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To what extent is the role of water the cause of present day desert landscapes?

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To what extent is the role of water the cause of present day desert landscapes? A desert is defined as an area that receives an annual rainfall of 250 mm of rainfall per year where evapo-transpiration equals or exceeds precipitation or in other terms where there is a permanent moisture deficit. Although the lack of water is a feature of deserts, it is present in terms of fogs, dew and underground supplies. Arid areas cover one third of the earth's land surface and there is an enormous amount of variety within in them. Most of these deserts lie in the tropical and subtropical belts between 20o and 30o north and south of the Equator. Geologically two types of deserts occur, shield deserts and mountain and basin deserts. Due to the lack of precipitation most of the year when rain does fall it is of high intensity and discharge increases as quickly as it decreases thus there is high run-off speed and sudden damage. ...read more.


The first and most obvious feature on deserts created by the role of water is a wadi. In the past the collective surface run-off flowing evenly along the ground from a storm has concentrated erosion into a deep, steep-sided ravine thus creating the relict feature of a wadi. Tunisia contains many wadis and shows the effects of a thick accumulation of debris washed down during flash floods. The pediment has several theories questioning its formation each with its own evidence. The first is based on wind undercutting the mountain excluding all mention of water. Sheet floods are another suggestion where eroding floodwater emerged for the uplands, no longer restricted by the valleys. This is questionable as, how could the sheet be created if there wasn't smooth rock surface present in the first place? The presence of alluvial deposits typical of a water eroded form suggests that water erosion must play a part in the development, however. ...read more.


These illustrate the role of water in the formation of present day desert landscapes clearly. Another feature produced by the actions of water is an alluvial fan, formed where wadis disgorge from highland areas depositing at the foot of the mountain front The last main landforms created by water are Playas receiving water through rivers and run-off. These show the role of water in the sense that where the surface consists of clay cracks appear after the waters of a storm dry up. Flat layers of clay, silt or salt can remain. Such examples of this 'flattest landform on land' can be seen in Bonneville salt flats where land-speed records are held. Water may not be ever present at the surface but periodic storms, where the conditions allow, much erosional work. It also generates run-off. The majority of features are a result of past wetter climates and it is therefore clear that water plays a significant role in the cause of present day landscapes even though it may not be producing dramatic changes at this moment in time. Miles Goodman ...read more.

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