Evidence for Increasingly Variable Drought Conditions in the United States Since 1895
Annual and summertime trends towards increasingly variable values of the Palmer Drought Severity Index (PDSI) over a sub-decadal period (five years) were investigated within the contiguous United States between 1895 and the present. For the contiguous U.S. as a whole, there is a significant increasing trend in the five-year running minimum-maximum ranges for the annual PDSI (aPDSI5yr(min| max range)). During this time frame, the average aPDSI5yr(min|max range) has increased by about one full unit, indicating a substantial increase is drought variability over short time scales across the United States. The end members of the running aPDSI5yr(min|max range) highlight even more rapid changes in the drought index variability within the past 120 years. This increasing variability in the aPDSI5yr(min|max range) is driven primarily by changes taking place in the Pacific and Atlantic Ocean coastal climate regions, climate regions which collectively comprise one-third the area of the contiguous U.S. Overall, interannual drought patterns are becoming more extreme and difficult to predict, posing a challenge to agricultural and other water-resource related planning efforts.