The Palmer Drought Severity Index (PDSI) has been the most commonly used drought index 
in the United States and was developed to measure intensity, duration, and spatial extent 
of drought. PDSI values are derived from measurements of precipitation, air temperature, 
and local soil moisture, along with prior values of these measures. Values range from 
-6.0 (extreme drought) to +6.0 (extreme wet conditions), and have been standardized to 
facilitate comparisons from region to region.

This drought index has been used to evaluate drought impact on agriculture. 
Because of the time scale built into this index, it is not suitable for the 
determination of longer-term hydrologic drought such as those that impact stream flow, 
reservoirs, and aquifers. The PDSI treats all precipitation as rain, so the index 
does not perform as well in higher elevations in the western U.S. in winter 
where much of the precipitation falls as snow.