The influence of seasonally frozen soil on the snowmelt runoff at two Alpine sites in southern Switzerland

In spring, snowmelt releases huge quantities of meltwater, affecting the hydrology of Alpine areas. Seasonal soil frost influences these water fluxes by locally decreasing the infiltration capacity of the soil, resulting in an increased amount of surface runoff. The main goal of this study was to investigate the spatial variability of the seasonal frost depth and to quantify by how much this seasonal soil frost affects the snowmelt discharge. For this purpose, an extensive field study was run for two winter seasons (2000/2001 and 2001/2002) at Gd St Bernard (2470 m) and Hannigalp (2090 m) in the southern Swiss Alps. The different components of the water balance (lateral runoff, deep percolation, liquid soil water content) were measured on delimited plots of 5 m2. The two winters investigated had opposing weather and soil frost conditions: in the first winter a thick snowpack prevented the formation of soil frost, whereas in the second winter little snow fell until January, which produced a deep and persistent soil frost. We classified the snowmelt events into several classes (mid-winter, late winter, spring and post-spring) and analysed the significance of the different water flow components for each melt situation. While 90-100% of melt water infiltrated into the ground during the first winter, 25-35% of melt water ran off laterally in the second, mainly during late winter and spring snowmelt events. In that second winter, the soil infiltration capacity was primarily reduced by the presence of a basal ice sheet after mid-winter melt events.