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What is a drought?

The definition of drought varies on the basis of the situation or area for which drought is being defined.  A simple definition of drought is a period of water shortage.  However, a period of water shortage can range from a few days or weeks for some crops to a few years for large reservoirs or ground-water aquifers.  Similarly, the water shortage can range from as little as an inch of precipitation for shallow root crops to as much as several feet of precipitation for water supplies that rely on streamflow or ground water.  Unlike the effects of other weather-related hazards, such as floods, the effect of a drought can develop slowly.

A drought is the result of many natural and human factors that affect the environment.  Matthai (1979, p. 5) stated that

"Among the natural factors are the climate of the area; antecedent conditions as exemplified by the amounts of soil moisture, rain, and snow; the distribution of rain and snow in time and space; water-table levels during the drought; water quality; and soil type.  Human factors include the degree of development of water storage and distribution systems; the number, location, and depths of wells; the patterns of water use and per capita consumption; the legal aspects relating to property rights, project operating rules, water-quality standards, and service contracts; economic considerations; and many more.  Therefore, a definition of a drought must be tailored to the conditions in an area at a given time."

The World Metrological Organization identified five general types of drought (Subrahmanyam, 1967):

1. Meteorologic drought--defined only in terms of precipitation deficiencies, in absolute amounts, for a given period.

2. Climatological drought--defined in terms of precipitation deficiencies, in percentages of normal values.

3. Atmospheric drought--defined not only in terms of precipitation deficiencies but possibly in terms of temperature, humidity, or windspeed.

4. Agricultural drought--defined principally in terms of soil moisture and plant behavior.

5. Hydrologic drought--defined in terms of reduction of streamflow, reduction in lake or reservoir storage, and lowering of ground-water levels.

An additional type of drought, the water-management drought, was added by Matthai (1979) to characterize water-supply shortages caused by the failure of water-management practices or facilities, such as an integrated water-supply system and surface or subsurface storage, to bridge normal or abnormal dry periods and equalize the water supply throughout the year.  In general, drought can be defined as an interval of time, generally in months or years, when the water supply at a given location is consistently short of the expected climatic and hydrologic norm.

A drought can only be described fully by the depiction of its numerous climatic and hydrologic elements.  Therefore, indices have been developed to simplify and present the climatic and hydrologic information and to characterize a drought spatially and temporally as to its intensity, duration, and severity.  Each of the indices has a different description of water shortage.  Some commonly used indices are departure from normal precipitation, departure from normal temperature, the Palmer Drought Severity Index, accumulated departure from normal streamflow, low-streamflow frequency, changes in water storage, ground-water levels and rates of decline, and lake levels.  Redmond (1991) reviewed the desirable properties of indices and stated that

It is important to remember that one index cannot describe everything about the original data.  Indices are intended to serve as approximations to real-world phenomena.  Decisions regarding which information to retain and incorporate in to the index are governed by the way the index is expected to be used.  This point should always be borne in mind when using any summarized quantity."

Source:  Williams-Sether, Tara, Macek-Rowland, K.M., and Emerson, D.G., 1994, Climatic and hydrologic aspects of the 1988-92 drought and the effect on people and resources of North Dakota: North Dakota State Water Commission Water Resources Investigation 29, 57 p.

References

Matthai, H.F., 1979, Hydrologic and human aspects of the 1976-77 drought: U.S. Geological Survey Professional Paper 1130, 84 p.

Redmond, Kelly, 1991, Climate monitoring and indices, in Wilhite, D.A., Wood, D.A., and Kay, P.A., eds., Drought management and planning: International Drought Information Center, Department of Agricultural Meteorology, University of Nebraska-Lincoln, IDIC Technical Report Series 91-1, p. 29-33.

Subrahmanyam, V.P., 1967, Incidence and spread of continental drought: World Meteorological Organization, International Hydrological Decade, Reports on WMO/IHD Projects, no. 2, Geneva, Switzerland.

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