What causes drought?
A drought is a period of drier-than-normal conditions that results in water-related problems. Precipitation (either rain or snow) falls in uneven patterns across the country. The amount of precipitation at a particular location varies from year to year, but over a period of years, the average amount is fairly constant. In the deserts of the Southwest, the average precipitation is less than 3 inches per year. In contrast, the average yearly precipitation in the Northwest is more than 150 inches.
When no rain or only a very small amount of rain falls, soils can dry out and plants can die. When rainfall is less than normal for several weeks, months, or years, the flow of streams and rivers declines, water levels in lakes and reservoirs fall; also the depth to water in wells increases. If dry weather persists and water-supply problems develop, the dry period can become a drought.
Moreland, J.A., 1993, Drought: USGS Open File Report 93-642
Drought monitoring with VegDRI (PDF, 1.6 MB)