangeland with over 80% in the arid and semiarid zones.[1] In these rangeland areas, there is usually sufficient water, primarily as precipitation, for plant growth in the form of grasses and small shrubs which are the primary foodstock for herbivores, both wildlife and domestic. While there is sufficient forage for the animals, many of these rangelands cannot be used for livestock production because of inadequate drinking water sources such as streams and springs. Traditionally, supplemental animal drinking water has been supplied by wells, ponds, and in some instances physical water transport. There are areas where even these supplemental water techniques are not available or otherwise unsuitable. One technique of water supply that can be used in most places in the world when other sources are unavailable is a process called water harvesting.

DEFINITION AND BACKGROUND Water harvesting is defined as the collection of precipitation from a prepared area for some beneficial use. Water harvesting for livestock and human drinking water supplies is an ancient practice dating back to the first-half of the Bronze Age, about 4000 yr ago.[2] It is probable that the first water harvesting system for drinking water was nothing more than a simple depression that filled with water running off a rock surface. Even today, in many arid regions, we can find rain-filled depressions in rock outcroppings that provide drinking water for wildlife. In the past 40 yr, there has been a renewed interest in water harvesting as a means of water supply for both livestock and domestic uses. There is no universally ‘‘best’’ method of water harvesting, since each site has its own unique characteristic features: soil type and topography; precipitation quantities and intensities; and water needs, timing, and quantities. The designer, installer, and ultimate user of a water harvesting facility should become as familiar as possible with the available techniques and adapt one that is best suited to the local environment, social, and economic conditions and site features. Many of the elements

of a water harvesting facility are interrelated and must be considered simultaneously. There is a considerable amount of technical literature, which describes or presents information concerning the various techniques of water harvesting. Unfortunately, much of this information is scattered in scientific or technical journals and proceedings of various meetings, and is written in a manner that is difficult to interpret for direct field application by farmers and technicians.[3]

WATER HARVESTING SYSTEM All water harvesting facilities for livestock watering have the same basic components (Fig. 1). The collection area (catchment area) can be a natural hillside, a smoothed soil area, an area treated or covered to reduce water infiltration and increase surface runoff, or even the roofs of buildings. The collected water is stored in some container, pond, or tank until it is needed. There are many ways the catchment area can be modified to reduce water loss by infiltration and increase the quantity of precipitation runoff. These can be separated into three general categories: 1) topography modification; 2) soil modifications; and 3) impermeable coverings or membranes. Table 1 presents a list of some of the more common catchment treatments with their estimated runoff efficiency and life expectancy. Generally, the lower runoff efficiency treatments require storms of higher intensities and total volume to produce significant quantities of runoff. For example, a sheet metal roof will have runoff from storms of lower rainfall intensities and quantities than a catchment of compacted earth. It is usually necessary to increase the size of catchments which have low runoff efficiency treatments compared with the size of a catchment with an impervious surface. Storage techniques for animal drinking water usually involve some form of container, tank, or lined pond. Unlined earthen pits or ponds are not usually satisfactory methods for water harvesting unless seepage losses are low or can be controlled. There are many types, shapes, and sizes of wooden, metal, and reinforced plastic or concrete storage containers. Costs and availability are primary factors for determining 731

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Livestock: Water Harvesting Methods

Karst–Neuse Fig. 1 Typical water harvesting system for livestock water. Source: From Ref.[4].

the suitability of these containers. Containers constructed from concrete wand plaster are relatively inexpensive, but their construction requires a significant amount of hand labor. One common type of storage is a steel rim tank with a concrete bottom.

Table 1 Potential water harvesting catchment treatments Treatment Topography modification Land smoothing and clearing Soil modification Sodium salts Water repellents and paraffin wax Bitumen Impermeable coverings Gravel-covered sheeting Asphalt-fabric membrane Concrete, sheet metal, and artificial rubber Source: Adapted from Ref.[5].

© 2008 by Taylor & Francis Group, LLC

Runoff efficiency (%) 20–35

Because water harvesting is a relatively expensive method of water supply, controlling evaporation losses is an important factor and should be an integral component of all water storage facilities. Although relatively expensive, roofs over the storage are commonly used. Evaporation control on sloping-sided pits or ponds is more difficult because the water-surface area varies with depth.

Estimated life (yr) 5–10

50–80 60–95

5–10 5–8

50–80

2–5

75–95

10–20

85–95

10–20

60–95

10–20

SOCIOECONOMIC CONSIDERATIONS Water harvesting techniques are practical methods of water supply for most parts of the world, but they are also a relatively expensive method of water supply. During the past few decades, there have been numerous water harvesting systems constructed worldwide. While many of the systems have been outstanding successes, others have failed. Some systems failed despite extensive efforts because of material and/or design deficiencies. Others have failed because of personnel changes, communication failures, or because the water was not perceived as needed by the local user. Word-of-mouth publicity of one failure will often spread more widely than all the publicity of 10 successful systems.

REFERENCES 1. Branson, F.A. Watershed Management on Range and Forest Lands, Proceedings of the Fifth Workshop of the United States/Australian Rangelands Panel, Boise Idaho, June 15–22, 1975; Heady, H.F., Falkenborg, D.H., Riley, J.P., Eds.; Utah Water Research Laboratory, Utah State University: Logan, UT, 1976; 196–209. 2. Anaya Garduno, M. Research methodologies for in situ rain harvesting in rainfed agriculture. In Rainfall Collection for Agriculture in Arid and Semiarid Regions, Proceedings of the Workshop University of Arizona, Tucson, Arizona and Chapingo Postgraduate College, Chapingo, Mexico; Dutt, G.R., Hutchinson, C.F., Anaya Garduno, M., Eds.; Commonwealth Agricultural Bureaux: Farnham House, Farnham Royal: Slough SL2 3BN, U.K., 1981; 43–47. 3. Frasier, G.W. Water harvesting for collecting and conserving water supplies. In Alfisols in the Semi-arid

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Tropics, Proceedings of the Consultants’ Workshop on the State of the Art and Management Alternatives for Optimizing the Productivity of SAT Alfisols and Related Soils, ICRISAT Center, India, Dec 1–3, 1983; Pathak, P., El-Swaify, S.A., Singh, S., Eds.; International Crops Research Institute for the Semi-Arid Tropics: Patancheru, Andhra Pradesh 502 324, India, 1987; 67–77. 4. Frasier, G.W.; Myers, L.E. Handbook of Water Harvesting, Agricultural Handbook 600; United States Department of Agriculture, Agricultural Research Service: Washington, DC 20402, 1983; 45 pp. 5. Frasier, G.W. Water for animals, man, and agriculture by water harvesting. In Rainfall Collection for Agriculture in Arid and Semi-arid Regions, Proceedings of the Workshop University of Arizona, Tucson, Arizona and Chapingo Postgraduate College, Chapingo, Mexico; Dutt, G.R., Hutchinson, C.F., Anaya Garduno, M., Eds.; Commonwealth Agricultural Bureaux: Farnham House, Farnham Royal: Slough SL2 3BN, U.K., 1981; 67–77.

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Livestock: Water Harvesting Methods

Karst–Neuse

Low-Impact Development Curtis H. Hinman School of Agricultural, Human and Natural Resource Sciences, Washington State University, Tacoma, Washington, U.S.A.

Derek B. Booth Center for Water and Watershed Studies, University of Washington, Seattle, Washington, U.S.A.

INTRODUCTION Low-Impact Development (LID) is a strategy for storm-water management that uses on-site natural features integrated with engineered, small-scale hydrologic controls to manage runoff by maintaining or closely mimicking predevelopment watershed hydrologic functions.[1] Planning for LID is most effective at the scale of an entire subdivision or watershed; engineering and site design elements, however, are implemented at the scale of individual parcels, lots, or structures. In combination, these actions seek to store, infiltrate, evaporate, or otherwise slowly release storm-water runoff in a close approximation of the rates and processes of the predevelopment hydrologic regime.

TYPICAL CHACTERISTICS OF LOW-IMPACT DEVELOPMENT Most applications of LID have several common components (Fig. 1):  Preserving elements of the natural hydrologic system that are already achieving effective storm-water management, recognized by assessment of a site’s watercourses and soils; channels and wetlands, particularly with areas of overbank inundation; highly infiltrative soils with undisturbed vegetative cover; and intact mature forest canopy.  Minimizing the generation of overland flow by limiting areas of vegetation clearing and soil compaction; incorporating elements of urban design such as narrowed streets, structures with small footprints (and greater height, as needed), use of permeable pavements as a substitution for asphalt/concrete surfaces for vehicles or pedestrians;[3] and using soil amendments in disturbed areas to increase infiltration capacity.  Storing runoff with slow or delayed release, such as in cisterns or distributed bioretention cells, across intentionally roughened landscaped areas or on 734

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vegetated roofing systems (‘‘green roofs’’).[4,5] Runoff storage in LID differs from traditional storm-water management, notably the latter’s use of detention ponds, primarily by its scale—small and distributed in LID, large and centralized in traditional approaches. Native soils play a critical role in storage and conveyance of runoff, particularly in humid regions. In such regions, one to several meters of soil, generally high in organic material and relatively permeable, overlay less permeable substrates of largely unweathered geologic materials. While water is held in this soil layer, solar radiation and air movement provide energy to evaporate surface soil moisture and contribute to the overall evapotranspiration component of the water balance. Water not evaporated, transpired, or held interstitially moves slowly downslope or down gradient as shallow subsurface flow (also called interflow) over many hours, days, or weeks before discharging to streams or other surface-water bodies. In arid regions with relatively lower organic content soils and vegetation cover, precipitation events can produce rapid overland flow response naturally; however, the principles of LID remain—retain native soils, vegetation, topography, and hydrologic regime to preserve aquatic ecosystem structure and function. The transition from a native landscape to a built environment increases the coverage of impervious surface from roads, parking areas, sidewalk, rooftops, and landscaping. The upper soil layers that evaporate, store, or infiltrate storm water are compacted or covered altogether. As a result, the watershed area contributing direct overland flow to streams, lakes, and wetlands increases; hence, precipitation will reach receiving waters much more rapidly and in greater volumes.[6] Typical storm-water management focuses on flood control and thus emphasizes the efficient collection and rapid conveyance of precipitation away from residential and commercial development, commonly to central control ponds. Several factors have led to this

Fig. 1 Schematic of typical LID applications at the scale of a residential building lot. Source: Modified from Ref.[2].

approach: Storm water has been perceived as a liability and applications have evolved from wastewater technology, hard conveyance structures and central control ponds are considered reliable and relatively simple to maintain, and the conveyance and collection approach is relatively simple to model for regulatory requirements. Although newer conveyance and pond strategies, if properly designed and maintained, can manage predevelopment peak flows under some conditions, a number of problems (e.g., increased runoff volume and extended flow durations, conversion of dispersed flow to point discharges) will continue to challenge this traditional strategy of storm-water management.[7]

LIMITATIONS Although the goal of LID to mimic the predevelopment hydrologic regime is laudable, it cannot feasibly be achieved everywhere or at all times. The hydrologic system evolved from, and is dependent on, the characteristics of undisturbed watersheds—mature vegetative cover, uncompacted soils, ungullied hillslopes—whose function cannot be expected to remain unchanged where half or more of the landscape has been appropriated for human uses. Thus the objectives of any given LID must be strategically chosen, recognizing both the opportunities and the limitations of any given site. The limitations are not simply those of subdivision design and development density; they are also limitations of site topography, soil permeability and depth, and groundwater movement. They are likely to be most prominent during periods of extended rainfall, where the distributed on-site infiltration reservoirs common to most LID designs will experience their highest water levels and soil layers approach or reach full saturation. Under such conditions, the downstream impacts of uncontrolled runoff—be it flooding, channel erosion,

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or aquatic habitat disruption—could be as severe as with conventional storm-water control. Regulatory requirements, typical zoning and housing type, and costs of sophisticated control technology required on sites with higher development density and soils with low infiltration rates also create significant challenges for reducing or eliminating hydrologic impacts from development sites. The potential failure of LID to control all flows from all storms, no matter how severe, is not an indictment of the approach as a whole. Indeed, LID probably represents the best opportunity to maintain a ‘‘natural,’’ or at least minimally disrupted, flow regime in an urban watershed. Where downstream flooding or channel erosion are concerns, however, a comprehensive drainage design will need to consider more traditional stormwater management approaches (such as detention ponds or bypass pipelines) in addition to LID applications. Although such ponds would be smaller, they may still be required to achieve human or ecosystem protection at storm recurrence intervals that may be relatively long but still fall within regulatory thresholds.

SITE DESIGN AND MANAGEMENT OBJECTIVES FOR LOW-IMPACT DEVELOPMENT The goals for LID are achieved through the following site objectives:  Maximize retention of mature vegetation cover and restore disturbed vegetation to intercept, evaporate, and transpire precipitation.  Preserve permeable, native soil and enhance disturbed soils to store and infiltrate storm flows.  Retain and incorporate topographic site features that slow, store, and infiltrate storm water.  Retain and incorporate natural drainage features and patterns.  Locate buildings and roads away from critical areas and soils that provide effective infiltration.  Minimize total impervious area and eliminate effective impervious surfaces (‘‘effective’’ impervious surfaces are the subset of the total imperviousness that has a direct hydraulic connection to the stream or wetland).  Manage storm water as close to its origin as possible by utilizing small-scale, distributed hydrologic controls.  Create a hydrologically rough landscape that slows storm flows and increases the time of concentration.  Increase reliability of the storm-water management system by providing multiple or redundant points of control.

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 Integrate storm-water controls into the development design and utilize the controls as amenities, creating a multifunctional landscape.  Utilize an interdisciplinary approach that incorporates planners, engineers, landscape architects, and architects working together from the initial phases of the project.  Reduce the reliance on traditional conveyance and pond technologies.  Provide community education and promote community participation in the protection of LID systems and receiving waters.

Minimization Techniques Minimization techniques reduce impacts of development on the hydrologic regime by reducing the amount of disturbance when preparing a site for development. Instead of grading an entire development site, only the lots and roads are graded while the rest of the ground is left undisturbed. Impervious surfaces are limited to areas where they are absolutely required. Cluster design is used to decrease the amount of the site developed and increase the amount of open space. Graded soils with high infiltration-capacity soils are stockpiled and reused.

These objectives can be grouped into five basic elements that constitute a ‘‘complete’’ LID design:[8] 1. 2. 3. 4. 5.

Conservation measures. Minimization techniques. Flow attenuation. Distributed integrated management practices. Pollution prevention measures.

Although these five elements can be applied to any development, the manner in which they are used must be determined by the local climate and soils.

Flow Attenuation By slowing runoff velocity, the opportunity for stormwater infiltration increases and the magnitude of peak discharges decline. Whereas traditional storm-water management directs water from a site as quickly as possible, LID holds runoff on-site as long as possible without causing flooding or other potential problems.

Conservation Measures

Distributed Integrated Management Practices

Conservation measures maintain as much of the natural landscape as possible. This includes retaining forests and other native vegetation, not filling wetlands, and providing buffers around wetlands and streams. These natural areas can then provide passive stormwater management opportunities, and they also double as open space.

LID incorporates a range of integrated best management practices throughout a site, commonly in sequence. An example of this is connecting a bioretention area to a natural area by conveyance through a grass swale. During high flow events, excess storm water is given the opportunity to continue to infiltrate while flowing from the site.

Fig. 2 Retrofitted residential block in north Seattle, displaying a variety of LID techniques: limited impervious area, amended soils, rain gardens, and bioswales. (Photo courtesy of Seattle Public Utilities.) Source: From Ref.[9].

© 2008 by Taylor & Francis Group, LLC

Pollution Prevention Measures Pollution prevention measures are accomplished through a variety of source control, rather than treatment, approaches. For example, community outreach activities, such as the publication of educational materials, can control pollution by not allowing contaminants to enter the watershed or to be released in the first place. This strategy complements other elements of LID design that also reduce the downstream delivery of pollutants by minimizing stormwater runoff volumes and by promoting filtration through soil.

CONCLUSION Low-Impact Development uses on-site natural features and small-scale hydrologic controls to manage runoff, closely mimicking predevelopment watershed hydrologic functions. Challenges for more widespread use of this approach include uncertainty in its application on relatively non-infiltrative soils, its role in mitigating high-intensity and (or) large-volume storms, and its construction in new or previously developed areas where high urban densities are desired or already present (Fig. 2). Although additional research will be needed to characterize fully the performance of LID practices across different physical settings and development scenarios, available information indicates that LID can fully control flows associated with lowintensity storms, improve water-quality treatment by increasing storm-water contact with soils and vegetation, and significantly reduce flows from largervolume storms on sites with relatively permeable soils. LID is a significant conceptual shift from conventional storm-water management; its broader application should encourage designers to incorporate native

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hydrologic processes as important organizing principles in the development of the urban and suburban landscape.

REFERENCES 1. Environmental Protection Agency. Low Impact Development—A Literature Review: EPA-841-B-00-005; 2000; 35 pp. (available at http://www.epa.gov/owow/ nps/lid/lid.pdf). 2. Puget Sound Action Team. Low Impact Development (Brochure); Olympia: Washington, 2001; 4 pp. (available at http://www.psat.wa.gov/Programs/LID/lid_cd/ brochure.pdf). 3. Brattebo, B.O.; Booth, D.B. Long-term stormwater quantity and quality performance of permeable pavement systems. Water Res. 2003, 37, 4369–4376. 4. Konrad, C.P.; Burges, S.J. Hydrologic mitigation using on-site residential storm-water detention. J. Water Resour. Plan. Manage. 2001, 127, 99–107. 5. Penn State Center for Green Roof Research; College of Agriculture, Pennsylvania State University: State College, PA, 2002. (available at http://hortweb.cas.psu. edu/research/greenroofcenter/). 6. Booth, D.B. Urbanization and the natural drainage system—impacts, solutions, and prognoses. Northwest Environ. J. 1991, 7, 93–118. 7. Booth, D.B.; Jackson, C.R. Urbanization of aquatic systems—degradation thresholds, stormwater detention, and the limits of mitigation. Water Resour. Bull. 1997, 33, 1077–1090. 8. Coffman, L.S. Using low-impact development in stormwater management. Water Environ. Res. Foundation, Progress Newslett. Winter 2001, 12 (1). (available at http://www.werf.org/press/winter01/01w_low.cfm). 9. Seattle Public Utilities Natural Drainage Systems Program. (available at http://www.ci.seattle.wa.us/util/ NaturalSystems/default.htm).

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Manure Management: Beef Cattle Industry Requirements Brent W. Auvermann Texas A&M System Agriculture Research and Extension Center, Amarillo, Texas, U.S.A.

John M. Sweeten Agricultural Research and Extension Center, Texas Agricultural Experiment Station (TAES), Amarillo, Texas, U.S.A.

INTRODUCTION Although cattle drinking water requirements are significant, averaging 10.8 gal hd
1day
1,[1] the beef cattle sector of America’s $100 billion yr
1 animal agriculture industry uses relatively little water for manure management. The vast majority (perhaps 98%) of the nation’s average of 10.3 million head of cattle on feed for slaughter and beef processing are fed in open, soilsurfaced feedyards. As a result, nearly all feedyard manure is collected in solid form. With an average turnover rate of 2–2.3 times per year, over 20 million head of cattle are fed in this manner, generating roughly a ton of as-collected manure per head fed and harvested. In the typical beef cattle feedyard, all manure deposited on the feedlot surface undergoes concurrent processes of: a) partial evaporative drying (from 75 þ % wet basis as-excreted down to 20–50% wet basis as-collected); b) partial decomposition of volatile organic solids; and c) atmospheric release of gaseous compounds that include carbon dioxide, volatile organic compounds, and ammonia.

MANURE COLLECTION AND HANDLING Manure collection practices from open feedyards include use of wheel loaders, box scrapers, dozers, or elevating scrapers.[2] Transportation of collected manure is provided in open top manure spreader trucks to farmland. Intermediate storage may be needed in temporary stacks within feedpens or in stockpiles adjacent to the cattle feedpens. This intermediate storage should be located within the envelope of containment of storm water runoff in accordance with state and federal requirements. The manure mechanically collected in air-dry form is in its least voluminous state and usually has good cash market demand from nearby farmers for use as bulk fertilizer. Collection, marketing, and/or distribution to farmers are handled by contractors in most cases. Most manure is sold and applied within 10–20 mi of the feedyard. Consequently, there is no incentive to add water 738

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to liquefy manure from beef cattle feedlots during collection, storage, treatment, and/or distribution. Because the limiting factor for marketing manure is usually the hauling cost, added water lowers the manure’s net fertilizer value.

WATER USE IN MANURE HANDLING Exceptions to the normal practice of handling of cattle feedlot manure in solid form may include these situations or considerations: 1. Intermittent water additions to compost windrows to raise moisture content to approximately 50% wet basis to initiate or restore active composting. 2. Spillage or leakage of water trough overflow onto a feedyard surface (e.g., water line leaks or trough overflow), which can carry very small quantities of manure solids (e.g., 100 mg as CaCO3 L1 should be removed by preliminary distillation

Acid (needs to be removed by preliminary distillation)

Automated phenate method applicable from 20 mg up to 2 mg NH3-N L1 without dilution

Refrigeration at 4 C if analyzed within 24 hr; Refrigeration at pH ¼ 2 (by H2SO4) or freezing at 20 C for upto 28 days

Does not require preliminary distillation. Known addition can be used when no calibration is needed

Refrigeration in tightly sealed glass or HDPE container in the dark works for up to 3 hr. Concentrated samples >20m L1 can be preserved for 2 wk

Based on the indophenol reaction

Phenate

Drinking, surface, saline waters, domestic and industrial wastewaters

10–600 mg NH3-N L

Magnetic stirrer and spectrophotometer (630 nm and a light path of at least 1 cm)

Titration

Used only after preliminary distillation

For samples >5 mg NH3-N L1

Distillation apparatus

Selective electrode

Drinking and surface waters, domestic and industrial wastewaters

0.03–1400 mg NH3-N L1. Longer response times needed for concentrations 2 mg S L1; turbidity needs to be eliminated

0.3 m L1 to 4.0 mg L1

Sample preservation

Comments

(Continued) © 2008 by Taylor & Francis Group, LLC

Nitrogen Measurement

Equipment

Nitrogen Measurement

Table 2 Methods for measurement of ammonium-nitrogen in waters and wastewaters (Continued) Method Nesslerization (not recommended as standard method)

Applicability Purified drinking water, natural waters, and highly purified wastewater effluents

Range of applicability 1

20 mg to 5 mg NH3-N L

Equipment pH meter and spectrophotometer (400–500 nm, light path of at least 1 cm), or filter photometer (light path of at least 1 cm; violet filter with max transmittance at 400–425 nm), or Nessler tubes

Interferences Turbidity, color, Mg, and Ca can be removed via preliminary distillation or by precipitation by zinc sulfate and alkali

Sample preservation

Comments

Dechlorination, 0.8 mL H2SO4 L1 sample, and storage at 4 C. The pH should be between 1.5 and 2 when acid is used and samples need to be neutralized immediately before analysis

This traditional method is not recommended because of potential problems with mercury disposal

Source: From Ref.[3].

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Nitrogen–Plant Method Ultraviolet spectrophotometric screening

Applicability

Range of applicability

Equipment

Interferences

Spectrophotometer (220 and 270 nm, light path of at least 1 cm)

Dissolved organic matter, surfactants, NO 2, Cr6þ, chlorite, chlorate

Measurement at 270 nm is used to correct for dissolved organic matter that may interfere at 220 nm. U.S. EPA method 0354.1

From 0.1 mg L1

Ion chromatograph, anion separator column, guard column, fiber or membrane suppressor, conductivity detector

Bromide can coelute

U.S. EPA method 0300.0

Used for screening only of uncontaminated natural and drinking waters, i.e., waters with low organic N content

Ion chromatography

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Table 3 Methods for measurement of nitrate-nitrogen in waters and wastewaters Comments

Drinking water

2 mg L1 to 1 1000 NO 3 -N L

Double-junction reference electrode, nitrate ion electrode, pH meter, magnetic stirrer

Chloride and bicarbonate when their weight ratios to NO 3 -N are >10 and >5, respectively;  2  NO 2 , CN , S , Br ,  3 4 I , ClO , ClO

Ionic strength adjustments can remove interferences

Cadmium reduction

All waters

1 0.01–1 mg NO 3 -N L ; For automated method 1 0.5–10 mg NO 3 -N L

Reduction column and spectrophotometer (543 nm, light path of at least 1 cm) or filter photometer (a filter with maximum transmittance near 540 nm and light path of at least 1 cm)

Suspended matter can restrict column flow; concentrations of metals >1 mg L1, oil, grease, residual chlorine can decrease reduction efficiency

This method is also applicable for determination of NO 2 when the reduction step is omitted. This method can also be automated and also combined with flow injection method. U.S. EPA method 0353.3

Gas segmented continuous flow colorimetric analysis

Estuarine and coastal waters, applicable also for nitrite determination

0.075 m L1 to 5.0 mg L1

Automatic sampler, analytical cartridge, open tubular cadmium reactor or cadmium reduction column proportioning pump, spectrophotometer or photometer with a 540 interference filter, nitrogen gas

Hydrogen sulfide >2 mg S L1; turbidity needs to be eliminated

Samples should be analyzed within 3 hr. U.S. EPA method 0353.4

Titanous chlorine reduction

All waters

1 0.01 to 20 mg NO 3 -N L

pH meter, ammonium gas sensing electrode, magnetic stirrer

NH3, NO 2

Proposed method

Automated hydrazine reduction

All waters

1 0.01 to 10 mg NO 3 -N L

Automated analytical equipment

Sample color that absorbs in the photometric range used

Proposed method

Source: From Refs.[3,4]. © 2008 by Taylor & Francis Group, LLC

Nitrogen Measurement

Nitrate electrode

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Table 4 Methods for measurement of organic nitrogen in waters and wastewaters Range of applicability

Equipment

Interferences

Sample preservation

MacroKjeldahl

Either low or high concentrations that require large volume for low concentrations

800 mL digestion apparatus, distillation apparatus, apparatus for ammonium determination

Nitrate in excess of 10 mg L1, large amounts of inorganic salts and solids, large amounts of organic matter

Lowering pH to 1.5–2 with concentrated H2SO4 and refrigeration at 4 C

SemimicroKjeldahl

High concentrations and that sample volume containing organic plus ammonium N between 0.2 and 2 mg

100 mL digestion apparatus, distillation apparatus, pH meter

Nitrate in excess of 10 mg L1, large amounts of inorganic salts and solids, large amounts of organic matter

Lowering pH to 1.5–2 with concentrated H2SO4 and refrigeration at 4 C

Block digestion and flow injection analysis

All waters and wastewaters

Block digestor, digestion tubes, injection valve, multichannel proportioning valve, flow injection manifold, absorbance detector (660 nm, 10-nm band path)

Large and fibrous particles; ammonium

Source: From Refs.[3,4].

nitrate–nitrite using gas segmented continuous flow colorimetric analysis). Organic Nitrogen A summary of methods for determination of organic nitrogen in waters and wastewaters is presented in Table 4. These include macro or semimicro-Kjeldahl method, and block digestion combined with flow injection analysis. Kjeldahl methods do not account for N in the form of azide, azine, azo, hydrazone, nitrate, nitrite, nitrile, nitro, nitroso, oxime, and semicarbazone.[3] If ammonium is not removed in the initial digestion, the result is the ‘‘Kjeldahl nitrogen’’ often called ‘‘total Kjeldahl nitrogen’’ that is defined as organic plus ammonium N.

CONCLUSION Measurements of N content in waters and wastewaters are of great interest because they relate to many natural and anthropogenic processes and their effects including human, animal, and plant health and wellbeing. Standard analytical methods available for the determination of various forms of N, including total N, ammonium, nitrite, nitrate, and organic N, are briefly discussed in this article. Nitrogen detection limits and applicable concentration ranges for these methods cover the typical levels encountered in water and wastewater. The reader is encouraged to use the full standard method description for all described methods in this article.[4–6]

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ACKNOWLEDGMENT The author would like to thank Drs. Robert Schwartz and N. Andy Cole (USDA-ARS-Bushland) and Drs. David Parker and Jim Rogers (West Texas A&M University) for useful discussions of N measurement methods.

REFERENCES 1. Budavari, S.; Ed. The Merck Index: An Encyclopedia of Chemicals, Drugs, and Biologicals, 12th Ed.; Merck Research Laboratories: Whitehouse Station, NJ, 1996. 2. Greyson, J. Carbon, Nitrogen, and Sulfur Pollutants and Their Determination in Air and Water; Marcel Dekker, Inc.: New York, NY, 1990. 3. Clesceri, L.S.; Greenberg, A.E.; Trussell, R.R. Selected Physical and Chemical Standard Methods for Students: Based on Standard Methods for Examination of Water and Wastewater, 17th Ed.; American Public Health Association, American Water Works Association, Water Pollution Control Federation: Washington, DC, 1990. 4. American Public Health Association, American Water Works Association, Water Environment Federation, Standard Methods for Examination of Water and Wastewater, 20th Ed.; American Public Health Association, Washington, DC, 1999. 5. U.S. Environmental Protection Agency, Methods for Chemical Analysis of Water and Wastes; EPA/6/4-79020; U.S. Government Printing Office: Washington, DC, 1993. 6. U.S. Environmental Protection Agency. Test Methods for Evaluating Solid Waste, Physical/Chemical Methods. Accessed http://www.epa.gov/epaoswer/hazwaste/test/ sw846.htm on May 18, 2002.

Nitrogen–Plant

Method

Nonpoint Source Pollution (NPSP)

Nitrogen–Plant

Ray Correll Peter Dillon Rai Kookana Mallavarapu Megharaj Ravendra Naidu Commonwealth Scientific and Industrial Research Organisation (CSIRO), Adelaide, South Australia, Australia

Walter W. Wenzel University of Agricultural Sciences, Vienna, Austria

INTRODUCTION Non-point source pollution (NPSP) has no obvious single point source discharge and is of diffuse nature (Table 1). An example of NPSP includes aerial transport and deposition of contaminants such as SO2 from industrial emissions leading to acidification of soil and water bodies. Rain water in urban areas could also be a source of NPSP as it may concentrate organic and inorganic contaminants. Examples of such contaminants include polycyclic aromatic hydrocarbons, pesticides, polychlorinated biphenyls that could be present in urban air due to road traffic, domestic heating, industrial emissions, agricultural treatments, etc.[1–3] Other examples of NPSP include fertilizer (especially Cd, N, and P) and pesticide applications to improve crop yield. Use of industrial waste materials as soil amendments have been estimated to contaminate thousands of hectares of productive agricultural land in countries throughout the world.

CONTAMINANT INTERACTIONS Non-point pollution is generally associated with lowlevel contamination spread at broad acre level. Under these circumstances, the major reaction controlling contaminant interactions are sorption–desorption processes, plant uptake, surface runoff, and leaching. However, certain contaminants, in particular, organic compounds are also subjected to voltalization, chemical, and biological degradation. Sorption–desorption and degradation (both biotic and abiotic) are the two most important processes controlling organic contaminant behavior in soils. These processes are influenced by both soil and solution properties of the environment. Such interactions also determine the bioavailability and/or transport of contaminants 778

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in soils. Where the contaminants are bioavailable, risk to surface and groundwater and soil, crop, and human health are enhanced.

IMPLICATIONS TO SOIL AND ENVIRONMENTAL QUALITY Environmental contaminants can have a deleterious effect on non-target organisms and their beneficial activities. These effects could include a decline in primary production, decreased rate of organic matter break-down, and nutrient cycling as well as mineralization of harmful substances that in turn cause a loss of productivity of the ecosystems. Certain pollutants, even though present in very small concentrations in the soil and surrounding water, have potential to be taken up by various micro-organisms, plants, animals, and ultimately human beings. These pollutants may accumulate and concentrate in the food chain by several thousand times through a process referred to as biomagnification. Urban sewage, because of its nutrient values and source of organic carbon in soils, is now increasingly being disposed to land. The contaminants present in sewage sludge (nutrients, heavy metals, organic compounds, and pathogens), if not managed properly, could potentially affect the environment adversely. Dumping of radioactive waste (e.g., radium, uranium, plutonium) onto soil is more complicated because these materials remain active for thousands of years in the soil and thus pose a continued threat to the future health of the ecosystem. Industrial wastes, improper agricultural techniques, municipal wastes, and use of saline water for irrigation under high evaporative conditions result in the presence of excess soluble salts (predominantly Na and Cl ions) and metalloids such as Se and As in soils.

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Table 1 Industries, land uses, and associated chemicals contributing to non-point source pollution Type of chemical

Agricultural activities

Metals/metalloid

Cadmium, mercury, arsenic, selenium

Non-metals

Nitrate, phosphate, borate

Salinity/sodicity

Sodium, chloride, sulfate, magnesium, alkalinity

Pesticides

Range of organic and inorganic pesticides including arsenic, copper, zinc, lead, sulfonylureas, organochlorine, organophosphates, etc., salt, geogenic contaminants (e.g., arsenic, selenium, etc.)

Irrigation

Sodium, chloride, arsenic, selenium

Dust

Lead, arsenic, copper, cadmium, zinc, etc.

Gas

Sulfur oxides, carbon oxides

Automobile and industrial emissions

Rainwater

Associated chemicals

Metals

Lead and lead organic compounds

Organics

Polyaromatic hydrocarbons, polychlorbiphenyls, etc.

Inorganic

Sulfur oxides, carbon oxides acidity, metals and metalloids

Source: (From Barzi, F.; Naidu, R.; McLaughlin, M.J. Contaminants and the Australian Soil Environment. In Contaminants and the Soil Environment in the Australasia-Pacific Region; Naidu, R., Kookana, R.S., Oliver, D., Rogers, S., McLaughlin, M.J., Eds.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1996; 451–484.)

Salinity and sodicity affect the vegetation by inhibiting seed germination, decreasing permeability of roots to water, and disrupting their functions such as photosynthesis, respiration, and synthesis of proteins and enzymes. Some of the impacts of soil pollution migrate a long way from the source and can persist for some time. For example, suspended solids can increase water turbidity in streams, affecting benthic and pelagic aquatic ecosystems, filling reservoirs with unwanted silt, and requiring water treatment systems for potable water supplies. Phosphorus attached to soil particles, which are washed from a paddock into a stream, can dominate nutrient loads in streams and down-stream water bodies. Consequences include increases in algal biomass, reduced oxygen concentrations, impaired habitat for aquatic species, and even possible production of cyanobacterial toxins, with series impacts for humans and livestock consuming the water. Where waters discharge into estuaries, N can be the limiting factor for eutrophication; estuaries of some catchments where fertilizer use is extensive have suffered from excessive sea grass and algal growth. More insidious is the leaching of nutrients, agricultural chemicals, and hydrocarbons to groundwater. Incremental increases in concentrations in groundwater may be observed over long periods of time resulting in initially potable water becoming undrinkable and then some of the highest valued uses of the resource may be lost for decades. This problem is most severe on tropical islands with shallow relief and some deltaic arsenopyrite deposits, where wells cannot be deepened to avoid polluted groundwater because

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underlying groundwater is either saline or contains too much As.

SAMPLING FOR NON-POINT SOURCE POLLUTION The sampling requirements of NPSP are quite different from those of the point source contamination. Typically, the sampling is required to give a good estimate of the mean level of pollution rather than to delineate areas of pollution. In such a situation, sampling is typically carried out on a regular square or a triangular grid. Furthermore, gains may be possible by using composite sampling.[4] However, if the pollution is patchy, other strategies may be used. One such strategy is to divide the area into remediation units, and to sample each of these. The possibility of movement of the pollutant from the soil to some receptor (or asset) is assessed, and the potential harm is quantified. This process requires an analysis of the bioavailability of the pollutant, pathway analysis, and the toxicological risk. The risk analysis is then assessed and decisions are then made as to how the risk should be managed.

MANAGEMENT AND/OR REMEDIATION OF NON-POINT SOURCE POLLUTION The treatment strategies used for managing NPSP are generally those that modify the soil properties to decrease the bioavailable contaminant fraction.

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This is particularly so in the rural agricultural environment where soil–plant transfer of contaminants is of greatest concern. Soil amendments commonly used include those that change the ion-exchange characteristics of the colloid particles and those that enhance the ability of soils to sorb contaminants. An example of NPSP management includes the application of lime to immobilize metals because the solubility of most heavy metals decreases with increasing soil pH. However, this approach is not applicable to all metals, especially those that form oxyanions—the bioavailability of such species increases with increasing pH. Therefore, one of the prerequisites for remediating contaminated sites is a detailed assessment of the nature of contaminants present in the soil. The application of a modified aluminosilicate to a highly contaminated soil around a zinc smelter in Belgium was shown to reduce the bioavailability of metals thereby reducing the Zn phytotoxicity.[5] The simple addition of rock phosphates to form Pb phosphate has also been demonstrated to reduce the bioavailability of Pb in aqueous solutions and contaminated soils due to immobilization in the metal.[6] Nevertheless, there is concern over the long-term stability of the processes. The immobilization process appears attractive currently given that there are very few cheap and effective in situ remediation techniques for metal-contaminated soils. A novel, innovative approach is using higher plants to stabilize, extract, degrade, or volatilize inorganic and organic contaminants for in situ treatment (cleanup or containment) of polluted topsoils.[7]

PREVENTING WATER POLLUTION The key to preventing water pollution from the soil zone is to manage the source of pollution. For example, nitrate pollution of groundwater will always occur if there is excess nitrate in the soil at a time when there is excess water leaching through the soil. This suggests that we should aim to reduce the nitrogen in the soil during wet seasons and the drainage through the soil. Local research may be needed to demonstrate the success of best management techniques in reducing nutrient, sediment, metal, and chemical exports via surface runoff and infiltration to groundwater. Production figures from the same experiments may also convince local farmers of the benefits of maintaining nutrients and chemicals where needed by a crop rather than losing them off site, and facilitate uptake of best management practices.

© 2008 by Taylor & Francis Group, LLC

Nonpoint Source Pollution (NPSP)

GLOBAL CHALLENGES AND RESPONSIBILITY The biosphere is a life-supporting system to the living organisms. Each species in this system has a role to play and thus every species is important and biological diversity is vital for ecosystem health and functioning. The detection of hazardous compounds in Antarctica, where these compounds were never used or no man has ever lived before, indicates how serious is the problem of long-range atmospheric transport and deposition of these pollutants. Clearly, pollution knows no boundaries. This ubiquitous pollution has had a global effect on our soils, which in turn has been affecting their biological health and productivity. Coupled with this, over 100,000 chemicals are being used in countries throughout the world. Recent focus has been on the endocrine disruptor chemicals that mimic natural hormones and do great harm to animal and human reproductive cycles. These pollutants are only a few examples of contaminants that are found in the terrestrial environment.

REFERENCES 1. Chan, C.H.; Bruce, G.; Harrison, B. Wet deposition of organochlorine pesticides and polychlorinated biphenyls to the great lakes. J. Great Lakes Res. 1994, 20, 546–560. 2. Lodovici, M.; Dolara, P.; Taiti, S.; Del Carmine, P.; Bernardi, L.; Agati, L.; Ciappellano, S. Polycyclic aromatic hydrocarbons in the leaves of the evergreen treeLaurus Nobilis. Sci. Total Environ. 1994, 153, 61–68. 3. Sweet, C.W.; Murphy, T.J.; Bannasch, J.H.; Kelsey, C.A.; Hong, J. Atmospheric deposition of PCBs into green bay. J. Great Lakes Res. 1993, 18, 109–128. 4. Patil, G.P.; Gore, S.D.; Johnson, G.D. Manual on Statistical Design and Analysis with Composite Samples; Technical Report No. 96-0501; EPA Observational Economy Series Center for Statistical Ecology and Environmental Statistics; Pennsylvania State University, 1996; Vol. 3. 5. Vangronsveld, J.; Van Assche, F.; Clijsters, H. Reclamation of a bare industrial area, contaminated by non-ferrous metals: in situ metal immobilisation and revegetation. Environ. Pollut. 1995, 87, 51–59. 6. Ma, Q.Y.; Logan, T.J.; Traina, S.J. Lead immobilisation from aqueous solutions and contaminated soils using phosphate rocks. Environ. Sci. Technol. 1995, 29, 1118–1126. 7. Wenzel, W.W.; Adriano, D.C.; Salt, D.; Smith, R. Phytoremediation: a plant-microbe based remediation system. In Bioremediation of Contaminated Soils; Soil Science Society of America Special Monograph No. 37, Adriano, D.C., Bollag, J.M., Frankenberger, W.T., Jr., Sims, W.R., Eds.; Soil Science Society of America: Madison, USA, 1999; 772 pp.

Nutrients: Best Management Practices Keith A. Kelling Nitrogen–Plant

Department of Soil Science, University of Wisconsin–Madison, Madison, Wisconsin, U.S.A.

Scott J. Sturgul Nutrient and Pest Management, University of Wisconsin–Madison, Madison, Wisconsin, U.S.A.

INTRODUCTION

Establish Nutrient Application Rates

Soil nutrients need to be managed properly to meet the fertility requirements of crops without adversely affecting the quality of water resources. The nutrients of greatest concern relative to water quality are nitrogen (N) and phosphorus (P). Nitrogen not recovered by crops can add nitrate to groundwater through leaching. Nitrate is the most common groundwater contaminant found in the United States.[1,2] Nitrate levels that exceed the established U.S. drinking water standard of 10 ppm nitrate-N have the potential to adversely affect the health of infants and livestock.[3] Surface water quality is the concern with P, as runoff and erosion from cropland add nutrients to water bodies that stimulate the excessive growth of aquatic weeds and algae. Of all crop nutrients, it is critical to prevent P from reaching lakes and streams since the biological productivity of aquatic plants and algae in fresh water environments is usually limited by this nutrient.[4] Consequences of increased aquatic plant and algae growth include reduced aesthetic and recreational value of lakes and streams as well as the seasonal depletion of water dissolved oxygen content, which may result in fish kills as well as other ecosystem disruptions.

The most important management practice for environmentally and economically sound nutrient management is the application rate.[5] Optimum nutrient application rates are identified through fertilizer response/calibration research for specific soils and crops. Economically optimum nutrient application rates provide maximum financial return, but as application rates near the economic optimum, the efficiency of nutrient use by the crop decreases and the potential for loss to the environment increases. Any nutrient application above this rate reduces profit and increases the likelihood of detrimental impact to the environment. Because of the overall importance of nutrient application rates, accurate assessments of crop nutrient needs are essential for minimizing threats to water quality while maintaining economically sound production. Soil testing is the most widely used method to accurately estimate nutrient needs of crops.

OVERVIEW Nutrient best management practices vary widely from one area to another due to cropping, topographical, environmental, and economic conditions. With the variety of factors to consider, no single set of best management practice can be recommended for all farms. Nutrient management practices for optimizing crop production while protecting water quality must be tailored to the unique conditions of individual farms. Practices that need to be considered in any nutrient best management program include the following.

Use Additional Tests for Fine-Tuning Nitrogen Applications The development of tests for assessing soil N levels provides additional tools for improving the efficiency of N fertilizer applications.[6] These tests allow fertilizer recommendations to be adjusted to site-specific conditions that can influence N availability. Tests include the preplant soil profile nitrate test,[7] the presidedress soil nitrate test,[8] plant analysis,[9] chlorophyll meters,[10] the basal stalk nitrate test,[11] and the end of season soil nitrate test.[12]

Use Calibrated Soil Tests for Phosphorus and Potassium In recent years, soil test recommendation programs for phosphorus (P), potassium (K), and other relatively 781

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immobile nutrients have tended to de-emphasize the soil build-up and maintenance philosophy in favor of a better balance between environmental and economic considerations by using a crop sufficiency approach.[13] These tests must be calibrated by field experiments to obtain predictable crop yield responses. Such an approach adds extra emphasis on regular soil testing. It is recommended that soil tests be taken at least every three to four years and more frequently on sands and other soils of low buffering capacity.[14]

Establish Realistic Yield Goals For many soil fertility programs, the recommendation of appropriate nutrient application rates is dependent on the establishment of realistic yield goals. Yield goal estimates that are too low will underestimate nutrient needs and can limit crop yield. Yield goal estimates that are too high will overestimate crop needs and result in soil nutrient levels beyond that needed by the crop which, in turn, has the potential to increase nutrient contributions to water resources.[15,16] Estimates should be based on field records and some cautious optimism—perhaps 10% above the recent three- to five-year average crop yield from a particular field.

Credit Nutrients from All Sources The best integration of economic return and environmental quality protection is provided by considering nutrients from all sources. In the determination of supplemental fertilizer application rates, it is critical that nutrient contributions from manure, previous legume crops grown in rotation, and land-applied organic wastes are credited. In many cases, commercial fertilizer application rates can be reduced when nutrient credits are accounted.

Time Nutrient Applications Appropriately Timing of application is a major consideration for the management of mobile nutrients such as nitrogen. The period between application and crop uptake of N is an important factor affecting the efficient utilization of N by the crop with the loss of N minimized by supplying it just prior to the period of greatest crop uptake.[17] However, several considerations, such as soil, equipment, and labor, are involved in determining the most convenient, economical, and environmentally safe N fertilizer application time. Although fall applications

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Nutrients: Best Management Practices

of N are commonly discouraged, they continue to be made primarily to ensure adequate time for spring planting. If fall applications of N are to be made, it is recommended that ammonium–nitrogen sources be used and that the applications be delayed until soil temperatures are below thresholds of biological activity (i.e., 50 F). Fall applications of N fertilizers are not recommended on coarse textured soils or on shallow soils over fractured bedrock. For less mobile nutrients, application timing is not a major factor affecting water quality protection. However, nutrient applications on frozen sloping soils or surface applications prior to periods likely to produce runoff events should be avoided to prevent P contributions to surface waters.

Use Nitrification Inhibitors When Appropriate Nitrification inhibitors are used with ammonium or ammonium-forming N fertilizers to improve N efficiency by slowing the conversion of ammonium to nitrate, thereby reducing the potential for losses of N that occur in the nitrate form. The effectiveness of a nitrification inhibitor depends greatly on soil type, time of the year applied, N application rate, and soil moisture conditions that exist between the time of application and the time of N uptake by plants. Research has shown that the use of nitrification inhibitors on medium- and fine-textured soils with fall N applications, or on poorly drained soils with fall or spring N applications, or on coarse-textured, irrigated soils with spring preplant N applications has the potential to increase corn yield and total crop recovery of N.[18,19] Fall applications of N with an inhibitor on coarse textured soils are not recommended.

Manage Manure to Maximize Benefits Manure applications to cropland provide nutrients essential for crop growth, add organic matter to soil, and improve soil physical conditions. The major concerns associated with manure applications are related to its potential for overloading soils with nutrients if manure applications exceed crop needs, or its application at times of the year when the risk of runoff losses are high. Recommended management practices include accounting for (or crediting) the nutrients supplied by manure, incorporating or injecting manure, distributing manure over numerous cropland fields, minimizing applications to frozen soils, avoiding fall applications to highly permeable soils, and avoiding applications to areas with direct access to surface

water (i.e., floodplains, waterways, etc.), or groundwater (i.e., shallow or permeable soils over fractured bedrock, etc.).[20] Manage Irrigation Water Overirrigation or rainfall on recently irrigated soils can leach nitrate and other contaminants below the root zone and into groundwater. Accurate irrigation scheduling that considers soil water holding capacity, crop growth stage, evapotranspiration, rainfall, and previous irrigation to determine the timing and amount of irrigation water to be applied can reduce the risk of leaching losses.[21] Use Soil Conservation Practices Land-use activities associated with agriculture often increase the susceptibility for runoff and sediment transport from cropland to surface waters. The key to minimizing nutrient contributions to surface waters is to reduce the amount of runoff and eroded sediment reaching them. Runoff and erosion control practices range from changes in agricultural land management (cover crops, diversified crop rotations, conservation tillage, contour farming, and contour strip cropping) to the installation of structural devices (diversions, grade stabilization structures, grassed waterways, and terraces). Recently, substantial emphasis is being placed on the benefits and installation of buffer strips which are effective in reducing contaminant transport to surface waters.[22]

CONCLUSION The previous text provides a brief summary of general nutrient management practices for crop production. This is not a complete inventory but rather an overview of soil fertility management options available to growers for improving farm profitability and protecting water quality. The selection of appropriate nutrient management practices for an individual farm needs to be tailored to the specific conditions existing at a given location.

REFERENCES 1. Hallberg, G.R. Agrichemicals and water quality. Proceedings of the Colloquium to Protect Water Quality, Board of Agriculture, National Research Council: Washington, DC, 1986.

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2. Madison, R.J.; Brunett, J.O. Overview of the occurrence of nitrate in groundwater of the United States. US Geological Service National Water Summary; Water-Supply Paper 2275; US Government Printing Office: Washington, DC, 1984; 93–105. 3. US Environmental Protection Agency. Water Programs—National Interim Primary Drinking Water Regulations, US Federal Register, v. 40, 1975. 4. Killorn, R.; Voss, R.; Baker, J.L. Plant Nutrients as Potential Pollutants; Iowa St. Univ. Coop. Extn. Serv. Bull.PM-; Iowa Cooperative Extension Service: Ames, IA, 1985; 6 pp. 5. Bundy, L.G. Nitrogen management for groundwater protection and efficient crop use. Proceedings of 1987 Wis. Fert., Aglime & Pest Mgmt. Conf., Madison, WI, Jan 1987; University of Wisconsin: Madison, WI, 1987; Vol. 26, 254–262. 6. Stanford, G. Assessment of soil nitrogen availability. In Nitrogen in Agricultural Soils; Stevenson, F.J., Ed.; American Society of Agronomy: Madison, WI, 1982; 651–688. 7. Bundy, L.G.; Malone, E.S. Effect of residual profile nitrate on corn response to applied nitrogen. Soil Sci. Soc. Am. J. 1988, 52, 1377–1383. 8. Bundy, L.G.; Meisinger, J.J. Nitrogen availability indices. Methods of Soil Analysis, Part 2. Microbiological and Biochemical Properties; Soil Science Society of America: Madison, WI, 1994; 951–984. 9. Westerman, R.E. Soil Testing and Plant Analysis, 3rd Ed.; Soil Science Society of America: Madison, WI, 1990. 10. Peterson, T.A.; Blackmer, T.M.; Francis, D.D.; Schepers, J.S. Using a Chlorophyll Meter to Improve N Management. Univ. Neb. Coop. Extn. Ser. Bull. 693-A; Nebraska Cooperative Extension Service: Lincoln, NE, 1993. 11. Binford, G.D.; Blackmer, A.M.; El-Hout, N.M. Tissue test for excessive nitrogen during corn production. Agron. J. 1990, 82, 124–129. 12. Combs, S.M.; Bundy, L.G.; Andraski, T.W. In-season tests for improving corn N management. Proceedings of the 1995 Wisconsin Fertilizer Dealer Meetings, Dec 1995; University of Wisconsin: Madison, WI, 1995; 7–95. 13. Olson, R.A.; Frank, K.D.; Grabowski, P.H.; Rehm, G.W. Economic and agronomic impacts of various philosophies of soil testing. Agron. J. 1982, 74, 492–499. 14. Kelling, K.A.; Bundy, L.G.; Combs, S.M.; Peters, J.B. Soil Test Recommendations for Field, Vegetable, and Fruit Crops; Univ. Wis. Coop. Extn. Ser. Bull. A2809; Wisconsin Cooperative Extension Service: Madison, WI, 1998. 15. Schepers, J.S.; Frank, K.D.; Bourg, C. Effect of yield goal and residual soil nitrogen considerations on nitrogen fertilizer recommendations for irrigated maize in Nebraska. J. Fert. Issues 1986, 3, 133–139. 16. Vanotti, M.B.; Bundy, L.G. Corn nitrogen recommendations based on yield response data. J. Prod. Agric. 1994, 7, 249–265.

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17. Jokela, W.E.; Randal, G.W. Fate of fertilizer nitrogen as affected by time and rate of application on corn. Soil Sci. Soc. Am. J. 1997, 61, 1695–1703. 18. Malzer, G.L.; Kelling, K.A.; Schmitt, M.A.; Hoeft, R.G.; Randall, G.W. Performance of dicyandiamide in the North Central States. Commun. Soil Sci. Plant Anal. 1989, 20, 2001–2022. 19. Hoeft, R.G. Current status of nitrification inhibitor use in U.S. agriculture. In Nitrogen in Crop Production; Hanck, R.D., Ed.; American Society of Agronomy: Madison, WI, 1984.

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20. Madison, F.W.; Kelling, K.A.; Massie, L.; Ward-Good, L. Guidelines for Applying Manure to Cropland and Pastures in Wisconsin; Univ. Wis. Coop. Extn. Ser. Bull. A3392; Wisconsin Cooperative Extension Service: Madison, WI, 1998. 21. Curwen, D.; Massie, L.R. Potato irrigation scheduling in Wisconsin. Am. Potato J. 1984, 61, 235–241. 22. Schmitt, T.J.; Bosskey, M.G.; Hoagland, K.D. Filter strip performance and processes for different vegetation, widths, and contaminants. J. Environ. Qual. 1999, 28, 1479–1489.

Observation Wells

Department of Geosciences, University of Arkansas, Fayetteville, Arkansas, U.S.A.

INTRODUCTION Observation wells are engineered openings constructed through the solid earth, usually circular in crosssection, that are drilled or otherwise excavated to allow human access to specific zones of underground water for the purpose of measuring attributes such as water levels or pressure changes that would otherwise not be observable at the Earth’s surface. Wells constructed to sample groundwater quality have been called observation wells by some hydrogeologists, but common accepted practice restricts the term monitoring wells to wells from which water samples may be collected for chemical analysis.[1] Groundwater-level monitoring networks incorporate multiple observation wells, and maps constructed from these water levels allow hydrogeologists and engineers to determine the direction of the subsurface flow. Observation wells are occasionally called ‘‘piezometers,’’ although most hydrogeologists consider piezometers to be a special type of observation well in which a simple tube emplaced in an aquifer, open at the top for access and measurement and open at the bottom to allow communication with the aquifer.[2] No matter what the name used for these features, observation wells are windows to the groundwater system, and they allow us to collect in situ information from which we can develop an understanding of the degree of interconnection of openings in the aquifer, calculate the amount of water that is stored in the subsurface openings, and determine the hydraulic head and pressure gradients present in the subsurface which control groundwater flow and rates of movement.

BASIC CONSTRUCTION Observation wells differ from groundwater monitoring and water-supply wells primarily in their objectives, and thus usually are constructed to be only large enough to allow accurate and rapid water-level measurements.[3–5] Observation wells typically are of small diameter, 2–10 cm, open to a discrete interval in an aquifer and have a shorter section of screen or other openings than would be desirable for a pumping, monitoring, or production well. Otherwise, all wells are constructed in the same general manner, by boring

a hole, inserting into the borehole a string of solid pipe (casing), on the bottom of which are openings (perforations, slots, or screen) that allow water from a specific subsurface zone into the well (Fig. 1). Porous-media filter pack, typically sand, is added to the annulus of the borehole and the screen, facilitating hydraulic connection between the aquifer and the observation well while minimizing the impact of the well on the aquifer. Above the filter pack, an impermeable material (typically bentonite, other clays, and concrete) is added for two reasons: to seal the casing to the rock material through which the well was drilled, and more importantly, to prevent water from filtering vertically up or down the borehole outside the casing. The casing is covered with a vented well cap, which keeps potential contaminants out of the well while allowing the air above the water in the well to maintain equilibrium with the atmosphere.[4,5] Inside a well-constructed observation well, the water level is free to fluctuate in response to natural and human-induced stresses on the aquifer system.

INFORMATION PROVIDED Observation wells allow scientists to gain knowledge of the energy and mass of water beneath the surface of the earth, the degree of void interconnection within the underlying rocks, and rates and directions of flow.[6] Knowledge of the energy and mass contained in a groundwater system is critical to understanding and managing that system as a resource. Hydraulic head is a measure of the energy available to move water in the subsurface and, when coupled with saturated thickness and other aquifer characteristics, the mass of water present may be determined. A water level measured in an observation well under static, non-pumping conditions is a measure of hydraulic head in the aquifer at the depth of the open or screened interval.[3] Hydraulic head is the height at which a column of water stands above a reference elevation, most commonly sea level (Fig. 1). Water moves from points of high head to points of low head. The difference in hydraulic head divided by the distance between two observation wells in which the heads are measured is the hydraulic gradient, and hydraulic gradient is a 785

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Phillip D. Hays John Van Brahana

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Observation Wells

Nitrogen–Plant Fig. 1 Typical observation well design.

controlling factor for calculating the rate at which water moves in the subsurface. Water-level measurements made in two or more observation wells at the same time can define local hydraulic gradients. Waterlevel measurements made in three or more wells at the same time can define water movement direction and gradient. Water-level measurements made at different times in the same observation well can define temporal trends in water level in response to short- or long-term stresses such as pumping, changing landuse, or natural variations in climate. Observation wells are a critical component of aquifer testing.[1,2,4,7–9] By relating the water-level response of the aquifer in the observation well to known stresses at differing distances, it is possible to compute hydraulic characteristics of the aquifer. These hydraulic characteristics, the interconnectivity of openings or ‘‘hydraulic conductivity,’’ the amount of water stored or ‘‘storativity,’’ and the amount of water that may leak vertically or ‘‘leakance,’’ allows us to quantitatively test our understanding of how the system works. This understanding allows us to evaluate suitability of the aquifer for potential uses, especially with respect to computing how much water the aquifer will yield. This computation requires knowledge of the distance between the pumped well and the observation well, measured radially as a length, r, and it allows reconstruction of the cone of depression. The cone of depression is a roughly conical area of decreasing water levels in an aquifer outward from the well from which water is being withdrawn (Fig. 2). The cone of depression results from pumping a specified discharge, Q from the aquifer, and is related to the ‘‘interconnectivity’’ of the openings of the aquifer, or in scientific terms, the hydraulic conductivity. During an aquifer test, water level is measured on a high-frequency periodic basis (late in the test measurements may be taken less frequently) using a steel or electrical tape or

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continuously using an automatic sensing device. Q, r, and h—hydraulic head or water level—are essential for computation of the storativity, S, a measure of the storage capacity of the aquifer and the ability of the aquifer to provide water. Depending on the hydrogeologic conditions of the flow system (confined and unconfined), the proper equation with the radius squared reflects the quantitative relation between pumping and the exact symmetry of the cone (Fig. 2). Without observation wells, such computations would not be possible. Observations wells are also critical components of local- to regional-scale groundwater-level monitoring networks.[1–3,6,10,11] Subsurface hydrologic systems typically are heterogeneous and complex. Comprehensive understanding of water-level conditions for an aquifer are derived through development of an observation well network in which information is gathered from many observation wells distributed across an area; water levels for locations between observation wells may then be derived by interpolation. These data provide the basis for generating 3-D maps of the water table or potentiometric surfaces, and are an important tool for water-resources planning and management. Because water levels vary with time, water-level network observation wells may be measured continuously or more commonly, only occasionally. Periodic waterlevel measurements are made at scheduled intervals, typically by manual means such as by steel or electric tape. Continuous groundwater-level data are measured by an automatic sensing device such as a pressure transducer or a float; these data are recorded by data loggers or recorders and retrieved periodically. Recently, acute responses of aquifers to intensive groundwater use and frequent drought have created a need for more rapid management decisions and action, and thus more timely data from observation wells. In response to this need, data from observation

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Observation Wells

Fig. 2 Drawdown and cone of depression resulting from pumping groundwater. The observation well at radial distance r from the pumping well allows computation of the shape and dimensions of the cone, as well as important hydraulic characteristics of the aquifer.

wells are increasingly continuously measured at the well, transmitted via telemetric equipment (typically satellite or land-line), and processed at a central location to be made available on a real-time basis over the Internet.[11]

REFERENCES 1. Freeze, R.A.; Cherry, J.A. Groundwater; Prentice-Hall, Inc.: Englewood Cliffs, NJ, 1979; 1–604. 2. Dominico, P.A.; Schwartz, F.W. Physical and Chemical Hydrogeology, 2nd Ed.; John Wiley and Sons: New York, 1998; 1–506. 3. Taylor, C.J.; Alley, W.M. Ground-water level monitoring and the importance of long-term water-level data. U.S. Geological Survey Circular 1217, 2001; 68 pp. 4. Driscoll, F.G.; Ed. Groundwater and Wells, 2nd Ed.; Johnson Division: St. Paul, MN, 1986; 547–549.

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5. Bohn, C.C. Guide for Fabricating and Installing Shallow Ground Water Observation Wells; Res. Note RMRS-RN-9; U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station: Ogden, UT, 2001; 5 pp. 6. Heath, R.C. Design of ground-water level observationwell programs. Ground Water 1976, 14 (2), 71–77. 7. Lohman, S.W. Ground-water hydraulics. U.S. Geological Survey Professional Paper 708, Washington, DC, 1972; 70 pp. 8. Walton, W.C. Selected analytical methods for well and aquifer evaluation. Illinois State Water Survey, Bulletin 49, Urbana, IL, 1962; 81 pp. 9. Ferris, J.G.; Knowles, D.B.; Brown, R.H.; Stallman, R.W. Theory of aquifer tests. U.S. Geological Survey Water Supply Paper 1536-E, 1962; 69–174. 10. http://etd.pnl.gov:2080/hydroweb.html. 11. Cunningham, W.L. Real-time ground-water data for the nation. U.S. Geological Survey Fact Sheet 090-01, 2001; 4 pp. url: http://pubs.water.usgs.gov/fs-090-01.

On-Farm Irrigation Flow Measurement Lawrence J. Schwankl Nitrogen–Plant

Department of Land, Air and Water Resources (LAWR), University of California–Davis, Davis, California, U.S.A.

INTRODUCTION

Propeller Flowmeters

Measurement of on-farm irrigation water plays a key role in irrigation water management. Efficient irrigation requires knowledge of both the crop water demand and the applied irrigation water. Measurement of the applied irrigation water at the farm level should, therefore, be done routinely as a component of good irrigation water management. On-farm water measurements can be conveniently discussed as measurement techniques under pipeline flow conditions and under open-channel flow conditions. The following sections describe practical, on-farm, flow measurement techniques to measure flow in pipelines and open-channel situations.

The most common on-farm, pipeline flowmeter is the propeller meter (Fig. 1).[3] A propeller, mounted in the pipeline, rotates at a rate proportional to the water flow velocity. The propeller is connected to the readout device via a cable or shaft. A gear system, appropriate for the pipe size, translates the flow velocity into a flow measurement. The readout device provides a totalized flow, an instantaneous flow rate, or both. Manufacturers set the display units (e.g., gallons, cubic meters, ac-ft, etc.) based on user preference. The flowmeter units can be purchased frequently as flangemounted or saddle-mounted devices. A propeller that occupies nearly the entire flow area is preferable to a smaller propeller, since it integrates the flow velocity across the pipe. Trash, such as weeds, in the water can entangle the propeller and adversely impact meter operation. A propeller meter should not be used under such water quality conditions.

PIPELINE FLOW MEASUREMENT Pipeline flow conditions most common at the farm level are pumped groundwater and pressurized water deliveries from water providers such as irrigation districts. In some situations, water will be pumped by the grower from a canal or similar open-channel delivery source. The discharge pipe from the pump becomes a convenient location to measure flow rate. Commercially available pipeline flow measurement devices most often require a full-pipe flow condition. They frequently measure flow velocity converted to a flow rate measurement using the pipe flow area. Flowmeter manufacturers recommend installment flow conditions. This is usually specified as the length of straight pipe upstream and downstream of the flowmeter. Bends, elbows, tees, valves, etc., located too close to the flowmeter, disrupt the flow path, and lower the accuracy of the flow measurement.[1] The general recommendation is that there be at least an 8- to 10pipe diameter length of straight pipe upstream of the meter, and at least 1-pipe diameter of straight pipe downstream of the meter. Always follow the manufacturer’s recommendations. The following are commonly used flow measurement devices for pipelines. There are numerous other pipeline flow measurement devices available.[2] 788

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Insertion Flowmeters Insertion meters are devices mounted at a point on the inside wall of the pipeline. The measurement device is usually a small rotor, turbine, or an electromagnetic flowmeter (discussed in greater detail later). The devices are often mounted to the pipe using a threaded insert. The insertion flowmeters are either calibrated by the manufacturer or field-calibrated by the user, using a flow velocity profiling technique. Insertion meters have the advantage of allowing relatively easy installation and removal, so they can be used at multiple pipeline locations. They also have the advantage of being usable on large pipe installations where other flowmeters (e.g., a propeller meter, electromagnetic meters) would be more expensive. A disadvantage of insertion meters is that since they measure flow velocity at a single point in the pipeline, they can be severely impacted by flow conditions (e.g., bends or valves), which disrupt the water flow path. Electromagnetic Flowmeters Electromagnetic flowmeters (magmeters)[3] induce and measure a voltage in the water flowing through the

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Fig. 2 Trajectory flow measurement method for a horizontal pipe.

Fig. 1 Schematic of a propeller meter.

acoustic signals and the returning signal’s frequency is also measured. A change in the frequency is a function of the velocity of particles in the water reflecting the signal. The Doppler meter will not be accurate when used with very high quality water, but it can be used on many groundwater and surface water sources. The major advantage of a Doppler meter is its portability. The sensor(s) mounts easily and quickly to the outside of the pipe and can, therefore, be used on even large diameter pipes. Flow in metal and plastic, but not concrete, pipe can be measured with a Doppler flowmeter. An external power source is required. Doppler flowmeters are more expensive than a propeller meter.

meter. The magnitude of the induced voltage is proportional to the water velocity. Most commonly, the meter is a tube (tube magmeter) through which the water flows. The tube magmeter contains magnetic coils to induce a voltage and electrodes to measure voltage, and electronics in the unit translate the voltage measurement to a flow reading. There are also electromagnetic meters that mount to a point on the inside pipe wall. These are insertion meters and can be moved to various pipeline measurement locations. Electromagnetic meters are very accurate and the tube magmeters have no obstructions within the pipe to cause a pressure drop or become entangled by trash in the water. Electromagnetic flowmeter installations do not require as long a section of straight pipe upstream and downstream of the meter (see manufacturer’s recommendations) when compared with propeller meters. Magmeters require an external power source, ranging from 240 V to battery-power, depending on the model and manufacturer. A disadvantage of electromagnetic meters has been their cost, substantially more than a similarly sized propeller meter, but the cost of some units has dropped significantly recently.

A low technology, but less accurate, method of estimating pipe flow rate is to measure the trajectory of the jet from the discharge end of a horizontal or vertical pipe. For a vertical pipe, the height of the jet above the end of the pipe is measured. For a horizontal pipe, the vertical and horizontal distances from the top of the pipe to the jet’s upper surface are measured (Fig. 2). Tables are available to convert the distance measurements into a flow rate estimate.[3] It is difficult to accurately determine the jet trajectory measurements. The flow trajectory measurement method is, therefore, only accurate as an estimate, but when no other flow measurement methods are available, it can provide useful information.

Acoustic Flowmeters

OPEN-CHANNEL FLOW MEASUREMENT

This group of flowmeters uses acoustic signals to measure flow.[3] Transit-time (ultrasonic) meters measure the alteration, caused by the flow velocity, in the transit time of an acoustic signal sent across the pipeline. Sensors are mounted to the outside of the pipeline, so the units are portable. Transit-time flowmeters, while accurate, are not widely used in on-farm applications due primarily to the cost. Doppler flowmeters measure the velocity of particles in the water. The unit transmits known frequency

On-farm flow measurement in open ditches and channels is more difficult, and often less accurate, than flow measurement in pipelines. Weirs and flumes can be installed in permanent ditches and channels, but flow measurement in temporary ditches is more problematic. Temporary ditch flow measurement options include using float-velocity or current meter measurements. The following are flow measurement options for use in on-farm open ditch and channel applications. Other flow measurement methods are also available.[3]

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Trajectory Methods

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On-Farm Irrigation Flow Measurement

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Float-Velocity Measurements

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Measurement of channel flow rate using the floatvelocity method[3] requires measuring both the velocity of an object, such as a stick or tennis ball, floating in the channel and the cross-sectional area of the ditch or channel. Since the floating object’s velocity is not the same as the channel velocity, correction factors[3] are used to determine an average channel flow velocity. Determining the velocity of the floating object is easily done, but determining the ditch cross-sectional area may be time-consuming. The resulting channel flow rate should be considered an estimate rather than an accurate measurement. Current Meter Measurements Current meters[2] can be used to determine the flow velocity at selected points in a channel. The velocity measurements, combined with cross-sectional area measurements, provide the channel discharge rate. The current meter is usually of one of the three types: (i) a mechanical meter with an anemometer or propeller device that rotates proportionally to the water flow velocity; (ii) a Doppler meter (see section on Acoustic Meters for pipelines); or (iii) an electromagnetic flowmeter. The current meter flow measurement is more accurate than the float-velocity method, but it is time-consuming. Weirs A weir[3–6] is a notch of particular shape through which water flows (Fig. 3). Generally, the notch shape is rectangular, trapezoidal, or triangular. A depth measurement a short distance upstream of the weir is recorded and a weir calibration equation translates the depth measurement to a flow rate. A disadvantage of weir use is that sediment and trash will gather on the weir’s upstream face. Weirs

Fig. 3 Trapezoidal weir installed in an earthen channel.

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Fig. 4 Flume installed in a small concrete channel.

can also be difficult to install in temporary ditches or channels. For these reasons, they are not widely used for on-farm flow measurements. Flumes A flume (Fig. 4)[2–8] is a device of particular crosssection, placed in the channel, constricting the channel sidewalls, raising the channel bottom, or both to force the water to accelerate. There are a large number of flume configurations commercially available, manufactured in metal or fiberglass, for installation in a channel. Installation ensuring that water does not erode and bypass the flume can be challenging in temporary ditch flow measurement applications. The flume flow depth, upstream of the ‘‘throat’’ section, is measured and the flow rate is determined using the flume’s calibration curve. The flume is often fitted with a stilling well to improve measurement accuracy. Continuous flume measurements can be recorded using a stage recorder or pressure transducer/data logger, installed on the stilling well.

Fig. 5 Ditch and siphon irrigation system irrigating an orchard.

Fig. 6 Ditch to furrow siphon pipe irrigation showing the head difference.

Flumes are able to pass sediment and some trash, an advantage in on-farm flow measurement. They have applications in measuring both irrigation water applied to the field and tailwater runoff from fields. There are flumes available in a wide range of flow capacities, from very large to small enough to measure the flow rate in individual field furrows. Siphon Flow Measurements Ditch and siphon irrigation systems (Fig. 5) are common. The siphon discharge rate can be determined by measuring the difference in water surface elevation (head) of the supply ditch and the discharge point in the field (Fig. 6), and measuring the diameter of the siphon pipe. Siphon discharge rate tables are available to interpret the measurements.[3]

CONCLUSION Methods and devices are available to measure irrigation flows under both pipeline and open-channel flow conditions. Pipeline flow measurement is often

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easier and more accurate than flow measurement in open ditches and channels, and pipelines usually provide a more permanent flow measurement installation location when compared with on-farm ditches. Selection of the flow measurement method is often based on the flow situation (e.g., pipeline vs. open channel), cost, and level of accuracy required.

REFERENCES 1. Hanson, B.R.; Schwankl, L.J. Error analysis of flowmeter measurements. J. Irrigation Drainage Eng. 1998, 124 (5), 248–256. 2. Jensen, M.E. Design and operation of farm irrigation systems. ASAE Monograph, No. 3; St. Joseph, Michigan, 1980. 3. U.S. Department of the Interior, Bureau of Reclamation. Water Measurement Manual, 1997. Also available at; http://www.usbr.gov/pmts/hydraulics_lab/pubs/manuals/ WMM_3rd_2001.pdf. 4. King, H.W.; Brater, E.F.; Wei, C.Y. Handbook of Hydraulics; McGraw-Hill Book Company: New York, NY, 1996. 5. Chow, V.T. Open-Channel Hydraulics; McGraw-Hill Book Company: New York, NY, 1959. 6. U.S. Department of Interior, Bureau of Reclamation. Design of small canal structures, 1983. 7. Bos, M.G.; Reglogle, J.A.; Clemmens, A.J. Flow Measuring Flumes for Open Channel Systems, 2nd Ed.; ASAE: St. Joseph, Michigan, 1991. 8. Clemmens, A.J.; Bos, M.G.; Replogle, J.A. FLUME: design and calibration of long-throated measuring flumes. Version 3.0; International Institute for Land Reclamation and Improvement, Publication 54, 1993.

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On-Farm Irrigation Flow Measurement

Open-Channel Flow Rate Measurements John E. Costa U.S. Geological Survey (USGS), Vancouver, Washington, U.S.A.

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Stephen F. Blanchard Office of Surface Water, U.S. Geological Survey (USGS), Reston, Virginia, U.S.A.

INTRODUCTION Open-channel flow measurements require specialized training and equipment, and they are expensive to obtain. The majority of continuous streamflow records are not based on direct measurement of river discharge—they are derived from continuous measurements of river elevations or stage.[1] These stage data are converted into discharge by the use of a stage/ discharge relation (rating) that is unique for each streamgaging location. Much of the effort and cost associated with streamgaging lies in establishing and updating this relation. Hydrologists visit streamgaging stations 6–10 times a year to make direct measurements of river depth, width, and velocity. From these data, they compute the open-channel flow rate. The range of measured flow rates and concurrent river stages are then used to build the rating curve for each site and to track changes to the rating curve. Measurement of open-channel flow rate, also known as discharge, is usually determined by measuring two components—the cross-sectional area of the flow and the water velocity. Open-channel flow rate is expressed in the equation: Q ¼ A
V, where Q is discharge, A is cross-sectional area, and V is water velocity. The cross-sectional area is fairly simple to determine and is usually measured by taking soundings at multiple points in the cross section to determine the depth. Width is usually measured with a tape. Water velocity can be measured using several types of instruments and techniques that include mechanical current meters, acoustic Doppler current profilers (ADCP), and low- and high-frequency radars. Several of these instruments and methods for measuring velocity and discharge are described below.

MECHANICAL METERS Mechanical current meters are instruments designed to measure the velocity of moving water at a point in a river. A typical current meter has a horizontally mounted bucket wheel that is rotated by the moving water. The rate of rotation of the bucket wheel on the 792

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current meter is proportional to the speed of the moving water at that particular point (Fig. 1).[1] The current meter is held at a point for about a minute (to average turbulent flow fluctuations) while the revolutions of the cups on the meter are counted. Usually, two point velocities (or a single velocity) are measured vertically to define an average velocity for the water column. The discharges from the individual columns are then summed to obtain the total discharge.

ACOUSTIC DOPPLER CURRENT PROFILERS An important development in open-channel flow measurement in the last ten years has been the deployment of ADCPs.[2] An ADCP uses acoustic energy— typically in the range 300–3,000 kHz—to measure water velocity throughout most of the water column by measuring the shift in the frequency of the acoustic signals reflected from materials suspended in and moving with the water. The ADCP determines water depth by measuring the time-of-travel of signals reflected from the channel bottom and measures boat velocity by using the Doppler shift of separate acoustic pulses reflected from the riverbed (Fig. 2). The channel width can then be computed using the instantaneous boat velocities and time between each measurement. The ADCP has made three important contributions to direct open-channel flow measurements in streams. First, discharge measurements using a mechanical current meter require a minimum of 20 individual measurements across the river. These measurements typically take about two hours to complete, but during floods they can take as long as several hours to complete. The ADCP measurement can be done in a matter of minutes rather than hours.[3,4] Second, the ADCP allows measurements in environments where mechanical current meters are inappropriate or unreliable, such as in tidally affected flows, highly unsteady flows, and flood flows. Third, ADCPs are used to measure continuous profiles of water velocity (the vertical velocity distribution is no longer assumed but rather measured for all but the near bed and near-surface), thereby providing more accurate measurements of streamflow.

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Fig. 1 Mechanical current meters— Price AA meter (left) and Pygmy meter (right).

ACOUSTIC FLOW MONITORS Scientists have observed that sediment-laden flows, such as debris flows and turbulent flow in steep mountain channels, generate ground vibrations that can be detected by geophones located near the channels. Systems of geophones and instrumentation designed to measure these ground vibrations are called acoustic flow monitors (AFM).[5,6] Most debris flows cause the ground around the channel to vibrate with a peak frequency of 20–50 Hz, and water floods cause ground vibrations with peak frequencies greater than 100 Hz. Geophones located near channels can be programmed to detect vibrations in these frequency ranges and capture the full spectrum of vibrations. These ground vibrations caused by water and sediment flow can then be related to discharge.[7,8]

LOW- AND HIGH-FREQUENCY RADAR U.S. Geological Survey (USGS) researchers are engaged in a series of proof-of-concept experiments to demonstrate the use of microwave and lowfrequency radars to measure discharge directly— without having to place any instruments in the water. Surface velocity can be measured at various points

Fig. 2 Hydrologist measuring streamflow using an acoustic doppler current profiler.

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across the river using the principal of Bragg scatter of a high-frequency (10 GHz) pulsed Doppler radar signal (Fig. 3). Cross-sectional area can be measured by suspending a conventional low-frequency (100 MHz), ground-penetrating radar (GPR) system over the water surface from a bridge or cableway and transiting it across the stream.[9] In the absence of a bridge or cableway, the GPR and radar systems have been mounted on helicopters and flown across the river, producing discharge values comparable to conventional discharge measurements.[10]

SPACE-BASED PLATFORMS Hydrologists have begun to consider the possibility of measuring and monitoring discharge from space.[11,12] Many important streams and rivers worldwide have no streamgages, and the ability to measure off-channel surface-water storage (wetlands, floodplains, lakes) and river discharge in virtually any location would provide new insight into the global hydrologic cycle and an understanding of the role of surface water in regulating the regional and global biogeochemical cycles. Such a space-based system might rely on radar altimetry for river stage and along-track interferometric synthetic aperture radar (SAR) measurements[13] of surface velocity along an entire reach of river. This requires some new thinking about the spatial utility of space-based remote sensing and the present in situ or cross-sectional basis for measuring river flow. Space-based technologies are unproven as tools for measuring discharge from great distances, but scientists are beginning to think about how to do it with support from the National Aeronautics and Space Administration (NASA).[14] Space-based instruments hold the promise to measure the elevations and perhaps even flow rates of the world’s largest rivers. But far smaller streams (e.g., draining less than 10,000 km2) around the world (including in the United States) present measurement needs that will challenge the resolution-cost trade-off capability of space-based sensors.

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Open-Channel Flow Rate Measurements

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Fig. 3 Pulsed Doppler radar on top of equipment shelter, and UHF radar antenna to left of shelter on bank of San Joaquin River, CA, are used for surface-velocity measurements. Cable across channel is used to suspend Ground Penetrating Radar (GPR) system over the water to measure channel cross section. Non-contact methods of streamgaging show substantial promise, but much remains to be learned.

REFERENCES 1. Rantz, S.E. Measurement and computation of streamflow: Volume 1, Measurement of stage and discharge: U.S. Geological Survey Water-Supply Paper 2175, 1982; 79–183, Chapter 5. 2. Simpson, M. Discharge measurements using a broadband acoustic Doppler current profiler, U.S. Geological Survey Open-File Report 01-01, 2001; 123 pp. 3. Morlock, S.E. Evaluation of acoustic Doppler current profile measurements of river discharge, U.S. Geol Survey Water-Resources Investigations Report 954218, 1996; 37 pp. 4. Mueller, D.S. Field evaluation of boat-mounted acoustic doppler instruments used to measure streamflow. Proceedings of the IEEE Seventh Working Conference on Current Measurement Technology, San Diego, March, 2003; 30–34. 5. LaHusen, R.G. Detecting debris flows using ground vibrations, U.S. Geological Survey Fact Sheet 236-96, 1996; 2 pp. 6. LaHusen, R.G. Debris-flow instrumentation. In DebrisFlow Hazards and Related Phenomena; Jakob, M., Hunger, O., Eds.; Springer: Berlin, 2005; 291–304. 7. Newman, J.D.; Bennell, J.D. A mountain stream flowmeter. Flow Meas. Instrum. 2002, 13, 223–229. 8. Marcial, S.S.; Melosantos, A.A.; Hadley, K.C.; LaHusen, R.G.; Marso, J.N. Instrumental lahar monitoring at pinatubo volcano. In Fire and Mud: Eruptions and Lahars of Mount Pinatubo; Newhall, C.G.,

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9.

10.

11.

12.

13.

14.

Punongbayan, R.S., Eds.; Philippine Institute of Volcanology and Seismology, Quezon City and University Press, Seattle: Philippines, 1996; 1015–1022. Costa, J.E.; Spicer, K.R.; Cheng, R.T. et al. Measuring stream discharge by non-contact methods: a proof-ofconcept experiment. Geophys. Res. Lett. 2000, 27 (4), 553–556. Melcher, N.B.; Costa, J.E.; Haeni, F.P. et al. River discharge measurements by using helicopter-mounted radar. Geophys. Res. Lett. 2002, 29 (22), 2084, doi:1010.29/2002GL015525. Alsdorf, D.E.; Lettenmaier, D.P.; Vorosmarty, C.; The NASA surface water working group the need for global, satellite-based observations of terrestrial surface waters. EOS 2003, 84 (22 July), 269, see also pages 275–276. Bjerklie, D.M.; Lawrence Dingman, S.; Vorosmarty, C.J.; Bolster, C.H.; Congalton, R.G. Evaluating the potential for measuring river discharge from space. J. Hydrol. 2003, 278 (1–4), 17–38. Goldstein, R.M.; Zebker, H.A. Interferometric radar measurement of ocean surface currents. Nature 1987, 328, 707–709. Vorosmarty, C.J.; Bririkett, C.; Dingman, L.; Lettenmaier, D.P.; Kim, Y.; Rodriguez, E.; et al. NASA Post-2002 Land Surface Hydrology Mission Component for Surface Water Monitoring: HYDRA-SAT HYDRlogical Altimetry SATellite. A report from the NASA Post2002 Land Surface Hydrology Planning Workshop, Irvine, CA, April 1999; 12–14.

Open-Channel Spillways Sherry L. Britton Gregory J. Hanson

INTRODUCTION Provision must be made in the design of almost every dam to permit the safe discharge of water downstream. The function of the spillway is to safely convey this discharge past the dam without unacceptable damage. The high velocities and large levels of energy involved in flow over spillways, particularly during major floods, make their design of considerable importance. The design and capacity of spillways play an important role in the layout and economics of every dam project. Spillways are selected for a specific dam and reservoir on the basis of discharge requirements, topography, geology, dam safety, and project economics.

TYPES OF SPILLWAYS The classifications of spillways are primarily based on their most prominent feature and/or function. This may include the type of discharge carrier or some other type of component.[1,2] Spillways are classified as controlled or uncontrolled depending on whether they are gated or ungated. Designation as a principal spillway generally indicates constant or frequent flow with an auxiliary spillway used to pass the infrequent larger flood events. Most spillways can be broken into two main categories: open-channel spillways and conduit spillways. Open-channel spillways include: a) straightdrop or free overfall spillways; b) chute spillways; c) cascade spillways; d) side-channel spillways; and e) unlined or vegetation lined earthen spillways. Conduit spillways include: a) siphon spillways; b) drop shaft or morning-glory spillways; c) tunnel spillways; and d) culvert spillways. The focus of this article is on open-channel spillways. Straight-Drop Spillways The U.S. Bureau of Reclamation[2] defines a straightdrop spillway as one in which the flow drops freely from the crest into a plunge pool or stilling basin (Fig. 1). Straight-drop spillways are used on thin arch

Nitrogen–Plant

Plant Science and Water Conservation Research Laboratory (PSWCRL), Agricultural Research Service (USDA-ARS), U.S. Department of Agriculture, Stillwater, Oklahoma, U.S.A.

or deck overflow dams, dams with a crest that has a nearly vertical downstream face, and with low earthfill dams.[3] Hydraulic concerns of the straight-drop spillway are with the control and dissipation of energy in the downstream plunge pool or stilling basin. A minimum depth of tailwater is required for effective dissipation of excess flow energy to prevent downstream scour. The tailwater level in the downstream channel should be at approximately the same level as the water surface in the stilling basin.

Chute Spillway A spillway that conveys water from a reservoir over a spillway crest into a steep-sloped open channel is known as a chute spillway[4] (Fig. 2). A smooth chute spillway conducts the overflow to an outlet energy dissipation basin. Chute spillways can be well adapted to earth or rock-fill dams when topographic conditions permit.[4] They are generally located through the abutment adjacent to the dam. However, they can be located in a saddle away from the dam structure. Such a location is preferred for earth dams to prevent possible damage to the embankment. The chute may be of constant width but is usually narrowed for economy and then widened near the end to reduce unit discharge.

Cascade Spillway The cascade or baffle chute spillway uses steps or other appurtenances to dissipate energy in the spillway channel and thereby, reduce energy dissipation requirements in the outlet basin (Fig. 3). The cascadespillway has greater flow depths than chute spillways requiring higher sidewalls. Spray action may be a concern due to air entrainment, and abrasion of the steps and other appurtenances can be a serious problem. Cascade-spillway use has increased recently due to the increased use of roller compacted concrete (RCC). A major concern for cascade spillways is to provide a smooth transition flow from the spillway crest to the first few steps. This transition is commonly attained using a smooth ogee crest. Transitions for flatter 795

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796

Open-Channel Spillways

Nitrogen–Plant Fig. 1 Straight-drop spillway.

Fig. 3 Roller compacted concrete cascade spillway.

sloped spillways, 2.5 H:1 V or less, can also be attained by an arrangement of smaller steps at the crest of the cascade chute.[5] The energy loss for a cascade spillway is strongly dependent on the slope length. The energy loss due to the steps depends primarily on the ratio of critical depth of flow to the step height and on the number of steps.

overflow weir discharging into a narrow channel in which the direction of flow is approximately parallel to the weir crest (Fig. 4). This type of spillway is used in circumstances similar to those of the chute spillway. Due to its unique shape, a side-channel spillway can be sited on a narrow dam abutment.

Side-Channel Spillway Earthen Spillway [6]

As defined by the U.S. Army Corp of Engineers, a conventional side-channel spillway consists of an

Fig. 2 Chute spillway.

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Earthen spillways are typically excavated in native materials and vegetated with grasses adapted to the local area (Fig. 5). Earthen spillways are often designed as auxiliary spillways with the perspective that damage may occur during infrequent operation, but not to the

Fig. 4 Side-channel spillway.

797

Nitrogen–Plant

Open-Channel Spillways

Fig. 5 Arial view of a vegetated-earthen spillway placed at dam abutment.

extent that it will lower the spillway crest or cause a catastrophic release of stored water. Often a saddle or low point on natural ground at the periphery of the reservoir will serve as the earthen spillway.

COMPONENTS OF SPILLWAYS Spillways consist of three major components: entrance structure, conveyance channel, and outlet structure. An entrance structure is designed to control the discharge and admits reservoir water to the spillway. The conveyance channel carries the discharge from the inlet structure to the outlet structure. The outlet structure dissipates the energy of the high velocity flow from the conveyance channel and discharges it to the channel downstream.[1] Entrance Structure The design of the spillway entrance structure for small dams is not usually critical, and a variety of simple crest patterns are used. In the case of large dams, it is important that the water be guided smoothly over the crest of the structure with a minimum of turbulence (Fig. 6). Therefore, an ogee-crest design is often used. The ogee crest takes the form of the underside of the nappe of a sharp crested weir when the flow rate corresponds to the maximum design capacity of the spillway. This results in near-maximum discharge efficiency. Correct design of the spillway entrance will minimize cost while providing sufficient crest length to pass the design discharge. Also, it will result in

© 2008 by Taylor & Francis Group, LLC

Fig. 6 Smooth flow over ogee spillway crest.

acceptable energy heads and pressure levels on the spillway crest, and acceptable unit discharges for the conveyance channel and outlet structure. Gates located at or near the crest are often used to control flow into the spillway. Types of gates may include flashboards, stop logs, lift gates, radial gates, rolling gates, and drum gates. A spillway using gates is often referred to as a controlled spillway. The use of gates allows for additional storage above the spillway crest and control of the timing and quantity of reservoir discharges. The disadvantages to gates are: they are expensive; they often require personnel at the structure for proper operation; and operational or structural failure may have catastrophic consequences. The inlet structure has a significant effect on the spillway discharge. There are cases where there is a need for increased flow capacity with upstream head elevation restrictions. A labyrinth weir or box inlet weir may be used to increase the inlet capacity for a given range of reservoir water surface elevations. A labyrinth weir is folded in plan view, increasing the total weir length to 3–5 times the spillway width. The labyrinth weir capacity is typically twice the standard overflow crest of the same width for low head ranges.[7] The capacity of the box inlet weir is directly related to the perimeter of the overflow box that extends into the reservoir[8] (Fig. 7). The discharge capacity of the standard ogee weir, labyrinth weir, and box weir, is given by the weir equation: Q ¼

pffiffiffiffiffiffiffiffii 2h Cd Lh 2gh 3

798

Open-Channel Spillways

Nitrogen–Plant Fig. 7 Chute spillway with notched box inlet.

in which Q is the discharge in cfs, Cd a dimensionless weir coefficient, L the length of weir in ft, g gravitational acceleration 32.2 ft sec
2, and h the total head on the crest in ft (vertical distance from the crest of the spillway to the reservoir level). The coefficient Cd, varies with spillway type and head. A typical range of Cd for the ogee crest is from 0.55 to 0.75. The typical entrance to earthen spillways as well as some other chute spillways consists of a forebay reach followed by a level crest prior to entering the conveyance channel. The discharge rating of these types of spillways is typically developed based on water surface profile calculation methods incorporating channel geometry and roughness conditions.

to pass infrequent flood flows. Damage may occur during operation, but it cannot be extensive enough to lower the spillway crest causing a catastrophic release of reservoir water (Fig. 8). The extent of damage that may occur in an unlined spillway is dependent on the duration and quantity of flow through the spillway, quality and maintenance of the vegetal cover, spillway geometry, and spillway geology.[9,10] Evaluation of expected erosion is the most difficult and critical problem encountered in the design of the earthen spillways. The designer must not only decide whether the channel materials will be eroded but also make reasonable estimates pertaining to the rate at which erosion will progress. Extensive exploration, testing of encountered materials, and geological profiles to a depth in excess of any anticipated scour are required to assist in the erosion estimates. Study of the history of erosion in the project area and research of erosion experiences at projects with similar facilities should be undertaken as part of the evaluation of expected erosion. The NRCS computer program ‘‘Sites,’’ uses a three-phase erosion model to predict erosion of vegetated-earthen spillways.[11]

Outlet Structure Water returned to the river below the dam must be kept from scouring or eroding the riverbed or dam foundation. Plunge pools or stilling basins are therefore required to reduce the velocity of the water before returning it to the downstream channel. These energy dissipation structures may be an integral part of the

Conveyance Channel With the exception of the straight-drop spillway that may be used for low dams, the water that passes the spillway inlet is carried to the downstream outlet structure by a conveyance channel. The material used to line the surface of the conveyance channel is dependent on frequency of use, erodibility of the natural materials, and overall spillway design. Principal spillway channels operate more frequently and are usually constructed of reinforced-concrete slabs 10–20 in. thick. Auxiliary spillways may also be lined to allow higher velocity flows and/or combination of the principal and auxiliary spillway functions into a single spillway. The high velocities and large levels of energy involved in flow over spillways may lead to problems with air entrainment, shock waves, cavitation, and abrasion. Abrasion may be a particular problem when entrained sediment is present. Earthen spillways are unlined or vegetation-lined channels that are most often used as auxiliary spillways

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Fig. 8 Erosion damage of an auxiliary spillway after experiencing a flow.

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floodwaters downstream of the dam and erodible material can have major consequences if not properly considered. In addition, trees, floating debris, and suspended sediment can influence the decision in selecting a spillway.[1] The required capacity (maximum outflow rate through the spillway) depends on the spillway design flow (inflow hydrograph to the reservoir), the normal discharge capacity of other outlet works, and the available storage. The selection of the spillway design flow is related to the degree of flood protection required that, in turn, depends on the type of dam, its location, and the consequences of a dam failure. A high dam storing a large volume of water located upstream of an inhabited area requires a much higher degree of protection from overtopping than a low dam storing a small quantity of water whose downstream reach is uninhabited. The probable maximum flood is commonly used for design of the former, while a smaller flood is suitable for the latter. Fig. 9 Energy dissipation basin.

dam or spillway. Two common structures used in dissipating the high energy of falling water are the apron-basin and the flip bucket. In the apron-basin type of structure, the high-velocity shallow flow coming from the dam is converted into a low-velocity deep flow by causing a hydraulic jump to occur on a horizontal or sloping concrete apron (Fig. 9). With a flip bucket, the toe of the dam is shaped to deflect the high-velocity flow upward away from the riverbed. The resulting ‘‘flip’’ breaks up the jet and dissipates the energy of the water. Additional energy is dissipated in a plunge pool downstream of the bucket.

FACTORS WHICH AFFECT SPILLWAY CHOICE AND DESIGN Several factors should be considered when choosing a spillway. Singh and Varshney[1] list several factors such as safety considerations, hydrologic and site conditions, type of dam, purpose of dam and operating conditions, conditions downstream of the dam, and nature and amount of solid material brought by the river. Safety is the most important factor to consider for a spillway because improper design of spillways or insufficient spillway capacity may result in dam failures. Spillway design and capacity depend on inflow discharge (frequency and shape of hydrograph), the elevation of the spillway crest, storage of the reservoir at various levels, and the geological site conditions, which include slope stability, steepness of the terrain, or possibilities of scour downstream. The type, purpose, and conditions, downstream of the dam also influence the spillway design and capacity. Rising

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REFERENCES 1. Singh, B.; Varshney, R.S. Spillways and embankment dams. In Engineering for Embankment Dams; Singh, B., Ed.; Balkema Publishers: Vermont, 1995. 2. U.S. Bureau of Reclamation Spillways. Design of Small Dams, 1st Ed.; U.S. Printing Office: Washington, DC, 1960; 247–343. 3. Donnelly, C.A.; Blaisdell, F.W. Straight drop spillway stilling basin. J. Hydr. Eng. ASCE 1965, 91 (HY3), 101–131. 4. Linsley, R.K.; Franzini, J.B. Spillways, gates, and outlet works. In Water-Resources Engineering, 2nd Ed.; Chow, V.T., Eliassen, R., Linsley, R.K., Eds.; McGraw-Hill, Inc.: New York, 1972; 231–268. 5. Rice, C.E.; Kadavy, K.C. Model study of a roller compacted concrete stepped spillway. J. Hydr. Eng. ASCE 1996, 122 (6), 292–297. 6. U.S. Army Corps of Engineers. Hydraulic Design of Spillways; Engineer Manual 1110-2-1630; U.S. Army Corps of Engineers: Washington, DC, 1990; 1(1)–7(15). 7. Tullis, J.P.; Amanian, N.; Waldron, D. Design of labyrinth spillways. J. Hydr. Eng. ASCE 1995, 121 (3), 247–255. 8. Gwinn, W.R. Model Study of a Box-Inlet Chute Spillway and SAF Stilling Basin, Agricultural Reviews and Manuals, Southern Series, No. 17; U.S. Department of Agriculture, Agricultural Research (Southern Region), Science and Education Administration: New Orleans, LA, 1981; 1–14. 9. Temple, D.M.; Hanson, G.J. Headcut development in vegetated earth spillways. Appl. Eng. Agric. 1994, 10, 677–682. 10. Temple, D.M.; Moore, J.S. Headcut advance prediction for earth spillways. Trans. ASAE 1997, 40, 557–562. 11. Temple, D.M.; Richardson, H.H.; Brevard, J.A.; Hanson, G.J. SITES: the new DAMS2. Appl. Eng. Agric. 1995, 11, 831–834.

Nitrogen–Plant

Open-Channel Spillways

Oxygen Measurement: Biological–Chemical Oxygen Demand Nitrogen–Plant

David B. Parker Division of Agriculture, West Texas A&M University, Canyon, Texas, U.S.A.

Marty B. Rhoades Feedlot Research Group, West Texas A&M University, Canyon, Texas, U.S.A.

INTRODUCTION Many aquatic organisms depend on dissolved oxygen (DO) for basic life functions. Dissolved oxygen is one of the factors that affect population diversity in surface waters. Aerobic bacteria, fungi, protozoa, and algae all carry out aerobic respiration and require DO to survive. Trout and other coldwater fish species require more DO than warm-water fish species. A DO concentration of 5 mg L
1 is generally recommended for warm-water fish, and 6 mg L
1 for coldwater fish.[1] Lower concentrations may be tolerated for short periods but result in stress on the fish. The solubility of oxygen in water varies with water temperature and atmospheric pressure. Dissolved oxygen concentrations range from 14.6 mg L
1 at 0 C to about 7 mg L
1 at 35 C.[2] Oxygen is less soluble in saline water than in pure water. Polluted waters usually have lower DO concentrations than pure water.

OVERVIEW In ponds and lakes, photosynthetic activity from aquatic plants such as algae produce DO. At night, these same plants compete with aquatic organisms for DO. Biodegradation of organic matter can decrease DO concentrations. Organic wastes are subject to further bacterial decomposition once they enter the environment. If domestic or agricultural organic wastes enter water that contains DO, the aerobic bacteria will utilize the organic material as a food and energy source. During this process, the bacteria respire DO and produce carbon dioxide. If enough DO is removed, then concentrations can fall below values required for survival of aquatic organisms. Fish kills are sometimes the result of spills or unauthorized releases of organic-laden waste to surface waters.[3] During warm conditions, fish kills can occur immediately after the release. When an organic waste release occurs during cold conditions, DO concentrations may not be affected for several months until water temperatures 800

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increase, making it difficult to assess the exact cause and nature of the fish kill. Fish kills are not always the direct result of organic releases. In some instances, the enrichment of waters by nutrients, a term called eutrophication, can lead to unwanted algal growth.[4] In many water bodies, phosphorus is the limiting nutrient for algal growth. A rapid release of dissolved phosphorus to surface waters can trigger algae blooms. Growing algae produces DO through photosynthesis, however, whenever the algae dies it is subject to decomposition that could lead to the decrease in DO.

DISSOLVED OXYGEN MEASUREMENT Dissolved oxygen was originally measured using the Winkler (iodometric) method. The original Winkler method was based on the fact that oxygen oxidizes Mn2þ under alkaline conditions. The higher valence manganese then oxidizes I
to free I2 under acidic conditions. The amount of free iodine released is proportional to the DO concentration. Because nitrite ions interfere with the DO determination, several modifications to the original Winkler method have been developed. Nitrite interference is overcome by using sodium azide (NaN3). Another modification includes using permanganate to oxidize any reducing agents present. In recent years, electronic DO membrane electrodes have become more common. Membrane electrodes can be either the polarographic or galvanic type.[5] Both types utilize a sensing element composed of two metal electrodes. A voltage is passed across the electrodes, and the measured current is converted to a DO concentration. Membrane electrodes are especially useful in field applications. When electrodes are attached to a long cord, DO can be measured at various depths in ponds and rivers. Dissolved oxygen electrodes are usually calibrated against water samples that have been analyzed by using the Winkler method. Because DO electrodes are sensitive to temperature, an accurate

Oxygen Measurement: Biological–Chemical Oxygen Demand

801

temperature measurement must be made at the same time so that a correction can be applied. There are many brands of DO electrodes available commercially.

Organic wastes can enter surface water bodies from a variety of sources, including raw sewage spills, permitted publicly owned treatment works (POTWs), animal feeding operations, runoff from land application areas, or improperly designed septic tanks.[6] Environmental regulations exist to protect surface water bodies from unwarranted discharges that could threaten aquatic life or human health. Environmental permits for discharge of wastewater or stormwater runoff to surface water bodies often include such items as temperature and biochemical oxygen demand (BOD). BOD is an empirical test in which standardized laboratory procedures are used to measure the biologically available organic matter in a water sample. It is based on the amount of oxygen that would be consumed if an abundant aerobic bacterial population were present. BOD is defined as ‘‘the quantity of oxygen used by bacteria while stabilizing decomposable organic matter under aerobic conditions.’’[2,7] The BOD test is widely used to determine the concentration of domestic and industrial wastes, and is helpful in evaluating the BOD removal efficiency of wastewater treatment plants. Miner, Humenik, and Overcash[3] presented the conceptual reaction as follows: BOD þ O2 ! CO2 þ H2 O þ Energy

ð1Þ

The standard BOD test is based on a 5-day incubation period, and is referred to as BOD5. As a ruleof-thumb, BOD5 is about 70–80% of the ultimate BOD (BODU).[2] Another reason for limiting the BOD test to 5 day is to avoid interferences from oxidation of ammonia to nitrite and nitrate, a process called nitrification. Nitrifying bacteria typically do not consume an appreciable amount of DO until 6–10 day after initiating the BOD test[8] (Fig. 1). With samples that have a BOD less than 7 mg L
1, the BOD is measured directly on the water sample.[2] For higher BODs, the wastewater sample is diluted because the BOD concentration exceeds the DO available. The dilution water reduces the toxicity of the wastewater and provides the DO needed for aerobic decomposition. Nutrients such as nitrogen, phosphorus, and trace metals are added to the dilution water, and it is saturated with oxygen before use. A buffer is added to maintain the pH in a range suitable for bacterial growth. The solution is ‘‘seeded’’ with the necessary microorganisms.[9] The seed source often consists of effluent from biological waste treatment

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Nitrogen–Plant

BIOCHEMICAL OXYGEN DEMAND

Fig. 1 The BOD curve showing (a) the normal curve resulting from oxidation of organic matter and (b) the curve if combined with nitrification of ammonia.

plants.[5] The sample is incubated at a temperature of 20 C, and DO is measured initially and after 5 day. The 5-day BOD is determined by subtracting the two values. The DO is typically measured using either the Winkler method or a membrane electrode. Because the BOD concentration is rarely known before the test, several different dilutions are performed to cover the range of possible BOD values.

CHEMICAL OXYGEN DEMAND The chemical oxygen demand (COD) test is used similarly to the BOD test to measure the concentration of a wastewater sample. In some cases, the COD data can be correlated with the BOD data, although this takes some experience to establish reliable correlation factors, which can range from 0.4 to 0.8. The COD test is used primarily to evaluate industrial waste, whereas the BOD test is used for biological waste.[2] Advantages of the COD test are that it can be performed in about 3 hr, as compared to 5 day for the BOD test, and it can be utilized in wastewater with high toxicity, where a BOD test cannot. The COD test utilizes a strong oxidizing agent (usually potassium dichromate) under heated acidic (usually sulfuric) conditions in the presence of chromium and silver salts to convert organic matter to water and carbon dioxide. Unlike the BOD test, the COD test oxidizes all organic matter, including biologically resistant organic matter such as lignin. As a result, COD values are greater than BOD values. The COD test can be very accurate and precise for those samples with a COD of 50 mg L
1 or greater.[2] Inorganic ions such as chloride, bromide, and iodide can cause erroneously high COD values. Mercuric sulfate can be added to the sample to remove the chloride interference. A major disadvantage of the COD test is that disposal of hazardous wastes such as mercury,

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Oxygen Measurement: Biological–Chemical Oxygen Demand

Table 1 Typical BOD5 and COD concentrations found in municipal, industrial, and agricultural waste products and wastewater BOD5

COD


1

200

450


1

100–300

250–1,000

—

1,400 77,500

Source Raw municipal wastewater

Units mg L

Nitrogen–Plant

Domestic sewage

mg L

Beef feedlot runoff pond effluent

mg L
1
1

Beef feedlot runoff pond sludge

mg L

—

Dairy lagoon effluent

mg L
1

350

1,500

Dairy lagoon sludge

mg L
1

—

52,000

Swine lagoon effluent

mg L
1

400

1,200

Swine lagoon sludge

mg L
1

—

64,600

Milking center wastewater

mg L
1

1,000

5,000

Cheese production wastewater

mg L
1

3,200

5,600

Candy production effluent


1

mg L

1,600

3,000

Synthetic textile effluent

mg L
1

1,500

3,300

Slaughterhouse processing effluent

kg/1000 kg raw product

5.8–8.5

—

Vegetable processing effluent

kg/1000 kg raw product

7–55

14–96

Papermill effluent

kg/1000 kg pulp

60

—

Dairy cow manure (fresh, as excreted)

kg/d/640 kg animal

1.0

5.7

Beef cattle manure (fresh, as excreted)

kg/d/450 kg animal

0.6

2.5

Swine manure (fresh, as excreted)

kg/d/100 kg animal

0.2

0.6

Source: From Refs.[8,10].

hexavalent chromium, sulfuric acid, and silver can be a problem.[5] Typical BOD and COD concentrations vary greatly among waste types. Waste streams from animal feeding operations have BOD concentrations greater than 5000 mg L
1 compared to 200 mg L
1 for typical municipal wastewater.[3] Typical BOD and COD concentrations for a variety of municipal, industrial, and agricultural waste sources are presented in Table 1. Chemical oxygen demand concentrations are typically about 2–5 times greater than BOD concentrations for the same waste product.

CONCLUSION Dissolved oxygen is one of the most important water quality parameters for aquatic life. Because many municipal, industrial, and agricultural sources discharge wastewater to surface water bodies containing aquatic life, environmental regulations have been promulgated to limit the amount of oxygen-consuming organic waste products that can be discharged to a stream or lake. Methods for measuring the strength and concentration of these oxygen consuming waste products include the 5-day biochemical oxygen demand (BOD5) and COD tests.

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REFERENCES 1. Wheaton, F. Aquaculture engineering. CIGR Handbook of Agricultural Engineering; American Society of Agricultural Engineers: St. Joseph, MI, 1999; Vol. II, Part II. 2. Sawyer, C.N.; McCarty, P.L. Chemistry for Environmental Engineering, 3rd Ed.; McGraw-Hill, Inc.: New York, NY, 1978. 3. Miner, J.R.; Humenik, F.J.; Overcash, M.R. Managing Livestock Wastes to Preserve Environmental Quality; Iowa State University Press: Ames, IA, 2000. 4. Hodges, L. Environmental Pollution; Holt, Rinehard and Winston, Inc.: New York, NY, 1973. 5. APHA. Standard Methods for the Examination of Water and Wastewater, 20th Ed.; American Public Health Association: Washington, DC, 1998. 6. Zoller, U. Groundwater Contamination and Control; Marcel Dekker, Inc.: New York, NY, 1994. 7. ASAE. Uniform Terminology for Rural Waste Management; ASAE Standard S292.5, American Society of Agricultural Engineers: St. Joseph, MI, 1999. 8. Lindeburg, M.R. Civil Engineering Reference Manual; Professional Publications, Inc.: Belmont, CA, 1989. 9. Csuros, M.; Csuros, C. Microbiological Examination of Water and Wastewater; CRC Press: Boca Raton, FL, 1999. 10. NRCS. Agricultural Waste Management Field Handbook, Part 651; United States Natural Resources Conservation Service: Washington, DC, 1992.

Pathogens: General Characteristics Soil and Water Cons Research Unit, U.S. Department of Agriculture (USDA), Lincoln, Nebraska, U.S.A.

INTRODUCTION The transmission of pathogens by water is a highly effective way of spreading infectious disease among large numbers of people. As early as 6000 years ago, the association of contaminated water and illness was documented in Sanskrit and Greek writings that described treatments for ‘‘impure water.’’[1] Today, waterborne disease outbreaks continue to be responsible for high morbidity and mortality worldwide (Table 1).[2,3]

CLASSES OF PATHOGENS ASSOCIATED WITH WATER Due to the introduction of chlorination and filtration of drinking water supplies in the early 1900s, the waterborne outbreak paradigm in the United States shifted from bacteria, as the primary agents causing waterborne disease, to protozoan parasites and enteric viruses. Currently, these groups of micro-organisms and others cause water-related disease worldwide. Table 2 differentiates between the different types of micro-organisms: bacteria, viruses, protozoan parasites, blue–green algae, and helminthes. For each microbial group, examples of water-related pathogens and associated diseases are also listed.

CATEGORIES OF WATER-ASSOCIATED DISEASES There are four disease categories related to water transmission of microbial pathogens: 1) waterborne; 2) water-washed; 3) water-based; and 4) water-related diseases.[4] Infectious agents that are excreted in the feces and transmitted by ingestion of contaminated water cause waterborne diseases. Classic examples of waterborne pathogens include Cryptosporidium parvum (cryptosporidiosis), poliovirus (polio), and Vibrio cholera (cholera). Water-washed infections occur in areas where personal hygiene and water availability is poor. Person-to-person transmission and contact with unclean household items effectively transmit waterwashed pathogens such as Trachoma (eye infections)

and Shigella (dysentery). Water-based diseases arise from infection with pathogens that spend all or a portion of their life in water. Examples of these include Dracunculus medinesis (dracontaiasis) and Schistosoma species (schistosomiasis). Unlike the other categories, water-related diseases are carried by water insects, as in the case of malaria, which is carried by mosquitoes.[2,4]

ENTERIC PATHOGENS Micro-organisms that are excreted in the feces and infect the gastrointestinal tract are called enteric pathogens. Enteric pathogens cause a wide range of illness from asymptomatic (no clinical signs but microbe can grow and may be transmitted to susceptible individuals) to mild intestinal symptoms (diarrhea, fever, malaise, etc.) to paralysis. Enteric pathogens are probably the most important causes of water-washed and waterborne infections worldwide. Approximately 31%, 15%, and 10% of reported waterborne outbreaks are caused by enteric protozoan (Giardia and Cryptosporidium), viral, and bacterial agents, respectively.[5] However, the etiological agents responsible for approximately half of all reported outbreaks go unrecognized. The unidentified agents causing these outbreaks are thought to be enteric viruses due to epidemiological and clinical similarities.[6]

PATHOGEN CHARACTERISTICS THAT ENHANCE PATHOGEN SURVIVAL AND TRANSMISSION Traits that may enhance pathogen survival and transmission in the environment may include numbers and location of pathogen reservoirs, concentration of pathogens excreted and mode of excretion, infectious dose, severity of illness, and individual pathogen traits. Because some pathogens, termed zoonotic, are able to infect humans and animals (domestic and wildlife) they can be distributed throughout the environment, contaminating land, air, and water (Table 3). Excretion of pathogens in the feces is another important characteristic since they can be released in very high 803

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Nitrogen–Plant

Jeanette Thurston-Enriquez

804

Pathogens: General Characteristics

Table 1 Worldwide waterborne outbreak(s) Country (year(s) of outbreak) United States (1993)

Disease (disease agent)

Number of cases (number of fatalities)

Cryptosporidiosis (Cryptosporidium parvum)

England (1971–1980)

Giardiasis (Giardia lamblia)

Czech Soviet Republic (1979–1982)

Shigellosis (Shigella)

>400,000 (>50) 60 (0) 287 (not indicated)

Nitrogen–Plant

Source: From Ref.[3].

concentrations. Table 4 lists the concentrations of enteric pathogens shed in feces.[7] Depending on the infectious dose, or the number of micro-organisms required to produce disease, numbers ranging from over 106 down to 1 infectious agent must be ingested. For example, ingestion of as little as 1 viral particle can induce illness in a susceptible individual compared to some bacterial pathogens that require ingestion of hundreds to thousands of organisms. Disease produced by enteric pathogens is generally mild, selflimiting, and in some cases, the infected individual may be shedding the pathogen without any symptoms (asymptomatic shedding). Fecally transmitted pathogens that produce mild symptoms, or are asymptomatic, enable the infected individual to effectively contaminate the environment, unlike other diseases that render the infected individual immobile. Genetically acquired microbial traits enable some microbial groups to be more resistant to heat, pH, or chemical (chlorine, ozone, etc.) and physical (ultraviolet light, gamma irradiation) disinfectants than others. Because of these differences, some pathogens survive longer in the environment than others. In general, enteric viruses and protozoan parasites survive longer in water subjected to environmental conditions and disinfected water than enteric bacteria.

ENVIRONMENTAL FACTORS AFFECTING PATHOGEN SURVIVAL Unlike pathogens that need to be transmitted by direct or close contact to infected individuals (i.e., gonorrhea, human immunodeficiency virus, and herpes), enteric pathogens are hardy enough to survive conditions outside the body for long periods. Their survival depends on environmental conditions such as temperature, cell aggregation, sunlight exposure, pH, and presence of predatory organisms, inorganic or organic matter, and chemicals that may affect their survival. As temperature increases, the inactivation or dying-off of enteric pathogens increases. For example, Giardia lamblia cysts remain viable in water for 77 days and 4 days at 8 C and 37 C, respectively.[8] Since enteric pathogens infect the gastrointestinal tract they can withstand fairly low pH values, although high pH conditions may inactivate enteric viruses. Aggregation or

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adsorption to organic or inorganic matter may serve to protect or shield enteric pathogens from the effects of sunlight, chemical treatments, or predatory organisms. Also, macro invertebrates (nematodes and amphipods) and protozoa have been shown to ingest pathogens and protect them from the effects of water treatment.[9] Sunlight may decrease pathogen survival due to the effects of ultraviolet light damage. Ultraviolet light produces nucleic acid damage that can be repaired by some micro-organisms (bacteria) but not others (viruses).

PATHOGEN WATER ROUTES OF TRANSMISSION Human and animal reservoirs of enteric disease can contaminate land, water, or air through several routes (Fig. 1). Fecal waste is deposited onto land in several ways: direct deposition by domestic animals and wildlife, piled for storage, piled (composting) or spread (land applied human biosolids) for treatment, and by agricultural practices (spraying, spreading, or injection of waste into/onto soil). Rain or other events that produce runoff from fecally contaminated sites increases the potential for transport of pathogens to surface waters that may be used for drinking, shellfish harvesting, irrigation, or recreational purposes. In addition, pathogens may be transported through soil and contaminate groundwater. Airborne transmission of pathogens via water droplets generated by human and animal activities or wind is another way by which pathogens in water may be transmitted. These water droplets may be transported naturally (surf droplets), by agricultural activities (spray irrigation), and other human practices (showers, cooling towers) through air and are capable of transmitting disease either through ingestion, inhalation, or contact.[2] Although considered foodborne, pathogens present in water used for harvesting vegetable crops or shellfish may increase the risk of illness through consumption of these products.

WATER TREATMENT There are many treatment practices for the reduction of pathogens in water and waste. These include

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Table 2 Types and characteristics of enteric and other pathogenic micro-organisms that can be transmitted by water

Bacteria

Viruses

Protozoa

Helminths

Microbial characteristics Prokaryotic, single-celled organisms Cell wall and membrane surrounding cellular components Reproduce via binary fission Susceptible to disinfectants and environmental conditions (compared to other water pathogens) Spores and dormant cells formed by some bacteria enable them to survive hash conditions (example Clostridium)

Disease or complication

Salmonella

Diarrhea, typhoid

Campylobacter

Bloody diarrhea

Enterohemorrhagic Escherichia coli

Bloody diarrhea, hemolytic uremic syndrome

Enteroinvasive E. coli

Dysentery

Enteropathogenic E. coli

Diarrhea

Enterotoxigenic E. coli

Diarrhea

Size range 0.1–10 mm

Shigella

Diarrhea

Obligate parasites (require a host for replication)

Hepatitis A and E

Liver disease

Composed of protein outer capsid surrounding nucleic acid core

Enteroviruses (polioviruses, coxsackieviruses, echoviruses, and enterovirus types 68–71)

Febrile and respiratory illness, meningitis, diarrhea, encephalitis, and others

Do not carry out metabolic functions

Rotaviruses (group A and B)

Diarrhea

Nucleic acid may be double or single stranded

Human caliciviruses

Vomiting, diarrhea

Long-term environmental survival due to simple structure and no requirement for nutrients

Astrovirus

Diarrhea

Size range 0.01–0.1 mm

Adenovirus

Diarrhea, eye and respiratory infections

Single-celled eukaryotic organisms

Giardia lamblia

Diarrhea

Complex structure

Cryptosporidium parvum

Diarrhea

Sexual or asexual replication

Cyclospora cayetanensis

Diarrhea

Some produce environmentally resistant stages (example Cryptosporidium oocysts and Giardia cysts)

Microsporidia

Diarrhea, kidney and respiratory infections

Size range 1–100 mm

Toxoplasma gondii

Flu-like (adults), encephalitis, and ocular disease (children)

Entamoeba histolytica

Amoebic dysentery

Naegleria fowleri

Meningoencephalitits

Multicellular, worms

Ascaris lumbricoides

Ascariasis

Complex life-cycles (one or more hosts required)

Necator americanus

Hookworm

Eggs are environmentally resistant

Trichuris trichiura

Whipworm

Taenia saginata

Beef tapeworm

Schistosoma mansoni

Schistosomiasis

Procaryotic single-celled organisms

Anabaena

Replicate by binary fission or fragmentation

Nodularia

All can produce toxins that can cause liver damage, neural damage, gastrointestinal symptoms

Some species produce toxins

Microcystsis

Some are resistant to extreme environmental conditions

Nostoc

Size range 1–100 mm

Alexandrium

9

Size range 1–10 mm Cyanobacteria (blue–green algae)

Pathogens associated with water disease

Source: From Ref.[4].

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Possible carcinogens

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Types of micro-organisms associated with water

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Pathogens: General Characteristics

Table 3 Examples of known and potential zoonotic enteric pathogens and their hosts

Table 4 Concentrations of enteric pathogens excreted in feces

Zoonotic pathogens

Enteric pathogen group

Pathogen hosts

Bacteria E. coli

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Salmonella

Range of pathogen concentration (per gram of feces)

Humans, domestic and wild animals

Bacteria

104–1010

Viruses

103–1012

Humans, domestic and wild fowl, and other animals

Protozoan parasites

106–107

[7]

Source: From Ref. .

Viruses Caliciviruses

Humans and potentially cattle and swine

Hepatitis E

Humans and potentially swine and rats

Protozoa Cryptosporidium

Humans, domestic and wild animals

Giardia

Humans, domestic and wild animals

filtration (sand, activated carbon, flora), coagulation and sedimentation, disinfection, composting, and constructed wetlands. A multibarrier approach is used for the removal or reduction of pathogenic microorganisms in municipal wastewater. The general steps include: 1) removal or sedimentation of large debris (primary treatment); 2) biological degradation (trickling filters, aeration tank, lagoon) and/or disinfection (secondary treatment); and 3) a combination of

physical (coagulation and filtration) and chemical (chlorine disinfection) steps in order to further reduce biological and chemical contaminants (tertiary treatment). Wastewater treatment has been shown to reduce bacteria by 99.99999%, viruses by 99.999% and protozoa by 99.993% (Giardia cysts) and 99.95% (Cryptosporidium oocysts).[10] Constructed wetlands have been applied as a low cost alternative to wastewater treatment. As water flows through the vegetated wetland absorption to flora, gravel or sand substrate, natural die-off, sedimentation, filtration, and predation occurs. For solid waste, compost piles can be used for the inactivation or reduction of pathogenic microorganisms. Temperature is the key factor controlling pathogen reduction in compost piles where it is suggested that 55 C must be maintained for at least 3 day and 15 day for static and aerated piles, respectively.[9] Difficulties in maintaining uniform temperature throughout the pile and regrowth of bacterial pathogens are two disadvantages for use of composting

Fig. 1 Environmental routes of pathogen transmission.

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for pathogen reduction.[9] Due to the different physical and genetic characteristics of pathogenic microorganisms, a combination of treatment or prevention practices may be necessary for their removal, death, or inactivation. For example, the ultimate barrier between pathogens and drinking water consumers is chemical or physical disinfection for inactivation of enteric viruses. Their small size (20–80 nm) enables them to bypass filtration processes, whereas filtration is required for the removal of protozoan parasites (>2 m in size) since they are highly resistant to most water disinfectants. EMERGING PATHOGENS Control of water transmission of pathogenic microorganisms continues to be a public health concern because there is an increasing immunocompromised (elderly, cancer, and AIDS patients) population and because a large percentage of waterborne outbreaks go unrecognized.[5,9,11] In fact, while less than 20 waterborne disease outbreaks are documented each year in the United States, it is estimated that the true incidence may be 10–100 times higher.[5] Even for wellestablished pathogens, the true incidence is unknown in the United States because: 1) reporting waterborne outbreaks and its agents is voluntary; 2) there is a lack of efficient detection methods for some important enteric pathogens; 3) contamination events in water are usually transient, thus etiological agents are not detected; and 4) individuals may not seek medical attention since acute gastrointestinal illness (AGI) is usually self-limiting and mild.[4,11] Furthermore, very little, if any, information exists on infectious agents that are newly recognized, or ‘‘emerging’’ in water supplies and their impact on waterborne disease outbreaks. CONCLUSION Water serves as a passive carrier for the transmission of disease. Human and animal populations may be exposed to pathogens through direct contact, ingestion, or inhalation of contaminated water. Enteric pathogens are the most important group of organisms

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807

relating to waterborne and water-washed diseases. They are excreted in high numbers in the feces and have traits that allow their survival and successful transmission in the environment. These traits should be considered when making decisions regarding strategies for limiting their transmission by water. A multibarrier approach may be best for efficient removal of bacterial, viral, and protozoan pathogens.

REFERENCES 1. Baker, M.N. The Quest for Pure Water; American Water Works Association: New York, 1948. 2. Gerba, C.P.; Rusin, P.; Enriquez, C.E.; Johnson, D. Environmentally transmitted pathogens. In Environmental Microbiology; Maier, R.M., Pepper, I.L., Gerba, C.P., Eds.; Academic Press: San Diego, 2000; 447–489. 3. Hunter, P.R. Waterborne Disease Epidemiology and Ecology; John Wiley and Sons Inc.: New York, 1998. 4. Moe, C.L. Waterborne transmission of infectious agents. In Manual of Environmental Microbiology; Hurst, C.J., Ed.; ASM Press: Washington, DC, 1997; 136–152. 5. Gerba, C.P. Pathogens in the environment. In Pollution Science; Pepper, I.L., Gerba, C.P., Brusseau, M.L., Eds.; Academic Press: San Diego, 1996; 278–299. 6. Barwick, R.S.; Levy, D.A.; Craun, G.F.; Beach, M.J.; Calderon, R.L. Surveillance for waterborne-disease outbreaks—United States, 1997–1998. Morb. Mortal. Wkly Rep. 2000, 49, 1–35. 7. Haas, C.N.; Rose, J.B.; Gerba, C.P. Quantitative Microbial Risk Assessment; John Wiley and Sons, Inc.: New York, 1999. 8. Bingham, A.K.; Jarroll, E.; Meyer, E. Giardia spp.: physical factors of exystation in vitro, and exystation vs. eosin exclusion as determinants of viability. Exp. Parasitol. 1979, 47, 284–291. 9. Bitton, G. Wastewater Microbiology; Wiley-Liss: New York, 1980. 10. Rose, J.B.; Carnahan, R.P. Pathogen removal by full scale wastewater treatment. Report to Florida Department of Environmental Regulation: Tallahassee, FL, 1992. 11. Waterborne Disease Studies and National Estimate of Waterborne Disease Occurrence. Notice of Data Availability and Request for Comments, 63 Federal Register 154 (11 August 1998), 42,849–42,852.

Nitrogen–Plant

Pathogens: General Characteristics

Pathogens: Transport by Water Graeme D. Buchan Centre for Soil and Environmental Quality, Lincoln University, Canterbury, New Zealand

Nitrogen–Plant

Markus Flury Center of Multiphase Environmental Research, Department of Crop and Soil Sciences, Washington State University, Pullman, Washington, U.S.A.

INTRODUCTION This article covers transport of pathogens from different sources (e.g., land-applied animal or human wastes), either over ground or through the subsurface (soil, sediments, or rock formations), and the possible arrival of contaminating pathogens at surface or groundwater. Topics covered include the following: characteristics of pathogens and of the transporting pore space; mechanisms of transport; pathogen interactions with soil; measurement and modeling of pathogen transport; and practical aspects, including conditions favoring transport to water and their avoidance. Transport and removal of pathogenic microorganisms are increasingly important, as humans intensify both their demands for freshwater and their disposal of wastes on land. Microbial transport is also important for the following processes: bioremediation of groundwater, e.g., the introduction of beneficial microbes to degrade chemical contaminants; filtration of wastewater, e.g., in buffer or filter strips along waterways, or in wetlands; and sand bed filtration of drinking water. Pathogens are released to the environment from two major sources: 1) agricultural activities involving the disposal of animal wastes as manure or effluents and 2) human wastes disposed of via septic tank systems or land-applied sewage sludge (biosolids). Pathogens include helminths (not covered here), protozoa, bacteria, and viruses, each of which has distinct transport characteristics in surface and subsurface waters.

larger groups by clumping of bacteria or formation of virus aggregates.[3] This increases the probability of their removal by straining (see below). Pathogens vary in their survivability in the environment. Bacteria are typically short-lived, but can reproduce and grow quickly as well. Oocysts of the protozoan Cryptosporidium (size 4–6 mm) are robust and persistent.[4] Viruses multiply only in host cells; for example, bacteriophages are viruses that infect and multiply inside bacteria, and human enteric viruses need humans to reproduce. The Pore Space In soil, the macropores (or drainable pores) have equivalent diameters d > 30 mm,[1] are air-filled at field capacity, and provide the main drainage pathways during heavy rainfall or irrigation. At the wilting point (water potential of 1.5 MPa), only pores with d < 0.2 mm remain water-filled (Fig. 1). Subsurface materials often have dual-porosity character,[4,5] with larger macropores (or fissures or conduits) embedded in a medium with finer pores, providing fast-track pathways (Fig. 2). Pathogen entrapment in soil can alter pore geometry by clogging of pores or formation of surface biofilms. An extreme example of this is the ‘‘biomat,’’ which forms in soils used for septic system disposal, made up of layers of intense microbial activity.

TRANSPORT AND FATE AT AND BELOW LAND SURFACES CHARACTERISTICS OF PATHOGENS AND PORES The Pathogens The size ranges of soil pores and soil biota vary over orders of magnitude[1,2] (Fig. 1). Viruses are in the range of nanometers, bacteria in the range of micrometers and protozoa in the range of micrometers to hundreds of micrometers. Pathogens may also form 808

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Overland Flow vs. Infiltration The first step controlling pathogen fate is at the land surface, where flow may separate into overland flow or infiltration (Fig. 3). Note that ‘‘overland flow’’ means (as it says) flow above the ground surface. ‘‘Runoff,’’ by contrast, may include both overland flow and subsurface flow submerged temporarily beneath the ground surface, which reemerges at lower

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compaction of soil such as by animals in a stockyard, or by the presence of frozen surface soil.

Pathogens can be filtered from overland flow by surface vegetation (e.g., in buffer or vegetated filter strips, which act as a ‘‘line of defense’’ between waterways and animal waste disposal sites). Die-off and waste sterilization result from prolonged surface exposure to desiccation, natural UV in sunlight, or freeze–thaw cycles, and predation by other microorganisms.

Fig. 1 Size ranges of soil particles and pores and of microorganisms. The vertical broken lines represent the equivalent diameters of pores, which empty at permanent wilting point (PWP), and at the transition between micropores and macropores. Under heavy rain or irrigation, water-filled macropores provide an efficient fast-track pathway for transport of pathogens or solutes (Fig. 2).

elevation—so-called ‘‘through flow’’ or ‘‘interflow’’. This distinction is important because the through flow is subject to the filtering action of soil, whereas overland flow is not (although it may be filtered by above-ground vegetation). Overland flow is most pronounced during heavy rainfall or irrigation and is encouraged by decrease in surface hydraulic conductivity, increase in slope or prior surface wetness,

Fig. 2 Illustration of macropore flow. This strongly structured, cultivated soil was flood irrigated with dye solution, causing preferential flow through macropores. This type of flow in ‘‘dual-porosity’’ soils, subsoils, or underlying rock (e.g., limestone with fissures or conduits) can fasttrack contaminants and pathogens to groundwater. (Photo courtesy of K.C. Cameron, Lincoln University, Canterbury, New Zealand.)

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Fate in the Subsurface: Transport, Entrapment, and Survival Within the soil, pathogens are present either in suspension in the aqueous phase (including the air–water interface), attached as single or multiple organisms on solid surfaces (including organic or clay particles in suspension in water), or in biofilms covering solid surfaces.[6]

Transport Processes The main transport mechanism is passive convection with local flow of the soil solution. The transported microbes are carried either free floating (singly or in clumps), or adsorbed onto colloid particles (e.g., organic particles or clay).[2] However, some microbes can move actively: chemotaxis is movement along a chemical gradient, e.g., toward a food substrate. The rate of transport increases with soil wetness and as mean pore size increases and pores become more continuous. Preferential flow can occur during heavy infiltration via macropores in soil (Fig. 2), or fissures or conduits in rock formations (e.g., limestone in karstic formations).[7] Microbes can travel faster than the mean pore-water velocity because of two differential flow effects. 1. Size exclusion: Microbes tend to be carried near pore centerlines and, because of their large size, cannot penetrate the ‘‘slow flow’’ zones next to pore walls, where flow is slowed by viscous drag. 2. Detouring effect: Microbes are swept preferentially through the larger pores, avoiding smaller pores. As a consequence, microbes can travel faster than the average speed of the moving water, and so can precede the arrival of solutes or tracers.[5]

Nitrogen–Plant

Fate of Pathogens in Overland Flow

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Pathogens: Transport by Water

Nitrogen–Plant

Fig. 3 Partitioning of water at the soil surface. Pathogens from agricultural or human wastes, or wild animals or birds, may be transported to surface or groundwater via overland flow, infiltration into soil followed by through flow, or drainage through soil. The partitioning is controlled by the rate of application of water or effluent vs. the soil’s infiltration rate. Transport in drainage may be fast-tracked to groundwater through soil or bedrock, which has dual-porosity character, with large macropores or fissures (Fig. 2). Fortunately, most infiltrating water is filtered and disinfected by soils and sediments, which act as defensive barriers.

Entrapment and Survival Microorganisms are subject to various processes that affect their transport and survival in the environment. These processes include the following:  Cell multiplication and death: Inactivation (death or loss of viability) increases at high temperatures or when food sources (organic matter and nutrients) are scarce or aeration conditions are unfavorable. Certain microorganisms, such as bacteria, can multiply rapidly when conditions become favorable again. Viruses may be inactivated at certain surfaces, for instance, in the presence of iron oxides, which are common in soils.  Predation: Microorganisms can feed on other microorganisms or otherwise inactivate them, e.g., some protozoans ingest bacteria and viruses.  Straining: Microorganisms can get trapped in pore throats that restrict passage. This effect tends to become important when the microbe (or clump) diameter is >5% of the average grain diameter. Straining is a weak effect in sandy or gravelly soils or aquifers, where the sizes of pores are large compared to microorganisms.  Filtration and attachment to solid surfaces: Microorganisms are removed from water by attachment (adhesion) to soil particle surfaces. Attachment is controlled by the surface properties of microbes and soil particles and the chemical composition of the water. Protozoa and bacteria can also actively bind to solid surfaces by using filaments or exudates.  Sedimentation: Microorganisms can settle out of water under gravity. This mechanism is usually

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minor because most microorganisms have a density close to that of water and are approximately neutrally buoyant. Swimming capabilities of some microorganisms will also prevent settling by gravity.  Detachment (release) from solid surfaces: Microorganisms can detach from solid surfaces when changes occur in the chemical or physical conditions that initially led to attachment, e.g., bacterial cells may be washed away from soil particle surfaces when water flow rates increase.  Air–water interface trapping: Certain microorganisms (e.g., viruses) tend to attach to air–water interfaces or air–water–solid interfaces, an event that occurs in surface waters or unsaturated subsurface environments, such as soils. Microbes are likely to be less efficiently removed from water moving in through flow than from deep percolation. First, through flow is likely to occur at higher soil water content, and indeed is often saturated flow, so there is less liquid–gas interface available for microbial removal. Also, travel is likely to be shorter for through flow than for deep percolation.

MEASURING PATHOGEN TRANSPORT Measuring transport is complicated by the difficulty of microbial detection methods, especially for viruses. Methods include direct sampling for protozoa, bacteria, and viruses;[8] using viruses or bacteriophages as tracers, e.g., with dye attachment;[3,9] or using surrogate particles (e.g., fluorescent latex microspheres).[3] Measuring microorganisms via samples taken from

Pathogens: Transport by Water

subsurface environments requires special care to maintain the environmental conditions of the sample location, in order to avoid alteration of microbial communities.

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understanding and avoiding pathogen transport to water are essential for sustainable development.

MODELING The difficulty of modeling transport processes in permeable media increases as we progress from water through solutes to microbes. For microbes, additional complexities include attachment and detachment (and their dependence on surface charges), multiplication and inactivation. In general, the modeling approaches consider convective and dispersive transport by moving water, coupled with terms for attachment/ detachment and survival,[9,10] and with water flow separated into macropore flow and matric flow.[2,5]

AVOIDING WATER CONTAMINATION A mechanistic understanding of the transport and fate of pathogens leads to the development of best management practices (BMPs) for avoiding water contamination. Historically, the single factor causing greatest risk of contamination is excess water from rainfall or snowmelt. This can fasttrack pathogens, either in overland flow to surface water, or via macropore or conduit flow to groundwater, bypassing the natural disinfection mechanisms present in the soil. Natural soils act as filters that protect groundwater from pathogens by removing them as described above. Degradation of soils by erosion or compaction will, however, reduce their natural disinfection capabilities.

CONCLUSION Earth’s natural subsurface materials play a critical ‘‘barrier’’ role in disinfecting water from pathogenic microorganisms. However, sometimes this barrier fails or is degraded, and pathogens migrate to and contaminate water resources. As humans intensify land use,

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Graeme Buchan is grateful for Research from the Winston Churchill Memorial and the OECD Co-operative Research (Biological Resource Management for Agriculture Systems).

Fellowships Trust (NZ) Programme Sustainable

REFERENCES 1. Cameron, K.C.; Buchan, G.D. Porosity and pore-size distribution. In Encyclopedia of Soil Science; Lal, R., Ed.; Marcel Dekker: New York, 2002; 1031–1034. 2. McGechan, M.B.; Lewis, D.R. Transport of particulate and colloid-sorbed contaminants through soil: part 1. General principles. Biosyst. Eng. 2002, 85, 255–273. 3. Gitis, V.; Adin, A.; Nasser, A.; Gun, J.; Lev, O. Fluorescent dye labelled bacteriophages—a new tracer for the investigation of viral transport in porous media: 1. Introduction and characterization. Water Res. 2002, 36, 4227–4234. 4. Morris, B.L.; Foster, S. Cryptosporidium contamination hazard assessment and risk management for british groundwater sources. Water Sci. Technol. 2000, 41, 67–77. 5. Unc, A.; Goss, M.J. Transport of bacteria from manure and protection of water resources. Appl. Soil Ecol. 2004, 25, 1–18. 6. Ginn, T.R.; Wood, B.D.; Nelson, K.E.; Scheibe, T.D.; Murphy, E.M.; Clement, T.P. Processes in microbial transport in the natural subsurface. Adv. Water Resour. 2002, 25, 1017–1042. 7. Conboy, M.J.; Goss, M.J. Natural protection of groundwater against bacteria of fecal origin. J. Contam. Hydrol. 2000, 43, 1–24. 8. Jin, Y.; Yates, M.V.; Yates, S.R. Microbial transport. In Methods of Soil Analysis: Part 4. Physical Methods; Dane, J., Topp, C., Eds.; Soil Science Society of America: Madison, WI, 2002; 1481–1509. 9. Jin, Y.; Flury, M. Fate and transport of viruses in porous media. Adv. Agron. 2002, 77, 39–102. 10. Yates, M.V.; Yates, S.R. Modeling microbial fate in the subsurface environment. CRC Crit. Rev. Environ. Control 1988, 17, 307–344.

Nitrogen–Plant

ACKNOWLEDGMENTS

Pesticide Contamination: Groundwater Roy F. Spalding Water Science Laboratory, University of Nebraska, Lincoln, Nebraska, U.S.A.

Nitrogen–Plant

INTRODUCTION Trace concentrations of most of the commonly used pesticides have been confirmed in groundwaters of the United States. Since groundwater is the source of 53% of the potable water, the more toxic pesticides and their transformation products are a concern from the standpoint of human health. Others are a risk to the environment in areas where contaminated groundwater enters surface water. Through toxicological testing, the USEPA has established Maximum Contaminant Levels (MCLs) or lifetime Health Advisory Levels (HALs) for several pesticides (Table 1). The EPA also has a separate list of unregulated compounds, including newly registered pesticides and their transformation products, such as acetochlor and alachlor-ESA, that are presently being evaluated or being considered for toxicological evaluation. Based on the results of the EPAs National Pesticide Assessment,[2] 10.4% of 94,600 community systems contained detectable concentrations of at least one pesticide. Evaluation of these results led to an estimated 0.6% of rural domestic wells containing one or more pesticides above the MCL.

PESTICIDE USE In the United States about 80% of pesticide usage is in agriculture. The remainder is used by industry, homeowners, and gardeners. About 500 million pounds of herbicide, 180 million pounds of insecticide, and 70 million pounds of fungicide were applied for agricultural purposes in 1993.[3] Several maps of the United States delineate usage patterns of several pesticides.[4] The majority of the triazine and amide herbicides are applied to fields in the north central corn belt states of Michigan, Wisconsin, Minnesota, Nebraska, Iowa, Illinois, Indiana, and Ohio. Commonly used organophosphorus insecticides are more heavily applied to fields in California and along the southeastern seaboard than in the northern corn belt. Carbamate and thiocarbamate pesticides are heavily used in potato growing areas of northern Maine, Idaho, the Delmarva Peninsula, and vegetable fields of California and the southeastern coastal states. Fungicide use is concentrated in high humidity and irrigated areas of the 812

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coastal states and to some extent along the Great Lakes and Mississippi River Valley. The fumigants carbon tetrachloride and ethylene dibromide (EDB) were used heavily in the past at grain storage elevators throughout the Midwest and elsewhere in the United States.

ASSOCIATED PESTICIDE BEHAVIOR IN SOILS AND WATER Although pesticide use is a dominant factor in groundwater contamination, leaching variability among pesticides exhibiting similar behaviors is striking and explains why several heavily used pesticides seldom if ever are detected in groundwater. In general, pesticides within a class have similar chemical characteristics upon which soil leaching predictions can be made based on persistence, solubility, and mobility. Pesticide class relationships with soils and water transport described in the following text are detailed in Ref.[5]. Individual frequencies of groundwater pesticide detection, in parenthesis next to commonly used products, are calculated from the Pesticide Groundwater Data Base (PGWDB)[4] and the National Water Quality Assessment (NAWQA) database.[6] High frequencies of detection identify those pesticides with a disposition to leach. Insecticides Chlorinated hydrocarbons are one of the oldest chemical classes of insecticides. Some of the best-known compounds include aldrin, dieldrin, DDE, DDT, endrin, and toxaphene. Although banned since the 1960s, their extremely persistent nature precludes their detection in very trace quantities in groundwater of the upper Midwest. On the other hand, heavily used organophosphates like malathion, methylparathion, disulfoton, and others have been extensively surveyed during several groundwater monitoring studies and have not been detected. The organophosphate insecticides, parathion (not reported (NR), http://fcn.state.fl.us/citytlh/water/apintro. htmlhttp://www.epa.gov/r5water/ordcom/ (accessed January 2001). 26. California Department of Health Services: Drinking Water Source Assessment and Protection (DWSAP) Program, Demonstration Project—Yosemite National Park, Yosemite Valley Water System: http://www.dhs. ca.gov/ps/ddwem/dwsap/dwsapdemos/yosemite.htm (accessed January 2001). 27. California Department of Health Services: Drinking Water Source Assessment and Protection (DWSAP) Program, Demonstration Project—City of Sebastopol: http://www.dhs.ca.gov/ps/ddwem/ dwsap/dwsapdemos/sebastopol.htm (accessed January 2001).

Wastewater– Yellow

8. U.S. Environmental Protection Agency. National Primary Drinking Water Regulations: Groundwater Rule; Proposed Rule 40 CFR Parts 141 and 142. Federal Register; 2000; (FR) 65(91) pp. 30194–30274 (May 10, 2000). Note Federal Register Can Be Accessed On Line: http://www.access.gpo.gov/su_docs/aces/aces140.html (accessed January 2001). 9. California Department of Health Services (DHS), Division of Drinking Water and Environmental Management, Drinking Water Source Assessment and Protection (DWSAP) Program; http://www.dhs.ca.gov/ ps/ddwem/dwsap/DWSAP_document.pdf (accessed January 2001). California Department of Health Services, Division of Drinking Water and Environmental Management: Sacramento, CA, 1999. 10. U.S. Environmental Protection Agency, National Water Quality Inventory: 1998 Report to Congress, EPA-816-R-00-013; U.S. EPA Office of Water (4606): Washington, DC, August, 2000. 11. U.S. Environmental Protection Agency, State Source Water Assessment and Protection Programs Guidance, Final Guidance EPA-816-R-97-009; U.S. EPA Office of Water: Washington, DC, 1997. 12. U.S. Environmental Protection Agency, Benefits and Cost of Prevention, Case Studies of Community Wellhead Protection, EPA/625/R-93/002; U.S. EPA Office of Water: Washington, DC, 1995. 13. National Research Council, Alternatives for Ground Water Cleanup; Committee on Ground Water Cleanup Alternatives, Water Science and Technology Board, Commission on Geoscience, Environment, and Resources, National Research Council, National Academy Press: Washington, DC, 1994. 14. Heath, R.C. Basic Groundwater Hydrology. U.S. Geological Survey Water-Supply Paper 2220; 1983. 15. U.S. Environmental Protection Agency, Guidelines for the Delineation of Wellhead Protection Areas; U.S. EPA Office of Ground-Water Protection: Washington, DC, 1987. 16. U.S. Environmental Protection Agency (EPA). Maps of States and Territories with Approved Wellhead Protection Programs; http://www.epa.gov/OGWDW/ wellhead.html (accessed January 2001). 17. California Department of Water Resources, California Well Standards, Bulletin 74-90; Department of Water Resources: Sacramento, CA, 1990.

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Wells: Drilling Thomas Marek Texas A&M System Agriculture Research and Extension Center, Amarillo, Texas, U.S.A.

INTRODUCTION There are many required processes entailed in the proper design and construction of a sand-free irrigation well. This article addresses the typical sequence of operations that should be used in the proper drilling of a new, sand-free irrigation well. Due to the number of operations involved in the process, the owner should outline and agree on them with prospective drillers before the operations begin.

CHOOSING A DRILLER The first consideration in drilling an irrigation well is selecting a driller. Some drillers have limited knowledge of advanced well design and development. The selection of a driller is most important and should be made from drillers who have kept up with technical advances. Experience does not overcome innovations in technology and should be viewed as complimentary to new, successful advances in drilling technologies. SELECTING A POTENTIAL WELL SITE

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Selection of a potential site(s) may include the relative availability of electricity or natural gas, but should be chosen according to the best available aquifer information. Estimates of the formation and saturated thickness of the aquifer can be made from existing or adjacent wells in relation to the proposed new site(s). The proposed well site(s) should be located far enough from existing irrigation wells to prevent drawdown interference.[1] Water district or authority rules typically govern the distance between wells. Every well, once pumping is initiated, establishes a drawdown curve and if wells are located too close together, the respective drawdown curves will begin to overlap, causing more drawdown and subsequently requiring more horsepower. Drilling of the test hole(s) is needed to gather information as to the anticipated production of the well and to provide a vertical formation map.

screened sections in the saturated zone, and for determining the proper sized gravel. This information is required for construction of the well screen prior to the main well drilling activities as the time period from well drilling initiation to gravel installation is limited. Sized gravel must be acquired prior to the main drilling activity. Without a test hole, the driller is essentially guessing at what is beneath the ground. Once the screen and gravel are in the borehole, next to nothing can be done to change either. The test hole drilling fluid should not be that of bentonite as bentonite is a clay-based compound that swells when wetted with water and seals the ‘‘pores’’ of the drilled borehole. Reasons for avoiding the use of bentonite is that it can mask the logging characteristics of some aquifer strata(s) and is difficult to remove from the borehole. A strongly suggested, preferred drilling fluid is that of a biological polymer. It is recommended that the test hole be drilled throughout the water bearing formation and that drilling samples are obtained as drilling progresses. While the number of samples can vary, a determination of the number of samples to be collected should be made based upon the anticipated saturated thickness of the aquifer. Once the test site borehole is drilled, a series of logs should be conducted to correlate strata data as to the availability and productivity of water within each strata of the formation. The three types of logs recommended are gamma log, specific conductivity, and spontaneous potential.[2]

PROPERLY PLUGGING THE TEST BOREHOLE Once the borehole logs have been completed, the test hole should be filled and sealed to prevent contaminants from entering the aquifer. Most water authorities have regulations regarding this process.

ANALYSIS OF CUTTING SAMPLES AND LOGS TEST HOLE PROCESSES A test hole is required to size the well screen slot size(s) and determine how and where to construct the 1344

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Analyzing test hole cutting samples consists of placing each sample in a stacked set of progressively smaller sized sieves and shaking them on a mechanical shaker.

Wells: Drilling

SELECTING A NEW WELL SITE A comparison of the sets of borehole data should be made and a site chosen as to where the best available water is potentially located. If the difference between the data sets is minimal, other factors such as energy or road accessibility can be considered.

SELECTING A GRAVEL PACK In a sand-free well, the gravel pack prevents the sand from entering the well casing but allows the water to flow efficiently through the gravel and into the wellbore. Simply put, the gravel stops the sand and the perforated well casing stops the gravel. If one uses an inadequate gravel pack, sand will destroy the pump impellers in a short time. In addition, if one pumps much sand over time and forms a cavity around the perforated section of the well, the potential for the lower part of the formation to collapse onto the casing is possible. The sieved data collected are necessary to determine the needed gravel size properly. Analysis and gravel sizing requires some expertise as there are ‘‘judgment’’ and experience factors that have to be applied. One should not compromise on the gravel, as the choice to do so is unwise. In addition to size, one wants the gravel to be uniform. Another factor that constitutes ‘‘better’’ gravel is the amount of quartz in the rock of the gravel.

SELECTING A WELL SCREEN A well screen is strongly suggested in the water bearing regions of the aquifer. It is typically a fabricated, continuously wound type casing reinforced by solid, vertical bars attached to the interior of the wound section. One of the most popular types uses a triangular shape. The smaller or tapered edge of the triangular shaped screen is to the inside of the wellbore. In this manner, any gravel or sand that makes it through the initial outer edge of the screen is allowed to be excavated when the well is being developed and presents no obtrusion to water intake.

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Next, using the test hole data, one decides as to where the perforated portion of the well screen needs to be located. In some cases, the screen may be scheduled in a skipped fashion to reduce screen costs and to promote water movement in a more laterally distributed mode. As the well screen location(s) is governed by the test hole strata data, the slot size of the well screen is governed by the size of the gravel selected. The selection is critical to not restrict water flow into the well bore, yet be smaller to prevent any gravel entrance after the well development process. SELECTING A DRILLING METHOD There are two basic drilling methods: the direct rotary and the reverse circulation drilling technique, and each has its advantages and application. The rotary method uses a drag type bit with a ‘‘centering collar’’ and can be from 4 in. to 24 in. in diameter. This drilling method involves using a hollow stemmed drilling shaft with drilling fluid being pumped through the interior of the shaft and allowing the drilled materials to be returned to the ground surface around the exterior of the drill stem. The reverse circulation drilling technique suctions the drilling mixture essentially through the drill stem. Thus, the water is returned to the borehole around the outside of the drill stem. This method requires more horsepower than that of the direct rotary method due to the suction pump. The advantages are that this method uses less water and less drilling mixture to stabilize the borehole, and holes greater than 24 in. can be achieved. SPECIFYING A CORRECT DRILLING FLUID The use of a biological polymer (organic) compound is strongly recommended. The typical borehole drilling process occurs within 24 hr and coincides with the initiation of the ‘‘natural breakdown’’ period of the organic drilling compound. After drilling and the casing setting process, the driller may use a small amount of chlorine solution to assist in the rapid breakdown of the compound but not damage the well screen slots. CHOOSING A CASING AND BORE HOLE SIZE Data indicate that drilling a bigger borehole and installing a bigger diameter well casing results in only marginally better well bore inflow. It should be noted that the aquifer, well pack and well screen, govern the amount of water inflow. Four inches of gravel (radially) is typical around the well screen and adds 8 in. to the diameter of the casing diameter. Thus, if one uses a 16-in. casing, a 24-in. well bore will be required.

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The results of the shaker data yield a distribution of curves from each sample zone.[3] The drilling coordinator can then plot the sand-sieved distribution information on a semi-logarithmic paper. In conjunction with the family of curves of the sand-sieved data, the three logs collected from the test borehole provide supplementary evidence as to the specific capacity of the respective formations.

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MAIN WELL DRILLING AND SAMPLES It is suggested that cutting samples be obtained during main drilling operations. Although the well screen and most of the materials would already have been prepared from the testhole data, if there is significant difference in formation detected, it is better to substantiate it during the main drilling phase. Also, the main drilling samples can be run later, if desired, to determine how they differ from the test hole stratas.

SETTING CASING, SCREEN, AND GRAVEL At this point, the borehole will be completed and the well screen assembly, casing and gravel should be on site. The casing should be installed. However, the author strongly encourages one to install a metal airline onto the exterior of the casing and screen to provide the depth to water reading directly with a small jet of compressed air.

WELL DEVELOPMENT

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Proper well development is essential and determines whether a well will be sand free and without it, maximum efficiency of the well cannot be achieved. The first operation involves bailing the well. This operation cleans out drilling materials that have settled to the bottom of the hole. It is cautioned that the rate of bailer withdrawal needs to be controlled and should not be excessive, especially in the water bearing portion of the aquifer. This is especially true of bailers that are sized close in diameter to the interior diameter of the well casing and screen. Rapid ascension of a bailer through the water portion of the formation is ‘‘harshly’’ pushing the gravel into the aquifer stratas ahead of the bailer (creating a positive pressure wave) and ‘‘slamming’’ the gravel back against the outside of the well casing as it passes (creating a negative pressure wave) behind the bailer. The development process begins with a cable rig utilizing a substantially weighted surge block. Even better is the utilization of a double-flanged surge block. This tool is lowered to the bottom of the hole and worked upward from the bottom to the top of the screened section of the well in short, rapid repetitive steps. This lower-to-upper direction is necessary because the progression of the sequence will draw sand into the well casing and it is unwise to risk getting a surge block ‘‘stuck’’ in the screen section due to sand atop the block from a top to bottom sequence. What is desired with this development operation is that the

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Wells: Drilling

surge block ‘‘puff,’’ not ‘‘punch’’ by excessive operation rates, the gravel pack. Through this ‘‘upsetting’’ and ‘‘closing’’ of the gravel pack (created in front and behind the surge block), drilling particles, gravel fines, and fine sands enter the casing. This process also orients the gravel against the casing and sets the adjacent sand of the formation against the gravel. The range of aquifer addressed in each sequence of the development depends on the saturated thickness of the site. If the thickness is large, a large range for each sequence may be appropriate and acceptable. The rate of the operation is more critical. Rates are typically suggested at 3 ft/sec by the author. Subsequently, any bailing operations associated with pump reworking later in time can have a significant impact on the gravel pack and formation stability. If a bail operator ‘‘runs’’ a bailer at too fast a rate within the screened region of the casing, one runs the risk of upsetting all the previous development efforts and the well may begin to pump sand after the bailing operation is complete.

COMPLETION OF WELLHEAD SITE After development, the well site should be completed to provide drainage away from the location. Additionally, all well logs should be forwarded to the appropriate water authority for registration of the well.

CLOSING COMMENTS Due to the number of items entailed in the drilling process, a tabulation of the expected operations should be submitted to potential drillers in the form of a bid. It should also be apparent that drilling a well properly will not produce more water than what is in the ground. It will, however, allow more efficient and feasible extraction of the water over time and provide one with feasible, long-term operation of the well.

REFERENCES 1. Kashef, A. Water wells. Ground Water Engineering; McGraw-Hill, Inc.: New York, 1986; 361–363. 2. Driscoll, F.G. Groundwater exploration. Groundwater and Wells, 2nd Ed.; Johnson Division: St. Paul, MN, 1986; 181–196. 3. Driscoll, F.G. Well screens and methods of sediment-size analysis. Groundwater and Wells, 2nd Ed.; Johnson Division: St. Paul, MN, 1986; 405–412.

Wells: Hydraulics Mohamed M. Hantush

INTRODUCTION Water wells have been used and continue to be used as devices for extracting groundwater from aquifers. The importance of wells is not limited to the development of groundwater resources. Wells are used for environmental purposes, among others, the removal of contaminants from groundwater and controlling salt-water encroachment in coastal areas.

CONCEPTS Hydraulics of water wells deals primarily with the application of Darcy’s law and continuity relations to solve problems related to groundwater flow toward wells. Productive wells are those that tap geological formations, called aquifers, which yield groundwater in significant quantities; i.e., capable of yielding (or adding into storage) and transmitting water in appreciable amounts. Wells that tap a confined aquifer, in which groundwater is under pressure greater than atmospheric, are called artesian wells, and whenever the hydraulic head is above the ground surface, they are referred to as flowing wells (Fig. 1). In these wells, groundwater flows freely under pressure without the need for pumping. A well that penetrates an unconfined aquifer (also referred to as a water table or phreatic aquifer) is called a water-table (or gravity) well (Fig. 1). The water level in this well corresponds approximately to the position of the water table (i.e., the surface of atmospheric pressure) at that location. Groundwater discharged from a well causes drawdown (i.e., lowering of the hydraulic head relative to its prepumping level) around the well, which decreases in the direction away from the well and forms what is known as the cone of depression (Fig. 2A). The hydraulic (or piezopmetric) head gradient formed by the cone of depression induces groundwater flow toward the pumped well, which in extensive aquifers is radially symmetric. This phenomenon is reversed for recharging wells where the hydraulic head buildup around the well decreases outwardly and causes a flow in that direction. The hydraulics of pumping wells applies also to recharging wells.[1] The mechanics of groundwater flow and yield (or storage) due to a

discharging (or recharging) well depend on the type of aquifer and the radius of influence of the well. The radius of influence R of a pumping well—the distance from the center of the well at which drawdown is practically zero—generally increases with time until it intercepts an external boundary (Fig. 2A). At which time the well discharge is partially derived from another source, if that external boundary represents an open water body, such as a stream or a lake. Elasticity of aquifers, including compressibility of water, and gravity drainage in unconfined aquifers—water released by drainage from the pore space through which the water table moves—are two primary mechanisms that account for the volumes of water released from or added into storage in aquifers. Leakage across semipervious confining layers (Fig. 4), also called aquitards or leaky units, overlying and/or underlying an aquifer can account for a significant fraction of the volume of pumped groundwater, or even sustains the total groundwater discharge rate when elastic storage is exhausted in seemingly extensive aquifers. The change of drawdown (or head buildup) with time and space around a pumped (or recharged) well depends on the aquifer hydraulic characteristics, such as its storage capacity and transmissibility. The latter is determined by the transmissivity parameter T [L2T
1], which measures the ability of a unit section of the aquifer to transmit flow throughout its entire thickness; it is the product of aquifer thickness B and the hydraulic conductivity K [LT
1]. The storage capacity of an aquifer is quantified by the storage coefficient S (also called storativity) [L3L
3], which is the volume of water released from (or added into storage of) a column of the aquifer of unit horizontal area per unit drop (or increase) of the head. In unconfined aquifers, the storage coefficient is approximated by the specific yield Sy, which gives the yield of an aquifer per unit area and unit drop of the water table. It is also defined as the drainable fraction of pore space in a unit volume of aquifer. An important parameter in the analysis of drawdown pffiffiffiffiffiffiffiffiffiffiffiffiffi in leaky aquifers is the leakage factor l ¼ Tb=K0 , which determines the areal distribution of the leakage [L]; where K0 and b, respectively, are the hydraulic conductivity and thickness of the semipervious confining layer. Another leaky aquifer parameter is the leakage coefficient[2] s ¼ b/K0 , 1347

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National Risk Management Research Laboratory, U.S. Environmental Protection Agency (US-EPA), Cincinnati, Ohio, U.S.A.

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Wells: Hydraulics

Fig. 1 Illustrative diagram of types of aquifers.

which is defined as the rate of flow across a unit (horizontal) area of the semipervious layer into (or out of) the aquifer under one unit hydraulic difference across the layer [T].

DARCIAN FLOW Under natural field conditions, groundwater percolates slowly through the porous aquifer material that, for all practical purposes, flow is laminar and provoked mainly by viscous forces. Near the well entrance

and inside the well, flow becomes turbulent and inertial forces can no longer be ignored. Darcy’s law applies to laminar flow where the specific discharge q [LT
1] (hypothetical flow rate per unit porous area normal to the flow direction) is proportional to the head gradient i and the constant of proportionality is the hydraulic conductivity K: q ¼ Ki

ð1Þ

Analysis of well hydraulics mainly combines Darcy’s law Eq. (1) and continuity relations to derive solutions

Wastewater– Yellow Fig. 2 Illustrative diagram: (A) a confined aquifer and cone of depression, (B) pore-water pressure and inter-granular (effective) stress.

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Eq. (2) under steady flow condition is called the Thiem equation,[3]   Qw R ln jðRÞ
jðrÞ ¼ ð3Þ 2pT r in which Qw is the well discharge rate [L3T
1]; and H is the hydraulic head at distance R from the center of the well [L]. The solution of Eq. (2) in an unconfined aquifer with T ¼ Kh is known as the Dupuit– Forchheimer well discharge formula,[3]   Qw R 2 2 ln h ðRÞ
h ðrÞ ¼ ð4Þ pK r Fig. 3 Illustrative diagram of an unconfined aquifer.

for drawdown in pumped aquifers (or head buildup in recharged aquifers). Based on these fundamental relations, the flow rate Q in an extensive confined aquifer across a cylinder of height equal to the thickness of the aquifer B and radius r is radially symmetric and can be expressed by the relationship (Fig. 2A): Qðr; tÞ ¼ 2prT

@jðr; tÞ @r

ð2Þ

Eqs. (3) and (4) describe the steady-state hydraulic head at distance r from the center of the well. Eq. (4) is based on Dupuit’s assumption[1] that the equipotential lines are nearly vertical and, thus, the flow is essentially horizontal. The hydraulics of gravity wells is largely dependent on this assumption, in which the governing flow equation can be linearized and solved easily. In most practical problems of flow around wells, the relationship between transient drawdown in the aquifer and rate of discharge of the well Qw can be expressed by the well-flow equation:[2,4–7] sðr; tÞ ¼

in which j is the hydraulic head [L]; T ¼ KB is the aquifer transmissivity; r is the radial distance from the well center; and t denotes time. This equation can also be applied to describe flow toward a well in an extensive water-table aquifer, but with j ¼ h and T ¼ Kh, where h is the elevation of the water table above the base of the aquifer at a distance r from the center of the well (Fig. 3). The transmissivity in unconfined aquifers therefore varies with time and distance, as the water table fluctuates in space and time in response to pumpage or recharge. The solution of

Qw WðuÞ 4pT

ð5Þ

where s is the drawdown in the main aquifer [L] (Fig. 4) at a distance r from the center of the well and time t; and W(u) is called the well function, which is related to aquifer hydraulic characteristics, r and t. The drawdown caused by wells operating near physical boundaries, such as streams cutting through alluvial valleys and impervious mountain ranges, can be estimated by using the method of images.[1–3,8] The method of superposition can be invoked to solve for the drawdown in a wells field.

Fig. 4 Illustrative diagram of a leaky-confined aquifer.

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The elastic properties of the aquifer matrix and water[9] is the primary mechanism for the release and storage of groundwater in confined aquifers. Prior to pumping, the total load (overburden) sT [MLT
2L
2] above the confined aquifer, including the atmospheric pressure, equilibrates with pore-water pressure p [MLT
2L
2] inside the aquifer and the intergranular pressure (or effective stress) seff [MLT
2L
2] exerted by the sediments on each other at the contact points; i.e., sT ¼ p þ seff (Fig. 2B). When groundwater is discharged from a well, the pore-water pressure decreases

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MECHANICS OF AQUIFER YIELD AND STORAGE

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and the effective stress increases by an equal magnitude; i.e., Dp ¼
Dseff, since the overburden sT remains constant. Consequently, the decreased porewater pressure results in the decompression and expansion of the water volume in storage, and the increased effective stress causes the compaction of the aquifer, somewhat reducing the pore space, and thus, the expulsion of additional volume of water. This elastic behavior of the aquifer and water is responsible for the release or taking into storage volumes of water in a pumped (or recharged) aquifer.[5] In wells of large diameters, the storage capacity in the well itself can be significant and impact drawdowns in pumped aquifers.[10] In water-table aquifers, water is derived from storage primarily by drainage of the pore space above the lowered water table (gravity drainage) and partly from elastic storage as in confined aquifers. The latter is ignored in practical applications and typically characterizes the early response of the aquifer to pumping. When groundwater is pumped from an unconfined aquifer, the well discharge is initially derived from elastic storage and the aquifer behaves as though it is confined. The induced average drawdown therefore creates a head gradient in the vicinity of the water table and causes vertical flow and the subsequent lowering of the water table. In which case, the bulk of the well discharge is accounted by the volumes of water released from storage by vertical displacement of the water table, and the initial decline in the average head, thus, slows down considerably for a period lasting minutes to a few hours. The drawdown appears to be flat during this period, which is referred to as the ‘‘delayed yield’’ period.[6,7] The water table can now keep pace with the declining average head and, as in the early stage, the reaction of the aquifer becomes equivalent to that in a confined aquifer where the flow is essentially horizontal, but with the storage coefficient equal to the specific yield. In a leaky aquifer groundwater is derived from: 1) elastic storage in the main aquifer; 2) gravity drainage if the aquifer is unconfined; and 3) elastic storage in the semipervious confining layers and induced vertical leakage across these units. In most aquifers, the hydraulic conductivity of the semipervious layer is smaller than that in the main aquifer by at least two orders of magnitude so that the flow across this layer can be assumed vertical. For all practical purposes, the flow in the main aquifer is horizontal, except in the vicinity of partially penetrating wells. When water is discharged from a well tapping the main aquifer, it is initially derived from elastic storage, and the average head is thus reduced as drawdown increases toward the discharging well forming a cone of depression. The vertical hydraulic head gradient formed by the drawdown at the interface between the upper and/or lower semipervious layer(s) and the main aquifer induces

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Wells: Hydraulics

vertical flow through the semipervious layer derived partly from elastic storage of this leaky layer and partly from leakage due to head differences across the layer(s). Elastic storage of a leaky formation is often neglected, unless it is extensively thick,[2] and leakage across this unit ql is usually assumed to be proportional to the head difference across the confining leaky unit(s), q1 ¼


j
j0 s or q1 ¼ s s

ð6Þ

in which j is the head in the main aquifer; j0 is the head in the aquifer(s) above the overlying and/or below the underlying leaky confining layer(s) (Fig. 4); and s is the coefficient of leakage, defined earlier. Contrary to completely confined aquifers, drawdown in leaky aquifers slows down in time and eventually levels off at steady state, as long as the head in the aquifer receiving or supplying leakage j0 is kept constant. Steady-state drawdowns occur when the discharge rate is at equilibrium with the total leakage rate through the semipervious layer. This behavior is similar to that displayed by the delayed yield phenomenon in unconfined aquifers, except that in the latter the discharge rate is derived entirely from drainage of the pore space above the water table rather than from leakage. The time-drawdown relation in fractured rock aquifers shows three distinct stages, which may be similar to the delayed yield response in unconfined aquifers. In fractured-rock aquifers, groundwater flow partly occurs through the interconnected fractures as though it is flowing through pipes, and partly by percolation through the unfractured porous blocks of the rock matrix. Storativity of the fractures (or fissures) accounts for the initial yield of a pumping well, and as pumping continues, the drawdown somewhat slows down as water in the porous matrix reaches the fractures. The delayed yield in fractured aquifers is, thus, the result of the low conductivity of the porous blocks relative to that of the open fractures. At the later stage, the well discharge is derived from both the fractures and the porous blocks as the cone of depression continues to expand.[11]

PARTIALLY PENETRATING WELLS A well whose screen (water entry section) length is smaller than the saturated thickness of the aquifer it penetrates is called a partially penetrating well. Flow is 3-D and no longer is horizontal in the vicinity of this well and can be turbulent. In fact, the vertical velocity components below and above the well screen can be very large. Partial penetration affects drawdown in the vicinity of the well and for large distances from the pumping well, the flow is essentially horizontal as

Wells: Hydraulics

AQUIFER TESTS Aquifer hydraulic characteristics, such as transmissivity, storativity, leakage factor, and leakage coefficient, are usually obtained from aquifer tests. In these tests, the aquifer is tested under natural field flow conditions, in which a well is pumped at a prescribed rate and the drawdown is measured therein and, preferably, in at least one observation well located at some distance from the pumped well. The Theis Type-Curve method for the estimation of aquifer transmissivity and storativity in a confined aquifer advanced the basic approach of solution to other aquifer flow scenarios.[2,6,7] In this method, a logarithmic plot of the well function W(u) against 1/u (called type curve) is superimposed over that of the drawdown s vs. t/r2 (called data curve) until a best match between the two curves is obtained. The hydraulic properties are then estimated from an arbitrarily chosen matching point and solving simple algebraic relations. In natural aquifers, the transmissivity (also the hydraulic conductivity) change with direction at a given location, in which case the aquifer is referred to as anisotropic. In these aquifers, the transmissivity along any direction can be determined uniquely in terms of its principal values, which are defined along two principal directions in the horizontal plane, and both the values and the principal directions can be estimated from aquifer tests, however, with three observation wells.[12]

WELL LOSSES The drawdown inside a discharging well sw is the sum of both formation head loss and well losses, sw ¼ Cf Qw þ Cw Qnw , in which Cf is the formationloss constant; Cw is the well-loss constant relating discharge to the well loss; and n is the exponent due to turbulence.[13] n ¼ 2[6] and may exceed 2,[8] and can be as high as 3.5.[14] The formation loss results from laminar flow through aquifer sediments and turbulent flow outside the well screen, and is linearly related to the well discharge Qw. Well losses are associated with friction losses, which occur when water moves into

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the well through the screen, and turbulent flow inside the well. Gravel packing (Fig. 2A) and the removal of fine aquifer sediments during well development reduce well losses outside the well. The formation losses and well-loss parameters Cf, Cw, and n can be estimated graphically from a step-drawdown well test, in which drawdown inside the well is measured in time and at incremental well discharge rates.[14,15] Notice: The U.S. Environmental Protection Agency through its Office of Research and Development funded and managed the research described here through in-house effort. It has been subjected to Agency review and approved for publication.

REFERENCES 1. Marin˜o, M.A.; Luthin, J.N. Seepage and Groundwater; Elsevier Scientific Publishing Company: New York, 1982; 489 pp. 2. Hantush, M.S. Hydraulics of wells. In Advances in Hydroscience; Ven Te Chow, Ed.; Academic Press: New York, 1964; 281–442. 3. Bear, J. Hydraulics of pumping and recharging wells. In Hydraulics of Groundwater; McGraw-Hill, Inc.: New York, 1979; 300–378. 4. Theis, C.V. The relation between lowering of the piezometric surface and the rate and duration of discharge of a well using ground-water storage. Am. Geophys. Union Trans. 1935, 16, 519–524. 5. Jacob, C.E. On the flow of water in elastic artesian aquifer. Trans. Am. Geophys. Union 1940, 22, 574–586. 6. Boulton, N.S. Unsteady radial flow to a pumped well allowing for delayed yield from storage. Int. Assoc. Hydrol. 1954, 37, 472–477. 7. Neuman, S.H. Theory of flow in unconfined aquifers considering delayed response of the water table. Water Resour. Res. 1972, 8, 1031–1045. 8. Jacob, C.E. Engineering Hydraulics; Rouse, H., Ed.; Wiley: New York, 1950; 321–386. 9. Meinzer, O.E. Compressibility and elasticity of artesian aquifers. Economic Geology 1928, 23, 263–291. 10. Papadopulos, I.S.; Cooper, H.H. Drawdown in a well of large diameter. Water Resour. Res. 1967, 3, 241–244. 11. Streltsova, T.D. Well pressure behavior of a naturally fractured reservoir. Soc. Pet. Eng. J. 1983, 23, 769–780. 12. Hantush, M.S. Analysis of data from pumping tests in anisotropic aquifers. J. Geophys. Res. 1966, 71, 421–426. 13. Bouwer, H. Well-flow systems. Groundwater Hydrology; McGraw-Hill Inc.: New York, 1978; 65–89. 14. Lennox, D.H. Analysis and application of stepdrawdown test. J. Hydraul. Div., Proc. Am. Soc. Civ. Eng. 1966, 92, 25–48. 15. Rorabaugh, M.I. Graphical and theoretical analysis of step-drawdown test of artesian well. Proc. Am. Soc. Civ. Eng. 1953, 79, 23.

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though the pumped well completely penetrated the aquifer.[2] Anisotropy of the hydraulic conductivity has impact whenever the flow is 3-D. In aquifers where the vertical conductivity is much smaller than the horizontal, the yield of partially penetrating wells may be appreciably smaller than that of an equivalent isotropic aquifer. The effect of the anisotropy increases as the well penetration decreases.[2]

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Wetlands and Petroleum John V. Headley Aquatic Ecosystem Research Protection Branch, National Water Research Institute, Saskatoon, Saskatchewan, Canada

Kerry M. Peru Environment Canada, National Hydrology Research Center (NWRI-NHRC), National Water Research Institute, Saskatoon, Saskatchewan, Canada

INTRODUCTION In recent years, the ecological importance of wetlands has become well established in the scientific community at large. There is growing acceptance to preserve wetlands, wherever possible, as these areas provide support functions for several natural and living resources. Wetlands mediate biogeochemical transformations and serve as buffer zones for assimilation and reduction of toxic petroleum hydrocarbons released to aquatic environments.

DEFINITIONS AND TERMS

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The terms ‘‘wetlands’’ and ‘‘petroleum’’ can have a variety of meanings; thus it is important to first define these terms as used in this review. As there is no universally accepted definition for wetlands, it should be noted that the term is used here to denote areas of ‘‘emergent land-forming transition zones between uplands and open water.’’[1] As pointed out by Catallo,[1] some wetlands such as potholes have discrete boundaries and do not provide gradual transitions. However, there are several general defining features of wetlands, including one or more of the following traits: 1) there are at least temporally waterlogged or saturated substrata; 2) there is a dominance of plants adapted to saturated soils; and 3) there is a scarcity of flood-intolerant plants. Examples of natural wetland areas include freshwater swamps and marshes, Prairie potholes, salt and brackish coastal marshes, tidal freshwater marshes, bottomland hardwood forests, and mangrove swamps. In addition to these natural habitats, there are vast regions of man-made lagoons and tailing ponds such as those found in the oilsands region of Alberta, Canada, that also fall under the general category of wetlands. The later and other engineered or constructed wetlands have been fully integrated into some management strategies for treatment and cleanup of petroleum hydrocarbon wastes.[2,3] Petroleum is used in this article to denote primarily petroleum hydrocarbons—ranging from light gases 1352

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such as methane, ethane, and butane, to volatile hydrocarbons such as benzene, toluene, ethylbenzene, xylene (BTEX); gas condensates hydrocarbons to heavier molecular weight components in the C5–30 range and beyond. The latter would include the asphaltenes and petroleum waxes as high as C60. Particular attention has been given in the literature to BTEX, polycyclic aromatic hydrocarbons, and their alkyl derivatives, primarily because of their importance as environmental contaminants.[4] Although petroleum industrial wastes contain heavy metals and other nitrogen and phosphorous nutrients, along with components with biological oxygen demand (BOD) and chemical oxygen demand (COD), their removal and treatment will not be the main focus of this review. Application of treatment wetlands for the removal of the latter has been well reviewed elsewhere.[3] Likewise, the many chemical reactions to form a diverse range of chlorinated and other halogenated hydrocarbons will not be covered here. Instead, the focus is on naturally occurring petroleum hydrocarbons. Specific attention is given to their fate and transport in wetlands.

CASE STUDIES—ISSUES AND CONCERNS There are some key questions and issues that the scientific community at large has to face regarding evaluating the effectiveness of wetlands for the treatment of petroleum hydrocarbon wastes. Provided the contaminant loading is not too severe to overload a given wetland’s capacity to assimilate and remove petroleum contaminants, wetlands can serve as a sink, rather than a source, for hydrocarbons to the environment. An exception is the offgassing of methane to the atmosphere and other light gases, or combustion of wetland vegetation. However, the majority of petroleum contaminants can be abated in the soil/water/vegetation environment of wetlands. In cases where there is damage to sensitive wetland ecosystems, there are difficult issues to tackle. For example, it is not established what actually constitutes ‘‘restoration of contaminated wetlands—and what are considered best management

Wetlands and Petroleum

strategies for wetland restoration.’’[5] Furthermore, there is the issue of generally accepted endpoints that are suitable for measurement of wetland damage or restoration.[4] These issues are difficult to address fully because of the fact that wetlands are dynamic systems governed largely by: 1) their hydrology; 2) succession of vegetation (types of plants/vegetation best adapted for a particular environment, such as freshwater, saline, or arctic conditions); 3) availability and type of nutrients (whether organic or inorganic); 4) type and physical–chemical characteristics of petroleum oil in contact with the wetland; 5) amount of petroleum exposed to the wetland; 6) whether the contaminants are aged or fresh, as this has major effects on bioavailability; and 7) the extent to which microbial communities at the site are adapted to the degradation of petroleum hydrocarbons.[2,5–8] In favorable cases, petroleum hydrocarbons can increase the abundance of hydrocarbon-degrading bacteria and fungi. Likewise, some wetland vegetations can sequester petroleum hydrocarbons and process chemicals. However, in view of the dynamics of wetland ecology, care and caution need to be exercised when making generalizations from case studies to other wetland systems for treatment of wastes. The successes and failures of wetlands for the abatement and treatment of petroleum hydrocarbons should be considered within the context of these factors above. It can be argued that the field is still relatively new and that many of the tools for general application of wetlands are still topics of research and development. However, a few case studies are selected below to illustrate how the various petroleum hydrocarbons can be successfully treated in either natural or engineered wetlands, based on reduction in concentration and toxicity in the outflow from wetlands compared with ambient levels in waste streams.

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wetland peat and underlying clay till. However, over the past 20 years, the lateral extent of contamination in the wetland has remained stable, and apparent free product thickness and BTEX concentrations have decreased over time. A number of natural processes have contributed to the containment of contaminants, including sorption, aerobic biodegradation, volatilization, and anaerobic biodegradation. Sorption and desorption processes were evaluated by laboratory testing of site soils. There was a significantly higher sorption to the wetland peat compared with clayey silt attributed to the peat’s higher organic content (40%) relative to the silt (1%). At this particular wetland, there was no significant resistance to desorption observed, indicating that benzene would remain mobile and bioavailable over time. Aerobic biodegradation and volatilization appeared to be the main removal processes. Anaerobic biodegradation occurred primarily in the clayey silt, based on geochemical indicator parameters, microbial analyses, and soil vapor sampling. Thus, overall, natural attenuation appeared to be a feasible remedial solution for this wetland by facilitating the containment and degradation of condensate components at the wetland site.

Treatment of Petroleum Hydrocarbons in Engineered or Constructed Wetland There are two general types of shallow vegetated ecosystems or constructed wetlands that are used for the treatment of petroleum hydrocarbon wastes. The wetlands

Results of a 5-year research study were reported by Moore et al.,[9] describing natural attenuation processes in a natural wetland, located downgradient of a sour gas processing plant in central Alberta, Canada. The investigation illustrated the utility of natural wetlands as a management option for the attenuation of condensate, which is primarily composed of C5 to C12 hydrocarbons, including BTEX compounds. The abatement in the natural wetland area (Fig. 1) was considered as a possible favorable remedial solution at the site in question.[9] It was established that both free-phase and dissolved-phase condensates have been discharging to the base of the wetland at 1 m below ground surface, resulting in contamination of the

© 2008 by Taylor & Francis Group, LLC

Fig. 1 Abatement of petroleum hydrocarbons in a natural wetland located downgradient of a sour gas processing plant in central Alberta, Canada. Source: From Ref.[9]. Courtesy of Marcel Dekker, Inc.

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Abatement of Petroleum Hydrocarbons in a Natural Wetland

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Fig. 2 Application of engineered or constructed wetlands for the treatment of petroleum hydrocarbons. Source: From Ref.[3]. #American Chemical Society, 1999.

have either: 1) free water surface with surface flow; or 2) subsurface flow with vegetated submerged bed systems, as illustrated in Fig. 2. The former more closely mimics the hydrologic regime of natural wetlands, whereas the latter serves to minimize the exposure of humans or wildlife to the petroleum wastewater. The use of engineered or constructed wetlands (Fig. 2) for the treatment of petroleum hydrocarbons has been well reviewed by Knight, Kadlec, and Ohlendorf.[3] In their review, the authors discussed and illustrated several good examples of treatment wetland applications, including the treatment of refinery effluents, spills and washing, oilsands processing water, and water produced from the processing of natural gas (Table 1). Several large-scale wetland projects currently exist at oil refineries for treatment of not only petroleum

hydrocarbons in wastes but also COD, biochemical oxygen demand, trace organics, metals, toxicity, total suspended solids, nitrogen, and phosphorus. As noted for natural wetlands, the removal of contaminants in the engineered wetlands is also a function of hydraulic loading and influent concentration and, to a lesser extent, is dependent on the diversity and nature of plant communities, water depth, and hydraulic efficiency. As pointed out by Knight, Kadlec, and Ohlendorf,[3] in most cases, data from petroleum industry wetland studies indicate that treatment wetlands are equally or more effective at removing contaminants from petroleum industry wastewaters than from other types of wastewater. However, for cost-effectiveness, it is critical to construct the treatment wetland with the appropriate size as too big a size leads to unnecessary

Table 1 Examples of petroleum industry full-scale and pilot treatment wetlands Site name/location

Purpose

Wastewater source

Total wetland size (ha)

Amoco, Mandan, ND, USA

Process water polishing

Refinery process water

16.6

Chevron, Richmond, CA, USA

Process water polishing

Refinery process water

36.4

Yanshan Petrochemical, Beijing, China

Process water polishing

Refinery process water

50

Yanshan Petrochemical, Beijing, China

Pilot facility

Refinery process water

1.5

Jinling Petrochemical, Beijing, China

Pilot facility

Refinery process water

0.75

Average flow (m3/day) 5700 9500 100,000

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Suncor, Inc., Alberta, Canada

Pilot facility

Oilsand process water

0.08

17.3

BP Petroleum, Port Everglades, FL, USA

Pilot facility

Contaminated groundwater

0.007

27

Shell Oil, Norco, LA, USA

Pilot facility

Refinery process water

0.02

547

Shell Oil, Bremen, Germany

Pilot facility

Tank farm effluent

Texaco, USA

Pilot facility

Refinery process water

0.04

Australia

Pilot facility

Oil terminal

0.06

Source: From Ref.[3]. #American Chemical Society, 1999.

© 2008 by Taylor & Francis Group, LLC

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Wetlands and Petroleum

cost overheads, whereas too small a size leads to the wetland being a source of contaminants rather than a viable treatment option.

CONCLUSION Although there is growing acceptance that wetlands provide critical buffer zones for the degradation of petroleum hydrocarbons released to aquatic environments, there remain a number of questions on their general use for treatment of wastes from one site to another. For example, there are still a number of knowledge gaps on how chemicals present in petroleum wastes affect microbial activities in wetland soils. Likewise, there is a need to better understand how to manage oil spills in wetland ecosystems. Physical removal of oil from wetlands may not always be advisable as the human foot traffic can cause severe and long-lasting damage. Furthermore, petroleum hydrocarbons mixtures in wetland environment can differ widely in their chemical makeup, toxicity, and metal content, leading to quite diverse and site-specific conditions. These factors, coupled with the dynamics of wetland ecology and the dependence of degradation rates on availability and type of nutrients, likely limit the ability to generalize treatment strategies from one wetland site to other locations.

ACKNOWLEDGMENTS

REFERENCES 1. Catallo, W.J. Ecotoxicology and wetland ecosystems— Current understanding. Environ. Toxicol. Chem. 1993, 12 (12), 2209–2224. 2. Williams, J.B. Phytoremediation in wetland ecosystems: progress, problems, and potential. Crit. Rev. Plant Sci. 2002, 21 (6), 607–635. 3. Knight, R.L.; Kadlec, R.H.; Ohlendorf, H.M. The use of treatment wetlands for petroleum industry effluents. Environ. Sci. Technol. 1999, 33 (7), 973–980. 4. Smits, J.E.; Wayland, M.E.; Miller, M.J.; Liber, K.; Trudeau, S. Reproductive, immune, and physiological end points in tree swallows on reclaimed oil sands mine sites. Environ. Toxicol. Chem. 2000, 12, 2951–2960. 5. Wilcox, D.A.; Whillans, T.H. Techniques for restoration of disturbed coastal wetlands of the Great Lakes. Wetlands 1999, 19 (4), 835–857. 6. Nyman, J.A. Effect of crude oil and chemical additives on metabolic activity of mixed microbial populations in fresh marsh soils. Microb. Ecol. 1999, 37, 152–162. 7. DeLaune, R.D.; Lindau, C.W.; Banker, B.C.; Devai, I. Degradation of petroleum hydrocarbons in sediment receiving produced water discharge. J. Environ. Sci. Health. Part A, J. Environ. Sci. Eng. Toxic Hazard. Substance Control 2000, 35 (1), 1–14. 8. Simon, M.A.; Bonner, J.S.; McDonald, T.J.; Autenrieth, R.L. Bioaugmentation for the enhanced bioremediation of petroleum in a wetland. Polycycl. Aromat. Compd. 1999, 14, 231–239. 9. Moore, B.J.; Headley, J.V.; Dupont, R.R.; Doucette, W.D.; Armstrong, J.E. Abatement of gas-condensate hydrocarbons in a natural wetland. J. Environ. Sci. Health. Part A, J. Environ. Sci. Eng. Toxic Hazard. Substance Control 2002, 37 (4), 425–438.

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Funding for the preparation of this review was provided, in part, by the Panel of Energy and Research Development (PERD), Environment Canada.

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Wetlands as Treatment Systems Kyle R. Mankin Department of Biological and Agricultural Engineering, College of Engineering, Kansas State University, Manhattan, Kansas, U.S.A.

INTRODUCTION Wetland ecosystems generally can be defined by the presence of saturated soils and plants that grow well under these conditions. These two features promote many processes that trap, transform, and utilize a variety of the materials that flow into a wetland system with the incoming water. Because wetlands possess this capacity to remove contaminants from water, they have been utilized and even constructed for the purpose of treating polluted waters. As constructed treatment wetlands become a more common feature in municipal, rural, agricultural, and industrial settings, it is important to understand the features, processes, and design considerations that make these systems attractive natural-treatment options. How Do Wetlands Work?

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Wetlands can be used to treat wastewater because they process contaminants. However, they treat wastewater more slowly than traditional treatment plants. Oxygen, and the manipulation of oxygen levels, is a primary concern for wastewater treatment because many of the necessary biological and chemical treatment processes require oxygen. Traditional treatment plants can easily manipulate oxygen levels by pumping air into the wastewater. Oxygen enters wetlands by slower, natural processes. Increasing oxygen concentration, by increasing wastewater contact with air, plant roots, or photosynthetic algae, often can enhance the processing ability of wetlands. When considering wastewater treatment by constructed wetlands, five contaminant groups are of primary importance: sediments, organic matter, nutrients, pathogenic microbes, and metals. Wetlands slow down water movement, allowing sediments to settle out of the water. Organic matter can be processed, or decomposed, by highly competitive microbes. Less competitive microbes called nitrifiers process nitrogen. Both microbe types require oxygen. Because the nitrifiers are less competitive, oxygen levels become very important to insure that both organic matter and nitrogen are fully processed. The other two, pathogenic microbes and metals, are more situational, related to the specific waste being treated. Wetlands treat pathogenic microbes by detaining 1356

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them until they naturally die off, are eaten by other predatory organisms in the wetland, or are exposed to UV radiation near the water surface. Metals are processed by being adsorbed to other particles and settling out of the water. The remainder of this entry further explains wetland processes and design considerations. References are provided for more in-depth information.

TREATMENT WETLAND TYPES Constructed vs. Natural Wetlands Wetlands constructed as treatment systems differ from natural wetlands in several important ways. Constructed wetlands usually are built with uniform depths and shapes designed to provide consistent detention times and maximize contaminant removal. In contrast, natural wetlands are irregular in depth and shape, which causes irregular flow, allows water to by-pass the shallow treatment zones by moving through the deeper channels, and leads to less effective treatment. In addition, water-quality regulations in the United States dictate that if a natural wetland is associated with an existing water body of the United States, as most are, wastewater discharges into the wetland must meet specific quality standards, similar to other water bodies. Wetlands constructed as wastewater treatment systems typically are located in uplands where wetlands did not exist before and are not subject to inflow water-quality regulations. Natural wetlands are not recommended for use as treatment wetlands. Constructed wetlands increasingly are being used for wastewater treatment in a variety of applications (Table 1). Examples can be found of wetlands being used to treat municipal sewage, urban runoff, onsite residential wastewater, animal feedlot and barnyard runoff, cropland runoff, industrial wastewater, mine drainage, and landfill leachate. Each application takes advantage of a combination of physical, chemical, and biological processes characteristic of natural wetlands to reduce the concentration of contaminants in water. Such contaminants include sediments, organic materials, nutrients (particularly nitrogen and phosphorus), metals, microbial pathogens, and pesticides.

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Table 1 North American wetlands as of 1994 Size (ha) Wastewater type

Quantity

Minimum

Agricultural

58

0.0004

Median

Maximum

0.1

47

Industrial

13

0.03

10

1093

Municipal

159

0.004

2

500

Stormwater

6

0.2

Other

7

3

8

42

376

1406

Source: From Ref.[1].

Free-Water vs. Submerged-Bed Wetlands Constructed wetlands have two common types. Freewater surface (FWS) wetlands (also called surface-flow wetlands) have plants that grow in a shallow layer of water over a soil substrate (Figs. 1 and 2). The location of the plants in the system can vary: the plants can float on the water surface with their roots suspended in the water (free-floating macrophyte systems); they can be rooted in the soil with the entire leaves and stems below the water surface (submerged-macrophyte systems); they can be rooted in the soil having leaves and stems that rise above the water surface (emergent macrophyte systems); or the wetland may use a combination of planted and open-water zones. About twothirds of existing wetlands as of 1994 were FWS.[1] In vegetated submerged-bed (VSB) wetlands (also called subsurface flow wetlands or rock-plant filters), plants are rooted in a porous media, such as sand or gravel, and water flows through the media in either horizontal or vertical direction (Figs. 3 and 4). About one-quarter of treatment wetlands were VSB systems.[1] However, these systems are currently used in thousands of smaller-scale, onsite residential applications in the United States that do not appear in this database.

TREATMENT PROCESSES

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Many wastewaters entering constructed wetlands must be pretreated to avoid excessive contaminant loading, particularly of mineral and organic solids.

Pretreatment technologies include septic tanks for onsite systems or anaerobic lagoons for animal waste, municipal, or mine-drainage treatment systems. In each case, the anaerobic condition in the pretreatment process reduces production of additional algae solids. Typical contaminant levels entering treatment wetlands are summarized in Table 2. The wetland type impacts the processes used to retain or remove contaminants. In a VSB system, wastewater flows through pore spaces of the media and comes into direct contact with the roots of plants. In a FWS system, water flows across the media surface and contacts plant stems and leaves. In either system, solid particles, including sediments (clay and silt particles and colloids) and organic matter (manure particles, organic residues, and algae or other phytoplankton), settle out of the water column or are trapped or filtered as water passes through a wetland. Contaminants that are adsorbed to sediments (e.g., P, NH4, fecal bacteria) or absorbed within organic solids (e.g., nutrients) are also removed. However, these constituents can be re-suspended or desorbed back into the wetland water. This natural cycling of materials is an important function of wetlands, although it makes system design and interpretation of treatment complex. Once entrapped, organic materials and associated contaminants are decomposed in wetlands by microbial and chemical transformations. In the degradation process, microbes use oxygen. The amount of oxygen used is related to the amount of organic material in the water. The controlled measurement of biochemical oxygen demand (BOD) is a common way to illustrate the amount of organic matter in water. When wastewater lacks oxygen, or is anaerobic, it requires the addition of oxygen to degrade organic matter. Oxygen is also required for transformation of ammonium to nitrite and nitrate (nitrification), whereas anaerobic conditions are required for transformation of nitrate to nitrogen gas (denitrification). Aerobic wetland conditions often remove metals by aerobic oxidation of iron; subsequently iron hydroxides and other metals precipitate in the wetland.[5] Although some oxygen diffuses into a wetland from the air, a common assumption is that oxygen also is transported through wetland plants and made available to microbes in close

Fig. 1 FWS wetland with emergent macrophytes.

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Fig. 2 A three-cell, FWS wetland for treating dairy wastewater. This system is in its first year of operation; plants were recently established. (Photo: Peter Clark.)

proximity to leaky roots.[6] This mechanism may be less important than once thought, though.[2] Treatment wetlands are thought to function effectively because they combine anaerobic zones in the water column with aerobic zones near the water interfaces with air and roots. However, because the microbes that break down organic carbon can out compete nitrifiers for oxygen, nitrogen removal in higher strength wastewaters is often low.

DESIGN CONSIDERATIONS Design and resulting effectiveness of constructed wetlands (Table 3) depend upon many factors: climate (precipitation, temperature, growing season, evapotranspiration), wastewater characteristics (constituents, loading, flow rate, and volume), topography, and wildlife activity. Wetland designs must specify total area; the number, depth, and size of wetland cells; hydraulic retention times; vegetation types and coverage; inlet and outlet configuration and location; and internal flow patterns.[2] Details for design can be found in

numerous references[2,7–11] and some elements are discussed here.

VSB Wetlands Properly designed VSB systems can achieve high removal rates. Treatment in a VSB wetland is governed by system residence time and wastewater contact with media and plant-root surfaces. Because of this, depth is a critical dimension and is often chosen according to the rooting depth of the selected plant (e.g., cattails: 30 cm; reeds: 40 cm; bulrush: 60 cm). Once depth is chosen, cross-sectional area (and thus wetland width) is selected to assure adequate flow rates. Then, volume (and thus wetland length) is determined from the retention time needed to treat the wastewater to the desired quality. Proper design of inlet and outlet control structures helps maintain uniform flow patterns and depth, avoids problems with clogging and freezing, and minimizes system operation and maintenance (O and M) problems. High loading from influent solids and clogging can lead to surface flows and poor treatment. VSB systems must receive influents that are pretreated

Wastewater– Yellow Fig. 3 VSB wetland with emergent macrophytes.

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governed by plant tolerance to standing water and treatment objectives. FWS vs. VSB Systems Selection of the most appropriate wetland system depends on wastewater characteristics, treatment requirements, and site constraints. VSB systems generally require less land area, are less susceptible to freezing and mosquito problems, and have no exposed wastewater at the surface (avoiding contact-related health problems). FWS systems are less expensive to construct (without the cost of media), have greater potential for wildlife habitat, and are easier to maintain if solids accumulate.

OPERATION AND MAINTENANCE O and M of treatment wetlands are relatively simple. The goal of an O and M plan is to assure that the wetland system continues to operate as planned, designed, and constructed. Several sources provide specific O and M guidance,[14] and most design manuals also contain such guidelines. Operation should be consistent with treatment objectives while maintaining structural integrity of the system, uniform flow conditions, and healthy vegetation as well as minimizing odors, nuisance pests and insects. Most maintenance plans require such items as checking water levels, checking for evidence of leaks or wildlife damage, and maintaining plant health on a weekly or monthly basis.

Fig. 4 VSB wetland for treating onsite residential wastewater. This system uses gravel media and variety of wetland plants. (Photo: Barbara Dallemand.)

to remove solids (e.g., septic tank and effluent filter or anaerobic lagoon).

FWS Wetlands Properly designed FWS systems also can achieve high removal rates. Design typically follows one of two methods. The areal loading approach allows a designer to select the wetland surface area according to the influent load and the desired effluent quality.[13] Another approach allows a designer to select the wetland area by knowing the biological reaction rate, wastewater concentration, and flow rate along with selected water depth and target outflow water quality.[7,8] Again, depth is a critical dimension and is

CONCLUSION Constructed wetlands are complex natural-treatment systems that are well suited for many applications. They are low in cost and maintenance, provide significant reductions of many contaminants, and offer an aesthetic appearance. More work is needed to

Wastewater type Residential-septic tank[2] Municipal-primary

[2]

Municipal-pond[2] [3]

Livestock

[avg.]

Livestock[3] [median] Landfill leachate

[4]

BOD5 (mg/L)

TSS (mg/L)

TN (mg/L)

NH4-N (mg/L)

NO3-N (mg/L)

TP (mg/L)

FC (per 100 mL)

129–147

44–54

41–49

28–34

0–0.9

12–14

105.4–106.0

40–200

55–230

20–84

15–40

0

4–15

105.0–107.0

11–35

20–80

8–22

0.6–16

0.1–0.8

3–4

100.8–105.6

263

585

254

122

3.6

24

1.6
105

81

118

274

60

1.1

20

1.7
103

312–729

241–7840

287–670

254–2074

0–3

0.9

—

Note: BOD5 ¼ 5-day biochemical oxygen demand, TSS ¼ total suspended solids, TN ¼ total nitrogen, NH4-N ¼ ammonium nitrogen, NO3-N ¼ nitrate nitrogen, TP ¼ total phosphorus, FC ¼ fecal coliform bacteria.

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Table 2 Wetland influent concentrations

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Table 3 Wetland treatment (%) Wastewater type Municipal[1] [avg.] Livestock

[3]

[avg.]

Landfill leachate[12] [range]

BOD5

TSS

TN

NH4-N

TP

FC

Metals

74

70

53

54

57

—

—

65

53

42

48

42

92

—

11–90

45–97

7–45

13–88

—

—

8–95þ

Note: BOD5 ¼ 5-day biochemical oxygen demand, TSS ¼ total suspended solids, TN ¼ total nitrogen, NH4-N ¼ ammonium nitrogen, TP ¼ total phosphorus, FC ¼ fecal coliform bacteria, metals ¼ Fe, Cu, Pb, Ni, or Zn.

characterize treatment processes in constructed wetlands and improve design procedures to account for variability in wastewater and climate.

ACKNOWLEDGMENTS The author extends his gratitude to Kristina M. Boone, Associate Professor of Agricultural and Environmental Communications, Kansas State University for her significant review contribution to this article, and to those at Kansas State University who have supported wetlands research and education.

REFERENCES 1. Kadlec, R.H.; Knight, R.L.; Reed, S.C.; Ruble, R.W.; Eds. Wetlands Treatment Database (North American Wetlands for Water Quality Treatment Database); EPA/600/C-94/002; U.S. Environmental Protection Agency: Cincinnati, OH, 1994. 2. US EPA. Constructed Wetlands Treatment of Municipal Wastewaters, EPA/625/R-99/010; U.S. Environmental Protection Agency: Cincinnati, OH, 2000. 3. CH2M Hill and Payne Engineering, Constructed Wetlands for Livestock Wastewater Management: Literature Review, Database, and Research Synthesis; Gulf of Mexico Program—Nutrient Enrichment Committee: Montgomery, AL, 1997. 4. Kadlec, R.H. Constructed wetlands for treating landfill leachate. In Constructed Wetlands for the Treatment of Landfill Leachates; Mulamoottil, G., McBean, E.A., Rovers, F., Eds.; Lewis Publishers: Boca Raton, FL, 1999; Chap. 2.

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5. US EPA, Engineering Bulletin—Constructed Wetlands Treatment; EPA/540/S-96/501; U.S. Environmental Protection Agency: Cincinnati, OH, 1996. 6. Brix, H. Do macrophytes play a role in constructed treatment wetlands? WaterSci. Technol. 1997, 35 (5), 11–17. 7. Kadlec, R.H.; Knight, R.L. Treatment Wetlands; Lewis Publishers: Boca Raton, FL, 1996. 8. Reed, S.C.; Crites, R.W.; Middlebrooks, E.J. Natural Systems for Waste Management and Treatment, 2nd Ed.; McGraw-Hill: New York, 1995. 9. Steiner, G.R.; Watson, J.T.; Eds. General Design, Construction, and Operation Guidelines: Constructed Wetlands Wastewater Treatment System for Small Users Including Individual Residences, Technical Report TVA/MW-93/10; Tennessee Valley Authority: Chattanooga, TN, 1993. 10. Powell, G.M.; Dallemand, B.L.; Mankin, K.R. RockPlant Filter Design and Construction for Home Wastewater Systems; MF-2340, www.oznet.ksu.edu/library/ h20ql2/mf2340.pdf Kansas State University Cooperative Extension Service: Manhattan, KS, 1998. 11. Sievers, D.M. Design of Submerged Flow Wetlands for Individual Homes and Small Wastewater Flows; Special Report 457; Missouri Small Wastewater Flows Education and Research Center, 1993. 12. Mulamoottil, G.; McBean, E.A.; Rovers, F.; Eds. Constructed Wetlands for the Treatment of Landfill Leachates; Lewis Publishers: Boca Raton, FL, 1999; 69, 86, 157, 212. 13. USDA-NRCS Constructed Wetlands for Agricultural Wastewater Treatment: Technical Requirements; U.S. Department of Agriculture–Natural Resource Conservation Service: Washington, DC, 1991. 14. Powell, G.M.; Dallemand, B.L.; Mankin, K.R. RockPlant Filter Operation, Maintenance, and Repair, MF-2337; www.oznet.ksu.edu/library/h20ql2/mf2337.pdf Kansas State University Cooperative Extension Service: Manhattan, KS, 1998.

Wetlands: Ecosystems Sherri L. DeFauw University of Arkansas, Fayetteville, Arkansas, U.S.A.

Wetlands perform key roles in the global hydrologic cycle. These transitional ecosystems vary considerably in their capacity to store and subsequently redistribute water to adjacent surface water systems, groundwater, the atmosphere, or some combination of these. Saturation in the root zone or water standing at or above the soil surface is key to defining a wetland. When oxygen levels in waterlogged soils decline below 1%, anaerobic (or reducing) conditions prevail. Most, but not all, wetland soils exhibit redoximorphic features formed by the reduction, translocation, and oxidation of iron (Fe) and manganese (Mn) compounds; the three basic kinds of redoximorphic features include redox concentrations, redox depletions, and reduced matrix.[1] Microbial transformations in flooded soils also impact other biogeochemical cycles (C, N, P, S) at various spatial and temporal scales. Several of the most rapidly disappearing wetland ecosystems in North America are profiled here, in terms of properties and processes.

HYDROLOGIC CONSIDERATIONS Wetland water volume and source of water are heavily influenced by landscape position, climate, soil properties, and geology. Wetlands may be surface flow dominated, precipitation dominated, or groundwater discharge dominated systems (Fig. 1). Surface flow dominated wetlands include riparian swamps and fringe marshes. In unregulated settings (i.e., no dams or diversions) these ecosystems are subject to large hydrologic fluxes, and vary the most in terms of soil development, sediment loads, and nutrient exchanges. Precipitation-dominated wetlands (e.g., prairie potholes and bogs) reside in landscape depressions and typically have a relatively impermeable complex of clay and/or peat layers that retard infiltration (or recharge) and also impede groundwater discharge (or inflow). Groundwater dominated wetlands (e.g., fens and seeps) may form in riverine settings, at slope breaks, or in areas where abrupt to rather subtle changes in substrate porosity occur. Groundwater contributions to wetlands are complex, dynamic, and rather poorly understood.[2]

Frequency and duration of flooding, and the longterm amplitude of water level fluctuations in a landscape are the three most important hydrologic parameters that ‘‘shape’’ the aerial extent of a wetland complex as well as determine the relative abundance of four intergrading wetland settings (i.e., swamps, wet meadows, marshes, and aquatic ecosystems[3]. Wetland hydrodynamics control soil redox conditions. The hydroperiod-redox linkage, in turn, controls plant macronutrient concentrations (N, P, K, Ca, Mg, S), micronutrient availability (B, Cu, Fe, Mn, Mo, Zn, Cl, Co), pH, organic matter accumulation, decomposition, and influences plant zonation. Oftentimes, the zonation of plants (as determined by competition and/or physiological tolerances) provides key insights into the hydrodynamics and biogeochemistry of an area.[3]

ECOSYSTEM PROFILES Four of the most rapidly disappearing wetland ecosystems in North America are summarized here in terms of key properties and processes. These profiles include comments on geographical extent, geomorphology, soils, hydrodynamics, biogeochemistry, vegetation structure, and/or indicator species, as well as recent estimates of ecosystem losses and ecological significance. Riparian Swamps Bottomland hardwood forests once dominated the river floodplains of the eastern, southern, and central United States. In the Mississippi Alluvial Plain, an estimated 8.6 million hectare area of bottomland hardwoods has been reduced to 2 million forested hectares remaining.[4] Although their true extents are not welldocumented (due to difficulties in determining upland edges), these hydrologically open, linear landscape features have been logged, drained, and converted to other uses (predominantly agriculture) at alarming rates. Between 1940 and 1980, bottomland hardwood forests were cleared at a rate of 67,000 ha yr
1.[5] Riparian wetlands are unique, vegetative zonal expressions of both short- and long-term fluvial processes. A widely used classification scheme relates flooding conditions (frequency and flood duration 1361

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Fig. 1 The relative contributions of groundwater discharge, precipitation, and surface flow determine main wetland types. Swamps and marshes are distinguished by the frequency and duration of surface flows. Source: Modified from Ref.[3].

during the growing season) with zonal associations of hardwood species.[6] Zone II (intermittently exposed ‘‘swamp’’—bald cypress and water tupelo usually dominate the canopy) and Zone III (semipermanently flooded ‘‘lower hardwood wetlands’’—overcup oak and water hickory are common) are accepted as wetlands by most, however, Zones IV (seasonally flooded ‘‘medium hardwood wetlands’’ that include laurel oak, green ash, and sweetgum) and V (temporarily flooded ‘‘higher hardwood wetlands’’—typically an oak– hickory association with loblolly pine) have been controversial when dealing with wetland management issues.[7] Typically the complexity of floodplain microtopography abates smooth transitions from one zone to the next, with unaltered floodplain levees oftentimes exhibiting the highest plant diversity.[8] Riparian wetlands process large influxes of energy and materials from upstream watersheds and lateral runoff from agroecosystems. As a result of these inputs, combined with decomposition of resident biomass, the organic matter content of these alluvial soils usually ranges from 2% to 5%.[8] Clay-rich bottomland tracts have higher concentrations of N and P as well as higher base saturations (i.e., Ca, Mg, K, Na) compared to upland areas. Watersheds dominated by riparian ecosystems export large amounts of organic C in dissolved and particulate forms.[9] Wastewater– Yellow

Prairie Potholes Prairie potholes comprise a regional wetland mosaic that includes parts of the glaciated terrains of the Dakotas, Iowa, Minnesota, Montana, and Canada. Originally, this region encompassed 8 million hectares of wetland prior to drainage for agriculture; an

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Wetlands: Ecosystems

estimated 4 million hectares remained at the close of the 1980s.[10] These landscape depressions are the most important production habitat in North America for most waterfowl.[11] Most prairie potholes are seasonally flooded wetlands dependent on snowmelt, rainfall, and groundwater. Four main hydrological groupings are recognized: ephemeral, intermittent, semi-permanent, and permanent.[11] Water level fluctuations are as high as 2–3 m in some settings. Measures of soil hydraulic conductivity demonstrate that groundwater flow is relatively slow (0.025–2.5 m yr
1); therefore, the potholes are hydrologically isolated from each other in the short-term.[12] Long-term ecological studies at the Cottonwood Lake Study Area, NPWRC-USGS[11] have revealed the intricacies of several prairie pothole phases. During periods of drought, marsh soils, sediments, and seed banks are exposed (i.e., dry marsh phase). Seed banks in natural sites have 3000–7000 seeds/m2. A mixture of annuals (usually the dominant group) and emergent macrophyte species germinate on the exposed mudflats and a wet meadow develops. As water levels increase, the annuals decline and emergent macrophytes rapidly recolonize (i.e., regenerating marsh). If the flooding is consistently shallow, emergent macrophytes will eventually dominate the entire pothole. Sustained deep water flooding results in extensive declines in emergent macrophytes (i.e., degenerating marsh), and intensive grazing by muskrats may culminate in a lake marsh phase as submersed macrophytes become established. When water levels recede, emergent macrophytes re-establish. The rich plant communities that develop in these dynamic marshland complexes are also controlled by two additional environmental gradients, namely, salinity and anthropogenic disturbances (involving conversion to agriculture and extensive irrigation well pumping).[11]

Northern Peatlands Deep peat deposits, in the United States, occur primarily in Alaska, Michigan, Minnesota, and Wisconsin, and are scattered throughout the glaciated northeast and northwest, as well as mountaintops of the Appalachians. The most extensive peatland system, in North America, is the Hudson Bay lowlands of Canada that occupies an estimated 32 million hectares.[13] The Alaskan and Canadian peatlands are relatively undisturbed, and the least threatened by developmental pressures. Elsewhere, peatlands have either been converted for agricultural use (including forestry) or mined for fuel and horticultural materials. Bogs and fens are the two major types of peatlands that occupy old lake basins or cloak the landscape.

Wetlands: Ecosystems

Pocosins The Pocosins region of the Atlantic coastal plain extends from Virginia to the Georgia–Florida border.[20] These non-alluvial, evergreen shrub wetlands are especially prevalent in North Carolina; in fact, pocosins once covered close to 1 million hectare in this state.[21] Derived from an Algonquin Indian word for ‘‘swamp-on-a-hill,’’ pocosins are located on broad, flat plateaus and sustained by waterlogged, acidic, nutrient-poor sandy, or peaty soils usually far removed from large streams. Wetland losses are high, with 300,000 ha drained for agriculture and forestry uses between 1962 and 1979.[22] Pocosins are characterized by a dense, ericaceous shrub layer; an open canopy of pond pine may be

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present or absent.[21] A typical low pocosin ecosystem [less than 1.5 m (5 ft) tall] includes swamp cyrilla (or titi), fetterbush, bayberry, inkberry, sweetbay, laurelleaf greenbrier, and sparsely distributed, stunted pond pine.[22] Pocosin soils may be either organic (with a deep peat layer—e.g., Typic Medisaprist) or mineral (usually including a water restrictive spodic horizon— e.g., Typic Endoaquod). As peat depth decreases, the stature of the vegetation increases. High pocosin [with shrubby vegetation 1.5–3.0 m (5–10 ft) tall and canopy trees approximately 5 m (16 ft) in height] usually occurs on peat deposits of 1.5 m (5 ft) or less in thickness or on wet sands.[20] The major natural disturbance to these wetlands is periodic burning (with a fire frequency of about 15–50 yr). Pocosin surface and subsurface waters are similar to northern ombrogenous bogs, but are more acidic with higher concentrations of sodium, sulfate, and chloride ions.[23] Carbon : Phosphorus (C : P) ratios increase sharply during the growing season; phosphorus availability limits plant growth and probably plays a crucial role in controlling nutrient export. Undisturbed pocosins export organic N and inorganic phosphate in soil water.[23]

CONCLUSIONS Despite existing preservation policies, U.S. wetland conversions are anticipated to continue at a rate of 290,000–450,000 acres (117,408–182,186 ha) annually.[24] It is widely known that wetlands are the product of many environmental factors acting simultaneously; perturbations in one realm (e.g., hydrology) not only impact local wetland properties and processes, but also have consequences in linked ecosystems as well. Wetlands are major reducing systems of the biosphere, transforming nutrients and metals, and regulating key exchanges between terrestrial and aquatic environments.

ACKNOWLEDGMENTS William J. Rogers, B. A. Howell, and Van Brahana commented on an earlier draft; their reviews are genuinely appreciated.

REFERENCES 1. Soil survey staff. Keys to Soil Taxonomy, 8th Ed.; USDA-NRCS: Washington, DC, 1998. 2. http://water.usgs.gov/nwsum/WSP2425/hydrology.html (accessed July 2002).

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The most influential, interdependent physical factors shaping these ecosystems include: 1) water level stability; 2) fertility; 3) frequency of fire; and 4) grazing intensity.[3] Northern bogs are dominated by oligotrophic Sphagnum moss species, and may be open, shrubby, or forested tracts. These predominantly rainfed (ombrogenous) systems have low water flow (with a water table typically 40–60 cm below the peat surface), are extremely low in nutrients (especially poor in basic cations), and accumulate acidic peats (pH 4.0– 4.5).[14] Fens are affected by mineral-bearing soil waters (groundwater and/or surface water flows), and possess water levels at or near the peat surface. Fens may be subdivided into three hydrologic types: soligenous (heavily influenced by flowing surface water); topogenous (largely influenced by stagnant groundwater); or limnogenous (adjacent to lakes and ponds).[14] These minerogenous ecosystems range from acidic (pH 4.5) to basic (pH 8.0); vegetation varies from open, sedgedominated settings to shrubby, birch-willow dominated associations to forested, black spruce-tamarack tracts. Nutrient availability gradients do not necessarily coincide with the ombrogenous–minerogenous gradient; recent investigations indicate higher P availability in more ombrogenous peatlands, and greater N availability in more minerogenous peatlands.[15] Northern peatlands represent an important, long-term carbon sink, with an estimated 455 Pg (1 Petagram ¼ 1015 g) stored worldwide.[16] An estimated 220 Pg of C is currently stored in North American peatlands, compared to about 20 Pg in storage during the last glacial maximum.[17] High latitude peatlands also release about 60% of the methane generated by natural wetlands.[18] In addition, sponge-like living Sphagnum carpets facilitate permanently wet conditions, and the high cation exchange capacity of cells retains nutrients and serves to acidify the local environment.[19]

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3. Keddy, P.A. Wetland Ecology: Principles and Conservation; Cambridge University Press: Cambridge, 2000. 4. Llewellyn, D.W.; Shaffer, G.P.; Craig, N.J.; Creasman, L.; Pashley, D.; Swan, M.; Brown, C. A decisionsupport system for prioritizing restoration sites on the Mississippi river alluvial plain. Conserv. Biol. 1996, 10 (5), 1446–1455. 5. Kent, D.M. Applied Wetlands Science and Technology, 2nd Ed.; Lewis Publishers: Boca Raton, FL, 2001. 6. Clark, J.R., Benforado, J., Eds.; Wetlands of Bottomland Hardwood Forests; Elsevier: Amsterdam, 1981. 7. Mitsch, W.J.; Gosselink, J.G. Wetlands; Van Nostrand Reinhold Company: New York, 1986. 8. Wharton, C.H.; Kitchens, W.M.; Pendleton, E.C.; Sipe, T.W. The Ecology of Bottomland Hardwood Swamps of the Southeast: A Community Profile; U.S. Fish and Wildlife Service, Biological Services Program FWS/ OBS-81/37: Washington, DC, 1982; 1–133. 9. Brinson, M.M.; Swift, B.L.; Plantico, R.C.; Barclay, J.S. Riparian Ecosystems: Their Ecology and Status; U.S. Fish and Wildlife Service, Biological Services Program FWS/OBS-81/17: Washington, DC, 1981; 1–151. 10. Leitch, J.A. Politicoeconomic overview of prairie potholes. In Northern Prairie Wetlands; van der Valk, A.G., Ed.; Iowa State University Press: Ames, 1989; 2–14. 11. http://www.npwrc.usgs.gov/clsa (accessed July 2002). 12. Winter, T.C.; Rosenberry, D.O. The interaction of ground water with prairie pothole wetlands in the cottonwood lake area, East-Central North Dakota, 1979–1990. Wetlands 1995, 15 (3), 193–211. 13. Wickware, G.M.; Rubec, C.D.A. Ecoregions of Ontario, Ecological Land Classification Series No. 26; Environment Canada, Sustainable Development Branch: Ottawa, Ontario, 1989. 14. http://www.devonian.ualberta.ca/peatland/peatinfo.htm (accessed July 2002).

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15. Bridgham, S.D.; Updegraff, K.; Pastor, J. Carbon, nitrogen, and phosphorus mineralization in Northern Wetlands. Ecology 1998, 79, 1545–1561. 16. Gorham, E. Northern Peatlands: role in the carbon cycle and probable responses to climatic warming. Ecol. Appl. 1991, 1 (2), 182–195. 17. Halsey, L.A.; Vitt, D.H.; Gignac, L.D. Sphagnumdominated Peatlands in North America since the last glacial maximum: their occurrence and extent. The Bryologist 2000, 103, 334–352. 18. Cicerone, R.J.; Ormland, R.S. Biogeochemical aspects of atmospheric methane. Global Biogeochem. Cycles 1988, 2 (2), 299–327. 19. van Breeman, N. How sphagnum bogs down other plants. Trends Ecol. Evol. 1995, 10 (7), 270–275. 20. Sharitz, R.R.; Gresham, C.A. Pocosins and Carolina bays. In Southern Forested Wetlands: Ecology and Management; Messina, M.G., Conner, W.H., Eds.; Lewis Publishers: Boca Raton, FL, 1998; 343–389. 21. Richardson, C.J. Pocosins: an ecological perspective. Wetlands 1991, 11 (S), 335–354. 22. Richardson, C.J.; Evans, R.; Carr, D. Pocosins: an ecosystem in transition. In Pocosin Wetlands: An Integrated Analysis of Coastal Plain Freshwater Bogs in North Carolina; Richardson, C.J., Ed.; Hutchinson Ross Publishing Company: Stroudsburg, PA, 1981; 3–19. 23. Wallbridge, M.R.; Richardson, C.J. Water quality of pocosins and associated wetlands of the Carolina coastal plain. Wetlands 1991, 11 (S), 417–439. 24. Schultink, G.; van Vliet, R. Wetland Identification and Protection: North American and European Policy Perspectives; Agricultural Experiment Station Project 1536; Michigan State University, Department of Resource Development: East Lansing, MI, 1997.

Wind Erosion and Water Resources R. Scott Van Pelt

INTRODUCTION Wind erosion is the result of wind impacting a dry, bare and loose soil surface. Approximately 500 million ha worldwide are susceptible to wind erosion. Plumes of fugitive dust are the most visible evidence of wind erosion and they may be lifted in the turbulent wind to heights in excess of a kilometer and transported hundreds or thousands of kilometers from the source before returning to the surface. The fine particles that become entrained in the wind and transported great distances contain higher concentrations of organic C, basic cations such as Na, Ca, and Mg, plant nutrients such as N, P, K, and Fe, trace metals, and soil contaminants than the soils from which they originated. Larger particles are often deposited onto surfaces including bodies of water by gravity close to the source area, while smaller particles may remain in the atmosphere for long periods of time and be transported into humid regions where they are often scavenged out of atmospheric suspension by cloud formation and rainfall. Fine atmospheric particles such as soil dust form condensation nuclei in the presence of near-saturated air. In addition to their role as condensation nuclei in the formation of rain droplets, other particles are intercepted and incorporated by the raindrops falling through the air. The soluble minerals in and adsorbed to the dust particles are dissolved and influence the chemical properties of the precipitation. In addition to the dust that is scavenged from the atmosphere by rainfall, the rain will tend to wash the dust that settled by gravity or by impact from vegetation and other surfaces, further increasing the concentration of dust and dust-borne solutes in the water exiting a watershed. In many nutrient deficient ecosystems, deposited soil dust is a crucial input to the nutrient cycle. Soil dust reacts with and catalyzes reactions with soil gasses and pollutants, thus reducing the deleterious effects of acid rain.

WIND EROSION AND DEPOSITION PROCESSES As wind blows over a surface, ephemeral gusts move particles in the sand to fine sand size range, which

either creep along the soil surface or become entrained in the wind. Most sand-sized particles do not rise more than 30 cm before returning to the surface, accelerated by the wind, with a significant horizontal component of motion. These saltating grains strike the surface with considerable force, releasing more saltating particles and abrading finer soil particles from soil aggregates and crusts resulting in plumes of dust. A thorough treatise on wind erosion processes is presented in Ref.[1]. Finer particles have a higher surface area to volume ratio and are easily entrained into the turbulent eddies to heights often exceeding 2–3 km. The median diameter of entrained particles decreases as altitude increases.[2] As the particles are transported downwind, the higher terminal velocity of the larger particles and shallower transport depth cause them to settle from the atmosphere first and thus closer to the source. Dust may also be deposited in the relative calm air to the lee of surfaces projecting into the wind, or they may impact and deposit on the surfaces. These processes are termed ‘‘dry’’ or ‘‘impact deposition,’’ respectively. Dry-deposited particles may be reentrained unless they are deposited on a free water surface. Free water surfaces are regarded as the most effective dust collectors.[3] Particles