SHORT-TERM TO LONG-TERM TRENDS IN HYDROLOGIC DROUGHT CONDITIONS IN THE CONTIGUOUS UNITED-STATES

Moisture trends are examined for the contiguous United States for recent 15, 30, 50 and 95 yr periods (1895 to 1989) using bivariate linear regression and annual values of the Palmer Hydrologic Drought Index. Linear trend slopes and corresponding p-values from Spearman rank correlations are mapped from a series of regressions to examine the spatial patterns of trends and how they change through time. For each mapped time series there are distinct core regions with strong positive and negative trends. Mean trends for the nation as a whole are positive for the 95, 50, and 30 yr time series, but switch to negative for the most recent 15 yr time series. The perception of climate changing toward either a wetter or drier regime is thus partially a function of temporal scaling.