Agricultural Drought and Its Monitoring and Prediction: Some Concepts - Monitoring and Predicting Agricultural Drought: A Global Study - page 3

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A gricultural Drought and Its Monitoring
a nd Prediction: Some Concepts
V IJENDRA K. BOKEN

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Droughts develop largely due to below-average precipitation over a land
area, and they adversely affect various economic sectors in a region. Some
of these adverse effects include reductions in agricultural production, hy-
dropower generation, urban and rural water supplies, and industrial out-
puts. These effects lead to other consequences, secondary and tertiary, that
further impact an economy. For instance, when agricultural production
declines, food and other commodities tend to cost more and cause eco-
nomic inflation in a society. Chain effects of persistent droughts can shat-
ter an economy and even cause famine and sociopolitical upheaval in some
countries.
How does one define a drought? Usually, either precipitation or a form
of drought impact is used to define a drought. Because precipitation and
drought impacts vary spatially, there is a geographical dimension to defini-
tions of drought. In Saudi Arabia or Libya, droughts are recognized after
two to three years without significant rainfall, whereas in Bali (Indonesia),
any period of six days or more without rain is considered drought (Dracup
et al., 1980; Sen, 1990). In Egypt, any year in which the Nile does not flood
is considered a drought year.
More than 150 definitions of drought are available in the literature
(Gibbs, 1975; Krishnan, 1979; Dracup et al., 1980; Wilhite and Glantz,
1987). For example, a drought can be characterized as climatological,
meteorological, water management, socioeconomic, absolute, partial, dry
spell, serious, severe, multiyear, design, critical, point, or regional (Palmer,
1965; Herbst et al., 1966; Joseph, 1970a, 1970b; Askew et al., 1971; Beard
and Kubik, 1972; Karl, 1983; Santos, 1983; Alley, 1984; Chang, 1990).
Often, the difference between an estimated water demand and an expected
water supply in a region becomes the basis to define a drought for that
region (Kumar and Panu, 1997). A few of the chapters in this topic provide

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