What is Drought?

Operational definitions help define the onset, severity, and end of droughts. No single operational definition of drought works in all circumstances, and this is a big part of why policy makers, resource planners, and others have more trouble recognizing and planning for drought than they do for other natural disasters. In fact, most drought planners now rely on mathematic indices to decide when to start implementing water conservation or drought response measures.

Research by Donald A. Wilhite, director of the National Drought Mitigation Center, and Michael H. Glantz, of the National Center for Atmospheric Research, in the early 1980s uncovered more than 150 published definitions of drought. The definitions reflect differences in regions, needs, and disciplinary approaches. Wilhite and Glantz categorized the definitions in terms of four basic approaches to measuring drought: meteorological, hydrological, agricultural, and socioeconomic. The first three approaches deal with ways to measure drought as a physical phenomenon. The last deals with drought in terms of supply and demand, tracking the effects of water shortfall as it ripples through socioeconomic systems.

Meteorological drought is usually an expression of precipitation’s departure from normal over some period of time. These definitions are usually region-specific, and presumably based on a thorough understanding of regional climatology. The variety of meteorologic definitions from different countries at different times illustrates why it is folly to apply a definition of drought developed in one part of the world to another:

United States (1942): less than 2.5 mm of rainfall in 48 hours
Great Britain (1936): 15 consecutive days with daily precipitation totals of less than .25 mm
Libya (1964): annual rainfall less than 180 mm
India (1960): actual seasonal rainfall deficient by more than twice the mean deviation
Bali (1964): a period of six days without rain

Meteorological measurements are the first indicators of drought.

Agricultural drought occurs when there isn’t enough soil moisture to meet the needs of a particular crop at a particular time. Agricultural drought happens after meteorological drought but before hydrological drought. Agriculture is usually the first economic sector to be affected by drought.

Hydrological drought refers to deficiencies in surface and subsurface water supplies. It is measured as streamflow and as lake, reservoir, and groundwater levels. There is a time lag between lack of rain and less water in streams, rivers, lakes, and reservoirs, so hydrological measurements are not the earliest indicators of drought. When precipitation is reduced or deficient over an extended period of time, this shortage will be reflected in declining surface and subsurface water levels.

Socioeconomic drought occurs when physical water shortage starts to affect people, individually and collectively. Or, in more abstract terms, most socioeconomic definitions of drought associate it with the supply and demand of an economic good.

Further Reading
For a more in-depth look at this subject, see Understanding and Defining Drought.
Dracup, J.A.; K.S. Lee; and E.G. Paulson, Jr. 1980. On the Definition of Droughts. Water Resources Research 16(2):297-302.
Wilhite, D.A.; and M.H. Glantz. 1985. Understanding the drought phenomenon: The role of definitions. Water International 10:111–120.