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 a

nnual 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.

Low humidity A high daily range of temperature Precipitation which is highly variable in

time & space The most extensive deserts lie astride the

tropics

The features?

Causes of aridity

Descending and dry stable air masses in the subtropical anti- cyclonic 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 areasdevelop strong high-

pressure cells, reducingthe 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, lo

w R.H. localized & sudden short-lived heavy downpour

(conventional in nature) – leading to flash flood & serious soil erosion Extreme climate: high temp high evapotranspir

ation 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