Desert Processes and Landscape Development I INTRODUCTION A B Landscape evolution in arid and semi arid regions of the earth can take on a distinctive characteristic of their own These areas are obviously marked by low annual precipitation distinctive floral faunal habitation and characteristic ephemeral seasonal erosion deposition processes which include both water and wind as principal driving agents This section will provide a notable contrast to the more humid fluvial glacial environments Definition Characteristics of Deserts 1 Dry Climate Rate Evaporation Rate of Precipitation a b c 2 Arid climate 10 to 15 inches year Desert Regime Semi arid 10 25 inches year Steppe Regime Nature of Rainfall precipitation overall year round precipitation is low in desert areas but is intense in instantaneous accumulation rates heavy storm downfall common flash floods are common As a result rain surface erosion is the dominant erosion process in deserts NOT WIND as one might expect Causal Factors of Dryness a Latitudinal Effects of Global Surface Heating Global Weathering patterns deserts often associated with lack of vertical air movement or lack of adiabatic cooling hence no air cooling no atmospheric condensation e g Sub tropical latitude deserts such as Sahara Arabian Australian 1 2 b Result of semi permanent high pressure systems High level temperature inversions Orographic Rain Shadow Effects on leeward sides of high mountain ranges or in remote continental interiors precipitation is concentrated on upwind side of mountains water condenses as air rises over mountains leaving dry air moving over leeward side creating desert conditions e g interior of Pacific Northwest Mohave desert of 162 California c 3 Lack of significant vegetative cover associated with inhospitable climate lacking water Plant species dominated by succulents waxy leaves stems to reduce evapotranspiration e g yucca aloe cactus and short brushy plants with small leaves and long tap roots e g sage brush live scrub oak High Landscape erosion Potential associated with lack of vegetative cover intense rainfall events flash floods weathering all couple together to present erosion potential a wind vs water as erosion agent although wind is a significant factor in desert conditions water is still the dominant agent of erosion capable of moving larger fragments and quantities of sediment at any given instant b Dry gulches arroyos and sediment laiden streams common in desert regions 4 Weathering Physical weathering predominates chemical weathering is at minimum in absence of water with such processes as salt wedging thermal exansion frost wedging and plant rooting Generally much slower weathering process than in humid chemically dominated terrains 5 Poor soil development pedogenesis is generally limited in arid areas but distinctive nonetheless Salts and pedogenic carbonate development is common in arid soils Often thin and lack extensive organic matter 6 Impermeable surface layers possible in the form of bedrock hardpans and salt beds Results in much surface runoff and little infiltration 7 Sand desert sand generated during the physical weathering process as opposed to clay dominated soils in humid chemically weathered areas Sand can be readily washed by surface runoff and is in the size range applicable for wind transport 8 Drainage Patterns Dominated by ephemeral streams flowing either seasonally or during storm periods Large volumes of sediment tend to be transported short distances during storm rainfall events 9 Closed or internal drainage basins are common in desert regions as opposed to open drainage basins in humid areas Due to ephemeral nature of stream 163 discharge due to high evaporation rates and infiltration II GLOBAL DISTRIBUTION OF DESERT ECOSYSTEMS Desert and Steppe conditions cover upwards of 25 30 of total continental land area forming the largest component of all climatic regimes A Subtropical Deserts at latitudes near or above the Tropics of Cancer and Capricorn 1 Tropic of Cancer includes Sahara Desert of N Africa the Arabian Desert of the Middle East 2 Tropic of Capricorn includes much of Australian interior and interior Pampas of Argentina a Atmospheric Pressure Differential derived from unequal heating of earth s surface by solar radiation 1 2 B C Equator direct radiation high temp air heating a warm air rises and moves poleward toward cooler air masses b Cool polar air moves toward equator replacing warm air circulation cells Air moving poleward gathers faster than can escape toward poles subtropical high pressure belts a High pressure pushes warm air downward and back toward equator forming Trade Winds b Warm descending air absorbs moisture to create drying effect producing subtropical desert belts of world Mid latitude Deserts the Gobi Desert of central Asia Mongolian and the Western interior of the United States Desert Facts and Figures 1 2 3 Largest Desert Sahara of Northern Africa 10 of total desert area Sandiest Desert the Arabian Desert of Arabian Peninsula Hottest Temperature Recorded 136 degrees F at Libya 164 4 5 6 III 1922 Sahara Desert Greatest 1 day temperature fluctuation 100 degrees F in Algeria N Africa 1927 Sahara Desert Highest Annual Avg Temperature 94 degrees F in Ethiopia Sahara Desert Least Annual Average Precipitation 0 03 inches in Arica Chile coastal Chile and Peru form some of the driest desert areas on earth FLUVIAL PROCESSES IN DESERT ARID REGIONS Running water is the most important agent of erosion and transportation in desert landscape development A Erosional Characteristics 1 Rain splash sheetwash rilling and flashflooding is common in surface drainage process owing to lack of vegetative cover 2 Although sporadic intense rainfall episodes result in great volumes of sediment being mobilized in a relatively short period of time 3 Stream gradients may be steep but ephemeral nature of flow results in unpredictable imbalance between erosion transporation and deposition 4 a Streams rapidly become choked with sediment during concentrated flow flood events b Sediment may be rapidly moved and stored under sporadic flow conditions Drainage basins tend to be closed i e not through flowing with internal drainage and basin sedimentation This follows in that the general lack of water does not allow extensive through flowing drainage 5 B Desert drainage systems tend to be bedload dominated by coarse sand and gravel finer silts clays and sands are readily transported and removed from the