Monitoring snowpack outflow volumes and their isotopic composition to better understand streamflow generation during rain-on-snow events
Rain-on-snow (ROS) events in mountainous catchments can cause enhanced snowmelt, leading to an increased risk of destructive winter floods. However, due to differences in topography and forest cover, the generation of snowpack outflow volumes and their contribution to streamflow are 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 elevations 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 in 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 of snowpack outflow occurred at our reference site (grassland 1220 m a.s.l. – metres above sea level), whereas snowpack outflow was 16 % less at the nearby forest site (1185 m a.s.l.), and 62 % greater at another grassland site located 200 m higher (1420 m a.s.l.). A detailed analysis of 10 ROS events showed that the differences in snowpack outflow volumes could be explained mainly by rainfall volumes and initial snow depths. The isotope signals of snowpack outflow were more damped than those of incoming rainwater at all three sites, with the most damped signal at the highest elevation site because its snowpack was the thickest and the residence times of liquid water in its snowpack were the longest, thus enhancing isotopic mixing in the snowpack. The contribution of snowpack outflow to streamflow, estimated with an isotope-based two-component end-member mixing model, differed substantially among the three lysimeter sites (i.e. between 7±4 and 91±21 %). Because the vegetation in our study catchment is a mixture of grassland and forest, with elevations ranging from 1000 to 1500 m a.s.l., our site-specific hydrograph separation estimates can only provide a range of snowpack outflow contributions to discharge from different parts of the study area. Thus, 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 data sets. This information may be useful for improving hydrological models in snow-dominated catchments.