Historical Palmer Drought Indices


Palmer Z Index: measures short-term drought on a monthly scale.

Palmer Drought Severity Index: attempts to measure the duration and intensity of the long-term drought-inducing circulation patterns. Long-term drought is cumulative, so the intensity of drought during the current month is dependent on the current weather patterns plus the cumulative patterns of previous months. Since weather patterns can change almost literally overnight from a long-term drought pattern to a long-term wet pattern, the PDSI can respond fairly rapidly.

Palmer Modified Drought Index: Operational version of the PDSI. Description available in Heddinghaus and Sabol (1991).

Palmer Hydrological Drought Index: measures hydrological impacts of drought (e.g., reservoir levels, groundwater levels, etc.) which take longer to develop and longer to recover from. This long-term drought index was developed to quantify these hydrological effects, and it responds more slowly to changing conditions than the PDSI.