Palmer Drought Severity Index (PDSI)

The Palmer Drought Severity Index (PDSI), developed by Wayne Palmer in 1965, was the first comprehensive water-balance drought index and remains a benchmark in drought monitoring. Rather than tracking precipitation alone, the PDSI runs a two-layer soil-moisture accounting that balances precipitation against evapotranspiration, runoff, and recharge to gauge whether the moisture supply is abnormally short for the prevailing conditions. It compares actual precipitation to the 'climatically appropriate for existing conditions' (CAFEC) precipitation, converts the departure into a standardized moisture anomaly, and accumulates it over time so that the index reflects the persistence and severity of drought, typically on a scale from about −4 (extreme drought) to +4 (extreme wetness). Because Palmer's original empirical constants were calibrated to particular U.S. regions and limited its spatial comparability, Wells, Goddard, and Hayes introduced the self-calibrating PDSI (sc-PDSI) in 2004, which derives those constants from local data and makes the index far more consistent across climates.