Snow cover runoff and stream discharge modelling during snow melt in Alpine terrain

The understanding of the role of snow cover runoff in complex terrain for the hydrological cycle is still limited. Water flow in snow is a complex process, because the strong layering of the snow cover causes strong vertical variation in hydraulic properties. It has already been shown that describing melt water flow through a snow cover using Richards equation for 1D unsaturated flow and taking into account the snow stratigraphy, improves snow cover runoff estimations locally. However, the small-scale spatial variability in snow cover height, snow stratigraphy and external influences such as incoming solar radiation and wind speed, is causing a complex relation between local snow melt and overall streamflow discharge. In this study, an advanced physically based snow cover model (SNOWPACK) is used in a spatially explicit mode for alpine terrain. The aim is to investigate whether the use of Richards equation in a distributed snowpack model (Alpine3D) can improve spatially explicit snow cover runoff estimations. The model setup simulates the snow cover development and runoff over a snow season for the Dischma catchment in Switzerland. The snow cover runoff is used as input for a stream discharge model and the modelled discharge is then compared to measured discharge at the catchment outlet. Solving Richards equation for snow yields better agreement than simpler (bucket) methods for liquid water flow in snow. It is also shown that the simulated snow cover runoff exhibits a strong spatial variability, which is a function of slope exposition and angle. This can be associated with different shortwave radiation input for snow melt. The results show that solving Richards equation for snow improves the estimation of the contribution of snow cover runoff to the hydrological cycle.