AQA AS Geography
4 Hot deserts and their margins
1 (a) In addition to low mean annual rainfall, aridity and the effectiveness of rainfall is
controlled by other factors such as evapotranspiration, rainfall seasonality and
rainfall intensity. In hot climates a large proportion of rainfall is evaporated before it
reaches the root zones of plants. If rainfall is concentrated in the summer months
(e.g. in the savanna) moisture losses to evapotranspiration will be much higher
than in climates with a winter rainfall maximum (e.g. Mediterranean). In climates
which experience frequent high-intensity rainfall in thunderstorms (e.g. temperate,
continental interiors), rapid run-off reduces the amount of rain available to
replenish soil moisture stores.
(b) Globally, the most extensive areas of aridity are in the tropics and sub-tropics.
The Sahara in North Africa, between latitudes 15ºN and 30ºN, is the world’s largest
expanse of desert. Also, in the northern hemisphere, the arid land of the Arabian
peninsula, Iran and the Sind Desert occupy roughly the same latitudes. In the
southern hemisphere there are large tropical and sub-tropical deserts in Australia
and the Kalarhari (southern Africa). Extreme aridity is also found in the topics and
sub-tropics in coastal deserts in Chile and Peru (Atacama) and southwest Africa
(Namib). In middle latitudes, aridity is most often associated with the rainshadow
effects of mountain barriers and isolation from the oceans. Examples include the
Mojave Desert in the southwest USA, Patagonia (Argentina) and the Gobi Desert
(China).
(c) Convective Hadley cells dominate the general circulation in the tropics and subtropics and are responsible for widespread aridity in the Sahara, the Arabian
peninsula and Australia. Air rising at the inter-tropical convergence zone is
transported polewards around the level of the tropopause. As this air cools and
becomes denser it slowly sinks back towards the surface between latitudes
15–25º. Sinking air is warmed dynamically (i.e. adiabatic warming due to the
increase in air pressure as it descends). This lowers humidity and prevents cloud
and rain from forming. The resulting clear skies and intense solar radiation create
arid conditions.
Cold ocean currents are the main cause of aridity in the coastal deserts of Namibia
and Chile and Peru. Along the coast, air masses cool through contact with the cold
surface waters of the ocean, and give rise to temperature inversions (i.e. cool air at
and near the surface and warmer air above). In this situation convection cannot
occur, thus inhibiting the formation of clouds and rain. The result is extreme aridity
and coastal deserts which are among the driest places on Earth.
(d) Arid environments are fragile. Over-exploitation of natural resources such as
arable land, pasture and woodland quickly leads to problems of land degradation,
soil erosion and ultimately, desertification. These problems can be reversed but
often only at great expense and over long periods. Most arid areas that have
suffered land degradation have been permanently damaged. Regeneration
through natural processes is unlikely to restore their biodiversity and former levels
of productivity.
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In northern China an ambitious reforestation programme aims to halt the spread of
desertification, and restore areas that have been badly degraded to sustainable
use. One of the worst affected regions is the Korqin Sandy Lands. Desertification
in this region has been driven by centuries of overgrazing, deforestation and more
recently, by human population growth. In northern China, the restoration strategy
centres on the planting of the ‘great green wall’, a massive area of reforestation
covering 350,000 km2. New forests and shelterbelts will protect farmland against
wind erosion, restore soil fertility and provide a sustainable source of timber. At the
same time, sustainable cultivation will be encouraged with livestock, supported by
fodder from the newly-planted forests, being integrated with crop growing. And for
the first time grazing will be controlled to ensure that carry capacities are kept to
sustainable levels. In regions like Korqin the project is already proving successful.
However, the project is a long-term one, and a complete assessment will not be
possible before its completion in 2050.
Salinisation is a type of environmental degradation in arid and semi-arid lands
caused by excessive irrigation of arable land. Waterlogging followed by
evaporation increases soil salinity and results in falling crop yields and, ultimately,
land abandonment. Large areas of salinised land are located in the Indus valley
in northern Pakistan. Within this area at Khushab, the World Bank funded the
reclamation of 360 km2 of salinised arable land. Rehabilitating the land includes
underdrainage with PVC pipes, the building of new surface water drains, and the
lining of irrigation canals. Completed in 1994, the project has proved successful.
The water table has been lowered and waterlogging and soil salinity reduced.
Improvements in land quality have restored the sustainability of farming, and
raised crop yields and incomes; developments which have effected a significant
improvements in lives of farming families in the Khushab district.
2 (a) The main types of mechanical weathering in hot deserts and their margins are salt
weathering, insolation weathering and freeze-thaw. Salt weathering results from
the percolation of soluble salts into surface rocks. As the salts precipitate out of
solution the salt crystals generate pressures which causes rock disintegration
(either as surface flaking or honeycombing). Insolation weathering occurs through
a combination of high surface temperatures and moisture (usually from dew). As a
result rock minerals expand and contract at different rates, weakening rocks which
slowly break down. Freeze-thaw occurs in tropical deserts, especially at altitude
and at night in winter, when frosts are not uncommon. Water, turning to ice in
confined cracks and crevices, leads to rock breakdown. The products of freezethaw weathering are most likely to be scree-sized particles and small boulders.
(b) Wind erosion and wind transport are more important in hot deserts and their
margins than in most other environments. There are two main reasons for this:
the lack of vegetation cover; and the dry climate. In humid and sub-humid
environments vegetation often provides a complete cover to the soil surface.
Vegetation lowers winds speeds near the ground surface, and plant roots bind the
soil particles together. In hot deserts and their margins, vegetation cover is at best
patchy, and there are often extensive areas of unvegetated ground. In these
circumstances, wind is a very effective agent of erosion and transport (by creep,
saltation and deflation). The dryness of soils also assists wind erosion and
transport. Moist soils are heavy and coherent, and not so easily eroded by the
wind.
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(c) Mesas, buttes and pediments are the result of mass movements and weathering
in hot arid environments and their margins. They are the remnants of horizontallybedded plateaux formed in sedimentary rocks. Mesas are table-like plateau
remnants, with steep sides and flat tops. Buttes are similar to mesas except that
the level summit surface is just a few tens of metres in width. Pediments are gently
sloping (concave) slopes which extend away from the foot of mesas and buttes.
Mesas and buttes form in the same way. After initial deep incision into a plateau
surface by a river (forming a canyon), the valley sides retreat parallel to
themselves by mass movement and weathering. The principal mass movement
and weathering processes are rockfall, debris avalanching, salt weathering,
freeze-thaw and isolation weathering. Parallel retreat explains why the profiles
and heights of mesas and buttes remain more or less constant. As parallel
retreat takes place, it leaves behind a gently sloping rock platform, covered in
water-deposited debris. The rock platform is known as a pediment.
(d) Hot deserts are fragile environments, easily degraded by human activity.
Currently over one-third of the world’s surface is threatened by desertification.
Most desertified and degraded land is found in arid and semi-arid environments
such as the Sahel in Africa. The concentration of desertification and land
degradation in such regions is explained by their ecological fragility. Severe
climatic conditions (high temperatures, low rainfall) result in slow rates of plant
growth and low primary production. Thus recovery from any damage to vegetation
is very slow. In addition, low biodiversity, with few species adapted to the harsh
conditions, means that the loss of a single species can rapidly destabilise food
webs and soils. Often the outcome is the degradation of entire ecosystems.
In the past 50 years rapid human population growth in hot arid and semi-arid
environments has put huge pressure on fragile ecological resources. Pastures
have been overgrazed as livestock numbers exceeded carrying capacities;
woodlands and forests have been lost due to excessive grazing, browsing and
clearance for firewood; and soils have been overcultivated. The end result is soil
erosion, the collapse of ecosystems, and poverty for indigenous populations. More
robust environments, such as the temperate deciduous forest, can absorb such
human impacts without the same level of degradation.
In the Nara district of Niger, severe desertification has been due to a combination
of drought and population pressure. Unsustainable exploitation of timber, soil,
grazing and water resources has led to environmental destruction, declining crop
yields and widespread poverty. Many indigenous people, no longer able to survive
as farmers, have been forced to migrate to the capital, Bamako.
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