Rainfall Variability

Rainfall in southern Africa varies temporally with annual, seasonal, and daily variations in the amount of rainfall received. Southern Africa, for example, experiences a high level of intraannual and intcrannual rainfall variability (Tyson, 1986). Rainfall occurs in the summer months across the central and northeastern parts of the country

* Precipitation usually includes the deposition of water in solid or liquid form including tain, dew, snow, hail (Goudie, 1994). In this chapter rainfall will be the main focus of the discussion.

with a winter rainfall area predominating to the southwestern part of the country. Summer rainfall is usually characterized by frequent thunderstorms over much of the central parts of the country, and winter precipitation is usually the result of frontal rains over the southwestern areas.

Assessments of rainfall records for South Africa indicate that interannual rainfall has been characterized by a series of wetter and drier years. An approximately 18-year cycle, for example, is evident over much of the summer rainfall area (Tyson, 1986; Mason and Jury, 1997) that extends into Zimbabwe (Torrance, 1972; Ngara et al., 1983) and Botswana. Definite wet and dry spells have been identified from meteorological records including the following dry spells: 1905-1906 to 19151916; 1925-1926 to 1932-1933 (the most consistently dry spell), 1944-1945 to 1952-1953; 1962-1963 to 1970-1971; 1980-1981 to 1990; 1991-1992 and 19941995 (Tyson et al., 1975; Tyson and Dyer, 1978, 1980; Tyson, 1981, 1984, 1986; Lindesay, 1998). Wetter spells occurred in the early 1920s, 1933-1934 to 19431944, 1953-1954 to 1961-1962 and 1971-1972 to 1980-1981. This latter wet spell was also the most persistently wet spell (Tyson and Preston-Whyte, 2000). Though drier spells tend to exhibit a greater spatial homogeneity than wet spells, the impression should not be gained that rainfall anomalies during the wet and dry spells are spatially uniform. Rather there is marked interannual and spatial variability in the distribution of wetter and drier conditions (Tyson, 1986; Lindesay, 1998). Available evidence suggests this pattern of wet and dry years has generally persisted since at least 1840 (Vogel, 1989).

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