1 Introduction: defi ning the desert system 2 Deserts of the world

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Todd Pierce Sullivan
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1 Preface xii 1 Introduction: defining the desert system Defining the desert system Physical, biological, and temporal components Evolution of deserts Global considerations Subtropical high-pressure belts Continental interiors Polar deserts Regional considerations Cold-current influences Rainshadow effect Edaphic environments Indices of aridity Desert surfaces Tectonically stable and unstable deserts Deserts of the past Changing human perspectives on deserts 12 2 Deserts of the world Introduction: the extent of global aridity Global deserts Africa North Africa: the Saharan Desert and the Sahel North Africa: the Somali-Chalbi Desert Southern Africa: arid Madagascar Southern Africa: the Karoo, Kalahari, and Namib Deserts Middle East and Arabia Negev and Sinai Deserts Deserts of Syria and Jordan The Arabian Peninsula Iran and Iraq Europe Asia Middle Asian deserts Deserts of India and Pakistan Deserts of China and Mongolia South America The west coast deserts: Peru Chile, Atacama, and Sechura deserts Altiplano/Puna Monte Desert Patagonian Desert 37 V

2 VI CONTENTS North America Chihuahuan Desert Sonoran Desert Mojave Desert The Great Basin deserts Australia 43 3 The climatic framework Introduction: classification of deserts by temperature Weather data Atmospheric controls: surface boundary layer Atmospheric water vapor and cloud cover Radiation Temperature of the air, surface, and subsurface Air temperature of hot deserts Surface temperatures Subsurface temperatures Albedo Precipitation Storm types and seasonality of precipitation Forms of precipitation other than rainfall: fog, dew, and snow Variability in precipitation Wind Effects of population growth and urbanization on desert climatology Air pollution Heat islands Temporal and spatial variability of climatic influences ENSO forcing of desert climates Expansion and contraction of the Sahara Desert The Sahel: land-surface atmosphere interactions 69 4 The hydrologic framework Introduction The water balance in deserts Water budgets Precipitation and its assessment: problems in gauging and network design Interception Evapotranspiration Introduction Evaporation Transpiration Infiltration and soil water Groundwater, subsurface flow, and springs Role of groundwater in arid environments Groundwater recharge Groundwater quality Surface runoff and floods Controls on runoff Runoff from slopes Runoff in channels Ephemeral channels Intermittent and perennial rivers Low-flow events and the ecological effects of drought 87

3 VII Transmission losses during floods The chemical quality of surface and soil water Water resources Groundwater Dams and reservoirs Long-distance transfer: canals and aqueducts Rainmaking Desalination Fog-water collection systems Case study: the waters of the Tigris-Euphrates Basin and the impact of modern water management 94 5 Lake systems: past and present Introduction to desert lakes Types of lake Perennial salt lakes Ephemeral lakes: playas and pans Wet (salt playas; discharge playas) and dry (recharge playas; claypans) systems Playa degradation Palaeolake systems: lakes as indicators of past climate changes Lakes of the global arid environment Western North America South America Australia Africa Asia China and Mongolia India and Pakistan Middle East Weathering processes and hillslope systems Introduction Weathering Insolation weathering Salt weathering Frost weathering Biological weathering Silt infiltration Weathering forms Cavernous weathering/tafoni Gnammas Duricrusts Terminology Silcrete Calcrete/caliche Gypcrete Salcrete: halite crusts Desert varnish Hillslope processes Rock slopes Hillslopes in massive rocks Scarp and cuesta forms 137

4 VIII CONTENTS Gravity-related activity: talus and scree slopes and related forms Badlands Case study: Borrego Badlands, California Composite surfaces (pediments) Desert soils and geomorphic surfaces Introduction The nature of soils in arid and semiarid regions Soil description and classification Soil characteristics of arid regions Physical characteristics The organic content of soils and nutrient availability Role of the past Role of relief and altitude Inorganic and biological soil crusts Inorganic soil crusts Biological/cryptobiotic surface crusts Spatial heterogeneity in soil properties and the ecohydrology of patterned vegetation zones Surface volume changes The properties and nature of swelling clay soils Patterned ground or gilgai Surface types: hamada and stone pavements Hamada Stone pavements Introduction Description of stone pavements Formation of pavements The aeolian aggradation theory of pavement development Pavement development as a relative-age dating tool Discussion Water as a geomorphic agent Introduction Groundwater sapping in slope and valley development Piping processes in channel and slope evolution Fluvial processes Channel morphology and channel flow Alluvium Sediment transport Sediment yields Fluvial landforms Alluvial fans Introduction Sediment production, transportation, and deposition Arroyos Gullies Landform assemblages Aeolian processes Introduction Near-surface flow 187

5 IX Variation in wind velocity with height Airflow and sediment transport over hills and dunes Wind processes Aeolian particles Particle sizes Processes of particle formation Particle entrainment (sand) Particle transport Modes of transportation Transport rates Landforms of accumulation: sand sheets, zibar, and sand stringers Landforms of accumulation: dunes Introduction The development of dune fields: palaeo-aeolian processes and evidence for multiple phases of activity Dune reactivation Interdune deposits and lakes Dune patterns and classification Dune accumulation influenced by topographic obstacles Lee dunes Climbing dunes, sand ramps, echo dunes, and cliff-top dunes Formation of self-accumulated dunes Dune initiation Crescentic dunes: barchans and transverse barchanoid ridges Linear dunes (seif dunes) Star dunes Dome dunes Vegetated dunes Hummock dunes, coppice dunes, or nebkhas Parabolic and elongate parabolic dunes Lunette dunes Vegetated linear dunes Ripples Landforms of aeolian erosion and desert dust Introduction Deflation features: desert depressions and pans Ventifacts Yardangs and ridge and swale systems Desert dust Definitions Environmental role and impacts of dust Effects on marine and terrestrial ecosystems Relationship to soil development and earth surface processes Impact of dust on climate, weather, and air quality Dust storms and vehicular accidents Dust entrainment, transport, and deposition Climatic factors in dust entrainment Surface factors: vegetation, crusts, and the availability of sand Anthropogenic activity Climatic events associated with blowing dust: scales of activity 233

6 X CONTENTS Frequency of blowing dust: interannual, seasonal, diurnal Dust-source areas Plant communities and their geomorphic impacts Introduction: characteristics of desert ecosystems Adaptations to desert conditions Adaptations to temperature Water use by plants Reproduction Nutrient cycling Salt adaptation Plant communities and ecotones Evergreen shrubs Drought-deciduous shrubs CAM succulents Perennial grasses Phreatophytes Desert annuals Poikilohydric plants Exotic plants Succession in desert plant communities Dune communities Vegetation type and density and relationship to geomorphic processes The role of vegetation in the erosion and deposition of sand The role of slope and aspect in plant distribution Effects of vegetation on stream-channel processes How floods and fluvial landforms affect vegetation How vegetation affects dryland river-channel processes and form Flow regulation and riparian communities in arid lands Animal communities Introduction: environmental requirements Adaptations to air and soil temperature, fire, and the gaseous environment Moisture parameters Effects on geomorphic processes Slope processes, surface stability, and soil development Surface movement and animal tracks Biopedturbation and burrowing Hydrologic impacts Effects of the geomorphic activity of animals on plant communities Desertification and the human dimension Desertification: introduction and terminology Climate change and desertification Anthropogenic causes of desertification The rural environment: overgrazing and woodcutting; devegetation and biological feedbacks Urbanization and technological exploitation Off-road vehicles and military vehicles Increases in dust-storm activity and the effect on humans and the environment 273

7 XI 13.4 Water resources: a rural and urban problem Groundwater withdrawal Depletion of river flow and loss of sediment Effects of irrigation: waterlogged soils and salinization Desert lakes affected by humans Case study: the Aral Sea Lake bottom exposure and salt and dust storms Ecosystem damage Climatic alteration Health concerns Discussion 283 References 285 Index 320

every continent has an extensive dry region! " deserts are as much as 1/3 of Earth s surface!

deserts! deserts! every continent has an extensive dry region! " deserts are as much as 1/3 of Earth s surface! Hollywood portrayal of vast stretches of sand dune! " Sahara has only 10% covered by sand!