Candidate Distributions for Climatological Drought Indices ( SPI and SPEI )

Abstract The Standardized Precipitation Index ( SPI ), a well‐reviewed meteorological drought index recommended by the World Meteorological Organization ( WMO ), and its more recent climatic water balance variant, the Standardized Precipitation‐Evapotranspiration Index ( SPEI ), both rely on selection of a univariate probability distribution to normalize the index, allowing for comparisons across climates. Choice of an improper probability distribution may impart bias to the index values, exaggerating or minimizing drought severity. This study compares a suite of candidate probability distributions for use in SPI and SPEI normalization using the 0.5° × 0.5° gridded Watch Forcing Dataset ( WFD ) at the…