Drought onset and recovery over the United States

This study employs precipitation (P) and ensemble soil moisture data sets from 1916 to 2007 to study the drought onset and demise over the United States. Both meteorological drought, classified using the standardized precipitation indices (SPIs), and agricultural drought, classified using the soil moisture percentiles, are studied. Drought onset is more predictable than drought demise. It takes 5-8 months for a region to accumulate enough P deficits to begin a drought, whereas a drought demise can come within one month to one season. A few strong rainfall episodes can end drought. The preferred season for the meteorological drought onset is at the beginning of the rainy season. Even though drought has a preferred season to begin, the ratio between the onset occurring in that season and the total drought events is less than 45%. For agricultural drought, the ratio is 60% or higher for the northwestern interior states and the Southwest. Over the Southern Plains, a cold El Nino-Southern Oscillation (ENSO) event occurs one season before the onset of drought. No drought events occur during the warm ENSO years. Therefore, the occurrence of a cold ENSO event can serve as an early warning for drought. For other areas influenced by ENSO, such as the lower Mississippi basin, the Ohio Valley, the Pacific Northwest, the upper Missouri basin, and the Southeast, most drought events occur during ENSO. The one-to-one correspondence between ENSO and drought is not good. For these areas, ENSO can only serve as a signal for intensive drought watch.