COMPARING THE PALMER DROUGHT INDEX AND THE STANDARDIZED PRECIPITATIONINDEX

The Palmer Drought Index (PDI) is used as an indicator of drought seve rity, and a particular index value is often the signal to begin or dis continue elements of a drought contingency plan. The Standardized Prec ipitation Index (SPI) was recently developed to quantify a precipitati on deficit for different time scales. It was designed to be an indicat or of drought that recognizes the importance of time scales in the ana lysis of water availability and water use. This study compares histori cal time series of the PDI with time series of the corresponding SPI t hrough spectral analysis. Results show that the spectral characteristi cs of the PDI vary from site to site throughout the U.S., while those of the SPI do not vary from site to site. They also show that the PDI has a complex structure with an exceptionally long memory, while the S PI is an easily interpreted, simple moving average process.