Arid lands are defined using annual rainfall and temperature as main criteria - form about 40% of land surface of the earth
Arid areas are defined as those where evaporation and transpiration exceed precipitation
There are 4 classes based on the ratio of precipitation to potential evapotranspiration
|
classification |
annual precipitation (mm) |
ppt/potential evapotranspiration |
|
hyperarid - Sahara, Mojave |
<100 |
<.03 |
|
arid |
100-250 |
.03-.2 |
|
semiarid |
250-500 |
0.2-0.5 |
|
dry subhumid |
500-750 |
0.5-0.75 |
Deserts are located in areas of the globe of descending air masses
belts of descending air occur at ~30oN and 30oS, the subtropical high pressure belts, and at the polar regions
Additional influences
warm and cold ocean currents
size, shape land and water masses
distribution of land masses
topography - orographic effect
Desert landforms -
Reflect aridity Þ lack of vegetation
Prime erosion agent = running water; secondary agent = wind
Fluvial landforms -
Ephemeral streams
alluvial fans
Saline and playa lakes
caliche
Wind erosion is common
Wind as erosion agent, in contrast to water, is independent of topographic controls so its effect is widespread, not channeled.
Erosion mechanisms = deflation and abrasion
sediments transported by wind show good sorting . They travel by
suspension
saltation - ~1 m from ground 75% of dune sands
surface creep 25% of dune sands
Constructional effects
Dunes - presence shows directions of dune-building winds
Loess - unconsolidated silt-sized wind deposited sediment - a resource - new soil