Deserts AS


The causes and distribution of deserts


  • Aridity is a lack of water and generally it can be classified by mean annual rainfall.
  • Temperature also has an effect as it can determine evapotranspiration.
  • Surprisingly many areas with similar rainfall regimes to Great Britain, may be classified as arid due to other factors at work.

The ‘rainfall definition’ of aridity

  • 250 – 500 Semi-Arid Sparse vegetation such as grassland, few trees grow
  • 25 – 250 Arid Plants only appear along river courses
  • < 25 mm/yr Extremely Arid Plant growth only after rainfall
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The causes and distribution of deserts

The causes of aridity


Deserts are found in 60+ countries of the world between 10° and 30° N and S. About one-third of the land surface of the world is classified as arid, semi-arid and/or dry.

Sand covers 20% of the Earth’s surface. Over 50% of this area is deflated desert pavements.

Cold ocean currents

Cold air present above such currents ensures that there is little moisture available to cool and form clouds. The coasts of Western, North and South America and Africa display such conditions. Both continents have west coast deserts just a little inland.

Rainshadow and continentality

Air descending from mountainous areas warms and dries by compression, little rainfall forms and aridity is the result. Central areas of continents are dry because air moving over landmasses does not absorb large amounts of water vapour. During the last ice age, conversion of water to ice resulted in larger continental areas. This extreme continentality is thought to have facilitated the spread of deserts during the ice age.

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The causes and distribution of deserts

World wide distribution of deserts

The majority of the world’s most arid areas lie between 15° and 30° North or South of the equator

distribution of deserts (

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Desert conditions and plant adaptations

Adaptations of plants

Many plants in the arid realm are physiologically specialised, adapted in form and structure. Within the desert there are innumerable niche locations for plants even though the soil is susceptible, skeletal, saline and immature.

These include:

  • a degree of ephemeralism, remaining dormant in the soil as fruits or seeds
  • unique dispersal systems, i.e. barbs and bristles
  • xerophytic, water-seeking
  • root adaptations, tap roots or heavy lateral branching
  • small leaves, with sunken or restricted openings
  • pale, reflective, leaves
  • hairs, spines or thick waxy–walled leaves, evolved to replace what might nominally exist
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Desert conditions and plant adaptations

Soil conditions

  • Soils that form in arid climates are predominantly mineral soils with low organic content.
  • The repeated accumulation of water in some soils causes distinct salt layers to form.
  • Calcium carbonate precipitated from solution may cement sand and gravel into hard layers called ‘calcrete’ that form layers up to 50 m thick.
  • Caliche is a reddish-brown to white layer found in many desert soils. Caliche commonly occurs as nodules or as coatings on mineral grains formed by the complicated interaction between water and carbon dioxide released by plant roots or by decaying organic material.
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Desert Processes

Desert surfaces

A range of desert surfaces exist:

  • Ergs – sandy deserts/sand seas, common only in about 30% of deserts. Their distribution seems to be climate linked (i.e. less than 150 mm of rain).
  • Pavements, gibber plains or reg – form as a result of wetting and drying. They are hard, rock covered areas.


Desert weathering is a controversial topic.

>Chemical weathering is limited by:

  • the lack of water
  • low rates of penetration into the rocks

>Salt weathering=Rocks in deserts often contain efflorescent salts which set up stresses in the rock and produce fractures. This process is seen in porous and poorly cohesive rocks.

>Exfoliation=Granular disintegration and chemical rotting also have an effect.

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Desert Processes

Aeolian processes
Winds that blow across deserts often produce an effect similar to fluid in motion. The lack of vegetation reduces surface roughness permitting smoother wind/land contact. The wind produces particulate sand, which is transported or deposited.

  • Abrasion – occurs when small particles are hurled by the wind against rock surfaces. This is only a minor erosional force and mostly occurs slightly above the ground. Ventifacts, rocks smoothed by wind abrasion, are common in deserts.
  • Deflation – wind blows away rock waste and lowers the desert.
  • Attrition – rock particles rub against each other and wear away.
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Desert Processes

Aeolian transport

  • Saltation moves small particles in the direction of the wind in a series of short hops and skips. It normally lifts sand-size particles no more than onecentimeter above the ground, and proceeds at one-half to one-third the speed of the wind. A saltating grain may hit other grains that jump up to continue the saltation. The grain may also hit larger grains that are too heavy to hop,but that slowly creep forward as they are pushed by saltating grains. This is called surface creep

Three processes have been recognised:

  • sedimentation – settling occurs and there is no further effect on other sand particles
  • accretion – occurs when sand grains come to a rest
  • encroachment – the process of continued growth of sand accumulations.

Once sand has accumulated it traps more and more sand, ripples turn into dunes, and dunes into ‘draa’.

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Desert Processes

Fluvial processes

Three main types of river are found in desert areas:

  • exogenous rivers – sources outside the desert
  • endoreic rivers – these form near the desert and never show beyond it
  • ephemeral rivers – these flow for only part of the year.
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Features produced by Water in Deserts

  • Rain does fall occasionally in deserts, and desert storms are often violent. A record 44 mm of rain once fell within 3 hours in the Sahara. Large Saharan storms may deliver up to 1 mm/minute.
  • Normally dry stream channels, called arroyos or wadis, can quickly fill after rain,and flash floods make these channels dangerous.
  • However, the evolution of arid landforms is often affected by events that occurred long ago. Past climatological conditions, reflected in many desert landforms, began to develop during pluvial periods several thousand years ago.
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Features produced by Water in Deserts

desert features formed by water (

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Features produced by Water in Deserts

desert features (

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Features produced by Wind Deposition

Sand sheets

These are flat areas of sand with sand grains that are too large to saltate. 45% of depositional surfaces are of this type, e.g. Selima in South Egypt.


The wind eventually blows sand into a network of troughs, crests and ripples that are perpendicular to the wind direction. They are the consequence of saltation.

sand dunes ( 

Accumulations of sand build into mounds and ridges, they become a dune when  the slip face is about 30 cm high. Dunes grow as sand particles move up the gentle upwind slope by saltation and creep. They fall onto the slipface inducing movement.

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Features produced by Wind Deposition

Barchan Dunes

  • Barchan means ‘active dune’. These dunes move.
  • Found in Northern Chad,Taklamakan Desert in China, Namib Desert in Namibia.

Barchan dunes (

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Features produced by Wind Deposition

Linear dunes (seif, transverse dunes or draa)

As to formation these dunes are either:

  • the result of obstacles getting in the way
  • an erosional phenomenon
  • as products of a vegetated landscape or,
  • as products of complex wind regimes, secondary wind flow patterns (and large amounts of sand)

linear dunes (

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Features produced by Wind Erosion

Rock pedestals

Wind sculpts stratified rock into pedestals by wind abrasion and weathering, e.g. Gava Mountains, Saudi Arabia

rock pedestal (

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Features produced by Wind Erosion


Wind abrasion turns the desert surface into a ridge and furrow landscape, e.g.various areas in Bahrain

zeugens (

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Features produced by Wind Erosion


  • (width to depth of 4 : 1)
  • A ridge and furrow landscape. Wind abrasion concentrates on weak strata; leaving harder material upstanding.
  • The Sphinx at Giza may be a modified yardang

yardangs (

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Features produced by Wind Erosion


  • Wind (and water) attacks the original surface leaving round-topped inselbergs (through exhumation).
  • The material removed has a deep-seated ‘decay’ origin and may display extensive ‘unloading’ (subsurface weathering).
  • There are two major forms: domed inselbergs (bornhardts) and boulder inselbergs (Kopjes, rubbins), e.g. Matopos, Zimbabwe
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Features produced by Wind Erosion

Bornhardt Formation

Bornhardt (

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Features produced by Wind Erosion

Kopje Formation

Kopje formation (

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Features produced by Wind Erosion

Deflation hollows

These are caused by the removal of fine particles by the wind, lowering the surface and creating a hollow, the best known example is the Qattara Depression.

deflation hollow (

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Man in arid lands

Settlement Patterns

The effects of people on the arid and semi-arid lands may be seen at three overlapping cultural/technological levels.

  • Small groups of people, like the bushmen of the Kalahari and the Australian aborigines, live a semi-nomadic food-gathering and hunting life, and have adapted to their environment with remarkable efficiency. Over the years, the hunter-gatherers have had little lasting impact on their environment.
  • By contrast, pastoral nomads seasonally cultivate selected areas. Pastoral nomads and shifting cultivators have greatly affected the natural flora and fauna. They respond to physical conditions through their mobility.
  • Many millions live a settled life in a relatively moist environment within arid zones near oases, on a flood plain or delta, or in an area irrigated by water brought from afar. Irrigation, with its associated land-use and settlement, has changed entire ecosystems, and ecological repercussions have been felt far beyond the irrigated areas.
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Man in arid lands



  • Traditionally the people who settled at oases used simple technology to raise and distribute water locally through aqueducts and underground channels or qanats. Evaporation losses from this system are minimal, though some water is lost by infiltration.
  • Other methods were to use shallow, gravity-fed channels, blocked by mud packing as required; cisterns; underground caverns; depressions dug to catch run-off and retain water moving by gravity through the sub-soil; and the creation of lakes during the rains by blocking stream beds with boulders and earth dams.
  • Today electric or diesel pumps raise groundwater from greater depths; massive dams create deep lakes stretching back hundreds of kilometres. Metal pipelines of decreasing diameter carry water from reservoir to the field, and release it through nozzles, or tiny drip feeds, to the plants.


  • Problems such as water logging and saline accumulation. Water not used by crops, lost by evapotranspiration, or drained away, accumulates as rising groundwater. In desert conditions soils rapidly acquire salt from the evaporation of dilute saline irrigation water. Water acquires more salt as it slowly moves up through the ground. The land eventually becomes too saline for crops to tolerate
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Man in arid lands

Pakistan: an ‘irrigated’ country
In Pakistan, the whole hydrological system of a huge river, the Indus, has been transformed, as engineering systems have evolved progressively to control and distribute its waters. Most of the flow in the Indus system is from catchments in the Himalayas and its foothills. Monsoon breaks in June and rivers reach peak flood-levels in the foothills in July–August, causing a flood wave to pass downriver. From September the river levels fall, then snowmelt brings another rise in March.

Irrigation has brought some problems for Pakistan.

  • Dams and diversions, have caused floodplains to be deprived of alluvium and its nutrients, and to need expensive fertiliser.
  • There is increased danger from infections like bilharzia.
  • Where year-round cultivation replaces cropping with a dry fallow period, crop pests may thrive on perennial food sources.
  • There are also many instances of plant pests and diseases being carried along water channels.
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