Drought Mechanisms and Megadrought Regions

Drought scientists conventionally distinguish four interrelated but analytically separate types: meteorological drought, a deficit of precipitation relative to a region's long-term average over a defined period; agricultural drought, insufficient soil moisture to support crop and vegetation growth regardless of the precise rainfall deficit driving it; hydrological drought, below-normal levels in streamflow, reservoirs, lakes, and groundwater, which typically lags meteorological drought by weeks to years because of the time needed for surface and subsurface water storage to respond; and socioeconomic drought, in which water shortage begins to affect the supply and demand of an economic good, tying the physical hazard to its human consequences. This typology, formalized in drought-monitoring literature including the U.S. National Drought Mitigation Center's widely used classification, underlies most operational drought indices, of which the Palmer Drought Severity Index, developed by meteorologist Wayne Palmer in 1965 for the U.S. Weather Bureau, and the Standardized Precipitation Index, introduced by Thomas McKee and colleagues at Colorado State University in 1993, remain among the most widely applied globally, alongside satellite-based soil-moisture and vegetation-health products now maintained by agencies such as NASA and the U.S. Department of Agriculture.