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

• 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