COMPARING THE PALMER DROUGHT INDEX AND THE STANDARDIZED PRECIPITATION INDEX¹

TLDR

The Palmer Drought Index (PDI) and the Standardized Precipitation Index (SPI) are drought severity indicators, with the PDI used to trigger contingency plans and the SPI designed to quantify precipitation deficits across multiple time scales. This study compares historical time series of the PDI with corresponding SPI series through spectral analysis. Spectral analysis reveals that PDI’s characteristics vary across U.S. sites and exhibit long‑memory complexity, whereas SPI’s characteristics are consistent site‑to‑site and behave as a simple moving‑average process.

Abstract

ABSTRACT: The Palmer Drought Index (PDI) is used as an indicator of drought severity, and a particular index value is often the signal to begin or discontinue elements of a drought contingency plan. The Standardized Precipitation Index (SPI) was recently developed to quantify a precipitation deficit for different time scales. It was designed to be an indicator of drought that recognizes the importance of time scales in the analysis of water availability and water use. This study compares historical time series of the PDI with time series of the corresponding SPI through spectral analysis. Results show that the spectral characteristics 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 SPI is an easily interpreted, simple moving average process.