Overview

The Palmer Drought Index (PDI) and Palmer Hydrological Drought Index (PHDI) were created by Wayne Palmer in the 1960s as a way of integrating water supply (precipitation) with water demand (evapotranspiration as computed from temperature) in a soil moisture model (Palmer, 1965). They are standardized indices where values of zero represent near normal conditions, negative values indicate drought, and positive values indicate wet spell conditions (Heim, 2002). Palmer (1965) developed the indices using monthly climate data, but he noted that they could be computed on a more frequent basis -- weekly or perhaps even daily. Monthly temperature and precipitation data are more consistent, more complete, and extend further back in time than daily data, so the PDI and PHDI have traditionally been computed on a monthly basis. Previous efforts to compute these indices on a more frequent basis (e.g., weekly) have led to results which were inconsistent with the historical record as represented by the monthly indices (Heim, 2005).

The PDI and PHDI shown on this web page represent a way of estimating the monthly PDI and PHDI on an operational weekly basis. If the date of the maps is before the end of the month, then observed temperature and precipitation through the date of the map are combined with 1971-2000 normals of temperature and precipitation for the remainder of the month to create an estimate of the monthly temperature and precipitation values. These estimated monthly temperature and precipitation values are fed into the Palmer model to compute the monthly PDI and PHDI values for the date in question (Heim, 2005).

Beginning September 4, 2021 the temperature and precipitation input for this product changed from CPC's station based data to NCEI's nClimGrid Daily data.

References