Historical Evolution of Snowpack Capacity to Buffer Rain-on-Snow Runoff in a Large Columbia River Headwaters Basin

Abstract. Rainfall during the snow season plays an increasingly important role in flood risk as climate warms and extreme events become more frequent. However, a given sized rain-on-snow (ROS) event can yield outcomes ranging from flooding to no runoff, depending partly on the snowpack’s antecedent cold content and capillary retention forces. Here, we analyze the seasonal evolution of the snowpack’s physical state over a 72-year period to assess long-term changes in its capacity to buffer runoff from liquid water input. We use ERA-5 Land data to force a snowpack model that tracks the layer-by-layer development of heat, mass, and structural framework of the snowpack throughout the snow season. We test our approach in a large Columbia River headwaters basin in NW Montana, USA. We evaluate cold content and total capillary retention of the snowpack to determine long term trends in Liquid Water Buffering Capacity (LW_bc) as it evolves throughout the snow season. The LW_bc of the snowpack exhibited robust long-term declines across all elevation bands, despite high intra- and interannual variability. The largest declines occurred during the Spring period, trending downward across the historical period by 43% to 80% depending on the elevation band. The core five weeks of mid-winter showed no trending change of LW_bc, and in fact demonstrated an increase in cold content over the 72 years. Our findings demonstrate that changes in the snowpack’s ability to buffer runoff, including dependencies on local basin factors related to snowpack seasonality and elevation, are a key component of evolving ROS risk.