North Dakota Water Science Center
The beginning of a drought is difficult to determine. Several weeks, months, or even years may pass before people know that a drought is occurring, and dry periods can last for 10 years or more. During the 1930's, most of the United States was much drier than normal. In California, the drought extended from 1928 to 1937, and in Missouri, the drought extended from 1930 to 1941. The extended dry period produced the "Dust Bowl" of the 1930's when dust storms destroyed crops and farms.
The first evidence of drought usually is seen in records of rainfall. Within a short period of time, the amount of moisture in soils can begin to decrease. The effects of a drought on flow in streams and rivers or on water levels in lakes and reservoirs may not be noticed for several weeks or months. Water levels in wells may not reflect a shortage of rainfall for a year or more after a drought begins.
Source: Moreland, Joe A., 1993, Drought: U.S. Geological Survey Open File Report 93-642, 2 p.