Drought differs from other natural hazards in several ways. First, drought is a slow-onset natural hazard often referred to as a creeping phenomenon (Gillette 1950). Because of the creeping nature of drought, its effects accumulate slowly over a substantial period of time, tterefore, the onset and end of drought is difficult to determine, and scientists and policy makers often disagree on the basis (i.e., criteria) for declaring an end to drought. Should drought's end be signaled by a return to normal precipitation and, if so, over what period of time does normal or above-normal precipitation need to be sustained for the drought to be declared officially over? Do precipitation deficits that emerged during the drought event need to be erased for the event to end? Do reservoir and ground water levels need to return to normal or average conditions? Impacts linger for a considerable period of time following the return of normal precipitation, so is the end of drought signaled by meteorological or climatological factors or diminishing impacts? tte first two examples above are natural elements of drought - i.e., they are part of the concept of meteorological drought. However, the latter examples are complicated by how water is used and managed - i.e., how we manage reservoirs and ground water will not only determine how rapidly water levels will decline in response to reduced inflows or infiltration but also how rapidly they will recover from an extended period of drought.
Second, unlike most other natural hazards, drought does not have a precise and universally accepted definition, ttis fact adds to the confusion about whether or not a drought exists and, if it does, its degree of severity. Realistically, definitions of drought must be region and application (or impact) specific, ttis is one explanation for the hundreds of definitions that exist. For this reason, the search for a universal definition of drought is of little value. Policy makers are often frustrated by disagreements among scientists on whether or not a drought exists and its degree of severity, ttis problem is largely a function of the different drought types - i.e., meteorological, agricultural, hydrological, and socioeconomic, each of which reflects a different disciplinary perspective.
ttird, drought impacts are nonstructural and spread over a larger geographical area than are damages that result from other natural hazards. Quantifying the impacts and providing disaster relief (what some would refer to as "political" drought) are far more difficult tasks for drought than for other natural hazards. Drought impacts are largely "invisible" and, therefore, difficult to quantify. Agricultural impacts are often unknown until the growing season is over and the harvest complete. Even during the latter stages of the growing season and in the post-harvest season, estimates of the economic impacts of drought may change to reflect changing prices, estimates of harvested hectares or land abandoned, or the percent of crop that may be cut for silage. Methodologies for estimating drought impacts are noticeably non-standardized from one estimator or estimate to another.
ttese characteristics of drought have hindered development of accurate, reliable, and timely estimates of severity and impacts - delaying the formulation of drought preparedness plans, ttey affect both the way we measure and perceive exposure to drought and our vulnerability to it.
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