Drought is an insidious natural hazard that results from a deficiency of precipitation from expected or "normal" that, when extended over a season or longer, is insufficient to meet the demands of human activities and the environment. Drought by itself is not a disaster. Whether it becomes a disaster depends on its impact on local people and the environment. Therefore, the key to understanding drought is to understand both its natural and social dimensions.

Drought is a normal part of climate, rather than a departure from normal climate (Glantz, 2003). The latter view of drought has often led policy and other decision makers to treat this complex phenomenon as a rare and random event. This perception has typically resulted in little effort being targeted toward those individuals, population groups, economic sectors, regions, and ecosystems most at risk (Wilhite, 2000). Improved drought policies and preparedness plans that are proactive rather than reactive and that aim at reducing risk rather than responding to crisis are more cost-effective and can lead to more sustainable resource management and reduced interventions by government (Wilhite et al., 2000a; see also Chapter 5).

The primary purpose of this chapter is to discuss drought in terms of both its natural characteristics and its human dimensions. This overview of the concepts, characteristics, and impact of drought will provide readers with a foundation for a more complete understanding of this complex hazard and how it affects people and society and, conversely, how societal use and misuse of natural resources and government policies can exacerbate vulnerability to this natural hazard. In other words, we are promoting a holistic and multidisci-plinary approach to drought. This discussion is critical to an understanding of the material presented in the science and technology section of this volume (Part II) as well as in the various case studies presented in Part III.

We use the term hazard to describe the natural phenomenon of drought and the term disaster to describe its negative human and environmental impacts.


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. Therefore, the onset and end of drought are difficult to determine, and scientists and policy makers 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 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 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 the end of drought signaled by meteorological or climatological factors, or by the diminishing negative human impact?

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. Definitions must be region specific because each climate regime has distinctive climate characteristics (i.e., the characteristics of drought differ significantly between regions such as the North American Great Plains, Australia, southern Africa, western Europe, and northwestern India). Definitions need to be application specific because drought, like beauty, is largely defined by the beholder and how it may affect his or her activity or enterprise. Thus, drought means something different for a water manager, an agriculturalist, a hydroelectric power plant operator, and a wildlife biologist. Even within sectors there are many different perspectives of drought because impacts may differ markedly. For example, the impacts of drought on crop yield may differ greatly for maize, wheat, soybeans, and sorghum because each is planted at a different time during the growing season and has different sensitivities to water and temperature stress at various growth stages. This is one explanation for the scores of definitions that exist. For this reason, the search for a universal definition of drought is a rather pointless endeavor. Policy makers are often frustrated by disagreements among scientists on whether a drought exists and its degree of severity. Usually, policy makers' principal interest is the impact on people and the economy and the types of response measures that should be employed to assist the victims of drought.

Third, drought impacts are nonstructural and spread over a larger geographical area than are damages that result from other natural hazards such as floods, tropical storms, and earthquakes. This, combined with drought's creeping nature, makes it particularly challenging to quantify the impact, and may make it more challenging to provide disaster relief than for other natural hazards. These characteristics of drought have hindered development of accurate, reliable, and timely estimates of severity and impacts (i.e., drought early warning systems) and, ultimately, the formulation of drought preparedness plans. 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 policy makers on the differentiation of these terms. 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 sig nificant portion of the northeast region, he indicated that drought occurred 81-100% of the time. Much of this region is arid, and drought is an inevitable feature of its climate. However, drought is a temporary feature of the climate, so it cannot, by definition, occur 100% of the time.

Nevertheless, it is important to identify trends over time and whether drought is becoming a more frequent and severe event. Concern exists that the threat of global warming may increase the frequency and severity of extreme climate events in the future (IPCC, 2001). 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 conflicts within and between countries are growing. Reducing the impacts of future drought events is paramount as part of a sustainable development strategy, a theme developed later in this chapter and throughout this volume.

Drought must be considered a relative, rather than absolute, condition. It occurs in both high- and low-rainfall areas and in virtually all climate regimes. Our experience suggests scientists, policy makers, and the public often associate drought only with arid, semiarid, and subhumid regions. In reality, drought occurs in most nations, in both dry and humid regions, and often on a yearly basis. The intensity, epicenter, and size of the area affected by drought will vary annually (see Chapter 12), but its presence is nearly always being felt. This reality supports the need for a national strategy (see Chapters 5 and 6).

A. Types of Drought

All types of drought originate from a deficiency of precipitation (Wilhite and Glantz, 1985). When this deficiency spans an extended period of time (i.e., meteorological drought), its existence is defined initially in terms of these natural characteristics. The natural event results from persistent large-scale disruptions in the global circulation pattern of the atmosphere (see Chapter 2). Exposure to drought varies spatially, and there is little, if anything, we can do to alter drought occurrence. However, the other common drought types (i.e., agricultural, hydrological, and socioeconomic) place greater

Decreasing emphasis on the natural event (precipitation deficiencies) -►

Increasing complexity of impacts and conflicts

Decreasing emphasis on the natural event (precipitation deficiencies) -►

Increasing complexity of impacts and conflicts

Time/duration of the event

Figure 1 Natural and social dimensions of drought. (Source: National Drought Mitigation Center, University of Nebraska, Lincoln, Nebraska, USA.)

Time/duration of the event

Figure 1 Natural and social dimensions of drought. (Source: National Drought Mitigation Center, University of Nebraska, Lincoln, Nebraska, USA.)

emphasis on human or social aspects of drought, highlighting the interaction or interplay between the natural characteristics of the event and the human activities that depend on precipitation to provide adequate water supplies to meet societal and environmental demands (see Figure 1). For example, agricultural drought is defined more commonly by the availability of soil water to support crop and forage growth than by the departure of normal precipitation over some specified period of time. No direct relationship exists between precipitation and infiltration of precipitation into the soil. Infiltration rates vary according to antecedent moisture conditions, slope, soil type, and the intensity of the precipitation event. Soils also vary in their characteristics, with some soils having a high water-holding capacity and others a low water-holding capacity. Soils with a low water-holding capacity are more drought prone.

Hydrological drought is even further removed from the precipitation deficiency because it is normally defined in terms of the departure of surface and subsurface water supplies from some average condition at various points in time. Like agricultural drought, no direct relationship exists between precipitation amounts and the status of surface and subsurface water supplies in lakes, reservoirs, aquifers, and streams because these components of the hydrological system are used for multiple and competing purposes (e.g., irrigation, recreation, tourism, flood control, hydroelectric power production, domestic water supply, protection of endangered species, and environmental and ecosystem preservation). There is also considerable time lag between departures of precipitation and when these deficiencies become evident in these components of the hydrologic system. Recovery of these components is also slow because of long recharge periods for surface and subsurface water supplies. In areas where the primary source of water is snowpack, such as in the western United States, the determination of drought severity is further complicated by infrastructures, institutional arrangements, and legal constraints. For example, reservoirs increase this region's resilience to drought because of the potential for storing large amounts of water as a buffer during dry years. However, the operating plans for these reservoirs try to accommodate the multiple uses of the water (e.g., protection of fisheries, hydroelectric power production, recreation and tourism, irrigation) and the priorities set by the U.S. Congress when the funds were allocated to construct the reservoir. The allocation of water between these various users is generally fixed and inflexible, making it difficult to manage a drought period. Also, legal agreements between political jurisdictions (i.e., states, countries) concerning the amount of water to be delivered from one jurisdiction to another impose legal requirements on water managers to maintain flows at certain levels. During drought, conflicts heighten because of limited available water. These shortages may result from poor water and land management practices that exacerbate the problem (e.g., see Chapters 10 and 12).

Socioeconomic drought differs markedly from the other types because it associates human activity with elements of meteorological, agricultural, and hydrological drought. This may result from factors affecting the supply of or demand for some commodity or economic good (e.g., water, grazing, hydro electric power) that is dependent on precipitation. It may also result from the differential impact of drought on different groups within the population, depending on their access or entitlement to particular resources, such as land, and/or their access or entitlement to relief resources. Drought may fuel conflict between different groups as they compete for limited resources. A classic example in Africa is the tension, which may become violent in drought years, between nomadic pas-toralists in search of grazing and settled agriculturalists wishing to use the same land for cultivation. The concept of socioeconomic drought is of primary concern to policy makers.

The interplay between drought and human activities raises a serious question with regard to attempts to define it in a meaningful way. It was previously stated that drought results from a deficiency of precipitation from expected or "normal" that is extended over a season or longer period of time and is insufficient to meet the demands of human activities and the environment. Conceptually, this definition assumes that the demands of human activities are in balance or harmony with the availability of water supplies during periods of normal or mean precipitation. If development demands exceed the supply of water available, demand may exceed supply even in years of normal precipitation. This can result in human-induced drought. In this situation, development can be sustained only through mining of groundwater and/or the transfer of water into the region from other watersheds. Is this practice sustainable in the long term? Should this situation be defined as "drought" or unsustainable development?

Drought severity can be aggravated by other climatic factors (such as high temperatures, high winds, and low relative humidity) that are often associated with its occurrence in many regions of the world. Drought also relates to the timing (i.e., principal season of occurrence, delays in the start of the rainy season, occurrence of rains in relation to principal crop growth stages) and effectiveness of the rains (i.e., rainfall intensity, number of rainfall events). Thus, each drought event is unique in its climatic characteristics, spatial extent, and impacts (i.e., no two droughts are identical). The area affected by drought is rarely static during the course of the event. As drought emerges and intensifies, its core area or epicenter shifts and its spatial extent expands and contracts. A comprehensive drought early warning system is critical for tracking these changes in spatial coverage and severity, as explained below.

B. Characterizing Drought and Its Severity

In technical terms, droughts differ from one another in three essential characteristics: intensity, duration, and spatial coverage. Intensity refers to the degree of the precipitation shortfall and/or the severity of impacts associated with the shortfall. It is generally measured by the departure of some climatic parameter (e.g., precipitation), indicator (e.g., reservoir levels), or index (e.g., Standardized Precipitation Index) from normal and is closely linked to duration in the determination of impact. These tools for monitoring drought are discussed in Chapter 3. Another distinguishing feature of drought is its duration. Droughts usually require a minimum of 2 to 3 months to become established but then can continue for months or years. The magnitude of drought impacts is closely related to the timing of the onset of the precipitation shortage, its intensity, and the duration of the event.

Droughts also differ in terms of their spatial characteristics. The areas affected by severe drought evolve gradually, and regions of maximum intensity (i.e., epicenter) shift from season to season. In larger countries, such as Brazil, China, India, the United States, or Australia, drought rarely, if ever, affects the entire country. During the severe drought of the 1930s in the United States, for example, the area affected by severe and extreme drought reached 65% of the country in 1934. This is the maximum spatial extent of drought in the period from 1895 to 2003. The climatic diversity and size of countries such as the United States suggest that drought is likely to occur somewhere in the country each year. On average 14% of the country is affected by severe to extreme drought annually. From a planning perspective, the spatial characteristics of drought have serious implications. Nations should determine the probability that drought may simultaneously affect all or several major crop-producing regions or river basins within their borders and develop contingencies for such an event. Likewise, it is important for governments to calculate the chances of a regional drought simultaneously affecting agricultural productivity and water supplies in their country and adjacent or nearby nations on which they depend for food supplies. A drought mitigation strategy that relies on the importation of food from neighboring countries may not be viable if a regional-scale drought occurs.


Drought, like all natural hazards, has both a natural and social dimension. The risk associated with drought for any region is a product of both the region's exposure to the event (i.e., probability of occurrence at various severity levels) and the vulnerability of society to the event. Vulnerability can be defined as "defenselessness, insecurity, exposure to risk, shocks and stress," and difficulty in coping with them (Chambers, 1989). It is determined by both micro- and macro-level factors, and it is cross-sectoral—dependent on economic, social, cultural, and political factors. Blaikie et al.'s (1994) disaster pressure model represents well the interaction of hazard with vulnerability (Figure 2). They explore vulnerability in terms of three levels. First, there are the root causes. These may be quite remote and are likely to relate to the underlying political and economic systems and structures. Second are the dynamic pressures, which translate the effects of the root causes into particular forms of insecurity. These pressures might include rapid population growth, rapid urbanization, and epidemics. As a result, unsafe conditions are created; for instance, through people living in dangerous locations and/or the state failing to provide adequate protection.

Understanding people's vulnerability to drought is complex yet essential for designing drought preparedness, mitigation, and relief policies and programs. At the micro level, determinants of vulnerability include:

The physical asset base of the household—for example, land, livestock, cash Human capital—for example, productive labor Social capital—for example, claims that can be made on other households within the community, perhaps for productive resources, food, or labor

Emergency Preparedness

Emergency Preparedness

Remember to prepare for everyone in the home. When you are putting together a plan to prepare in the case of an emergency, it is very important to remember to plan for not only yourself and your children, but also for your family pets and any guests who could potentially be with you at the time of the emergency.

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