Role of Water in Hot Deserts (AQA A Level Geography)

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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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