Leonard S Unganai And Tsitsi Bandason

Southern Africa lies between 0°S to 35°S latitude and 10°E to 41°E longitude. 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 affecting 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. Southern Africa has suffered recurrent droughts since record keeping began (Nicholson, 1989; Unganai, 1993). Severe drought periods included 180030, 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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12E 15E 18E 21E 24E 27E 30E 33E 36E 39E 42E 4E Figure 20.1 Mean annual rainfall for southern Africa.

the worst drought years were 1911-12, 1923-24, 1946-47, 1972-73, 1981-82, 1982-83, 1986-87, and 1991-92 (Zimbabwe Department of Meteorological Services, personal communication, 2002).

During the severe and recurrent droughts of the 1980s and 1990s, the impact on vulnerable communities and the environment was catastrophic. Agricultural production in the smallholder-farming sector fell by up to 70% during the 1991-92 drought in most countries in the subregion and threatened about 30 million people with starvation, requiring an estimated U.S. $4 billion in food aid (IFPRI, 1998). In Zimbabwe alone, 5.6 million people out of a population of 12 million received drought relief at a cost of about Zimbabwe $3 billion (U.S. $0.3 billion) (Ngara and Rukobo, 1999). Because of the strong dependence of Zimbabwe's economy on rain-fed agriculture, gross domestic product (GDP) for 1992 registered a negative growth rate of 6% (Franklin, 1998) as a direct result of the severe 1991-92 drought. In this chapter, the significance of drought and monitoring systems in southern Africa are reviewed with specific reference to Zimbabwe.

No objective, operational definition of drought exists in Zimbabwe. However, drought conditions are said to be occurring when rainfall is below 75% of the long-term average for a prolonged period during the rainfall season. Declaration of drought is made only when agricultural production and water supplies have been adversely affected to the extent that smallholder-farming communities cannot cope without state assistance.

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Agriculture is the backbone of southern Africa's economy. In Zimbabwe it contributes about 11-18% of the GDP, 40% of annual exports, and 50% of the country's industrial raw materials. About 70% of the country's population depend directly on agriculture for their livelihood, with 30% being formally employed in that sector (Ngara and Rukobo, 1999). Agriculture also provides a significant market for products from industry. Main crops grown in the subregion include maize (staple food), tobacco, cotton (cash crops), sorghum, pearl millet, sugar, and a variety of horticultural crops. With most of the subregion's agricultural production being rain-fed, climatic extremes, if unanticipated, can produce catastrophic downstream effects on the economy, as was the case during the recent droughts of 198283, 1986-87, 1991-92, and 1994-95, and 2000-01.

The total rainfall and maize yield in the smallholder-farming sector in Zimbabwe are strongly correlated (r = .72; figure 20.2). During the severe 1991-92 drought, maize yields dropped to nearly zero in that sector. The impact of rainfall on Zimbabwe's economic performance is evident in figure 20.3. The GDP shrunk by almost 6% in 1992 following the devastating 1991-92 drought (Franklin, 1998). The strong response of the country's GDP to rainfall fluctuations is a reflection of the country's strong dependence on the performance of the agricultural sector.

For all practical purposes, southern Africa's rainy season spans November-March. December, January, and February are the peak rainfall months. Significant contributors of rainfall are the Intertropical Convergence Zone (ITCZ) and tropical-temperate troughs and their associated cloud bands (Torrance, 1981; Tyson, 1986). Some of the heaviest rains over southern Africa are associated with the infrequent passage of tropical cyclones across the coastal margins of Mozambique. Tropical-temperate troughs are frequently responsible for floods during the second half of the austral summer but are generally shorter lived during the early summer. Total rainfall from individual trough events depends on the availability of atmospheric moisture, atmospheric stability, strength of upper-level divergence, and the speed of movement of the trough. Dry summers are dominated by confluent upper winds that reduce the potential for convection over southern Africa and are often accompanied by an upsurge of tropical disturbances in the southwest Indian Ocean, representing an eastward shift in the preferred location of summer convection (Mason, 1995).

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