The Signature of Hillslope Aspects on the Hydrologic Response of Snow-Covered Catchments

The hydrologic response of Alpine catchments embeds the geomorphologic signature of the dominant drivers influencing snowmelt runoff dynamics. The focus of this work is the role of aspect as the main control on incoming radiation. We investigate the link between hillslope aspects and discharge generation for a study catchment in the Swiss Alps, coupling a physical model for the simulation of snowmelt to an hydrologic response model accounting for geomorphology. Incoming radiation and topographical shading are explicitly accounted for in the computation of the snowpack energy balance at the local scale. The role of aspects is analyzed through virtual experiments where the coupled model is applied to rotations of the reference DEM. The distributed simulation results for each of these virtual experiments show that the hourly discharge during the snowmelt period exhibits a visible geomorphologic signature of aspects at sub-catchment scale, but this progressively fades out going from headwater streams to the outlet. Accordingly, the geomorphologic signature is scale-dependent: it is significant at small scales where the high aspect correlation generates predominant orientations but is lost at larger scales where aspects are de-correlated and different orientations are averaged out. The de-correlation of aspects is in fact a typical feature of Alpine regions, as suggested by the asymptotic flattening of the variograms computed for 4 selected Swiss catchments. The scale-dependence of the geomorphologic signature on snowmelt runoff explains why temperature-index snow models are effective in simulating the dynamics of snow accumulation and melting at catchment scale.