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.

 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 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 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 million deaths in Ethiopia  Now in Ethiopia 6 million require food aid, 15 million face starvation by the end of % 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