Palmer Drought Severity Index
The Palmer Drought Severity Index (PDSI, weekly index from CPC shown), devised in 1965, was the first drought indicator to assess moisture status comprehensively. It uses temperature and precipitation data to calculate water supply and demand, incorporates soil moisture, and is considered most effective for unirrigated cropland. It primarily reflects long-term drought and has been used extensively to initiate drought relief. It is more complex than the SPI and the Drought Monitor.
A variety of Monthly PDI values are available from the National Climatic Data Center (NCDC) as well as in the drought.gov Map Viewer. Monthly approximations, updated weekly, are also available from NCDC.