A drought is a period of drier-than-normal conditions that results in water-related problems.
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 precipitation in the Northwest is more than 150 inches per year.
When little or no rain falls, soils can dry out and plants can die. When rainfall is less than normal for a period of weeks to years, streamflows decline, water levels in lakes and reservoirs fall, and the depth to water in wells increases. If dry weather persists and water-supply problems develop, the dry period can become a drought.