PDSI

The Palmer Drought Severity Index (PDSI) is a meteorological drought index developed by Wayne Palmer in 1965. It is designed to quantify drought intensity by balancing moisture supply and moisture demand, using monthly precipitation and temperature data to estimate potential evapotranspiration and a simplified soil-moisture accounting system. The index provides a standardized measure of dryness or wetsness relative to a climate normal, with values traditionally spanning roughly from -4 (extremely dry) to +4 (extremely wet).

Calculation of PDSI involves a recursive moisture balance that combines a moisture supply term with a moisture demand term. Precipitation contributes to replenishing soil moisture, while temperature influences potential evapotranspiration and thus the rate of moisture loss. The resulting deficits or surpluses propagate through time, producing a single index value for each month that reflects the historical context of prior moisture conditions. Because of its design, PDSI emphasizes longer-term drought patterns rather than short, transient rain shortages, and it is often used with standardized interpretations of severity.

Applications and interpretation: PDSI is widely used in drought monitoring and climate research, particularly in the United States. It is related to other Palmer indices such as the Palmer Z-Index and the Palmer Hydrological Drought Index (PHDI), and it is frequently reported alongside regional drought outlooks. Negative values indicate dryness, with commonly used rough thresholds placing mild to moderate drought in the negative range and severe to extreme drought at more negative values; positive values indicate wet conditions.

Strengths and limitations: PDSI captures persistent moisture anomalies and integrates multi-month memory, making it useful for historical drought comparisons. Limitations include sensitivity to data quality and the baseline period, reliance on PET estimation, and reduced applicability in snow-dominated or complex terrains. Variants such as the Self-Calibrating PDSI (SC-PDSI) have been developed to address some calibration and climate-change-related issues.