The Palmer Drought Severity Index (PDSI) is a widely used meteorological drought indicator that uses temperature and precipitation data in a soil water balance model to assess drought severity on a scale from -4 to +4.
The Palmer Drought Severity Index (PDSI) was developed by Wayne Palmer in 1965 and remains one of the most widely used drought indicators in the United States and globally. The PDSI uses monthly temperature and precipitation data along with soil water-holding capacity information in a two-layer soil moisture model to calculate water supply and demand, then compares actual conditions to climatically appropriate conditions. The index produces values typically ranging from -4 (extreme drought) to +4 (extremely wet), with values near zero representing normal conditions. Negative values between -2 and -3 indicate moderate to severe drought, while values below -4 indicate exceptional drought. The PDSI is a standardized measure that allows comparison of drought severity across different regions and climate zones. However, it has limitations: it assumes all precipitation falls as rain, has a fixed temporal lag that makes it slow to respond to changing conditions, and uses a simplified model of evapotranspiration. The Self-Calibrating PDSI (scPDSI) addresses some of these issues by automatically calibrating to local climate conditions. The PDSI has been used to reconstruct historical drought patterns from tree-ring data, providing centuries-long perspectives on drought variability that are essential for understanding current drought risk in the context of natural climate variability.