Spatial patterns of multiple drought types in the contiguous United States: a seasonal comparison

The impacts of severe droughts on human activities, terrestrial ecosystems, and hydrologic systems are wide ranging and easily discernible. Because these impacts are felt at different times by water users, various forms of drought are recognized (e.g. agricultural, hydrological). The purpose of this study is to examine spatial patterns of drought in the contiguous United States on a seasonal basis using 4 measures of drought severity. Data used include the Palmer Moisture Anomaly Index (ZINX), Palmer Drought Severity Index (PDSI), Palmer Hydrologic Drought Index (PHDI), and standardized scores of monthly precipitation (PREZ) for the 344 climatic divisions in the contiguous United States, by month, for the period 1931 to 1985. Principal Components Analysis and a VARIMAX rotation are used to search for spatially coherent drought regions, i.e. clusters of climatic divisions that experienced similar moisture conditions during the study period. Regional drought patterns are presented using isoline maps of principal component loadings. Within a given season, the location, size and orientation of most drought reqons is largely consistent across the drought types represented by the PREZ, ZINX, PDSI, and PHDI. Although the fast responding index (ZINX, PREZ) patterns are more discrete (i.e. the drought regions are smaller and more numerous), the general location and orientation of the regions conform to those for the more slowly responding indices (PDSI, PHDI). For the fast responding indices there is a considerable amount of interseasonal variability in the regional drought patterns. The amount of variability decreases for both PDSI and PHDI, with most PHDI drought regions remaining stable throughout the year.