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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.

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Man land relationship

Man Land Relationship

In the Tropical Desert

What are deserts
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
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
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 continentalinteriors restricts the influence of moist oceanic air masses in summer

  • e.g. central Asian & African deserts

Causes of aridity

  • Large continental areas

    develop strong high-

    pressure 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 creatinga strong temperature inversion.

Constraints potentials

Constraints & Potentials

In the Tropical Deserts

Environmental constraints
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


  • 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


  • Most famine deaths in sub-Saharan Africa

Mid 1980s african drought
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


  • 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.


  • 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

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
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