What actually counts as a drought?

Drought is a deficit relative to what is normal for a region, not an absolute amount of rainfall, which is why a wet-by-Phoenix-standards year could be a severe drought in Seattle. Climatologists distinguish several kinds. Meteorological drought is a precipitation shortfall over a period. Agricultural drought describes soil moisture insufficient for crops, which can occur quickly if rain stops during a critical growth stage even after a wet winter. Hydrological drought refers to depleted reservoirs, streamflow, and groundwater, which lags precipitation by months or years. Socioeconomic drought describes water supply failing to meet demand, which depends as much on consumption as on rainfall.

Drought differs from other natural hazards in developing slowly and ending ambiguously. There is no moment of impact and often no clear end, which is part of why it receives less attention than storms despite causing enormous cumulative economic losses. In the United States, conditions are tracked and published weekly in the US Drought Monitor, which integrates precipitation, soil moisture, streamflow, and reservoir data into categories from abnormally dry through exceptional drought, and those designations trigger agricultural assistance and water restrictions.

Two complicating factors have grown more prominent. Higher temperatures intensify drought independently of rainfall by increasing evaporation from soil and reservoirs and demand from plants, producing what researchers describe as hot drought, which is why the same precipitation deficit is more damaging now than it was decades ago. And groundwater depletion means that in many agricultural regions, including parts of the American West and the Ogallala Aquifer region, water is being extracted faster than it recharges, which functions as a slow-motion drought that persists regardless of any single year's rain. Recovery from a multi-year drought typically requires several wet seasons rather than one, since a single heavy rainfall year can run off rather than recharge depleted aquifers.