1.1 Definition
The semiarid regions of the world are defined as transition
zones between arid and sub humid belts. semiarid regions
are also defined as areas where precipitation is less
than potential evaporation, characterized by high temperatures
(30oC-45oC) in the hottest months.
1.2 Characteristics
Arid and semiarid areas account for one third of the
earth's surface land area. ICRISAT (1998) estimates
that, semiarid areas especially within the tropics cover
most parts of the developing nations in the world including
Latin America; most parts of sub-Saharan Africa, large
portion of Eastern and Southern Africa and parts of
India and South East Asia.
semiarid regions are characterized by low and restricted
precipitation because of the inability of moisture bearing
winds to penetrate into, and cool down within such regions.
Also, zones of high pressure may prevent entry of the
winds resulting into meteorological phenomenon associated
with little or lack of rainfall in these areas.
Almost all precipitation in arid and semiarid regions
occurs as rain, except in higher altitudes where dew
and hoar frost can form due to great differences between
day and night temperatures.
semiarid lands of the world are also characterized
by basins of closed drainage due to inability of the
surface waters to maintain themselves against evaporation.
Both transpiration and evaporation are high because
abundant heat energy is supplied to change the limited
amounts of liquid water into water vapor either directly
or through biological processes thus maintaining the
heat balance of the area.
semiarid areas have been classified according to amount
of rainfall received, duration of the wet period and
the dominant vegetation. The wet period refers to a
period during which rainfall is higher than evapotranspiration
and is usually distributed over 3-4 month period in
a year. The vegetative cover is characterized mainly
by shrubs, scrubs and grass.
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