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Man Land Relationship
In the Tropical Desert
What are deserts?
These regions are characterized by very
low annual rainfall (usually less than 300
mm)
 Sparse vegetation
 Extensive areas of bare, rocky mountains,
plateau and alluvial plains.
 Sand dunes cover less than 1/3 of desert
regions.

What are deserts?

In general, a desert is a region in which
mean annual potential evapotranpiration
(Etp) exceeds mean annual precipitation
(P) by a factor of two or more.
Where are deserts?
Deserts cover approximately 1/3 of the Earth’s land surface.
The features?
Low humidity
 A high daily range of temperature
 Precipitation which is highly variable in
time & space
 The most extensive deserts lie astride the
tropics

Causes of aridity

Descending and dry
stable air masses in
the subtropical anticyclonic belts maintain arid conditions throughout
the year

Large land masses reinforce the effects of stable
air masses

Long distances to continental interiors restricts
the influence of moist oceanic air masses in
summer
e.g. central Asian & African deserts

Causes of aridity

Large continental areas
develop strong highpressure cells, reducing
the influence of frontal system in winter

Mountain barriers block rain-bearing winds and
create rain-shadow areas in their lee

e.g. Great Basin Desert of North America
The Himalayas in central Asia to prevent
penetration of the south-west monsoon to
the Gobi and Takla Makan deserts
Causes of aridity

Deserts located on the
western coast of South
America and southern
Africa (Atacame, Namib)
owe their hyperarid climates to the influence of
cold oceanic currents offshore.

These reinforce the subsidence-induced stability
of the atmosphere by cooling surface air masses
and creating a strong temperature inversion.
Constraints & Potentials
In the Tropical Deserts
Environmental Constraints
Low, unreliable & irregular annual ppt input,
low R.H.
 localized & sudden short-lived heavy
downpour (conventional in nature)
– leading to flash flood & serious soil erosion
 Extreme climate: high temp high
evapotranspiration rate
 Strong wind causes dust storms
 Drought – a limiting factor

Drought

MDCs:


Drought is costly, but not deadly
LDCs:


Drought is frequently deadly
food supplies are fragile, malnutrition is
“normal”, the poor can be killed quickly in
famine
Drought

Most famine deaths in sub-Saharan Africa
Mid 1980s African
Drought
Affected 20 countries,
150 million people
 30 million in urgent
need of food aid
 10 million refugees
seeking food and water
 100,000 to 250,00
deaths

Africa

Current drought conditions in southern
Africa


14 million in 6 countries face starvation
Botswana refusing food aid from US and EU:

fears about genetically modified food.
Victoria Falls, Dry Season
Ethiopia

Drought and war brought famine in 1984


1 million deaths in Ethiopia
Now in Ethiopia




6 million require food aid,
15 million face starvation by the end of 2002
10% of government revenues spent on foreign
debt repayments
Will require 200 million tonnes of food aid
1984 Ethiopian Famine
Effects of droughts
Permanent settlement cannot be
supported – nomadic existence of
indigenous people, except along
permanent river or in oases
 Extensive pastoral farming with
transhumance and nomadic grazing
 In oases / along permanent rivers:
sedentary / settled agriculture
 irrigation is essential for agriculture – the
source: underground water

Soil in deserts
The excess of evaporation over
precipitation
 gives rise to physical or mechanical,
 rather than chemical, weathering of rocks,
 and to upward movement of soil moisture
and near surface groundwater.

Soil in deserts

As a result, water-soluble salts (principally
sodium chloride, calcium carbonate, and calcium
sulphate) accumulate in desert soils

forming calcic and gypsic horizons in the subsoil.

Insolation weathering and salt weathering
dominate processes of rock breakdown.

On a regional scale, lack of water gives rise to
internal drainage and thus to playas and salt
lakes.
Environmental Opportunities




High temp
- high thermal input
Dry and sunny weather and climate
- long growing season
Clear skies
- favour aviation, satellite observation and
space industry
Dryness & sunniness
- retirement centres e.g. Mediterranean, Sahara
margin