CONTENTS

Preface xii

1 Introduction: defi ning the system 1 1.1 Defi ning the desert system 1 1.1.1 Physical, biological, and temporal components 1 1.2 Evolution of 2 1.2.1 Global considerations 2 1.2.1.1 Subtropical high-pressure belts 2 1.2.1.2 Continental interiors 4 1.2.1.3 Polar deserts 4 1.2.2 Regional considerations 4 1.2.2.1 Cold-current infl uences 4 1.2.2.2 Rainshadow effect 6 1.2.2.3 Edaphic environments 6 1.3 Indices of aridity 6 1.4 Desert surfaces 8 1.5 Tectonically stable and unstable deserts 8 1.6 Deserts of the past 9 1.7 Changing human perspectives on deserts 12

2 Deserts of the 14 2.1 Introduction: the extent of global aridity 14 2.2 Global deserts 14 2.2.1 Africa 14 2.2.1.1 North Africa: the Saharan Desert and the Sahel 15 2.2.1.2 North Africa: the Somali- 18 2.2.1.3 Southern Africa: arid Madagascar 18 2.2.1.4 Southern Africa: the Karoo, Kalahari, and Deserts 22 2.2.2 Middle East and Arabia 24 2.2.2.1 and Sinai Deserts 24 2.2.2.2 Deserts of Syria and Jordan 26 2.2.2.3 The Arabian Peninsula 26 2.2.2.4 Iran and Iraq 27 2.2.3 Europe 28 2.2.4 Asia 29 2.2.4.1 Middle Asian deserts 29 2.2.4.2 Deserts of India and Pakistan 30 2.2.4.3 Deserts of China and Mongolia 30 2.2.5 South America 33 2.2.5.1 The west coast deserts: Peru–, Atacama, and Sechura deserts 33 2.2.5.2 Altiplano/Puna 36 2.2.5.3 36 2.2.5.4 37

V VI CONTENTS

2.2.6 North America 37 2.2.6.1 39 2.2.6.2 40 2.2.6.3 41 2.2.6.4 The Great Basin deserts 42 2.2.7 Australia 43

3 The climatic framework 48 3.1 Introduction: classifi cation of deserts by temperature 48 3.2 Weather data 48 3.3 Atmospheric controls: surface boundary layer 50 3.3.1 Atmospheric water vapor and cloud cover 50 3.3.2 Radiation 51 3.3.3 Temperature of the air, surface, and subsurface 52 3.3.3.1 Air temperature of hot deserts 52 3.3.3.2 Surface temperatures 52 3.3.3.3 Subsurface temperatures 53 3.3.4 Albedo 54 3.3.5 Precipitation 54 3.3.5.1 Storm types and seasonality of precipitation 55 3.3.5.2 Forms of precipitation other than rainfall: fog, dew, and snow 58 3.3.5.3 Variability in precipitation 60 3.3.6 Wind 62 3.3.7 Effects of population growth and urbanization on desert climatology 63 3.3.7.1 Air pollution 63 3.3.7.2 Heat islands 64 3.4 Temporal and spatial variability of climatic infl uences 64 3.4.1 ENSO forcing of desert climates 64 3.4.2 Expansion and contraction of the Desert 67 3.4.3 The Sahel: land-surface–atmosphere interactions 69

4 The hydrologic framework 71 4.1 Introduction 71 4.2 The water balance in deserts 71 4.3 Water budgets 73 4.3.1 Precipitation and its assessment: problems in gauging and network design 73 4.3.2 Interception 74 4.3.3 Evapotranspiration 75 4.3.3.1 Introduction 75 4.3.3.2 Evaporation 76 4.3.3.3 Transpiration 78 4.3.4 Infi ltration and soil water 79 4.3.5 Groundwater, subsurface fl ow, and springs 81 4.3.5.1 Role of groundwater in arid environments 81 4.3.5.2 Groundwater recharge 82 4.3.5.3 Groundwater quality 83 4.4 Surface runoff and fl oods 84 4.4.1 Controls on runoff 84 4.4.2 Runoff from slopes 85 4.4.3 Runoff in channels 86 4.4.3.1 Ephemeral channels 86 4.4.3.2 Intermittent and perennial rivers 87 4.4.3.3 Low-fl ow events and the ecological effects of drought 87 CONTENTS VII

4.4.4 Transmission losses during fl oods 88 4.5 The chemical quality of surface and soil water 89 4.6 Water resources 90 4.6.1 Groundwater 91 4.6.2 Dams and reservoirs 92 4.6.3 Long-distance transfer: canals and aqueducts 92 4.6.4 Rainmaking 93 4.6.5 Desalination 94 4.6.6 Fog-water collection systems 94 4.7 Case study: the waters of the Tigris-Euphrates Basin and the impact of modern water management 94

5 Lake systems: past and present 98 5.1 Introduction to desert lakes 98 5.2 Types of lake 98 5.2.1 Perennial salt lakes 98 5.2.2 Ephemeral lakes: playas and pans 98 5.2.2.1 Wet (salt playas; discharge playas) and dry (recharge playas; claypans) systems 100 5.2.2.2 Playa degradation 103 5.2.3 Palaeolake systems: lakes as indicators of past climate changes 104 5.3 Lakes of the global arid environment 106 5.3.1 Western North America 106 5.3.2 South America 109 5.3.3 Australia 112 5.3.4 Africa 114 5.3.5 Asia 117 5.3.5.1 China and Mongolia 117 5.3.5.2 India and Pakistan 120 5.3.6 Middle East 120

6 Weathering processes and hillslope systems 122 6.1 Introduction 122 6.2 Weathering 122 6.2.1 Insolation weathering 123 6.2.2 Salt weathering 123 6.2.3 Frost weathering 125 6.2.4 Biological weathering 125 6.2.5 Silt infi ltration 125 6.3 Weathering forms 126 6.3.1 Cavernous weathering/tafoni 126 6.3.2 Gnammas 128 6.4 Duricrusts 128 6.4.1 Terminology 128 6.4.2 Silcrete 129 6.4.3 Calcrete/caliche 130 6.4.4 Gypcrete 133 6.4.5 Salcrete: halite crusts 133 6.5 Desert varnish 134 6.6 Hillslope processes 136 6.6.1 Rock slopes 136 6.6.1.1 Hillslopes in massive rocks 136 6.6.1.2 Scarp and cuesta forms 137 VIII CONTENTS

6.6.2 Gravity-related activity: talus and scree slopes and related forms 141 6.6.3 Badlands 142 6.6.3.1 Case study: Borrego Badlands, California 144 6.7. Composite surfaces (pediments) 145

7 Desert soils and geomorphic surfaces 148 7.1 Introduction 148 7.2 The nature of soils in arid and semiarid regions 148 7.3 Soil description and classifi cation 149 7.4 Soil characteristics of arid regions 150 7.4.1 Physical characteristics 150 7.4.2 The organic content of soils and nutrient availability 151 7.4.3 Role of the past 151 7.4.4 Role of relief and altitude 152 7.5 Inorganic and biological soil crusts 152 7.5.1 Inorganic soil crusts 152 7.5.2 Biological/cryptobiotic surface crusts 153 7.6 Spatial heterogeneity in soil properties and the ecohydrology of patterned vegetation zones 156 7.7 Surface volume changes 158 7.7.1 The properties and nature of swelling clay soils 158 7.7.2 Patterned ground or gilgai 158 7.8 Surface types: hamada and stone pavements 159 7.8.1 Hamada 159 7.8.2 Stone pavements 159 7.8.2.1 Introduction 159 7.8.2.2 Description of stone pavements 162 7.8.2.3 Formation of pavements 164 7.8.2.4 The aeolian aggradation theory of pavement development 165 7.8.2.5 Pavement development as a relative-age dating tool 166 7.8.2.6 Discussion 167

8 Water as a geomorphic agent 168 8.1 Introduction 168 8.2 Groundwater sapping in slope and valley development 168 8.3 Piping processes in channel and slope evolution 171 8.4 Fluvial processes 172 8.4.1 Channel morphology and channel fl ow 172 8.4.2 Alluvium 173 8.4.3 Sediment transport 174 8.3.4 Sediment yields 178 8.5 Fluvial landforms 179 8.5.1 Alluvial fans 179 8.5.1.1 Introduction 179 8.5.1.2 Sediment production, transportation, and deposition 180 8.5.2 Arroyos 182 8.5.3 Gullies 184 8.4.4 Landform assemblages 185

9 Aeolian processes 186 9.1 Introduction 186 9.2 Near-surface fl ow 187 CONTENTS IX

9.2.1 Variation in wind velocity with height 187 9.2.2 Airfl ow and sediment transport over hills and dunes 188 9.3 Wind processes 191 9.3.1 Aeolian particles 191 9.3.1.1 Particle sizes 191 9.3.1.2 Processes of particle formation 192 9.3.2 Particle entrainment (sand) 194 9.3.3 Particle transport 194 9.3.3.1 Modes of transportation 194 9.3.3.2 Transport rates 195 9.4 Landforms of accumulation: sand sheets, zibar, and sand stringers 195 9.5 Landforms of accumulation: dunes 197 9.5.1 Introduction 197 9.5.2 The development of dune fi elds: palaeo-aeolian processes and evidence for multiple phases of activity 197 9.5.3 Dune reactivation 199 9.5.4 Interdune deposits and lakes 200 9.5.5 Dune patterns and classifi cation 201 9.5.6 Dune accumulation infl uenced by topographic obstacles 204 9.5.6.1 Lee dunes 204 9.5.6.2 Climbing dunes, sand ramps, echo dunes, and cliff-top dunes 204 9.5.7 Formation of self-accumulated dunes 205 9.5.7.1 Dune initiation 205 9.5.7.2 Crescentic dunes: barchans and transverse barchanoid ridges 205 9.5.7.3 Linear dunes (seif dunes) 206 9.5.7.4 Star dunes 208 9.5.7.5 Dome dunes 209 9.5.8 Vegetated dunes 209 9.5.8.1 Hummock dunes, coppice dunes, or nebkhas 210 9.5.8.2 Parabolic and elongate parabolic dunes 211 9.5.8.3 Lunette dunes 212 9.5.8.4 Vegetated linear dunes 212 9.6 Ripples 213

10 Landforms of aeolian erosion and desert dust 216 10.1 Introduction 216 10.2 Defl ation features: desert depressions and pans 216 10.3 Ventifacts 218 10.4 Yardangs and ridge and swale systems 221 10.5 Desert dust 225 10.5.1 Defi nitions 226 10.5.2 Environmental role and impacts of dust 226 10.5.2.1 Effects on marine and terrestrial ecosystems 227 10.5.2.2 Relationship to soil development and earth surface processes 227 10.5.2.3 Impact of dust on climate, weather, and air quality 228 10.5.2.4 Dust storms and vehicular accidents 229 10.5.3 Dust entrainment, transport, and deposition 229 10.5.3.1 Climatic factors in dust entrainment 230 10.5.3.2 Surface factors: vegetation, crusts, and the availability of sand 230 10.5.3.3 Anthropogenic activity 232 10.5.4 Climatic events associated with blowing dust: scales of activity 233 X CONTENTS

10.5.5 Frequency of blowing dust: interannual, seasonal, diurnal 233 10.5.6 Dust-source areas 234

11 Plant communities and their geomorphic impacts 237 11.1 Introduction: characteristics of desert ecosystems 237 11.2 Adaptations to desert conditions 238 11.2.1 Adaptations to temperature 238 11.2.2 Water use by plants 240 11.2.3 Reproduction 242 11.2.4 Nutrient cycling 243 11.2.5 Salt adaptation 244 11.3 Plant communities and ecotones 245 11.3.1 Evergreen shrubs 245 11.3.2 Drought-deciduous shrubs 246 11.3.3 CAM succulents 246 11.3.4 Perennial grasses 247 11.3.5 Phreatophytes 247 11.3.6 Desert annuals 248 11.3.7 Poikilohydric plants 250 11.3.8 Exotic plants 250 11.4 Succession in desert plant communities 251 11.5 Dune communities 252 11.6 Vegetation type and density and relationship to geomorphic processes 253 11.6.1 The role of vegetation in the erosion and deposition of sand 253 11.6.2 The role of slope and aspect in plant distribution 255 11.6.3 Effects of vegetation on stream-channel processes 256 11.6.3.1 How fl oods and fl uvial landforms affect vegetation 256 11.6.3.2 How vegetation affects dryland river-channel processes and form 257 11.6.3.3 Flow regulation and riparian communities in arid lands 257

12 Animal communities 259 12.1 Introduction: environmental requirements 259 12.1.1 Adaptations to air and soil temperature, fi re, and the gaseous environment 262 12.1.2 Moisture parameters 263 12.2 Effects on geomorphic processes 264 12.2.1 Slope processes, surface stability, and soil development 264 12.2.1.1 Surface movement and animal tracks 264 12.2.1.2 Biopedturbation and burrowing 265 12.3 Hydrologic impacts 265 12.4 Effects of the geomorphic activity of animals on plant communities 266

13 Desertifi cation and the human dimension 267 13.1 Desertifi cation: introduction and terminology 267 13.2 Climate change and desertifi cation 268 13.3 Anthropogenic causes of desertifi cation 269 13.3.1 The rural environment: overgrazing and woodcutting; devegetation and biological feedbacks 270 13.3.2 Urbanization and technological exploitation 271 13.3.3 Off-road vehicles and military vehicles 272 13.3.4 Increases in dust-storm activity and the effect on humans and the environment 273 CONTENTS XI

13.4 Water resources: a rural and urban problem 275 13.4.1 Groundwater withdrawal 276 13.4.2 Depletion of river fl ow and loss of sediment 277 13.4.3 Effects of irrigation: waterlogged soils and salinization 278 13.4.4 Desert lakes affected by humans 279 13.5 Case study: the Aral Sea 280 13.5.1 Lake bottom exposure and salt and dust storms 282 13.5.2 Ecosystem damage 282 13.5.3 Climatic alteration 282 13.5.4 Health concerns 283 13.6 Discussion 283

References 285

Index 320