Classifying Droughts

With this background, the focus narrows to consider the character of droughts and drought impacts in South Africa in more detail. Defining what is meant by drought is not an easy task. Droughts can be classified as meteorological, agricultural, hydro-logical, or sociological (Wilhite and Glantz, 1985; Dent et al., 1987; Erasmus, 1987; Bruwer, 1989, 1990). Drought, moreover, is also a relative rather than an absolute condition. Droughts can be described as being either an agricultural drought (a condition where soil moisture is depleted such that yields are considerably reduced) or a hydrological drought (actual water supply being less than the minimum required for normal operations) (Wilhite and Glantz, 1985).

In South Africa, drought is broadly defined as occurring when 75% or less of normal precipitation is received (Laing, 1992), being classed as severe if it extends over two consecutive seasons. Other indices such as the Palmer Drought Severity Index (PDSI), include inputs of soil moisture, runoff, evaporation, and temperature (Zucchini and Adamson, 1984; Erasmus, 1987; Wilhite, 1987; Bruwer, 1990). Classification of a dry spell and/or drought depends on the definition, criteria, and statistical methods used.

The timing of rainfall is also critical in determining the progression and consequences of a drought situation. Although most of the country, for example, experienced good rainfalls in 1987-1988 and 1988-1989, the individual rain events were short, torrential downbursts. As these incidences show, timing and adequate rainfall, balanced against evaporation and infiltration, determine the amount of rainfall available for surface and groundwater usage. Opportune rainfall is also essential for crop growth, and several improvements in determining drought severity and crop response have been developed including the Agricultural Catchments Research Unit (ACRU) model (Schulze, 1984, 2000; Schulze et al., 2001) and the Crop Environment Resource Synthesis (CERES-MAIZE) model (Du Pisani, 1987).

Using such methods, together with other descriptive criteria including rainfall over three seasons, grassland condition, availability of water for stock, stock condition/deaths, availability and volume of fodder purchased (Bruwer, 1989, 1990), one is able to divide the country into areas that are more or less drought prone. Records for a 30-year period, for example (1936 to 1986), show-that 27% of the country has been declared a disaster drought area for more than 50% of the time (Bruwer, 1989, 1990).

Periods of drought, usually spanning at least 2 or more seasons, occurred in the late 1920s and early 1930s, much of the 1960s and 1980s, and more recently in the early and mid-1990s. These drought periods have brought with them and have compounded a variety of problems (including access to water in rural poor areas and water availability and use in agricultural and industrial sectors). Droughts have also highlighted several prevailing factors that predispose certain areas and groups to heightened drought risk (e.g. poor water infrastructure, land degradation, globalization) (Vogel, 1994; Scoones et al, 1996; Benson and Clay, 2000; Dilley, 2000).

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