Hydrologic drought in the contiguous United States, 1900–1989: Spatial patterns and multiple comparison of means

Abstract

A basic question in drought climatology is whether there has been a shift toward drier or wetter conditions within the period of operational climatic records. This question is examined for the contiguous United States by analyzing mean values of the Palmer Hydrologic Drought index over the 90‐year period 1900–1989. Using isoline maps and difference of means tests, moisture conditions in the 1900–44 period are compared to conditions in the 1945–89 period, and multiple comparisons are made among the three discrete climatologically standard 30‐year periods (1900–29; 1930–59; 1960–1989). For each mapped time series there are distinct core regions with above‐ and below‐normal moisture conditions. Comparisons of the 45‐year means reveal significant differences at 39% of the nations' climatic divisions, with clear inverse relationships in the most abnormally wet or dry regions. Comparisons among the 30‐year mean periods show that the largest changes are in the interior, stretching from the northcentral Rocky Mountains into the northern Great Plains. A transition from a wetter to a drier moisture regime occurs between the early and middle 30‐year periods in this region, with drier conditions persisting through the last 30‐year period. The degree of regional heterogeneity in mean moisture conditions, coupled with the changing moisture status among the discrete time periods, suggests that the perception of trends in hydrologic drought conditions is subject to both spatial and temporal scaling.