Role of Water in Hot Deserts (AQA A Level Geography)
Revision Note
Desert Sources of Water
Rainfall is sporadic, however, rare substantial, intense flash floods generate considerable runoff
These episodic flash floods arise from convectional storms in mountain ranges and flow as sheet floods or are confined within channels
Despite being short-lived, large amounts of sediment are washed down from the mountains to be deposited on alluvial lowland plains
There is a limited vegetative cover to absorb water and this limits humus levels, lowering plant root disturbance, making the soil dense and compact
Rain is therefore, able to beat down on the surface with maximum impact, dislodging fine particles and blocking pore spaces, thereby further reducing the infiltration rate
This minimal infiltration rate means that slopes of less than 2° experience extensive overland flow
Ephemeral rivers
Snowmelt and episodic flash floods provide the water for these intermittent rivers and are also known as wadis, barrancas, washes or arroyos
More typical of desert areas, streams flow intermittently or seasonally after rainstorms and are powerful forces of erosion
Endoreic rivers Exogenous rivers
This is when a river flows into the desert but terminates in an inland lake or sea
River Jordan into the Dead Sea, the Aral Sea, The Bear into the Great Salt Lake
Exogenous rivers
Large, perennial rivers with an external source
They have sufficient discharge to flow continuously, despite high evaporative rates as they cross arid land
The Blue Nile, the Nile's main tributary, originates in the Highlands of Ethiopia
Water Action in Hot Deserts
Fact: more people die of drowning in hot arid and semi-arid regions than die of thirst!
The impact of water is significant in shaping desert landscapes
Water processes
Erosion
Hydraulic action
Abrasion
Corrosion
Attrition
Transportation
Traction
Saltation
Suspension
Solution
Deposition
Takes place when velocity drops
Usually on the inside of meander bends and where streams/channels flow out of the mountain edges onto flat desert plains
Sheetwash processes
Studies suggest that surface runoff is likely to occur within 10 minutes of the start of the downpour
Firstly, sheet flood occurs – water flows evenly over land and is not confined to channels
Secondly, collective runoff becomes concentrated into deep, steep-sided wadis or arroyos
The average occurrence in semi-arid margins of the Sahara is once a year, and once a decade in extremely arid interiors
This infrequency of floods compared to the great number of wadis suggests that they were created when storms were more frequent – they are a relict feature
Other water processes
Splash erosion
Rill erosion
Gully erosion
Bank erosion
Examiner Tips and Tricks
Remember that it rains infrequently in deserts, and the influence of water in shaping desert landscapes is often overlooked. You need to know that water’s effectiveness in erosion, is due to factors such as vegetation cover, slopes and the permeability of surfaces as well as rainfall amounts.
Also, many desert landforms may be relict features, formed during wetter climatic periods.
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