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DESERTS AND DESERT ENVIRONMENTS

Environmental Systems and Global Change Series Series Editors: Professor Antony Orme (UCLA), Professor Olav Slaymaker (University of British Columbia), and Dr Tom Spencer (University of Cambridge) The development of this new series of advanced undergraduate and graduate textbooks has been stimulated by three widely recognized trends in the teaching of earth and environmental sciences at university level. Firstly, the systems approach is now well established in university physical geography and earth/environmental science curricula around the world, at both undergraduate and graduate levels. Secondly, concerns about the pace and extent of global change have increasingly informed – and given an urgent social relevance to – a wide range of course offerings in these subjects. Lastly, implicit in the environmental systems approach is the importance of integrating findings and methodologies from a wide range of disciplines, including ecosystems science, geomorphology, hydrology, geophysics, oceanography, climatology, archaeology, and environmental planning. The ESGC Series is explicitly designed to reflect these educational trends. It is an ambitious new venture resulting from the merging of two existing publishing initiatives – Blackwell’s Environmental Systems and Pearson’s Understanding Global Environmental Change Series – and its objectives may be simply stated: • to create an awareness and understanding of the way key environmental systems operate and interact; • to explore the pace and extent of global (and regional) environmental change and to show how environmental systems respond to change over a variety of scales in time and space; • to attract students from a range of disciplines and to encourage students to think in new ways that transcend traditional discipline boundaries; • to underline the relevance of these studies to social/environmental problems, and to encourage students to bring a scientific approach to solving such problems. Books in the series are aimed at advanced undergraduates and graduates taking degree courses in physical geography, earth science, environmental science, ecology, and archaeology. Titles in the series will have an international relevance, with examples and case studies taken from varied environments around the world. 1 The Cryosphere and Global Environmental Change, Olav Slaymaker and Richard E.J. Kelly 2 Deserts and Desert Environments, Julie Laity Forthcoming 3 The Pace of Environmental Change, Antony Orme 4 Oceans and Global Environmental Change, Tom Spencer 5 Water in a Changing World, John Pitlick, James Wescoat, and Harihar Rajaram

DESERTS AND DESERT ENVIRONMENTS

Julie Laity

A John Wiley & Sons, Inc., Publication

This edition first published 2008, © 2008 by Julie Laity Blackwell Publishing was acquired by John Wiley & Sons in February 2007. Blackwell’s publishing program has been merged with Wiley’s global Scientific, Technical and Medical business to form Wiley-Blackwell. Registered office: John Wiley & Sons Ltd, The Atrium, Southern Gate, Chichester, West Sussex, PO19 8SQ, UK Editorial offices: 9600 Garsington Road, Oxford, OX4 2DQ, UK The Atrium, Southern Gate, Chichester, West Sussex, PO19 8SQ, UK 111 River Street, Hoboken, NJ 07030-5774, USA For details of our global editorial offices, for customer services and for information about how to apply for permission to reuse the copyright material in this book please see our website at www.wiley. com/wiley-blackwell The right of the author to be identified as the author of this work has been asserted in accordance with the Copyright, Designs and Patents Act 1988. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, except as permitted by the UK Copyright, Designs and Patents Act 1988, without the prior permission of the publisher. Wiley also publishes its books in a variety of electronic formats. Some content that appears in print may not be available in electronic books. Designations used by companies to distinguish their products are often claimed as trademarks. All brand names and product names used in this book are trade names, service marks, trademarks or registered trademarks of their respective owners. The publisher is not associated with any product or vendor mentioned in this book. This publication is designed to provide accurate and authoritative information in regard to the subject matter covered. It is sold on the understanding that the publisher is not engaged in rendering professional services. If professional advice or other expert assistance is required, the services of a competent professional should be sought. Library of Congress Cataloguing-in-Publication Data Laity, Julie. Deserts and desert environments / Julie Laity. p. cm. Includes bibliographical references and index. ISBN 978-1-57718-033-3 (pbk. : alk. paper) 1. Deserts. 2. Geomorphology. I. Title. GB611.L25 2008 551.41′5–dc22 2008015536 A catalogue record for this book is available from the British Library. Set in Meridien 9.5 on 11.5 pt by SNP Best-set Typesetter Ltd., Hong Kong Printed in Singapore by Markono Print Media Pte Ltd

CONTENTS Preface

xii

1 Introduction: defining the desert system 1.1 1.2

1.3 1.4 1.5 1.6 1.7

Defining the desert system 1.1.1 Physical, biological, and temporal components Evolution of deserts 1.2.1 Global considerations 1.2.1.1 Subtropical high-pressure belts 1.2.1.2 Continental interiors 1.2.1.3 Polar deserts 1.2.2 Regional considerations 1.2.2.1 Cold-current influences 1.2.2.2 Rainshadow effect 1.2.2.3 Edaphic environments Indices of aridity Desert surfaces Tectonically stable and unstable deserts Deserts of the past Changing human perspectives on deserts

2 Deserts of the world 2.1 2.2

Introduction: the extent of global aridity Global deserts 2.2.1 Africa 2.2.1.1 North Africa: the Saharan Desert and the Sahel 2.2.1.2 North Africa: the Somali-Chalbi Desert 2.2.1.3 Southern Africa: arid Madagascar 2.2.1.4 Southern Africa: the Karoo, Kalahari, and Namib Deserts 2.2.2 Middle East and Arabia 2.2.2.1 Negev and Sinai Deserts 2.2.2.2 Deserts of Syria and Jordan 2.2.2.3 The Arabian Peninsula 2.2.2.4 Iran and Iraq 2.2.3 Europe 2.2.4 Asia 2.2.4.1 Middle Asian deserts 2.2.4.2 Deserts of India and Pakistan 2.2.4.3 Deserts of China and Mongolia 2.2.5 South America 2.2.5.1 The west coast deserts: Peru–Chile, Atacama, and Sechura deserts 2.2.5.2 Altiplano/Puna 2.2.5.3 Monte Desert 2.2.5.4 Patagonian Desert

1 1 1 2 2 2 4 4 4 4 6 6 6 8 8 9 12

14 14 14 14 15 18 18 22 24 24 26 26 27 28 29 29 30 30 33 33 36 36 37 V

CONTENTS

VI

2.2.6

2.2.7

North America 2.2.6.1 Chihuahuan Desert 2.2.6.2 Sonoran Desert 2.2.6.3 Mojave Desert 2.2.6.4 The Great Basin deserts Australia

3 The climatic framework 3.1 3.2 3.3

3.4

Introduction: classification of deserts by temperature Weather data Atmospheric controls: surface boundary layer 3.3.1 Atmospheric water vapor and cloud cover 3.3.2 Radiation 3.3.3 Temperature of the air, surface, and subsurface 3.3.3.1 Air temperature of hot deserts 3.3.3.2 Surface temperatures 3.3.3.3 Subsurface temperatures 3.3.4 Albedo 3.3.5 Precipitation 3.3.5.1 Storm types and seasonality of precipitation 3.3.5.2 Forms of precipitation other than rainfall: fog, dew, and snow 3.3.5.3 Variability in precipitation 3.3.6 Wind 3.3.7 Effects of population growth and urbanization on desert climatology 3.3.7.1 Air pollution 3.3.7.2 Heat islands Temporal and spatial variability of climatic influences 3.4.1 ENSO forcing of desert climates 3.4.2 Expansion and contraction of the Sahara Desert 3.4.3 The Sahel: land-surface–atmosphere interactions

4 The hydrologic framework 4.1 4.2 4.3

4.4

Introduction The water balance in deserts Water budgets 4.3.1 Precipitation and its assessment: problems in gauging and network design 4.3.2 Interception 4.3.3 Evapotranspiration 4.3.3.1 Introduction 4.3.3.2 Evaporation 4.3.3.3 Transpiration 4.3.4 Infiltration and soil water 4.3.5 Groundwater, subsurface flow, and springs 4.3.5.1 Role of groundwater in arid environments 4.3.5.2 Groundwater recharge 4.3.5.3 Groundwater quality Surface runoff and floods 4.4.1 Controls on runoff 4.4.2 Runoff from slopes 4.4.3 Runoff in channels 4.4.3.1 Ephemeral channels 4.4.3.2 Intermittent and perennial rivers 4.4.3.3 Low-flow events and the ecological effects of drought

37 39 40 41 42 43

48 48 48 50 50 51 52 52 52 53 54 54 55 58 60 62 63 63 64 64 64 67 69

71 71 71 73 73 74 75 75 76 78 79 81 81 82 83 84 84 85 86 86 87 87

CONTENTS

4.5 4.6

4.7

4.4.4 Transmission losses during floods The chemical quality of surface and soil water Water resources 4.6.1 Groundwater 4.6.2 Dams and reservoirs 4.6.3 Long-distance transfer: canals and aqueducts 4.6.4 Rainmaking 4.6.5 Desalination 4.6.6 Fog-water collection systems Case study: the waters of the Tigris-Euphrates Basin and the impact of modern water management

5 Lake systems: past and present 5.1 5.2

5.3

Introduction to desert lakes Types of lake 5.2.1 Perennial salt lakes 5.2.2 Ephemeral lakes: playas and pans 5.2.2.1 Wet (salt playas; discharge playas) and dry (recharge playas; claypans) systems 5.2.2.2 Playa degradation 5.2.3 Palaeolake systems: lakes as indicators of past climate changes Lakes of the global arid environment 5.3.1 Western North America 5.3.2 South America 5.3.3 Australia 5.3.4 Africa 5.3.5 Asia 5.3.5.1 China and Mongolia 5.3.5.2 India and Pakistan 5.3.6 Middle East

6 Weathering processes and hillslope systems 6.1 6.2

6.3

6.4

6.5 6.6

Introduction Weathering 6.2.1 Insolation weathering 6.2.2 Salt weathering 6.2.3 Frost weathering 6.2.4 Biological weathering 6.2.5 Silt infiltration Weathering forms 6.3.1 Cavernous weathering/tafoni 6.3.2 Gnammas Duricrusts 6.4.1 Terminology 6.4.2 Silcrete 6.4.3 Calcrete/caliche 6.4.4 Gypcrete 6.4.5 Salcrete: halite crusts Desert varnish Hillslope processes 6.6.1 Rock slopes 6.6.1.1 Hillslopes in massive rocks 6.6.1.2 Scarp and cuesta forms

VII

88 89 90 91 92 92 93 94 94 94

98 98 98 98 98 100 103 104 106 106 109 112 114 117 117 120 120

122 122 122 123 123 125 125 125 126 126 128 128 128 129 130 133 133 134 136 136 136 137

CONTENTS

VIII

6.6.2 6.6.3

Gravity-related activity: talus and scree slopes and related forms Badlands 6.6.3.1 Case study: Borrego Badlands, California 6.7. Composite surfaces (pediments)

7 Desert soils and geomorphic surfaces 7.1 7.2 7.3 7.4

7.5

7.6 7.7

7.8

Introduction The nature of soils in arid and semiarid regions Soil description and classification Soil characteristics of arid regions 7.4.1 Physical characteristics 7.4.2 The organic content of soils and nutrient availability 7.4.3 Role of the past 7.4.4 Role of relief and altitude Inorganic and biological soil crusts 7.5.1 Inorganic soil crusts 7.5.2 Biological/cryptobiotic surface crusts Spatial heterogeneity in soil properties and the ecohydrology of patterned vegetation zones Surface volume changes 7.7.1 The properties and nature of swelling clay soils 7.7.2 Patterned ground or gilgai Surface types: hamada and stone pavements 7.8.1 Hamada 7.8.2 Stone pavements 7.8.2.1 Introduction 7.8.2.2 Description of stone pavements 7.8.2.3 Formation of pavements 7.8.2.4 The aeolian aggradation theory of pavement development 7.8.2.5 Pavement development as a relative-age dating tool 7.8.2.6 Discussion

8 Water as a geomorphic agent 8.1 8.2 8.3 8.4

8.5

Introduction Groundwater sapping in slope and valley development Piping processes in channel and slope evolution Fluvial processes 8.4.1 Channel morphology and channel flow 8.4.2 Alluvium 8.4.3 Sediment transport 8.3.4 Sediment yields Fluvial landforms 8.5.1 Alluvial fans 8.5.1.1 Introduction 8.5.1.2 Sediment production, transportation, and deposition 8.5.2 Arroyos 8.5.3 Gullies 8.4.4 Landform assemblages

9 Aeolian processes 9.1 9.2

Introduction Near-surface flow

141 142 144 145

148 148 148 149 150 150 151 151 152 152 152 153 156 158 158 158 159 159 159 159 162 164 165 166 167

168 168 168 171 172 172 173 174 178 179 179 179 180 182 184 185

186 186 187

CONTENTS

9.3

9.4 9.5

9.6

9.2.1 9.2.2 Wind 9.3.1

Variation in wind velocity with height Airflow and sediment transport over hills and dunes processes Aeolian particles 9.3.1.1 Particle sizes 9.3.1.2 Processes of particle formation 9.3.2 Particle entrainment (sand) 9.3.3 Particle transport 9.3.3.1 Modes of transportation 9.3.3.2 Transport rates Landforms of accumulation: sand sheets, zibar, and sand stringers Landforms of accumulation: dunes 9.5.1 Introduction 9.5.2 The development of dune fields: palaeo-aeolian processes and evidence for multiple phases of activity 9.5.3 Dune reactivation 9.5.4 Interdune deposits and lakes 9.5.5 Dune patterns and classification 9.5.6 Dune accumulation influenced by topographic obstacles 9.5.6.1 Lee dunes 9.5.6.2 Climbing dunes, sand ramps, echo dunes, and cliff-top dunes 9.5.7 Formation of self-accumulated dunes 9.5.7.1 Dune initiation 9.5.7.2 Crescentic dunes: barchans and transverse barchanoid ridges 9.5.7.3 Linear dunes (seif dunes) 9.5.7.4 Star dunes 9.5.7.5 Dome dunes 9.5.8 Vegetated dunes 9.5.8.1 Hummock dunes, coppice dunes, or nebkhas 9.5.8.2 Parabolic and elongate parabolic dunes 9.5.8.3 Lunette dunes 9.5.8.4 Vegetated linear dunes Ripples

10 Landforms of aeolian erosion and desert dust 10.1 10.2 10.3 10.4 10.5

Introduction Deflation features: desert depressions and pans Ventifacts Yardangs and ridge and swale systems Desert dust 10.5.1 Definitions 10.5.2 Environmental role and impacts of dust 10.5.2.1 Effects on marine and terrestrial ecosystems 10.5.2.2 Relationship to soil development and earth surface processes 10.5.2.3 Impact of dust on climate, weather, and air quality 10.5.2.4 Dust storms and vehicular accidents 10.5.3 Dust entrainment, transport, and deposition 10.5.3.1 Climatic factors in dust entrainment 10.5.3.2 Surface factors: vegetation, crusts, and the availability of sand 10.5.3.3 Anthropogenic activity 10.5.4 Climatic events associated with blowing dust: scales of activity

IX

187 188 191 191 191 192 194 194 194 195 195 197 197 197 199 200 201 204 204 204 205 205 205 206 208 209 209 210 211 212 212 213

216 216 216 218 221 225 226 226 227 227 228 229 229 230 230 232 233

CONTENTS

X

10.5.5 Frequency of blowing dust: interannual, seasonal, diurnal 10.5.6 Dust-source areas

11 Plant communities and their geomorphic impacts 11.1 Introduction: characteristics of desert ecosystems 11.2 Adaptations to desert conditions 11.2.1 Adaptations to temperature 11.2.2 Water use by plants 11.2.3 Reproduction 11.2.4 Nutrient cycling 11.2.5 Salt adaptation 11.3 Plant communities and ecotones 11.3.1 Evergreen shrubs 11.3.2 Drought-deciduous shrubs 11.3.3 CAM succulents 11.3.4 Perennial grasses 11.3.5 Phreatophytes 11.3.6 Desert annuals 11.3.7 Poikilohydric plants 11.3.8 Exotic plants 11.4 Succession in desert plant communities 11.5 Dune communities 11.6 Vegetation type and density and relationship to geomorphic processes 11.6.1 The role of vegetation in the erosion and deposition of sand 11.6.2 The role of slope and aspect in plant distribution 11.6.3 Effects of vegetation on stream-channel processes 11.6.3.1 How floods and fluvial landforms affect vegetation 11.6.3.2 How vegetation affects dryland river-channel processes and form 11.6.3.3 Flow regulation and riparian communities in arid lands

12 Animal communities 12.1 Introduction: environmental requirements 12.1.1 Adaptations to air and soil temperature, fire, and the gaseous environment 12.1.2 Moisture parameters 12.2 Effects on geomorphic processes 12.2.1 Slope processes, surface stability, and soil development 12.2.1.1 Surface movement and animal tracks 12.2.1.2 Biopedturbation and burrowing 12.3 Hydrologic impacts 12.4 Effects of the geomorphic activity of animals on plant communities

13 Desertification and the human dimension 13.1 Desertification: introduction and terminology 13.2 Climate change and desertification 13.3 Anthropogenic causes of desertification 13.3.1 The rural environment: overgrazing and woodcutting; devegetation and biological feedbacks 13.3.2 Urbanization and technological exploitation 13.3.3 Off-road vehicles and military vehicles 13.3.4 Increases in dust-storm activity and the effect on humans and the environment

233 234

237 237 238 238 240 242 243 244 245 245 246 246 247 247 248 250 250 251 252 253 253 255 256 256 257 257

259 259 262 263 264 264 264 265 265 266

267 267 268 269 270 271 272 273

CONTENTS

13.4 Water resources: a rural and urban problem 13.4.1 Groundwater withdrawal 13.4.2 Depletion of river flow and loss of sediment 13.4.3 Effects of irrigation: waterlogged soils and salinization 13.4.4 Desert lakes affected by humans 13.5 Case study: the Aral Sea 13.5.1 Lake bottom exposure and salt and dust storms 13.5.2 Ecosystem damage 13.5.3 Climatic alteration 13.5.4 Health concerns 13.6 Discussion

XI

275 276 277 278 279 280 282 282 282 283 283

References

285

Index

320

PREFACE

The desert environment, owing to it complexity, is best appreciated through a long period of discovery. Its nature and beauty are subtle and illusory. Over time, the observer sees a very complex environment: a mosaic that retains the imprint of the past and offers glimpses into the present. It is my hope that this book will be a reference source and provide a coherent and informative basis for desert study. On a practical level, the desert is of interest because humans increasingly exploit these regions, as technology allows us to expand into a realm that hitherto posed strong limitations. Given the recent explosive population growth in arid areas, the increasing problems of environmental degradation, and a need to make informed decisions in desert management, this book fills a niche for those seeking to understand the desert environment as a whole: its climate, hydrology, geomorphology, and basic biology. Throughout the book, there are specific references to human interactions and environmental concerns. I hope that the ideas and information presented herein play a role in informing management decisions, as the desert recovers very slowly from harm. In particular, it must be recognized that water is a precious resource, which should not be exploited to the extent that the natural biological environment is impaired. As case studies in this book show, only a few decades of mismanagement for short-term gain can destroy an ecosystem that has evolved over thousands of years. From an academic perspective, the book is concerned with the nature, origin, and evolution of desert environments. In recent years, increasingly detailed studies, the incorporation of new dating techniques, and an expansion of the global scope of scientific inquiry have helped us to better understand the desert environment. The approach in Deserts and Desert Environments differs from that of many other excellent books by exploring broader themes, discussing in depth global deserts, providing more details on both the surface and subsurface XII

climatic environment, considering all aspects of the hydrologic environment, and examining geomorphic linkages to the biological environment. Thus, the book aims to provide a comprehensive view of deserts, through intertwining themes, and exploring the interactions between the geologic, geomorphic, biological, climatic, and hydrologic spheres. The book emphasizes many basic principles, and discusses some details in more depth. Examples are given from many areas of the world and, where appropriate, more extensive case studies are presented. The book may be used as a reference or as a text for a class in desert environments or desert geomorphology. By and large, the chapters in the book can be considered independently and read in any order. Following an introduction to the nature of deserts and the imprint of past climates in Chapter 1, the book explores aspects of physical geography. The second chapter introduces the reader to the deserts of the world. Although this chapter can be read later in order, its placement near the beginning of the book provides an overview of the great diversity of desert landscapes and gives a sense of the different foci of research. Chapter 3 provides an essential introduction to desert climatology. This chapter explores weather data, the surface boundary layer, the effects of urbanization on desert climatology, the variability of climatic influences in both time and space (El Niño Southern Oscillation forcing and shifting desert boundaries), and the impact of climate on the biological environment. The fourth chapter reviews the hydrologic framework (precipitation, interception, evapotransiration, infiltration, and groundwater), explores surface runoff and flooding, and briefly reviews the water resources available to humans. Although lakes are rare in modern deserts, they were much more widespread in the past, and Chapter 5 examines palaeolakes and lakebed surfaces, as well as contemporary lakes, such as Mono Lake. In an environment in which vegetation is sparse, surface runoff infrequent, and salts are

PREFACE

common, weathering processes become important, and this theme is considered in Chapter 6. Additionally, the nature of hillslope processes in deserts is examined. Chapter 7 introduces desert soils and surfaces, emphasizing how these differ from their counterparts in more humid environments. Water as a geomorphic agent is explored in Chapter 8, where both subsurface and surface processes are examined, as well as some of the most significant fluvial landforms and landform assemblages. Aeolian processes are examined in two chapters (9 and 10), which are subdivided into sections on basic wind processes, landforms of accumulation (dunes), landforms of erosion, and desert dust. Dust has received increasing attention in recent years, as it is globally disseminated by the winds, affecting not only deserts, but environments as far away as the Amazon basin. Anthropogenic factors in sand destabilization and dust production are also discussed in these chapters. Chapters 11 and 12 examine how the biological community responds to the limitations of water, high temperatures and radiation, and the presence of salt in desert soils; and emphasizes the important hydrologic and geomorphic impacts of plants and animals. Finally, in Chapter 13, the human dimension of deserts is developed more fully, with particular attention drawn to issues of desertification and problems associated with excessive groundwater withdrawal and depletion of surface water. One of the goals of this book is to provide the readers with imagery designed to enhance their understanding of global deserts. High-quality maps and annotated orbital imagery illustrate the spatial dimensions and regional topography of deserts. Many of the maps and graphs in this book were provided by David Deis, whose exemplary attention to design and detail is evident throughout. Some of the photographs in this book were graciously provided by Robert Howard, Tony and Amalie Orme, Steve Adams, and Lloyd Laity. At the end of the book, there is a long list of references to enable the reader to further explore the topics discussed. The research in arid lands continues to expand rapidly, and it has not been possible to read or cite all of this work. During the process of writing this book, I have had the very great pleasure of exploring many new themes and ideas in desert research. I realize that I may have insufficiently cited some work or missed important contri-

XIII

butions to the field, and for that I apologize to the authors. I would like to sincerely thank an anonymous reviewer who provided useful comments on the content and organization of the book. Freelance editor Nik Prowse oversaw the editing, proofreading, and indexing stages of the book and provided invaluable assistance and a push to the finish line. I am fortunate to have a desert environment so close to my home, which has made it possible to explore large areas over many decades. Throughout this time, many people have been instrumental in inspiring or supporting me. My initial interest in geography was stimulated by my parents. My father drove us around large areas of Australia, Canada, and the United States and, with camera in hand, encouraged my interest in photography. My mother never found a pebble, a feather, or a flower that was not her friend, and always walked with her eyes trained on the ground. Although she had no formal training, her enthusiasm for the beauty of nature was my strongest influence. The desert was formally introduced to me by Antony Orme of UCLA, who first encouraged me in my academic pursuits, and later provided long-term inspiration through his own lifetime of enthusiasm for geomorphic study. During many of my desert travels, I have been accompanied by graduate students and my family, who were invariably cheerful and useful companions. In particular, I would like to thank Tim Boyle, Aaron Davis, Mark Kuhlman, and Alaric Clark. They have helped to fix flat tires, cajole transmissions, help other stranded motorists, and dig and tow vehicles out of the sand. My daughter, Kelsey Laity-D’Agostino, who played in sand dunes while in diapers and suffered greatly from saltating sand until she grew sufficiently tall, matured to be an asset to my field explorations, able to quickly erect a tent in the darkest and windiest conditions. Furthermore, her zeal for learning sustains my own interests. My husband, Saverio D’Agostino, is the glue that holds my life together. Over the years, he has served as field assistant, mechanic, laboratory technician, horse caretaker, fellow scientist, and chief cook and bottle washer. This book would not have been possible without his extraordinary generosity and the many cups of tea he has provided. Julie Laity

1 INTRODUCTION: DEFINING THE DESERT SYSTEM 1.1 DEFINING

THE DESERT SYSTEM

Deserts and semideserts are the most extensive of the Earth’s biomes, occupying more than one-third of the global land surface. Of this area, approximately 4% is classified as extremely arid (hyperarid), 15% arid, and about 14.6% semiarid (Meigs 1953, 1957). In total, about 49 million km2 are affected by aridity. If dry-subhumid areas are included in the classification, then drylands comprise about 47% of the Earth’s land surface (United Nations Environment Program 1992). “True” deserts are considered to be the warm hyperarid and arid regions, and semiarid and dry-subhumid regions the desert fringes. Collectively, the dry areas of the world occupy more land than any other major climatic type.

1.1.1 PHYSICAL, BIOLOGICAL, AND TEMPORAL COMPONENTS

Deserts are characterized by their great aridity and may share in common features of climate, weather, geomorphology, hydrology, soils, and plant and animal life. However, defining a desert is not a simple matter, as witnessed by the many attempts at a systematic characterization based on aspects of climate (precipitation, evaporation, and temperature), geomorphic features, and flora and fauna. Although high temperatures, winds, and shifting sands may be present in some deserts, they are not components of all arid environments. In an attempt to define a desert, Shreve (1951) describes regions of “low and untimely distributed rainfall, low humidity, high air temperatures, strong wind, soil with low organic content and high content of mineral salts, violent erosional work by water and wind, sporadic flow of streams and poor development of nominal dendritic drainage.” Although this definition is a good fit for many North American deserts, it poorly constrains others. For example, the Atacama and Namib Deserts, with their low average temperatures and high coastal

humidities, do not conform to Shreve’s description. However, it has proven difficult to arrive at a universally accepted definition, perhaps because deserts themselves show considerable individuality, and because of the existence of continuous transitions between the different types of deserts. Considered from a biological standpoint, deserts may be considered to be areas where the availability of water is low. “True” deserts result from a deficiency in the amount of precipitation received relative to water loss by evaporation. For organisms, aridity may be a relative condition, as the amount of water available may be a function of several interacting variables, including precipitation, temperature, soil texture, groundwater seepage, and slope and aspect. Furthermore, some organisms obtain their moisture from fog or dew, a source of water that is not regularly measured. Climate, vegetation, and fauna have all been used to delimit desert boundaries. Deserts may be divided into categories based on their temperature (hot; temperate; coastal) or moisture (hyperarid; arid; semiarid) characteristics. In many instances, the drier areas of the Earth are simply divided into two groups: arid and semiarid (or, synonymously, desert and semidesert; or desert and steppe). Plant and animal components are commonly incorporated in desert classification systems. Vegetative criteria of Shreve (1942) include floristic content, physiognomy and life forms, and structure and social organization. Herpetofauna and climate were used to set boundaries for the Chihuahuan Desert (Morafka 1977); and herpetofauna and plants for the eastern Sonoran Desert boundary (Lowe 1955). The delimitation of desert areas is difficult, particularly the location of the outer boundaries. This is the case even for relatively well-studied deserts in populated areas. For example, the boundaries assigned to the Sonoran Desert vary widely according to the criteria used by individual researchers (MacMahon & Wagner 1985). Additionally, desert boundaries are often considered as shifting zones of

2

CHAPTER

transition rather than lines clearly demarcated by climate or by abrupt changes in species or associations. Transitional boundaries may result from human impact or from decadal climatic fluctuations. Satellite imagery has allowed an annual examination of fluctuating boundaries, most notably the southern boundary of the Sahara (Tucker et al. 1991; Nicholson et al. 1998). Understanding desert climates is essential because of strong linkages between climate, biological processes, and geomorphology. Indeed, studies of desertification suggest that human-induced ecological and geomorphic changes may induce climate change, or at least prolong natural drought episodes. These themes are explored more fully in Sections 3.4.2, 3.4.3, and 13.2. Although all deserts are characterized by aridity, other climatic factors, such as temperature and humidity or season of precipitation, show considerable variation. The following climatic characteristics are common to many interior tropical and subtropical deserts: (1) high summertime temperatures, (2) an excess of potential evaporation over precipitation as a result of high temperatures, wind, and clear skies, (3) high variability of precipitation totals, distribution, and intensity, (4) a more prominent role for wind than in other zones, (5) clear skies prevailing over 70% of the time, and (6) low humidity (commonly 15–30% for inland deserts and as low as 5% in the Saharan Desert). Winter temperatures show large variations from place to place, largely a reflection of continentality and latitude. For coastal deserts, conditions are considerably different than for interior deserts, as proximity to cool ocean currents and the occurrence of frequent fogs give rise to cooler maximum and average temperatures and very high relative humidities for areas immediately adjacent to the sea. As may be inferred from the difficulties in defining and delimiting deserts, they do not present a homogeneous landscape. Long-term differences in climatic, tectonic, biological, and geologic history cause deserts to be individually distinct. Each tends to have a unique assemblage of landscape elements and processes. Geomorphologically, the North American deserts are dominated by the erosional and depositional effects of surface water; the eastern Sahara by aeolian processes; and the Atacama by extreme aridity, barren landscapes, saline deposits, and mass wasting that is enhanced by earthquake activity (Oberlander 1994). Deserts also vary tremendously in their tectonic settings. The stability and age of

1

Australian deserts contrasts sharply with the youth and tectonic instability of arid North America. Furthermore, unique landscape assemblages bear the imprint of climatic change over long time periods. The Earth’s climate has changed profoundly during the Quaternary and Tertiary, so many desert landscapes are palimpsests; that is, composed of relict elements produced under the influence of past climates and modern elements formed in the present climatic regime. Thus, it is impossible to understand modern desert environments without a consideration of earlier climatic, hydrologic, tectonic, geomorphic, and biological conditions. Deserts are superb repositories of these past legacies, as aridity and, in some cases, relative inactivity of the surface, act to preserve landscape assemblages. As noted by Williams (1994, p. 644), “one intriguing outcome of the polygenetic nature of desert landscapes is...the frequent juxtaposition of very old elements of the landscape with others that are very new.” This book aims to examine the many forces that have shaped and continue to influence desert landscapes, and to provide a broad appreciation of how the legacy of the past informs the processes of the present.

1.2 EVOLUTION

OF DESERTS

1.2.1 GLOBAL

CONSIDERATIONS

The arid regions of the world, other than those in the high Arctic, owe their origin to climatic, topographic, and oceanographic factors that prevent the incursion of moisture-bearing weather systems. Although the causes of aridity are discussed separately below, it should be noted that most deserts are arid because of a combination of factors. For example, west of the Andes in the Peru–Chile desert, aridity is a result of subtropical atmospheric subsidence, reinforced by upwelling of the cold Humboldt Current, and by the Andean rainshadow.

1.2.1.1 Subtropical high-pressure belts The world’s arid and semiarid regions are mainly subtropical in distribution, covering about 20% of the Earth’s land surface (Glennie 1987) (Fig. 1.1). As shown in Fig. 1.1, the Equator is flanked to the north and south by Hadley cells, each composed of a rising branch in the rainy equatorial zone, and a descending branch near 30° North and South in the arid subtropical zones. In the equatorial zone, opposing

ARCTIC CIRCLE

A S I A N ORT H

EUROPE

AMERICA DRY, SUBSIDING AIR

Mojave Sonoran

30°N

Great Basin

1

TROPIC OF CANCER

Karakum

Syrian

3

Sahara

Chihuahuan

Thar

S

Atl anti c Ocean

CELL

Gobi Taklimakan

Iranian

P a c i fi c Ocean

Arabian

AFRICA

HA

DLEY

EQUATOR

DRY, SUBSIDING AIR

EQUATOR

SOUTH

P a c i fi c Ocean

AMERICA

Indi an Ocean

Caatinga Scrubland

Namib

Atacama TROPIC OF CAPRICORN

Simpson

30°S

California Current

2

Peru (Humboldt) Current

3

Canaries Current

4

Benguela Current

5

West Australian Current

Kalahari

2

A UST R AL IA

5

4

1

Great Sandy Gibson

Patagonia

Great Victoria

ARID & SEMIARID REGIONS

ANTARCTIC CIRCLE

A

N

T A

R C

T

I C

A

FIG. 1.1 Map of the global deserts. The world’s deserts are largely subtropical in distribution. Cold currents are labeled and shown with arrows.

4

CHAPTER

winds converge in the intertropical convergence zone or ITCZ, and feed air into the rising branches, favoring convective rainfall. The descending branch forms zones of elevated sea-level pressure, referred to as the subtropical high-pressure belt (McIlveen 1992). Although localized vertical motions of the atmosphere occur in this zone of subsidence, larger-scale vertical motion is suppressed by persistent thermodynamic stability, leading to a general lack of rainfall. Relative humidities throughout the troposphere are very low in the zone of subsidence, and only rarely do active disturbances penetrate to break the normal aridity. The subtropical high-pressure belt is broken up into anticyclonic cells, so that the associated subsidence is discontinuous and aridity is not present in all longitudes. In the Northern Hemisphere, air moving clockwise around the equatorial sides of cells brings moist air to the eastern continental margins including the Caribbean, East Africa, and south and central China. Although subsidence dominates North India and Pakistan for much of the year, the summer monsoon brings 3 months of abundant rainfall except in the far northwest.

1.2.1.2 Continental interiors Continental interiors are arid owing to the distance from the sea and sources of moisture. In central Asia, for example, a great mountain arc prevents the deep incursion of moisture-laden air of the summer monsoon. In winter, a vast high-pressure cell develops, with very dry subsiding air. Interior deserts have a much greater range in annual temperature than those closer to a coast. Despite the high latitudes of Asian deserts, the summers are extremely hot, with maximum temperatures in excess of 38°C at all lowland stations. Winters are very cold, with minima ranging from −30 to −50°C (Fig. 1.2). The Great Basin Desert of North America also experiences great annual ranges of temperature and freezing conditions and snow in winter. The average temperature in January is −2°C. Nonetheless, summer temperatures in the southern Great Basin can be very hot (Laity 2002). The highest temperature ever recorded in the USA (57°C) was in Death Valley.

1.2.1.3 Polar deserts Low levels of solar radiation at the poles result in very cold temperatures. Because of this, the atmosphere contains little moisture for precipitation, and

1

although precipitation may be frequent, it is very light, with the depth of precipitable water not exceeding 10 mm at any time (Hidore & Oliver 1993). In Antarctica, mean annual precipitation ranges from 51 mm on the plateau to as much as 510 mm at some peninsular locations. Relative humidity values may be as low as 1%, with both humidity and cloud cover decreasing inland. Winds are also a predominant factor in polar deserts, making blizzards and drifting snows a common occurrence. For reasons of space, polar deserts and periglacial environments cannot be addressed in this book. It is interesting to note, however, that they share several geomorphic processes in common with warm deserts, particularly as the paucity of vegetation cover allows the free sweep of the wind. Thus, aeolian features such as dunes and ventifacts are shared by both desert types.

1.2.2 REGIONAL

CONSIDERATIONS

1.2.2.1 Cold-current influences Cool coastal deserts form adjacent to cold ocean currents on the western margin of continents. They are often long and narrow in form, and may be bounded to the east by north–south-oriented mountain ranges. The climate is greatly moderated by the proximity to cold waters and is characterized by rainlessness, fog and dew, and cold temperatures. Coastal deserts include the Atacama, along the coast of Chile and Peru and adjacent to the Humboldt Current; the Namib on the coast of southwest Africa along the Benguela Current (Fig. 1.3); and the desert along the Pacific coast of Baja California, Mexico, adjacent to the California Current. Other deserts with coldcurrent influences are the coastal Sahara in northwest Africa, the Arabian Peninsula and Horn of Africa, and the western coast of Australia (Warner 2004). Subsidence from the subtropical highpressure belt reinforces the effect of the cold coastal waters. Orographic barriers, such as the Andes Mountains, may prevent the incursion of moisture bearing systems from continental interiors. The movement of water from polar latitudes to low latitudes, with associated upwelling of deep cold waters, produces cold currents. Warm air from the high-pressure cells is cooled by contact with the water and layers of fog form. The air that crosses the land is foggy (relative humidity at or near 100%) and chilled nearly to the temperature of the water, normally from 15 to 18°C. The affected layer is thin

K A Z A K H S T A N

Sy r

D a r ’y a

w

l

a

n

d

s

Aral Sea

U

Z B E K I S T oA N

L

n T

u

r

a Am u

ya

T URKMENISTAN

Da

r’

Kara Kum

FIG. 1.2 The deserts of continental interiors are arid owing to their distance from the sea and sources of moisture. Such deserts have a much greater range of temperatures than those close to the coast. Winters in the Asian deserts can be very cold, with minima ranging from −30 to −50°C. In this image, the shrinking Aral Sea appears in 2002, filling with seasonal ice, and the deserts of the Kyzylkum and Karakum (also known as Kara Kum) to the southeast and south of the lake, respectively, are blanketed in snow. The diversion of freshwater inflows to the saline Aral Sea for agriculture has led to a considerable loss of lake volume and quality. Source: NASA MODIS Rapid Response Team, NASA/GSFC. See Plate 1.2 for a color version of this image.

6

CHAPTER

Etosha Pan

1

summer. During the winter the sea breeze may weaken somewhat, but in general conditions vary little throughout the year.

1.2.2.2 Rainshadow effect

N a m i b Walvis Bay

e r t D e s

Atl ant i c Ocean

N A M IBIA

FIG. 1.3 Cool coastal deserts form adjacent to cold ocean currents on the west coasts of continents. The climate is moderated by the proximity to cold waters, which tend to impede convection. Moisture is largely provided by fog, shown here along the southern coast of the Namib Desert. True desert conditions with intense aridity occur in the Namib Desert, a strip 80–150 km wide along the Atlantic coast. To the east, on the right side of the satellite image, is the inland Kalahari Desert. Source: NASA Aqua/MODIS sensor. See Plate 1.3 for a color version of this image.

(150–600 m), and above these levels, hot and dry subsiding subtropical air prevails, causing a temperature inversion. Rainfall amounts are very low because: (1) the air aloft lacks moisture, (2) the layer of cold air near the surface impedes convection, and (3) the moist surface air is too small in volume to provide an adequate moisture source. Condensation, however, occurs nightly and all exposed objects are wetted. The seasons have little impact, with winter temperatures averaging only 3–6°C below those of

When air crosses mountain barriers, it rises on the windward side, and subsides on the lee side. Subsidence prevents convection and causes adiabatic heating that results in a pronounced drying effect. Thus, many of the world’s arid areas lie to the lee of mountainous regions. In Australia, the Great Divide and other mountains on the east coast lie in the path of the prevailing southeasterly trade winds, creating a rainshadow effect that accentuates the aridity of the central continent. Aridity in the Patagonian Desert of South America results from the Andes mountain range, which blocks rain-bearing westerly air masses and gives rise to strong, dry adiabatic winds and dust storms. Little moisture results from cold air masses from the South Atlantic. Cool winters and mild summers characterize the Patagonian Desert, with temperatures decreasing southward. Similarly, the dry belts of Canada and the USA owe their origin to rainshadow effects resulting when westerly winds cross high-elevation mountains aligned north–south. Lying in the rainshadow of the Sierra Nevada-Cascade chain, the Great Basin Desert receives from 100 to 300 mm of precipitation annually (Fig. 1.4).

1.2.2.3 Edaphic environments Edaphic deserts result in large measure from the influence of the soil. For example, in the Kalahari Desert, high evaporation rates and sandy soils that absorb rainfall produce a region that lacks surface water despite a rainfall range from 200 mm in the south to 500 mm in the north. Edaphic factors enhance the apparent aridity of this region, which is also fostered by continentality and atmospheric subsidence.

1.3 INDICES

OF ARIDITY

Defining the margins of deserts and the boundary between arid and semiarid regions is difficult. Indices to determine aridity are based on rainfall alone, on water balance (the relationship that exists in a given area between precipitation (P), losses due to evapotranspiraton (ET), and changes in storage (S)), on soil type, or on vegetation.

DEFINING THE DESERT SYSTEM

7

FIG. 1.4 The subsidence of air to the lee of mountain barriers creates deserts in the rainshadow. In this image, the Sierra Nevada of California, which rises to over 4400 m, creates a rainshadow to the west in the Owens Valley. The decline in rainfall on the lee slopes can be seen as a decrease in vegetation. The Olancha Dunes are a small dune field formed in the southern section of Owens Valley. Vegetation within the dunes probably grows by exploiting groundwater. Alluvial fans can be seen along the basal slope of the Sierra.

On the basis of moisture indices, deserts are commonly divided into three categories: hyperarid, arid, and semiarid. Hyperarid regions have at least 12 consecutive months without rainfall and no regular seasonal cycle of rainfall. The Western Desert of Egypt and the Atacama Desert are hyperarid. Arid regions receive between 25 and 200 mm of rainfall annually, whereas semiarid lands have between 200 and 500 mm (Grove 1977). Semiarid grasslands are generally referred to as steppes, although this term is often ill-defined. Most climatic systems used to define aridity are based on the concept of water balance. An example is the Thornthwaite Moisture Index (Im) (Thornthwaite 1948). Im =

s − 0.6d × 100 e

In this equation, s is the sum of monthly surpluses of precipitation above the estimated potential evaporation; d is the sum of monthly deficits in precipitation; and e is the estimated annual potential evaporation based on mean monthly values of temperature, with an adjustment for season of rainfall, and including a factor for soil-moisture storage. Thornthwaite moisture value indices of 0 to −20 are

considered dry subhumid; values of −20 to −40 are semiarid (200–500 mm precipitation); and values of −40 to −56 are arid (25–200 mm precipitation). Below −57, the region is considered hyperarid (generally