A drought index is a numerical indicator that quantifies drought severity based on meteorological, hydrological, or agricultural data. Common indices include the PDSI, SPI, and USDM categories.
Drought indices are quantitative tools used to characterize the severity, duration, and spatial extent of drought conditions by integrating one or more variables such as precipitation, temperature, soil moisture, streamflow, reservoir storage, and snowpack. They transform complex data into a single numerical value that can be compared across space and time, facilitating drought monitoring, early warning, and response planning. The Palmer Drought Severity Index (PDSI) uses temperature and precipitation data in a soil water balance model, with values ranging from -4 (extreme drought) to +4 (extremely wet). The Standardized Precipitation Index (SPI) calculates precipitation anomalies at multiple time scales, making it useful for assessing meteorological, agricultural, and hydrological drought. The U.S. Drought Monitor combines multiple indicators and expert judgment to produce a weekly map classifying conditions from D0 (abnormally dry) through D4 (exceptional drought). Other notable indices include the Standardized Precipitation Evapotranspiration Index (SPEI), which adds an evapotranspiration component, and the Vegetation Health Index (VHI) derived from satellite imagery. No single index captures all aspects of drought, so operational drought monitoring programs typically use a suite of complementary indices.