Deserts and Desert Environments

Taking a global perspective, this book provides a concise overview of drylands, including their physical, biological, temporal, and human components. Examines the physical systems occurring in desert environments, including climate, hydrology, past and present lakes, weathering, hillslopes, geomorphic surfaces, water as a geomorphic agent, and aeolian processes Offers an accessible introduction to the physical, biological, temporal, and human components of drylands Investigates the nature, environmental requirements, and essential geomorphic roles of plants and animals in this stressful biological environment Highlights the impact of human population growth on climate, desertification, water resources, and dust storm activity Includes an examination of surface/atmosphere interactions and the impact of ENSO events.

Preface xii 1 Introduction: defining the desert system 1 1.1 Defining the desert system 1 1.2 Evolution of deserts 2 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 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 3.4 Temporal and spatial variability of climatic influences 64 4 The hydrologic framework 71 4.1 Introduction 71 4.2 The water balance in deserts 71 4.3 Water budgets 73 4.4 Surface runoff and floods 84 4.5 The chemical quality of surface and soil water 89 4.6 Water resources 90 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 5.3 Lakes of the global arid environment 106 6 Weathering processes and hillslope systems 122 6.1 Introduction 122 6.2 Weathering 122 6.3 Weathering forms 126 6.4 Duricrusts 128 6.5 Desert varnish 134 6.6 Hillslope processes 136 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 7.5 Inorganic and biological soil crusts 152 7.6 Spatial heterogeneity in soil properties and the ecohydrology of patterned vegetation zones 156 7.7 Surface volume changes 158 7.8 Surface types: hamada and stone pavements 159 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 8.5 Fluvial landforms 179 9 Aeolian processes 186 9.1 Introduction 186 9.2 Near-surface flow 187 9.3 Wind processes 191 9.4 Landforms of accumulation: sand sheets, zibar, and sand stringers 195 9.5 Landforms of accumulation: dunes 197 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 11 Plant communities and their geomorphic impacts 237 11.1 Introduction: characteristics of desert ecosystems 237 11.2 Adaptations to desert conditions 238 11.3 Plant communities and ecotones 245 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 12 Animal communities 259 12.1 Introduction: environmental requirements 259 12.2 Effects on geomorphic processes 264 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 13.4 Water resources: a rural and urban problem 275 13.5 Case study: the Aral Sea 280 13.6 Discussion 283 References 285 Index 320