Drought as Hazard: Concepts, Definition, and Types
Drought differs from other natural hazards in several ways. First, drought is a slow-onset hazard, often referred to as a creeping phenomenon (Gillette 1950). Figure 1.2 further illustrates the hypothetical life cycle of a typical drought and the compounding aspects of its development and impacts. Because of the creeping nature of drought, its effects accumulate slowly, usually over several months or longer. Therefore, the onset and end of drought are difficult to determine, and scientists and policymakers often disagree on the bases (i.e., criteria) for declaring an end to drought. Tannehill (1947) notes:
We may truthfully say that we scarcely know a drought when we see one. We welcome the first clear day after a rainy spell. Rainless days continue for some time and we are pleased to have a long spell of fine weather. It keeps on and we are a little worried. A few days more and we are really in trouble. The first rainless day in a spell of fine weather contributes as much to the drought as the last, but no one knows how serious it will be until the last dry day is gone and the rains have come again ... we are not sure about it until the crops have withered and died.
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
Increasing recognition of the multiple dimensions and timescales of drought. (Courtesy of World Meteorological Organization's Climatology and Agrometeorology Commissions.)
need to be sustained for the drought to be declared officially over? Is the drought event over only when precipitation deficits that accumulated during the drought event are erased? Do reservoirs and groundwater levels need to return to normal or average conditions? Impacts linger for a considerable time following the return of normal precipitation, so is drought terminated by meteorological or climatological factors, or by the diminishing negative impact on society and the environment?
Second, the absence of a precise and universally accepted definition of drought adds to the confusion about whether a drought exists and, if it does, its degree of severity. Realistically, definitions of drought must be region and application (or impact) specific (Wilhite and Glantz 1985). Definitions must be region specific because each climate regime has distinctive climatic characteristics (i.e., the characteristics of drought differ significantly between regions such as the North American Great Plains, Australia, eastern and southern Africa, western Europe, and northwestern India). Definitions need to be application specific because drought, like beauty, is defined by the beholder and how it affects his or her activity or enterprise. Thus, drought means something different for a water manager, a commodities producer, a hydroelectric power plant operator, a subsistence farmer, and a wildlife biologist. Even within sectors, such as agriculture, there are many different perspectives of drought because impacts may differ markedly for crop and livestock producers and agribusiness. For example, the impacts of drought on crop yield will differ for maize, wheat, soybeans, and sorghum because each crop is planted at a different time during the growing season and has different sensitivities to water and temperature stress at various growth stages. Management factors also play a significant role in crop yields. This is one of the reasons why numerous definitions of drought exist. For this reason, the search for a universal definition of drought is a rather pointless endeavor. Policymakers are at times frustrated by disagreements among scientists on whether a drought exists and its degree of severity. A policymaker is trying to determine if government should respond and, if so, through what types of response measures. The suite of responses employed is often based on those used for past drought events, with little or no consideration of whether these measures were actually effective. This book strives to change the paradigm for drought management from being reactive to proactive, the latter being an approach focused on risk reduction and, thus, reduced societal vulnerability.
Third, drought impacts are nonstructural and spread over larger geographical areas and temporal scales than are damages that result from other natural hazards such as floods, tropical storms, and earthquakes. These features of drought, combined with its creeping nature, make it particularly challenging to quantify and attribute specific impacts, and, therefore, more challenging to provide disaster relief in a timely and effective manner for drought than for other natural hazards.
These three characteristics of drought have hindered development of accurate, reliable, and timely forecasts; estimates of severity and impacts; and, ultimately, the formulation of drought preparedness plans and the implementation of appropriate risk reduction strategies or measures. Similarly, emergency managers, who have the assignment of responding to drought, struggle to deal with the large spatial coverage usually associated with drought.
Drought is a temporary aberration, unlike aridity, which is a permanent feature of the climate. Seasonal aridity (i.e., a well-defined dry season) also must be distinguished from drought. Considerable confusion exists among scientists and policymakers on the differentiation of these terms, especially in arid and semiarid regions. For example, Pessoa (1987) presented a map illustrating the frequency of drought in northeastern Brazil in his discussion of the impacts of and governmental response to drought. For a significant portion of the northeast region, he indicated that drought occurred 81-100 percent of the time. Much of this region is arid, and drought is a recurrent feature of its climate. However, drought is a temporary feature of the climate, so it cannot, by definition, occur 100 percent of the time. Similarly, researchers have defined a relative minimum during the Central American and Caribbean rainy season as a "midsummer drought" even though it occurs as part of the annual rainfall cycle each year (Magana et al. 1999).
Nevertheless, it is important to identify trends over time and whether drought is becoming a more frequent and severe event. Today, concern exists that the threat of a warming climate may increase the frequency and/or severity of extreme climate events for some regions in the future (IPCC 2012; Melillo et al. 2014). As pressure on finite water supplies and other limited natural resources continues to build, more frequent and severe droughts are cause for concern in both water-short and water-surplus regions where tensions within (e.g., upstream vs. downstream) and between countries are growing. Anticipating and reducing the impacts of future drought events is paramount. It must be part of a sustainable development strategy, a theme developed later in this chapter and throughout this book.
Drought is a relative, rather than absolute, condition occurring in virtually all climate regimes. Our experience suggests scientists, policymakers, and the public associate drought primarily with arid, semiarid, and subhumid regions. For example, while drought has been traditionally associated with the southwestern United States and other parts of the western United States, the relatively humid Apalachicola-Chattahoochee-Flint Basin in the southeastern region of the United States has been among the most contentious watersheds in the country, owing to the combination of drought and water extraction. In reality, drought occurs in most nations, in both dry and humid regions, and often on a yearly basis, especially in larger countries characterized by multiple climate zones that result from different climatic controls. Drought characteristics and management will vary across each of these climatic zones. Drought is increasingly realized as having major impacts not only on agriculture but also on water supplies affecting health, energy, transportation, and recreation. This reality supports the need for a national strategy or policy that emphasizes drought risk reduction (Wilhite et al. 2014; WMO and GWP 2014; Chapters 2, 3, and 4).