Winter Rainfall Buffers Streamflow Sensitivity to Snow Losses in the Salt River Watershed in Semi-Arid Southwest US

In continental interior watersheds in the western United States, significant declines in snowpack have not consistently led to declines in streamflow, potentially due to the strong influence of precipitation variability. The Salt River watershed in the semi-arid southwestern United States is a promising location to explore the combined influences of warming temperatures and highly variable precipitation on streamflow. We examined trends in temperature, precipitation, snow, and streamflow in the Salt River basin and its gauged tributaries in central Arizona and evaluated whether shifts from snow to rain have changed streamflow patterns. To separate rainfall and snowmelt inputs at the watershed scale, we developed a gridded model that separates snow and rain inputs and simulates snow accumulation and melt using temperature, precipitation, and relative humidity. Results show that in the last 50 years, temperatures increased significantly in all sub-watersheds and led to simulated declines in the fraction of precipitation falling as snow and increases in the fraction falling as rain in most sub-watersheds. While annual streamflow levels also declined, these declines were primarily associated with variability in annual precipitation. After adjusting for precipitation, annual streamflow did not decline over this period and may have increased in several watersheds. This increase may have been caused by greater winter streamflow, as streamflow magnitudes and runoff ratios generated in winter precipitation events were significantly higher than those for monsoonal summer events. Of the top 10% of events that generated streamflow, the majority occurred in winter months across 8 of the 9 sub-watersheds. 20-40% of these high streamflow events were associated with atmospheric rivers, which can lead to heavy winter rainfall, rain on snow, and snowfall over the Salt River basin. We are exploring the extent to which precipitation-adjusted streamflow increases are related to higher streamflow generation in winter months when warmer temperatures potentially generate more streamflow from more frequent melt, rain on snow, or winter rainfall. These findings imply that snow losses from climate warming may be somewhat ameliorated by increased winter streamflow due to warmer temperatures in winter months.