Using nonparametric Mann-Kendall tests, we assessed long-term (1953-2012) trends in streamflow
and precipitation in Northern California and Southern Oregon at 26 sites regulated by dams and 41 “unregulated” sites. Few (9%) sites had significant decreasing trends in annual precipitation, but September precipitation declined at 70% of sites. Site characteristics such as runoff type (groundwater, snow, or rain) and dam regulation influenced streamflow trends. Decreasing streamflow trends outnumbered increasing trends for most months except at regulated sites for May-September. Summer (July-September) streamflow declined at many sites, including 73% of unregulated sites in September. Applying a LOESS regression model of antecedent precipitation vs. average monthly streamflow, we evaluated the underlying streamflow trend caused by factors other than precipitation. Decreasing trends in precipitation-adjusted streamflow substantially outnumbered increasing trends for most months. As with streamflow, groundwater-dominated sites had a greater percent of declining trends in precipitation-adjusted streamflow than other runoff types. The most pristine surface-runoff dominated watersheds within the study area showed no decreases in precipitation-adjusted streamflow during the summer months. These results suggest that streamflow decreases at other sites were likely due to more increased human withdrawals and vegetation changes than to climate factors other than precipitation quantity