“Drought” is challenging to define. A drought is defined by a shortage of expected precipitation, often, but not always, coupled with other meteorological phenomena like abnormally high temperatures. A certain degree of departure from some climate, soil, or hydrological norm over some period of time must be recorded for an area to be considered “in drought.” Droughts are generally considered to be a departure from “normal” precipitation over some discrete period of time [7].

Practically, the definition of drought depends on a combination of policy, scientific, and public perception considerations and is region-specific. For example, some definitions characterize drought as a period of time with precipitation below a certain threshold. Such a definition would not be appropriate for climates such as that found in Omaha, Nebraska, where long periods of time without any rainfall are common [7].

Droughts may be defined by supply-type triggers. Meteorological droughts are defined by a “degree of dryness” due to changes in the timing, amount, or intensity of precipitation. Other drivers include evaporation and transpiration (the loss of water from vegetation), driven by changes in temperature, wind speed, relative humidity, and cloud cover [8]. Meteorological drought indices can be simple or complicated, short- or long-term [9].

Droughts may also be defined by demand-type triggers [7]. A common example is agricultural drought. Agricultural drought is defined by the impact of meteorological drought conditions on agricultural conditions, such as the effects of decreased precipitation and reduced soil moisture on crop yields and crop vulnerability [8]. Plant water demand depends on a variety of factors, including the biophysical properties of the soil, current weather conditions, the biological condition of the specific plant, and the stage of plant growth. Agricultural drought indices thus typically account for crop vulnerability at different stages during their life cycle [9].

Hydrologic drought is another demand-type trigger. Hydrologic drought relates changes in precipitation (including snowfall) to decreases in subsurface or surface water availability. Hydrologic drought typically lags meteorological and agricultural drought, although the duration of the lag depends on the shape and size of the watershed in question [8]. Hydrologic droughts can be further subdivided into stored water droughts and natural water droughts. Stored water droughts are triggered by deficits in stores of water held in manmade reservoirs, natural lakes, and groundwater aquifers. These storages are only depleted by long, unusually low levels of precipitation. In contrast, natural water droughts—decreases in streamflow—typically occur within a few weeks or months of below-normal rainfall [7].

Some scientists believe that parts of the West are not in drought, per se, but rather that these areas are aridifying—become more arid on a relatively permanent basis [2]. The distinction is important for water resource management regarding whether a return to “normal” precipitation can be expected over the next several generations.

Water scarcity occurs when the demand for water exceeds the supply. Like drought, scarcity can occur over different time scales, different places, and with different intensities. Scarcity is often related to drought but can also be completely independent of it. As an example, the Colorado River basin in the Southwest is considered to be over-allocated even under “normal” conditions, though drought and aridification intensify problems in the basin [10].

[2] J. Overpeck and B. Udall, “Climate change and the aridification of North America,” PNAS, vol. 117, no. 22, pp. 11856–11858, 2021. 

[7] National Drought Policy Commission, “Preparing for Drought in the 21st Century,” U.S. Department of Agriculture, Washington, D.C., 2000.

[8] National Drought Mitigation Center, “Types of Drought,” 2022. [Online]. Available: https://drought.unl.edu/Education/DroughtIn-depth/TypesofDrought.aspx. [Accessed June 30, 2022].

[9] World Meteorological Organization and Global Water Partnership, “Handbook of Drought Indicators and Indices,” Integrated Drought Management Programme (IDMP), Geneva, Switzerland, 2016.

[10] Bureau of Reclamation, “Colorado River Basin Water Supply and Demand Study: Executive Summary,” U.S. Department of Interior, Washington, D.C., 2012.