DESERTS AND
DESERT ENVIRONMENTS
Julie Laity
Wiley-Blackwell.
A John Wiley & Sons, Inc., Publication
2008
2008
Preface xii
1 Introduction: defining the desert system 1
1.1 Defining the desert system 1
1.1.1 Physical, biological, and temporal components 1
1.2 Evolution of deserts 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 influences 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 world 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-Chalbi Desert 18
2.2.1.3 Southern Africa: arid Madagascar 18
2.2.1.4 Southern Africa: the Karoo, Kalahari, and Namib Deserts 22
2.2.2 Middle East and Arabia 24
2.2.2.1 Negev 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–Chile, Atacama, and Sechura deserts 33
2.2.5.2 Altiplano/Puna 36
2.2.5.3 Monte Desert 36
2.2.5.4 Patagonian Desert 37
V
2.2.6 North America 37
2.2.6.1 Chihuahuan Desert 39
2.2.6.2 Sonoran Desert 40
2.2.6.3 Mojave Desert 41
2.2.6.4 The Great Basin deserts 42
2.2.7 Australia 43
3 The climatic framework 48
3.1 Introduction: classification 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 influences 64
3.4.1 ENSO forcing of desert climates 64
3.4.2 Expansion and contraction of the Sahara 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 Infiltration and soil water 79
4.3.5 Groundwater, subsurface flow, 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 floods 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-flow events and the ecological effects of drought 87
VI CONTENTS
4.4.4 Transmission losses during floods 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
CONTENTS VII
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 classification 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 flow 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
VIII CONTENTS
9.2.1 Variation in wind velocity with height 187
9.2.2 Airflow 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 fields: 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 classification 201
9.5.6 Dune accumulation influenced 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 Deflation 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
CONTENTS IX
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 floods and fluvial 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, fire, 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 Desertification and the human dimension 267
13.1 Desertification: introduction and terminology 267
13.2 Climate change and desertification 268
13.3 Anthropogenic causes of desertification 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
X CONTENTS
13.4 Water resources: a rural and urban problem 275
13.4.1 Groundwater withdrawal 276
13.4.2 Depletion of river flow 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
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