2.2.1 Sources of Energy & Sediment | AQA A Level Geography Revision Notes 2018

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 nighttime 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	Winds are formed by air moving from areas of high pressure to areas of low pressure. 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 as short, intense storms - causing 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

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

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