Spatial Variability of Long‐Term Streamflow Trends in the Southeastern United States

ABSTRACT We assessed long‐term discharge trends for 189 streams and rivers from the Mountain, Piedmont, Southeastern Plains, and Coast ecoregions of Georgia, South Carolina, and North Carolina, U.S.A. Trends over time of average annual discharge volumes (gages with 50+ years of data), average annual groundwater levels (gages with 30+ years of data; n = 143), total annual precipitation ( n = 275 stations), average annual temperature maximums ( n = 207 stations), and average annual temperature minimums ( n = 204 stations) (all with 50+ years of data) were statistically assessed using Mann–Kendall analyses. Decreasing trends were observed at 72% of sites, and 22% of those sites had significant decreases in streamflow. Patterns of streamflow were significantly associated with ecoregion ( X 2 = 34.3, df = 6, p < 0.01) and stream size ( F 3 , 196 = 9.1, p < 0.01). Using contingency tests, we assessed the relationships between discharge and factors that might be influencing changes in discharge (including groundwater, precipitation, and temperature). The Mountain ecoregion showed unchanging streamflow conditions that were somewhat associated with precipitation patterns. In contrast, the Piedmont, Southeastern Plains, and Coast ecoregions showed drying streamflow conditions that were associated with a combination of groundwater and/or climate factors, depending on the area. In the southeastern U.S., streamflows are broadly declining, but changes are spatially complex, likely from a range of causative factors, and impacts are ecoregion specific.