When rainfall falls well below normal for an extended period, drought develops. The shortage can last weeks, months, or years. Impacts cascade from dry soil and stressed vegetation to reduced river flows, lowered groundwater, and restrictions on water use.

Meteorological drought is defined by precipitation deficit. Agricultural drought focuses on soil moisture available to crops. Hydrological drought appears later as reservoirs and aquifers decline. Severity depends on both the length of the dry spell and the demand for water in the region.

Monitoring combines rain gauges, satellite soil-moisture estimates, and streamflow data. Early warnings allow farmers and water managers to adjust practices. Some droughts end with a single heavy rain event; others require a prolonged return to wetter patterns.

Historical droughts have reshaped societies and ecosystems across continents.