Spatial patterns of drought frequency and duration in the contiguous USA based on multiple drought event definitions

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Abstract
This study examines patterns of drought frequency and duration in the contiguous USA based on multiple definitions of drought events. Patterns are examined using data from the three Palmer indices: monthly moisture anomaly index (ZINX), drought severity index (PDSI), and hydrologic drought index (PHDI). The data span a 94‐year period and are spatially aggregated to the climatic division level. The stimulus for this study is the lack of systematic examination of the possibility of varying spatial patterns of drought frequency and duration among the drought types represented by these indices.Isoline maps of mean drought frequency indicate that the index used to define a drought has a major impact on the spatial patterns. Patterns identified using the monthly moisture anomaly index are nearly inverse of those for the drought severity and hydrologic drought indices. Patterns of drought duration also vary among the indices, although previous findings of greater drought persistence in the interior regions of the USA were upheld by all three indices. Results also indicate that varying the parameters (intensity, minimum duration) of an index‐specific drought event definition has a minor impact only on the spatial patterns.