Arid and Semi-Arid Arid and Semi-Arid Landscapes Landscapes (Desert) has Precipitation (mm) < 10 x Temperatu (Desert) has Precipitation (mm) < 10 x Temperatu C Desert < 25 cm = 250 mm rain C Desert < 25 cm = 250 mm rain ESCO index ESCO index t Precipitation/Potential Evaporation < 0.25 t Precipitation/Potential Evaporation < 0.25 ty Index = Potential Evaporation/ Precipitation > 4 ty Index = Potential Evaporation/ Precipitation > 4 About 10% of a About 10% of a non-polar non-polar desert is dune desert is dune covered covered Skip Lawson's Theory pp. 283-284
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Arid and Semi-Arid Arid and Semi-Arid LandscapesLandscapes
1. Arid (Desert) has Precipitation (mm) < 10 x Temperature in 1. Arid (Desert) has Precipitation (mm) < 10 x Temperature in ooCC 2525oo C Desert < 25 cm = 250 mm rain C Desert < 25 cm = 250 mm rain
2. UNESCO index 2. UNESCO index Desert Precipitation/Potential Evaporation < 0.25Desert Precipitation/Potential Evaporation < 0.25 Aridity Index = Potential Evaporation/ Precipitation > 4.0Aridity Index = Potential Evaporation/ Precipitation > 4.0
About 10% of a About 10% of a non-polar desert is non-polar desert is dune covereddune covered
Skip Lawson's Theory pp. 283-284
World's DesertsWorld's Deserts about 1/5th of land
Semi-AridSemi-AridSemi-arid (Steppe, Savannah, dry grassland) has Semi-arid (Steppe, Savannah, dry grassland) has Precipitation (mm) > 10 x Temperature in Precipitation (mm) > 10 x Temperature in ooC but < than 20xC but < than 20x
(c) 2004-2013 Charles L. Smart. All rights reserved
Flash FloodsFlash Floods
Role of Desert Varnish, lack of plant cover
Stream Flood
(c) 2004-2013 Charles L. Smart. All rights reserved
Desert Landforms Produced By Desert Landforms Produced By WaterWater
The physics of wind generated The physics of wind generated landforms is studied using sand landforms is studied using sand
grains in wind tunnelsgrains in wind tunnels
Eolian Processes Eolian Processes
Wind storm, Lothagam Hill dunesWind storm, Lothagam Hill dunes
Physics of Wind TransportPhysics of Wind Transport
• Air has low density, so wind is a relatively minor Air has low density, so wind is a relatively minor agent of geomorphic changeagent of geomorphic change
• Rock density 2000 times density same volume of airRock density 2000 times density same volume of air so rock fragments are difficult to lift with air.so rock fragments are difficult to lift with air.• A one meter per second (1 m/sec) flow of water lifts A one meter per second (1 m/sec) flow of water lifts
the same particle as about 30 m/sec flow of airthe same particle as about 30 m/sec flow of air
Sandstorm
Water and wind formed erosional features such as this toadstool
ToadstoolsToadstools• "Undercut, mushroom shaped pedestal rocks in "Undercut, mushroom shaped pedestal rocks in
desert areas are commonly attributed to wind desert areas are commonly attributed to wind erosion"erosion"
• Ignore Bloom's comments p291 right 1 Ignore Bloom's comments p291 right 1 following the line above. following the line above.
• This area is frequently hit with high sandstorms. This area is frequently hit with high sandstorms. These remove the paint from our trucks.These remove the paint from our trucks.
Wind-Borne Sediment causes Wind-Borne Sediment causes deflation and abrasiondeflation and abrasion
Sahara to CaribbeanEffect on Hurricanes
Surface creep
Desert Pavements or RegDesert Pavements or Reg
Note desert varnish, lack of sand, Iron and Note desert varnish, lack of sand, Iron and Manganese oxides over everythingManganese oxides over everything
Infiltration is slow – Flash Floods, and most water Infiltration is slow – Flash Floods, and most water leaves the arealeaves the area
Deposition Landforms of Eolian Deposition Landforms of Eolian SandsSands
• Reduced wind velocity results in sediments Reduced wind velocity results in sediments depositiondeposition
• Dunes are hills of loose wind-born sandDunes are hills of loose wind-born sand
• Size, shape, and orientation of dunes are Size, shape, and orientation of dunes are determined by available sand, vegetation, and determined by available sand, vegetation, and windwind
Sand drift in Lee Of Mountains
Only 10% of desert surface
Dunes form in a leeDunes form in a lee
Dunes are accumulations in the lee of local obstructions, possibly an upwind dune