2 - Droughts
Published online by Cambridge University Press: 18 December 2009
Summary
HISTORICAL DROUGHTS
Drought may be defined as an extended period of rainfall deficit within an otherwise higher rainfall regime. However, this definition is subjective, for the length of period without rain varies between locations depending on major climatic regimes. Periods of rainfall deficit deemed to be a drought are also, in many cases, based upon rainfall patterns observed over relatively short historical records. In a country with modern records extending back only one to two centuries, a period of five years with a rainfall deficit may be termed a drought despite the fact that such events may have extended naturally to 50 years or more prior to the start of the modern record. The severity of a drought, therefore, is largely determined by its impact on humans rather than an arbitrary measure such as the Richter scale with earthquakes. Droughts affect both agricultural and urban communities but it is often the former that feel the effects before and more severely than the latter.
Drought is one of four classes of water scarcity (Robinson, 1993). These are:
aridity, a permanent shortage of water caused by a dry climate;
drought, an irregular phenomenon occurring in exceptionally dry years;
desiccation, a drying of the landscape, particularly the soil, resulting from activities such as deforestation and over-grazing; and
water stress, due to increasing numbers of people relying on fixed levels of run-off.
Drought may also be seen to be a temporary form of water scarcity, aridity a permanent form and desertification a human-induced form (that may be either temporary or permanent).
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- Extreme EventsA Physical Reconstruction and Risk Assessment, pp. 17 - 50Publisher: Cambridge University PressPrint publication year: 2006