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close this bookPreparing for Drought: A Guidebook for Developing Countries (UNEP, 1992, 80 p.)
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"We have no good definition of drought. We may say truthfully that we scarcely know a drought when we see one. We welcome the first clear day after a rainy spell. Rainless days continue for a time and we are pleased to have a long spell of such 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." (Tannehill, 1947)

Drought as a Natural Hazard

Drought differs from other natural hazards (e.g., floods, tropical cyclones, and earthquakes) in several ways. First, since the effects of drought often accumulate slowly over a considerable period of time, and may linger for years after the termination of the event, a drought's onset and end are difficult to determine. Because of this, drought is often referred to as a "creeping phenomenon" (Tannehill, 1947). Second, the absence of a precise and universally accepted definition of drought adds to the confusion about whether or not a drought exists and, if it does, its severity. Realistically, definitions of drought must be region and application (or impact) specific. This is one explanation for the scores of definitions that have been developed. Unfortunately, many of these definitions have done a less than adequate job of defining drought in meaningful terms for scientists and policy makers. This is the result, at least in part, of misunderstandings of the concept by those formulating definitions. Third, drought impacts are less obvious and are spread over a larger geographical area than are damages that result from other natural hazards. Drought seldom results in structural damage. For these reasons the quantification of impacts and the provision of disaster relief are far more difficult tasks for drought than they are for other natural hazards. These characteristics have hindered the development of accurate, reliable, and timely estimates of drought severity and impacts.

Drought is a normal part of climate for virtually all climatic regimes. It occurs in high as well as low rainfall areas. Drought differs from aridity in that the latter is restricted to low rainfall regions and is a permanent feature of the climate. Many people associate the occurrence of drought with the Great Plains of North America, East Africa, West African Sahel, India, or Australia; they may have difficulty visualizing drought in Southeast Asia, Brazil, Western Europe, or the eastern United States, regions perceived by many to have a surplus of water. For example, residents of many humid regions often refer to "green droughts" (i.e., droughts associated with apparent ample rainfall but reduced agricultural productivity because of poor timing of rains or ineffective precipitation). Thus, the character of drought is distinctly regional, reflecting unique meteorological, hydrological, agricultural, and socioeconomic characteristics.

Drought is the consequence of a natural reduction in the amount of precipitation received over an extended period of time, usually a season or more in length, although other climatic factors (such as high temperatures, high winds, and low relative humidity) are often associated with it in many regions of the world and can significantly aggravate the severity of the event. Drought is also related 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 the effectiveness of the rains (i.e., rainfall intensity, number of rainfall events).

Drought severity is dependent not only on the duration, intensity, and geographical extent of a specific drought episode, but also on the demands made by human activities and vegetation on a region's water supplies. The characteristics of drought along with its far-reaching impacts make its effects on society, economy, and environment difficult, though not impossible, to identify and quantify. The significance of drought should not be divorced from its societal context. The impact of a drought depends largely on societal vulnerability at that particular moment. Subsequent droughts in the same region will have different effects, even if they are identical in intensity, duration, and spatial characteristics.

Drought Types and Definitions

Because drought affects so many economic and social sectors, scores of definitions have been developed by a variety of disciplines. In addition, because drought occurs with varying frequency in nearly all regions of the globe, in all types of economic systems, and in developed and developing countries alike, the approaches taken to define it also reflect regional differences as well as differences in ideological perspectives. Impacts also differ spatially and temporally, depending on the societal context of drought. A universal definition of drought is an unrealistic expectation.

Definitions of drought can be categorized broadly as either conceptual or operational. Conceptual definitions are of the "dictionary" type, generally defining the boundaries of the concept of drought, and thus are generic in their description of the phenomenon. For example, the American Heritage Dictionary (1976) defines drought as "a long period with no rain, especially during a planting season." Operational definitions attempt to identify the onset, severity, continuation, and termination of drought episodes. Definitions of this type are often used in an "operational" mode. These definitions can also be used to analyze drought frequency, severity, and duration for a given historical period. An operational definition of agricultural drought might be one that compares daily precipitation to evapotranspiration (ET) rates to determine the rate of soil water depletion and then expresses these relationships in terms of drought effects on plant behavior at various stages of development. The effects of these meteorological conditions on plant growth would be reevaluated continuously by agricultural specialists as the growing season progresses.

Many disciplinary perspectives of drought exist. Each discipline incorporates different physical, biological, and/or socioeconomic factors in its definition of drought. Because of these numerous and diverse disciplinary views, considerable confusion often exists over exactly what constitutes a drought (Glantz and Katz, 1977). Research has shown that the lack of a precise and objective definition in specific situations has been an obstacle to understanding drought, which has led to indecision and/or inaction on the part of managers, policy makers, and others (Wilhite et al., 1986). It must be accepted that the importance of drought lies in its impacts. Thus definitions should be region and impact or application specific in order to be used in an operational mode by decision makers. A comprehensive review of drought definitions and indices can be found in a technical note published by the World Meteorological Organization (WMO) (1975). Consult Subrahmanyam (1967), Glantz and Katz (1977), Sandford (1979), Dracup et al. (1980), and Wilhite and Glantz (1985) for a thorough discussion of the difficulties in defining drought.

Drought can be grouped by type as follows: meteorological, hydrological, agricultural, and socioeconomic (Wilhite and Glantz, 1985). Meteorological drought is expressed solely on the basis of the degree of dryness (often in comparison to some "normal" or average amount) and the duration of the dry period. Definitions of meteorological drought must be considered as region specific since the atmospheric conditions that result in deficiencies of precipitation are highly variable from region to region. For example, some definitions differentiate meteorological drought on the basis of the number of days with precipitation less than some specified threshold. Extended periods without rainfall are common for many regions; such a definition is unrealistic in these instances. Other definitions may relate actual precipitation departures to average amounts on monthly, seasonal, water year, or annual timescales. Definitions derived for application to one region usually are not transferrable to another since meteorological characteristics differ. Human perceptions of these conditions are equally variable. Both of these points must be taken into account in order to identify the characteristics of drought and make comparisons between regions.

Hydrological droughts are related more with the effects of periods of precipitation shortfall on surface or subsurface water supply (i.e., stream flow, reservoir and lake levels, groundwater) rather than precipitation shortfalls (Dracup et al., 1980; Klemes, 1987). Hydrological droughts are usually out of phase or lag the occurrence of meteorological and agricultural droughts. Meteorological droughts result from precipitation deficiencies; agricultural droughts are largely the result of soil moisture deficiencies. More time elapses before precipitation deficiencies show up in components of the hydrological system (e.g., reservoirs, groundwater). As a result, impacts are out of phase with those in other economic sectors. Also, water in hydrological storage systems (e.g., reservoirs, rivers) is often used for multiple and competing purposes (e.g., power generation, flood control, irrigation, recreation), further complicating the sequence and quantification of impacts. Competition for water in these storage systems escalates during drought, and conflicts between water users increase significantly.

The frequency and severity of hydrological drought is often defined on the basis of its influence on river basins. Whipple (1966) defined a drought year as one in which the aggregate runoff is less than the long-term average runoff. Low-flow frequencies have been determined for many streams. If the actual flow for a selected time period falls below a certain threshold, then hydrological drought is considered to be in progress. However, the number of days and the level of probability that must be exceeded to define a hydrological drought period is somewhat arbitrary. These criteria will vary between streams and river basins.

Agricultural drought links various characteristics of meteorological and hydrological drought to agricultural impacts, focusing on precipitation shortages, differences between actual and potential evapotranspiration, soil water deficits, and so forth. A plant's demand for water is dependent on prevailing weather conditions, biological characteristics of the specific plant, its stage of growth, and the physical and biological properties of the soil. An operational definition of agricultural drought should account for the variable susceptibility of crops at different stages of crop development. For example, deficient subsoil moisture in an early growth stage will have little impact on final crop yield if topsoil moisture is sufficient to meet early growth requirements. However, if the deficiency of subsoil moisture continues, a substantial yield loss may result.

Finally, socioeconomic drought associates the supply and demand of some economic good or service with elements of meteorological, hydrological, and agricultural drought. Some scientists suggest that the time and space processes of supply and demand are the two basic processes that should be included in an objective definition of drought (Yevjevich, 1967). For example, the supply of some economic good (e.g., water, hay, electric power) is weather dependent. In most instances, the demand for that good is increasing as a result of increasing population and/or per capita consumption. Therefore, drought could be defined as occurring when the demand exceeds supply as a result of a weather-related supply shortfall (Sandford, 1979). This concept of drought supports the strong symbiosis that exists between drought and human activities. Thus, the incidence of drought could increase because of a change in the frequency of the physical event, a change in societal vulnerability to water shortages, or both. For example, poor land use practices such as overgrazing can decrease animal carrying capacity and increase soil erosion, which exacerbates the impacts of and vulnerability to future droughts. This example is especially relevant in semiarid regions and in areas of hilly or sloping terrain (e.g., Lesotho).

Drought Characteristics and Severity

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 index from normal and is closely linked to duration in the determination of impact. The simplest index in widespread use is the percent of normal precipitation. With this index, actual precipitation is compared to "normal" or average precipitation for time periods ranging from one to twelve or more months. Actual precipitation departures are normally compared to expected or average amounts on a monthly, seasonal, annual, or water year (October-September) time period. One of the principal difficulties with this (or any) index is the choice of the threshold below which the deficiency of precipitation must fall (e.g., 75 percent of normal) to define the onset of drought. Thresholds are usually chosen arbitrarily. In reality, they should be linked to impact. Many indices of drought are in widespread use today, such as the decile approach (Gibbs, 1967; Lee, 1979; Coughlan, 1987) used in Australia, the Palmer Drought Severity Index and Crop Moisture Index (Palmer, 1965; 1968) in the United States, and the Yield Moisture Index (Jose et al., 1991) in the Philippines. For a comparison of several popular meteorological indices, see Olidapo (1985).

Another distinguishing feature of drought is its duration. Droughts usually require a minimum of two to three months to become established but then can continue for several consecutive 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. The five-year (1979-83) drought in northeast Brazil is a good case in point. In this series of years, 1979 and 1980 were both drought years in the classic sense (i.e., a significant deficiency during the principal rainy reason). In 1981, the seasonal rainfall totals were slightly above normal but the temporal distribution resulted in agricultural drought. In 1982, the opposite pattern occurred (meteorological drought) and the results were less adverse for agriculture. These four "drought" years were followed by 1983, the most severe drought year of the previous 25 years (Magalh et al., 1988).

Droughts also differ in terms of their spatial characteristics. The areas affected by severe drought evolve gradually, and regions of maximum intensity shift from season to season. In larger countries, such as Brazil, China, India, the United States, or Australia, drought would rarely, if ever, affect the entire country. During the severe drought of the 1930s in the United States, for example, the area affected by severe drought never exceeded 65 percent of the country (see Figure 1). In India, the droughts of this century have rarely affected more than 50 percent of the country. An exception occurred in 1918-19, when 73 percent of the country was affected (Sinha et al., 1987). On the other hand, it is indeed rare for drought not to exist in a portion of these countries in every year. For example, Figure 1 illustrates that in the United States the percent area affected by drought is usually greater than 10 percent. Thus, the governments of these larger countries are more accustomed to dealing with water shortages and have established an infrastructure to respond, albeit reactively. For smaller countries, it is more likely that the entire country may be affected since droughts are usually regional phenomenon - they result from large-scale anomalies in atmospheric circulation patterns that become established and persist for periods of months, seasons, or longer.

Figure 1. Percent area of the United States experiencing severe/extreme drought, 1895-1991 (Heim, 1991).

From a planning perspective, the spatial characteristics of drought have serious implications. A large-scale regional drought may significantly influence a nation's ability to import food, a potential impact mitigation strategy, from neighboring countries that may be affected equally. Likewise, the occurrence of drought worldwide or in the principal grain exporting nations, such as during the ENSO event of 1982-83, may alter significantly a developing country's access to food from donor governments.

Impacts of Drought

The impacts of drought are diverse and often ripple through the economy. Thus, impacts are often referred to as direct or indirect, or they are assigned an order of propagation (i.e., first-, second-, or third-order) (Kates, 1985). Conceptually speaking, the more removed the impact from the cause, the more complex its link to the cause. In other words, a loss of yield resulting from drought is a direct or first-order impact of drought. However, the consequences of that impact (e.g., loss of income, farm foreclosures, outmigration, government relief programs) are secondary or tertiary impacts. First-order impacts are usually of a biophysical nature while higher-order impacts are usually associated with socioeconomic valuation, adjustment responses, and long-term "change." Asfaw (1989) succinctly summarized drought impacts as "direct or indirect, either singular or cumulative, immediate or delayed."

Because of the number of affected groups and sectors associated with drought, the geographic size of the area affected, and the difficulties connected with quantifying environmental damages and personal hardships, the precise determination of the financial costs of drought is an arduous task. Average annual estimates of the direct losses attributable to drought are misleading. Although some drought-related costs and losses may occur each year in some countries, in most instances they tend to occur in clusters around major single or multiple year events. Therefore, direct and indirect losses may be extremely large for one or two consecutive years and then negligible for several years. This clustering of drought-related costs and losses is repeated over and over again. For example, northeast Brazil experienced a severe drought from 1979 to 1983 that was preceded and followed by a series of favorable or wet years. The same can be said for Kenya, 1983-84 (Downing et al., 1987); Zimbabwe, 1981-84 (Makarau and Marume, 1989); Botswana, 1979-80 (Moremi, 1987); and India, 1980-82 (Sinha et al., 1987) and 1988-89 (Venkateswalu, 1992). The ebb and flow of dry and wet years (and thus the drought-related costs and losses) hinders the preparedness process in all countries. Human nature is to assume that next year will be a "good" year.

The impacts of drought can be classified into three principal sectors: economic, environmental, and social. Table 1 illustrates the principal impacts associated with each of these sectors. The economic impacts of drought are numerous, ranging from direct losses in the broad agricultural and agriculturally related sectors, including forestry and fishing, to losses in recreation, transportation, banking, and energy. Other economic impacts would include added unemployment, increases in food prices and overall disruption of food supply, strain on financial institutions because of farm foreclosures, increased costs of new or supplemental water resource development, and loss of revenue to local, state, and federal government. Environmental losses are the result of damages to plant and animal species, wildlife habitat, and air and water quality; forest and range fires; degradation of landscape quality; and soil erosion. These losses are difficult to quantify, but growing public awareness and concern for environmental quality has forced public officials to focus greater attention on these effects. Increasing levels of environmental regulation (e.g., water quality, preservation of wildlife habitat) have imposed a new layer of constraints on water managers during water-short periods. This trend is likely to continue. Social impacts mainly involve public safety, health, conflicts between water users, inequities in the distribution of impacts and disaster relief programs, loss of life, increased social unrest, depopulation of rural areas, and reduced quality of life.

TABLE 1. Classification of drought-related impacts (modified from Wilhite, 1992b).

Problem Sectors



· loss from crop production

annual and perennial crop losses; damage to crop quality
reduced productivity of cropland (wind erosion, etc.)
insect infestation
plant disease
wildlife damage to crops

· loss from dairy and livestock production

reduced productivity of rangeland
forced reduction of foundation stock
closure/limitation of public lands to grazing
high cost/unavailability of water for livestock
high cost/unavailability of feed for livestock
high livestock mortality rates
increased predation
range fires

· loss from timber production

forest fires
tree disease
insect infestation
impaired productivity of forest land

· loss from fishery production

damage to fish habitat
loss of young fish due to decreased flows

· loss of national economic growth, retardation of economic development
· income loss for farmers and others directly affected
· loss from recreational businesses
· loss to manufacturers and sellers of recreational equipment
· increased energy demand and reduced supply because of drought-related power curtailments
· costs to energy industry and consumers associated with substituting more expensive fuels (oil) for hydroelectric power
· loss to industries directly dependent on agricultural production (e.g., machinery and fertilizer manufacturers, food processors, etc.)
· decline in food production/disrupted food supply

increase in food prices
increased importation of food (higher costs)

· unemployment from drought-related production declines
· strain on financial institutions (foreclosures, greater credit risks, capital shortfalls, etc.)
· revenue losses to federal, state, and local governments (from reduced tax base)
· revenues to water supply firms

revenue shortfalls
windfall profits

· loss from impaired navigability of streams, rivers and canals
· cost of water transport or transfer
· cost of new or supplemental water resource development


· damage to animal species

wildlife habitat
lack of feed and drinking water
increased vulnerability to predation (e.g., from species concentration near water)

· wind and water erosion of soils
· damage to fish species
· damage to plant species
· water quality effects (e.g., salt concentration)
· air quality effects (dust, pollutants)
· visual and landscape quality (dust, vegetative cover, etc.)


· food shortages (decreased nutritional level, malnutrition, famine)
· loss of human life (e.g., food shortages, heat)
· public safety from forest and range fires
· conflicts between water users
· health-related low flow problems (e.g., diminished sewage flows, increased pollutant concentrations, etc.)
· inequity in the distribution of drought impacts/relief
· decreased living conditions in rural areas
· increased poverty
· reduced quality of life
· social unrest, civil strife
· population migration (rural to urban areas)


Drought is, indeed, a complex and poorly understood natural hazard. Impacts are far-reaching and may linger for months or even years beyond the termination of the event. The impacts of drought result from complex interactions between physical and social systems. An understanding of the characteristics of drought and an appreciation of the magnitude of economic, social, and environmental impacts must precede the establishment of a viable assessment and response strategy that will lead to a reduction of societal vulnerability. This has been and continues to be a difficult assignment, but progress is being made. Chapter 2 will document, through case studies, the approach being taken in several drought-prone countries. The experiences of these countries can provide valuable lessons for other governments.