Monitoring Agricultural Drought in Southern Africa - Monitoring and Predicting Agricultural Drought: A Global Study - page 268

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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 pop-
ulation 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, cli-
matic extremes, if unanticipated, can produce catastrophic downstream
effects on the economy, as was the case during the recent droughts of 1982-
83, 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

[268

.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 Conver-
gence 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 diver-
gence, and the speed of movement of the trough. Dry summers are dom-
inated 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).

=

Line
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11.
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Norm
PgEn

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C auses of Agricultural Drought

In terannual Rainfall Variability

Southern Africa is a predominantly semiarid region with high interannual
variability in rainfall (figure 20.4) and a pronounced annual cycle (Nicol-
son, 1986; Tyson, 1986). The coefficient of variation of annual rainfall