In the context of municipal water resource management, drought refers to a water shortage resulting from a lack of precipitation. In some circumstances supplies may become insufficient to meet customer demands. Water supplies can be adversely affected by conditions like hot and dry summers, or winters with low snowfall and inadequate snowpack. Droughts can vary greatly in severity, and the duration can be short-term or persist for years.
Droughts are defined by three different stages:
- Meteorological drought is when there is a prolonged period with less than average precipitation. Meteorological drought usually precedes the other kinds of drought.
- Agricultural drought affects crop production or the ecology of the range, usually caused by an extended period of below average precipitation.
- Hydrological drought is when the water reserves available in sources such as aquifers, lakes and reservoirs falls below the statistical average.
Drought conditions and appropriate response to drought is specific to a region and the local conditions. The most definitive source in the United States for measuring drought is the Drought Monitor, run by the National Drought Mitigation Center of the University of Nebraska-Lincoln. In Canada, the most definitive source for measuring drought is the Canadian Drought Monitor, which is run by the Canadian Government. These Drought Monitor maps detail drought intensity throughout the United States, Canada and their territories and they are updated on a weekly basis.
The Alliance for Water Efficiency has also published three resources to help guide drought policy: