Agriculture Reference
In-Depth Information
CHAPTER TWENTY
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Southern Africa lies between 0°S to 35°S latitude and 10°E to 41°E longi-
tude. In this region, annual rainfall ranges from below 20 mm along the
western coastal areas of Namibia to as high as 3000 mm in some highland
areas of Malawi (figure 20.1). Rainfall generally increases from south to
north in response to topography and the main rain-bearing systems af-
fecting the subregion. In the southwest sections of the sub-region, annual
rainfall averages below 400 mm, whereas the high-altitude areas receive up
to 3000 mm due to orographic enhancement.
Two important features that control the climate of southern Africa are
the semipermanent subtropical high-pressure cells centered in the southeast
Atlantic and the southwest Indian Ocean. These subtropical high pressure
cells are associated with widespread and persistent subsidence (Lockwood,
1979). Part of southern Africa is under the downward leg of the Hadley
Cell, superposed on the zonal Walker cell. The complex interaction of
these cells, particularly during warm El Niño/Southern Oscillation (ENSO)
episodes, is usually associated with drier than normal austral summers over
much of southern Africa. Much of southern Africa is therefore semiarid and
prone to recurrent droughts. In South Africa, for operational purposes,
a drought is broadly defined as occurring when the seasonal rainfall is
70% or less of the long-term average (Bruwer, 1990; Du Pisani, 1990).
It becomes a disaster or severe drought when two or more consecutive
rainfall seasons experience drought.
Drought affects some part of southern Africa virtually every year. South-
ern Africa has suffered recurrent droughts since record keeping began
(Nicholson, 1989; Unganai, 1993). Severe drought periods included 1800-
30, 1840-50, 1870-90, 1910-15, 1921-25, 1930-50, 1965-75, and
1980-95. During some of these drought periods, rivers, swamps, and wells
dried up and well-watered plains turned into barren lands. For Zimbabwe,
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