Monitoring snowpack outflow volumes and their isotopic composition to better understand streamflow generation during rain-on-snow events

Abstract

Abstract. Rain-on-snow (ROS) events in mountainous catchments can cause enhanced snowmelt, leading to destructive winter floods. However, due to differences in topography and vegetation cover, the generation of snowpack outflow and its contribution to streamflow is spatially and temporally variable during ROS events. In order to adequately predict such flood events with hydrological models, an enhanced process understanding of the contribution of rainwater and snowmelt to stream water is needed. In this study, we monitored and sampled snowpack outflow with fully automated snowmelt lysimeter systems installed at three different locations in a pre-Alpine catchment in Central Switzerland. We measured snowpack outflow volumes during the winters of 2017 and 2018, as well as snowpack outflow isotopic compositions for winter 2017. Snowpack outflow volumes were highly variable in time and space reflecting differences in snow accumulation and melt. In winter 2017, around 815 mm snowpack outflow occurred at our reference site (grassland 1220 m above sea level, m asl), whereas snowpack outflow was 16 % less at the nearby forest site (1185 m asl), and 62 % greater at another grassland site located 200-meter higher (1420 m asl). A detailed analysis of ten ROS events showed that the snowpack outflow volumes could be explained mainly by rainfall volume and initial snow depth. The isotope signal of snowpack outflow was more damped than that of incoming rainfall at all three sites, with the most damped signal at the high-elevation site because its snowpack was the thickest and residence times of liquid water in the snowpack were the longest, thus enhancing isotopic mixing in the snowpack. The contribution of snowpack outflow to streamflow, estimated by isotope-based two-component end member mixing analysis, differed substantially among the three lysimeter sites. Because the study catchment vegetation is a mixture of grassland and forest and altitudes range from 1000 to 1500 m asl, the catchment-average contribution of snowpack outflow to stream discharge is likely to lie between the end member mixing estimates derived from the three site-specific datasets. Thus, our hydrograph separation estimates based on the measurements from the three lysimeter sites provide a range of snowpack outflow contributions to discharge from different parts of the study area. This information may be useful for improving hydrological models in snow-dominated catchments.