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AQA GEOG1: Hot Desert Environments and their margins
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Location and characteristics of hot deserts and their margins (arid
and semi-arid) - climate, soils and vegetation.
Causes of aridity - atmospheric processes relating to pressure,
winds, continentality, relief and cold ocean currents.
Arid geomorphological processes: mechanical weathering.
The effect of wind - erosion: deflation and abrasion;
transportation; suspension, saltation, surface creep; deposition.
The effect of water - sources: exogenous, endoreic and ephemeral.
The role of flooding.
Landforms resulting from - wind action: yardangs, zeugen and sand
dunes - water action: pediments, inselbergs, mesas and buttes, salt lakes,
alluvial fans, wadis and badlands.
Desertification: distribution of areas at risk, physical and human
causes, impact on land, ecosystem and populations.
Case study of desertification in the Sahel - the struggle for
survival to include the energy/fuel wood crisis, water supply, the impact
on food supply/farming and livelihoods and coping/management strategies
including external aid.
Managing hot desert environments and their margins - to consider
and evaluate the strategies adopted with regard to land use and
agriculture in areas such as the Sahel and contrast with the development
of areas such as south-western USA or southern Spain. Implications and
potential for sustainability.
A desert is an arid region which receives less than 250mm rain per year. A
semi arid region receives between 250-500mm of rain per year.
Location of Deserts
Most of the world’s deserts hot deserts are
found;
a) In two bands; 15-30 degrees latitude
North, and 15-30 degrees latitude
South.
b) The middle of continents
c) Next to mountain ranges
d) Near to cold ocean currents
30N
15N
15S
30S
They are found in these places because…
1. Atmospheric Circulation – the effect of the high pressure cell
Example: Sahara
Desert, North Africa
Hadley Cell
30 North
Sub Tropical High
Equator
Trade Winds
30 South
Sub Tropical High
The trade winds blow
from the areas of high
pressure to the equator;
the trades are South
Easterly in the Southern
Hemisphere and North
Easterly in the Northern
hemisphere.
o The sun’s rays are most concentrated at the equator. The air lying above the ground’s surface is
heated, and this warm air carries lots of water vapour.
o The heated air rises, creating an area of low pressure in the Inter Tropical Convergence Zone. The
convection currents cause the air to rise and cool, causing thick clouds and torrential rainfall.
o The rising air then reaches the tropopause, a layer in the atmosphere. The air is forced to diverge
towards the North and South poles.
o As the air moves away from the equator, it cools. Cooling makes the air denser and so it subsides
back towards Earth at 30 degrees North or South. The sinking air creates the area of high pressure
known as the Subtropical High.
o The high pressure sinking air is warmed by compression. It is dry because all of its moisture was
dropped as convectional rainfall near to the equator. The high pressure prevents convectional
rainfall, because the air cannot rise. Maritime air currents from the sea are blocked because winds
cannot blow into an area of high pressure.
2. The rain shadow effect
Mountain ranges act as
orographic barriers – they
force the moist maritime
winds to rise and cool.
This creates rain on the
windward slopes – relief
rainfall. The air then
moves inland and
descends. It is warm and
dry – this is the rain
shadow effect.
Example:
Atacama, South
America: prevailing
south east trades
bring moist air off the
Atlantic. This is
forced to rise over
the Andes mountain
range.
3. The continental interior effect
Moist air off the sea will drop its moisture as it hits land. By
the time it reaches the continental interiors, all of the
moisture will have been dropped, so the winds are dry, and
there will be little rain.
Example: The Gobi and Iranian deserts
4. The cold current effect
Cold ocean currents can cause
maritime winds to be cooled as
they travel over the ocean.
Their capacity for moisture is
reduced and this causes
precipitation over the ocean
itself. Sometimes, dense
advection fog is formed. By the
time the winds reach land, they
are carrying little moisture, so it
rains. The fog soon evaporates
when it comes into contact
with the warm surface.
Example: The Peruvian
Humboldt and the Atacama:
The Peruvian Humboldt current cools
any Westerly Winds blowing onto the
West Coast of South America,
contributing to the aridity of the
Atacama.
Peruvian Humboldt
Cold Current, Pacific
Ocean
Characteristics of Hot deserts:
Climate
o Mean annual temperature between 20 and 30
degrees Celsius
o Large seasonal variations in temperature – up
to 50 degrees in summer and below 0 in winter
o Large diurnal (daily) ranges of up to 50 degrees,
because the lack of cloud cover allows heat to
radiate away and temperatures fall
considerably at night – allows for formation of
dew and mist
o Less than 250mm rain per year; this can be in
the form of torrential storms – when convection currents occasionally overcome
the downward pressure of descending air, they can rise and lead to storms.
o Strong desert winds and sandstorms – hot air convection currents and significant
variation in temperature can call small twisters.
o High sunshine values due to the absence of cloud in the area of high pressure,
where air is subsiding.
o Thunderstorms can be triggered by intensive convective activity
Soils
Desert soils are thin, and can be rocky (in Hammada and Reg
deserts) or sandy (in Erg deserts). There is little vegetation,
and the lack of water stops the scarce vegetation from
producing humus when it rots, so soil remains undeveloped. If
the water table is close to the surface, evaporation and
capillary action is likely to bring the moisture to the surface,
causing salts to be deposited near to the surface, producing
an alkaline soil. The soils are highly porous and permeable,
leading to poor moisture retention. However, due to lack of leaching due to minimal rainfall,
irrigation and management can mean that crops can be successfully grown in the desert.
Vegetation
Although there is little water in the desert, plants have adaptations to allow them to
survive:
o Ephemeral plants avoid droughts by producing seeds which can lie dormant for
long periods between rain events and which germinate, flower and produce
seeds very quickly when rain does arrive.
o Xerophytes resist droughts by having thin, tough and spiky leaves,
so there is less surface area from which water can be lost by
transpiration. Thick bark means that water will not be lost through
evaporation. Hairs on the leaves also reduce water loss.
o Water storage in plants, including succulents in plants like cacti
which store water in roots and stems
o Phreatophytes send out long roots to tap water deep under ground.
o Oasis vegetation exists where an aquifer rock layer meets the surface. Most
oases are settled and cultivated – little natural vegetation remains.
Examples of Desert Vegetation
Saguaro Cactus, Sonoran Deseret, USA – phreatophytic roots
spread out to collect water over a large area; spines intercept
the rain and direct to roots; concertina pleats in stem allow
expansion to store up to 5 tonnes of water. The cactus can lose
82% of its water before it dies of dehydration.
Creosote Bush, Sonoran Desert, USA – Phreatophytic
adaptations, long roots to catch rainwater and deep tap roots to
access ground water supply, as rains are unreliable; Small
leaves to reduce evapotranspiration, with stomata on underside;
Leaves have varnish to reduce water loss.
Ocotillo plant, Sonoran Desert, USA – Ephemeral adaptations;
dormant in drought, then germinates and flowers quickly if rain
arrives. Leaves are shed in drought to reduce water loss by
evapotranspiration.
Desertification is the degradation of semi arid land by human activities
and change in climate. It is an
environmental degradation process which
leads to the decline of productive land. A
third of land worldwide is at risk from
desertification, and 250 million people
are already affected. 46% of Africa is at
risk from desertification. Desert is
encroaching into semi arid desert
margins.
Physical causes of desertification – climatic change
o Lower rainfall – in subtropical areas where most semi arid areas are, surface
water and groundwater will be reduced, because it is used up or evaporated. The
volume of water for vegetation is reduced, so the vegetation dies. There are
fewer plants and trees, so there are fewer roots to anchor the soil. This leads to
soil erosion.
o Higher Temperatures - global surface temperature has risen by 0.6 degrees over
the last century and is predicted to continue to rise. As temperatures increase, the rate
of evapotranspiration also increases. This dries out soils, lowering surface water levels.
Vegetation dies, so there is less root anchorage and more soil erosion.
Human activity – the main cause of desertification
o Overgrazing – if the number of animals increases beyond
o
o
o
o
the carrying capacity, over grazing can become a problem.
Overgrazing reduces the amount of vegetation, so there is more
soil erosion. Trampling by animals compresses and breaks down
the structure of soil making erosion more likely.
Population growth – increases pressure on land and more food is needed to meet
the growing demand. This leads to further overgrazing, over cultivation, deforestation
and irrigation.
Over cultivation – increases in population through high birth rates and immigration
means that there is an increased demand for food. Farmers leave behind traditional land
use because more land is required to grow crops. Exploitation of the soil leaves it
without enough nutrients to support plants, leaving it unproductive. The soil is then
easily eroded.
Deforestation – If population rises, there is a greater demand for wood; it
is needed for building materials and fuel. Cutting down areas of woodland also
increases the amount of land available for agriculture. This leads to more trees
being cut down, so fewer roots mean that soil is more vulnerable to erosion.
Irrigation – Depletes surface water and may involve unsustainable pumping
of aquifers. As water levels decrease, water availability for plants decreases leading to
less vegetation cover and soil erosion. Some irrigation techniques erodes soil directly –
surface irrigation washes top soil away. If too much water is used to irrigate the crops,
excess can sink into the soil and raise groundwater levels. If the aquifer is saline, high
concentrations of salt may be brought to the surface, increasing the salinity of the soil.
This is known as Salinisation, and plants may be unable to survive.
Impacts…
Land
As fertile top soil is eroded, land
becomes unproductive.
Ecosystems
Less plant life can be supported by
the soils. This means that fewer
animals can be sustained, and so
biodiversity decreases.
Populations
If agricultural production decreases to the point where
farmers cannot feed their families or earn a living, they
may have to migrate from the area. The land which people
move to will be put under increased pressure. If people are
unable to move, a community’s inability to produce the
food they require may lead to famine.
Desertification in the Sahel
The Sahel is a 3900km long belt that runs East to West across
Africa. It separates the hyper-arid Sahara from the wetter
Savannah to the South. The Sahel runs through 10 countries,
some of which are the poorest in the world, including Niger
and Mali.
The causes of Desertification in the Sahel…
1. Climate Change – between 1968 and 1997, rainfall in the Sahel decreased by
between 29-49%. A 5 year drought was caused beginning in 1968, and
droughts were common then throughout the 1990s. The average
temperature of the Sahel has also increased over the past century. This
increases evapotranspiration, reducing ground and surface water supplies.
2. Increased population = more intensive agriculture - The population increase
in the Sahel is 3% per year; between 1968-1998, population increased from
274 million to 628 million. Food production increases by only 2% per year, so
additional pressure is being put on land. This has lead to over cultivation,
over grazing and over-irrigation. All of these factors increase desertification
3. Deforestation – More space is needed for agriculture, and so large areas of
forest have been cleared to create more space for agriculture, usually by
slash and burn. Wood is also needed for energy supply – 82% of all energy
used in the Sahel comes from wood. With fewer trees, soil is more vulnerable
to erosion.
The Struggle for Survival – effects of desertification in the Sahel
o Erosion of topsoil has reduced the area of productive agricultural land in the
Sahel. In Mauritania, all that is left in a 200km wide strip running across the
country. In Niger, 2500 square kilometres is lost each year to desertification.
o As land area decreases, so does the amount of food produced. Niger can now
produce less than a 20th of the food they could 40 years ago. This causes loss of
livelihoods and famine.
Management strategies in the Sahel
o Contour Ploughing – ploughing along contours rather than through them to limit
soil erosion from wind
o Magic Stones – “Water Harvesting” – rows of stones placed along contours,
slowing surface run off and protecting top soil. Water contained long enough for
it to soak into soil. Increased yields by 40% in Burkino Faso.
o Careful management of irrigation and the implementation of drip irrigation,
which reduces waste and prevents excessive Salinisation. In Senegal, farmers are
being educated in the use of drip irrigation. Concrete dams have been built next
to rock outcrops to catch rainwater and condensed dew. Storage tanks then hold
the water, allowing it to be used for irrigation.
o Promoting the use of solar ovens, to reduce the need for fire wood and limit
deforestation. Introduction of an efficient version of the traditional Mogogo
stove in Eritrea, Sudan and Senegal, which needs 50% less wood than the older
versions, reducing deforestation.
o Growing the Jatropha curcas plant in Mali, around food crops. It can grow in
poor quality soil and is not eaten by animals. Its roots help to bind soil together
and therefore protects against water and wind erosion. Oil from the plant can be
sold providing a valuable source of income. The plant can also be burnt, reducing
need for deforestation.
Recent developments relying upon external aid…
Tal Rimah Rangeland Rehabilitation Project,
funded by USAID and US Forestry Service, in
the Badia, Jordan. Stone Walls were built to
retain the limited water, called water
harvesting. Drought tolerant shrubs like
atriplex were planted to provide grazing for
animals, encourage biodiversity and hold soil
together to prevent erosion.
The Eden Reforestation Project – charity planted 1
million trees in Ethiopia’s Rift Valley to reduce dust
storms after years of desertification.
Projet Agro-Forestiere, funded by Oxfam, in
Burkino Faso. Stone lines constructed along the
contours of the land, and grasses and tree seedlings
are planted alongside the stones during the rains.
The contours channel the rain water to the plants,
helping them to grow.
Evaluating these strategies;
o Projects funded by external aid can have negative impacts. During the drought of
1968-73, thousands of wells were built. Farmers used this to increase the size of
their herds, but the land could not cope with the overgrazing that followed.
Large areas of land became desertified as a result.
Managing Desert Margins: Southwest USA
The great plains of the South West, including Texas, Kansas and New Mexico, have
experienced desertification. Drought conditions caused the Dust Bowl Effect. This
was enhanced by years of bad farming practices which had left the top soil dry and
susceptible to Aeolian erosion. Ploughing and growing crops in rows up and down
the slopes encouraged channelling and gullying. Overgrazing reduced vegetation
cover and so increased soil erosion. In order to control soil erosion in the semi arid
ecosystem, techniques have been employed;
Drip irrigation minimises the use of water
and fertiliser by allowing water to slowly
drip onto the roots of plants through a
network of valves, pipes, tubing and
emitters. Liquid fertiliser is mixed with the
irrigation water – this is called fertigation.
Chemigation can also take place
(application of pesticides and other
chemicals). Drip irrigation can achieve
water conservation; less water is wasted,
and more crops are produced for the same
amount of water used.
Drip irrigation has advantages:
o Minimised fertiliser and nutrient loss due to localised application
o High water application efficiency
o Minimised soil erosion
…and disadvantages:
o Expensive – high initial cost
o Takes lots of time to install, as factors such as topography, soil, agro-climatic
conditions must be studied.
Crop rotations– 4 to 6 year rotations of maize, wheat, hay and clover, increasing the
water-absorbing capacity of the soil
Preventing cattle from grazing – trampling of soil reduced ground cover and
reduced the soil’s capacity to hold water. Grazing also delayed tree growth, which
means that rain water can more easily wash top soil away, as there is no protection.
Contour Ploughing - ploughing slopes horizontally not vertically to reduce soil
erosion by water and wind
Desertification in Southern Spain
Southern Spain is the driest country in Europe – 50% of the country is classified as
arid. It is estimated that 37% of Spain is at risk of desertification. The desertification
is caused by climate change, agriculture and development.
Climate Change
Average temperature in
Spain has risen by 1.5
degrees. Rainfall has fallen by
9.5%. As climate gets hotter
and drier, there is an
increased risk of forest fire
causing deforestation and
desertification.
Agriculture
Over cultivation reduces the
nutrient content of the soil so
much so that it is no longer
productive. Overgrazing reduces
vegetation cover. Groundwater
resources are used to irrigate
crops, lowering groundwater
levels.
Development
Increases in tourism – an
estimated 180,000 holiday
homes being built along the
coast every year. Increased
pressure on water supplies for
water parks, swimming pools
and golf courses.
o Conflicts have developed over water – a black market has developed supplied by
water from illegal bore holes.
o Exploitation of groundwater has led to loss of habitats such as wetlands and
reduced biodiversity in the region.
o As groundwater levels lower, the water can become salinised. When this
happens, the water is no longer suitable for drinking or irrigation.
o As aquifers become salinised so do the soils around them leading to further
desertification.
Almeria is the driest area in Europe and has rainfall less than 250mm per year. It was
an impoverished agricultural area, but is now home to the largest concentration of
greenhouses in Europe. The semi arid land which was once almost worthless has
been transformed into an intensive agricultural production using polythene tunnels.
Polytunnels also use hydroponic methods, growing crops without soil. Concerns
have been raised over the sustainability of this agricultural system; desertification
threatened 37% of Spain and most is due to human mismanagement.
o The National Hydrological Plan was adopted in 2001 to ensure that water supply
meets demands by; transferring water from areas with plentiful supply to areas
with little; desalinisation plants to produce fresh water from saline aquifers;
reducing the water wasted during irrigation, by improving infrastructure –
investing in technology for sprinkling and drip irrigation.
o Areas at high risk of forest fires are being identified and defence plans drawn
up so that fires can be dealt with quickly. Areas that are destroyed have to be
reforested. This reduces soil erosion and so reduced desertification.
o An official water trading scheme has been set up that allows farmers to buy
water, in an attempt to reduce illegal exploitation of aquifers.
Types of Desert
1
2
3
1) Erg – undulating sand seas formed by the accumulation of sand.
2) Reg – a transition zone between erg and hammada – a vast stony
plain, created when deflation has removed lighter sand and
sediment.
3) Hamada – barren rocky highlands consisting of barren plateaus and
little sand.
Mechanical and Chemical Weathering
Thermal expansion – exfoliation and granular disintegration – Thermal expansion is
caused by the extreme diurnal (daily) temperature ranges. The constant heating and
cooling is known as insolation – and this causes the rock to repeatedly heat up and
cool down. Outer layers off rock expand and contract faster than those on the inside,
forming stresses that crack the rock. The outer layers may peel off – this is
exfoliation. Individual grains may fall off because different minerals heat and cool
ant different rates – this is granular disintegration.
Frost Shattering (freeze-thaw) – this occurs in deserts where there is moisture and
with temperatures which fluctuate above and below freezing. Water will enter joints
and pores in rocks, and at night, if the temperature falls below 0, the water will
freeze and expand. Over time, repeated freeze-thaw action weakens the rock and
causes pieces to fall off.
Salt weathering (chemical) – salts in the ground percolate upwards by capillary
action. The saline water enters cracks in rocks, and the high temperature causes the
water to evaporate, forming salt crystals. As the salt crystals form, they expand,
exerting pressure on the rock. As the pressure builds, pieces fall off the rock.
Hydration – when moisture, often in the form of dew, combines with minerals in the
rock causing it to swell and the outer layer to peel off.
Effect of Wind – Aeolian Processes
Transport
Particles are more likely to be moved by wind if they
are dry, unconsolidated and if no vegetation is
attached to it. Winds transport best when they are
over 20km/h in speed and blow steadily in one
direction. Suspension is when material less than
0.15mm in diameter us picked up by the wind and
raised to considerable heights. This can cause a dust
storm, which can carry material to the edge of the desert. Saltation occurs when
wind speeds exceed the threshold velocity. Particles are picked up, lifted into the air
and then bounce back to the ground. Surface creep is when particles being carried
by Saltation bounce to the ground and land on larger stones, pushing them forwards.
The constant bombardment gradually moves small stones and pebbles over the
desert surface.
Wind Erosion
Deflation – refers to the action of the wind on the desert floor, when the
sedimentary rocks containing pebbles have been
naturally cemented together with sand and silt.
They are progressively removed, leaving the reg,
pebble strewn desert and a lower land surface.
Deflation can also cause the formation of
deflation hollows; the areas where the land has
been lowered allow cool air to sink into them at
night. The cool air condenses forming dew, and due to mechanical and chemical
weathering, the depression becomes deeper.
Abrasion
Abrasion is sand blasting – particles being carried by the wind in suspension hitting
the surface of rock. Abrasion at its smallest scale produces ventifacts or dreikanter –
3 sided pebbles with a highly polished flat surface.
Water in deserts
Exogenous rivers have a source outside of the desert margin, and flow throughout
the year, despite evaporation reducing their volume. Exogenous rivers erode just like
streams at more humid latitudes, but because much of the river’s energy is focussed
on downward vertical erosion, and less on widening the valley, the typical steep
sided canyons are formed. Eg. River Colorado, Grand Canyon, USA, flows through
Sonoran Desert.
Endoreic Rivers terminate inland in the form of an inland sea or lake. The River
Jordan is an example, which terminates in the Dead Sea.
Ephemeral Rivers flow intermittently or seasonally after rainstorms, for example,
the Todd River in the Simpson Desert, Australia.
The role of flooding
The torrential nature of desert rain exceeds the infiltration capacity of the ground, so
most water must drain away by overland flow. High temperatures and the presence
of duricrust (a hard crust created by the accumulation of silicon dioxide, aluminium
oxide and iron oxide) combine to give a hard impermeable surface which inhibits
infiltration. A lack of vegetation means no interception and soils are not broken up by
root systems, so there are few pore spaces for water to be absorbed into.
Flash Floods are sudden, strong, rapid flows of water
though a channel. They occur because heavy rainfall cannot be
absorbed by the dry, hard desert soil, so run off collects in
channels and flows rapidly downhill. Flash floods have enough
energy to transport large pieces of desert rock by traction.
They also transport pebbles, gravel and sand by suspension
and Saltation. The material carried by the flash floods erodes
the channel by abrasion, making it deeper.
A sheet flood is a slow moving even flow of water over land
– it isn’t confined to a channel. After a period of intense
rainfall, water collects across the dry impermeable desert
floor and flows down gentle slopes as a sheet of water.
Pebbles, gravel and sand can be carried short distances, and
material carried erodes the desert surface by abrasion.