Types of Drought

A drought is a prolonged shortage of water relative to the normal conditions and needs of an area. Meteorological, agricultural and hydrological droughts describe this shortage at different points in the water cycle: rainfall, moisture available to crops, and water stored in rivers, reservoirs and groundwater.

Meteorological drought: a rainfall deficit

Meteorological drought occurs when rainfall is substantially below the normal level for a particular area over a specified period. Its assessment depends on the local climate and season, rather than on a single rainfall amount applicable everywhere.

• The principal indicator is the departure of actual rainfall from normal rainfall for the period being assessed.
• The timing and distribution of rainfall matter because prolonged dry spells can occur even when seasonal rainfall totals conceal the shortage.
• Rainfall monitoring and weather-based early warning help identify the developing hazard.

Agricultural drought: insufficient moisture for crops

Agricultural drought occurs when available soil moisture is insufficient to meet crop water requirements. It connects the rainfall shortage with its effects on farming, rather than measuring rainfall alone.

• The key concern is root-zone soil moisture relative to the needs of the crop at its particular growth stage.
• Its severity depends on rainfall distribution, soil water-holding capacity, evaporation, crop characteristics and access to irrigation.
• A dry spell during a critical growth stage can damage crops even when the seasonal rainfall deficit is not large.
• Soil-moisture conservation, suitable crop choices and protective irrigation can reduce agricultural losses.

Hydrological drought: depleted water resources

Hydrological drought occurs when surface-water and groundwater availability falls below normal levels. It concerns the water resources that support drinking-water supply, irrigation and other uses.

• Indicators include streamflow, reservoir storage and groundwater levels, rather than rainfall alone.
• It generally develops more slowly than meteorological drought because rivers, reservoirs and aquifers respond to accumulated deficits.
• It can persist after rainfall improves because depleted water stores take time to recover.
• Water budgeting, demand management and protection of recharge areas help reduce vulnerability.

How the three types are related

The three types are connected but are not interchangeable. A rainfall deficit may first reduce soil moisture and later diminish streamflow, reservoir storage and groundwater recharge.

• The progression is not automatic: irrigation and stored water can buffer crops against deficient rainfall.
• Drought preparedness therefore requires separate monitoring of rainfall, crop and soil-moisture conditions, and water-resource availability.