Decadal trends in seasonal streamflow and stream temperature across the United States
Abstract. Streamflow influences water supply and hydrologic extremes, whereas stream temperature controls thermal conditions critical for aquatic species. Changes in streamflow and stream temperature have far reaching ecological and socioeconomical implications; thus, it is essential to understand their interconnected seasonal trends and relation to external forcing factors. In this analysis, we (1) documented and compared decadal trends (1980–2020, 1990–2020, and 2000–2020) in seasonal and annual streamflow and stream temperature metrics at sites across the conterminous United States (CONUS), Alaska, Hawaii, and Puerto Rico; (2) assessed land cover, precipitation, air temperature, and baseflow fraction effects on streamflow and stream temperature; and (3) selected three hydrologic regions (Midwest, California-Nevada, and Atlantic Coast) within CONUS as regional case studies to better understand the effects of climate and land-use drivers on streamflow and stream temperature. Trends were evaluated using the following likelihood categories: likely (pt ≤ 0.1), somewhat likely (0.33 ≥ pt > 0.1), uncertain (0.67 ≥ pt > 0.33), and unlikely (0.67 > pt), where pt is the p-value determined from the Mann-Kendall trend test. Most trends were uncertain or unlikely, but some significant changes were found, such as positive correlations between air temperature and rising stream temperatures and between precipitation and increasing streamflow. Streamflow increased across most of CONUS for the 2000–2020 period, but decreases were more prominent for the 1980–2020 and 1990–2020 trend periods across the southern and western CONUS. During the 2000–2020 trend period, seasonal trends in increasing stream temperature were attributed to warmer summer and fall air temperature. Increasing seasonal streamflow was attributed to increases in precipitation in the winter and spring and decreases in baseflow fraction during the spring and summer seasons. We show streamflow trends vary in direction across different trend periods for multiple U.S. regions, which poses challenges for water resources planning and management. Regional results indicated that increasing streamflow in the Midwest region was attributed to increasing precipitation and decreasing baseflow fraction across all seasons. Decreasing streamflows in the California-Nevada region were associated with increasing spring baseflow fraction and increasing annual air temperature. Increasing and decreasing stream temperatures in the Atlantic Coast region were attributed to both increasing and decreasing air temperature, respectively. This study emphasizes the importance of studying streamflow and stream temperature together and demonstrates how comparisons of seasonal and annual metrics provide a more nuanced analysis of streams than annual metrics alone.
