Sources of Energy & Sediment (AQA A Level Geography)
Revision Note
Written by: Jacque Cartwright
Reviewed by: Bridgette Barrett
Desert Energy Sources
Energy comes from insolation, wind and precipitation runoff in a desert system
Table Showing Different Sources of Energy in Deserts
Source of energy | Features |
|---|---|
Insolation | Insolation is strong in hot deserts - due to a lack of cloud cover, and the angle of insolation Diurnal range is large, with high daytime temperatures and low night-time temperatures. Changes in temperatures create steep pressure gradients, which create desert winds High insolation rates mean high levels of evaporation, leading to dry, mobile surfaces and insolation weathering such as exfoliation |
Wind | As air moves from areas of high pressure to areas of low pressure, winds are created. If the pressure gradient is large, then the winds will be strong Prevailing winds will erode and transport more sediment than frequently changing wind direction Winds will carry and flow long distances, increasing power and erosion due to a lack of obstructions (vegetation or landforms) Harmattan winds from the Sahara create powerful dust (loess) storms that blow over West Africa between Nov and April |
Precipitation runoff | Precipitation is infrequent and irregular, with most of it being short, intense storms - cause high energy inputs in localised areas With few plants to intercept, surface runoff is prevalent Surfaces are hard (baked by the Sun) or have exposed rock, making infiltration rates poor and increasing surface runoff |
Deserts are considered active, mobile landscapes due to the inputs of wind, water and insolation
Desert Sediment Sources
Sediment in hot deserts is derived from:
Weathering of the exposed and underlying rock
Fluvial deposits from past and present rivers which have brought sediments into the desert regions
Aeolian (wind) transfers and deposits sediments from beyond the margins of the desert
Mass movement
Erosion of exposed rock on desert plains
Sediment cells
Sediment movement is a sub-system of the hot desert system
Inputs are the sources of sediment
Transfers are the movement of sediment by wind and water within the system
Stores (sediment sinks) are the areas of deposition
Outputs removal of sediment by wind or water out of the system or to another sub-system
Large deserts can be a significant source of global sediment
Large clouds of loess from the Sahara desert have been transferred and deposited thousands of miles around the world
Sediment budgets
Deserts can be dominated by processes of erosion or deposition making them either sources or recipients of sediment
Where erosion dominates there is a net sediment loss
In deserts of deposition, there is a net sediment gain
However, most deserts have areas of gains and losses and the sediment budget considers the relative amount of sediment within that particular desert system or sub-system/cell
Example of a Possible Sediment Budget for a Hot Desert
| Est. Average Rates (tonnes/km²/yr.) | % of Total Eroded Material |
|---|---|---|
Mass movement | 35.3 | 0.6 |
Gully/wadi erosion | 76. 8 | 1.2 |
Slope wash erosion | 5438.2 | 98.2 |
Total erosion | 5550.3 | 100 |
Channel deposition | 536.65 | 9 |
Sediment trapped in the reservoir | 648.85 | 13 |
Total Deposition | 1185.5 | 22 |
In the example above, water is the major process of erosion in hot deserts, despite the lack of regular precipitation
The majority of desert sediment derives from ancient pluvial processes
E.g. the Sahara Desert was once the ancient Tethys Sea, but the movement of the tectonic plates forced the African plate to collide with the Eurasian plate, forming the Alps and the Himalayas but closing off the Tethys Sea's water source
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