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Man Land Relationship

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  1. Man Land Relationship In the Tropical Desert

  2. 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.

  3. 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.

  4. Where are deserts? Deserts cover approximately 1/3 of the Earth’s land surface.

  5. 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

  6. 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

  7. 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

  8. 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.

  9. Constraints & Potentials In the Tropical Deserts

  10. 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

  11. 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

  12. Drought • Most famine deaths in sub-Saharan Africa

  13. 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

  14. 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.

  15. Victoria Falls, Dry Season

  16. 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

  17. 1984 Ethiopian Famine

  18. 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

  19. 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.

  20. 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.

  21. 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

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