Photo Credit: Montana USGS, http://mt.water.usgs.gov/pub/waterissues.htm
Drought originates from a deficiency of precipitation over an extended period of time, usually a season or more. This deficiency results in a water or soil moisture shortage for some activity, group, or environmental sector. Drought is a normal, recurrent feature of climate, although many erroneously consider it a rare and random event. It occurs in virtually all climatic zones, but its characteristics vary significantly from one region to another.
Drought is usually considered relative to some long-term average condition of balance between precipitation and evapo-transpiration perceived as “normal”. Drought is related to the timing (i.e., principal season of occurrence, delays in the start of the rainy season, occurrence of rains in relation to principal crop growth stages) and the effectiveness (i.e., rainfall intensity, number of rainfall events) of the rains. The effects of drought become apparent with a longer duration because more and more moisture-related activities are affected. (The State of Montana Multi-Hazard Mitigation Plan and Statewide Hazard Assessment, Drought Effects October 2004).