University of Arizona


What Is Drought?

Drought generally is thought of as a period of time with abnormally low precipitation that leads to a lack of water. But, what represents a lack of water depends on one's perspective, whether it be meteorological, agricultural, hydrological, or ecological. Thus, more explicit definitions of drought tend to be driven by activities of particular interest and the measurements typically used and applied therein. Consideration of the region in which activities occur can help to further clarify the definition of drought.

Most everyone knows drought from a meteorological, or weather, perspective. For example, if an area has not had rain for an extended period of time, people might start to say that the region is experiencing a drought. According to the National Weather Service, a meteorological drought is measured by comparing the amount of precipitation to an average for a period of time. Such a measure can indicate when a drought is starting. Meteorological drought is defined differently from region to region as the normal amount of precipitation and the seasons during which it typically falls for an area often differs from others. For example, drought in Arizona and other parts of the arid and semi-arid Southwest may by driven by a longer period of dryness than for areas in the northeastern United States that receive more precipitation more frequently.

Agricultural drought shares characteristics with meteorological drought, but additionally is measured by soil moisture and reservoir levels, as many growers rely on stored water for irrigation. The National Drought Mitigation Center includes crop damage and yield reductions in their conceptual definition of drought.

Hydrological drought also reflects meteorological drought, but links periods of limited precipitation to impacts on water storage and supply. Interestingly, a meteorological drought in one region can lead to a hydrological drought in another through reduced streamflow. For instance, a lack of snowfall in Utah and western Colorado can lower the amount of runoff that ultimately ends up in Lake Powell and Lake Mead, important reservoirs for Arizona, Nevada, and California. Hydrological drought additionally can be influenced by human water consumption if water use exceeds capture or recharge and leads to substantial declines in storage.

Periods of time with below-average precipitation also can cause ecological drought, when a lack of rain or snow leads to impacts on natural systems. Consequences may include reduced growth or death of vegetation and smaller populations of wildlife.

Different people pay more attention to certain types of drought than to others. For example, farmers typically are concerned about agricultural and hydrological drought, urban planners and water managers about hydrological drought, and ranchers about ecological drought. Consequences of drought can affect many aspects of society, with monetary losses that often are higher than those brought about by other weather disasters.

There are many online resources for more drought information, including those from the U.S. Drought Portal, U.S. Drought Monitor, North American Drought Monitor, and National Weather Service Climate Prediction Center.