The link is to the NOAA Climate Prediction Center site.

The Drought Severity Index (Palmer 1965) was developed to measure the departure of the moisture supply at specific locations. The index is based on the supply-and-demand concept of the water balance equation, taking into account precipitation and temperature data, as well as the local Available Water Content (AWC) of the soil. The objective of the Palmer Drought Index was to provide measurements of moisture conditions that were standardized so that comparisons using the index could be made between locations and between months.