Global Desert Systems (AQA A Level Geography)

• Deserts can be hot or cold with high levels of aridity
• They are defined as:

Areas that receive less than 250mm of precipitation per annum 

• Deserts are described in terms of their deficiencies:
  • Water
  • Soil
  • Vegetation
  • Population
• Deserts produce the smallest amounts of organic matter and have the lowest net primary production (NPP)
• Desert environments are natural open systems with inputs, outputs, transfers/flows and stores of sediment, water and energy

Table of Desert System Elements

• Desert systems are generally in dynamic equilibrium, where the inputs and outputs are balanced, for instance:
  • Seasonal winds lead to small, short-term changes in a sand dune's profile
  • However, overall the shape of the dune mostly remains the same
• Any changes to an input or output can cause positive or negative feedback loops
  • Positive feedback results in the change being enhanced 
    • Less vegetation reduces evaporation and lowers humidity which reduces precipitation rates leading to less vegetation etc. 
  • Negative feedback restores balance - it checks any change
    • Temperature increase raises evaporation rates and dries the surrounding surface, making it easier to erode
    • Dust clouds form, which blocks some insolation, reducing temperatures and slowing evaporation

The hot desert water cycle showing the depth of water stores and high rates of potential evapotranspiration (PET).

1. Rain is low and irregular, interspersed with heavy downpours
2. Actual evapotranspiration is low, but the potential is high
3. Water from aquifers is capable of reaching the surface naturally or abstracted through wells

• Drylands stretch across 41.3% of Earth's land surface and are present on every continent
• They make up 77% of Australia’s land area, 66% of Africa, 40 % of Asia, 24% of Europe, and 15% of Latin America
• They are home to approximately 20% of the world's population
• Arid areas are located along the mid and low latitudes of the Tropic of Cancer and the Tropic of Capricorn
• Semi-arid areas lie along the margins of these arid areas, for example, the Sahara desert is an arid area and the Sahel would be classified as a semi-arid area
• Some of these areas are also located adjacent to mountain ranges, like the Atacama Desert which is alongside the Andes
• Other patterns for the distribution of arid and semi-arid areas are the way they are situated on the western margins of their relative land masses where cold ocean currents run along the coast - the Namib desert, Africa is due to the Benguela Current running up the west coast of Africa
• More arid areas lie in continental interiors where moisture is lost before reaching the area, such as the Turkestan desert situated in central Asia

Distribution of arid and semi-arid regions; note that most semi-arid regions fringe arid areas

• Semi-arid areas receive between 250 – 500mm of precipitation; they have sparse vegetation such as grassland, with few trees 
• Arid regions receive 25 – 250mm; plants only appear along river courses
• Hyper-arid areas receive (less) < 25 mm/yr. with plant growth occurring only after rainfall
• Many hot deserts have potentially fertile soils – evidenced by irrigation schemes
• All have plant and animal life – special adaptations enable them to survive the conditions
• Some are populated by humans; seasonally or permanently – Namibia, Cairo, or Karachi
• Precipitation is usually small, infrequent but intense
• Landforms are distinct in each area of deserts and develop over long periods of time, through form-changing processes
• Landscape can be divided into 3 types:
  • Hamada - most of the sand has been removed, leaving behind a landscape of gravel, boulders and bare rock plateaus
  • Reg - are hard impermeable surfaces composed of rock fragments set in sand or clay
  • Erg - sandy deserts/sand seas, common only in about 30% of deserts.  Their distribution appears to be climate-linked (i.e. less than 150 mm of rain)

• Desert landscapes are diverse and due to:
  • Geological factors – tectonics and rock type
  • Climate – temperature, rainfall and wind
  • Weathering and mass movement processes
  • Time 

Figure showing the impact of climate on the physical landscape of deserts