The effect of seasonal soil frost on the alpine groundwater recharge including climate change aspects

In alpine areas, the snow cover plays an important role as a water reservoir. Water is stored as snow over the winter and released in spring, recharging mountain aquifers through infiltration. These aquifers are essential, especially for supplying water for human activities during dry seasons. Numerous studies have shown that locally soil frost can drastically reduce the water infiltration. However, we know much less about the hydrological impact of soil frost at a larger scale, in particular with regard to groundwater recharge. This was our motivation for initiating an extensive field experiment in the southern Swiss Alps. Several methods at different spatial scales were adapted to explore (a) the local effect of a partially frozen ground on the snowmelt discharge in an alpine area, (b) the key processes influencing groundwater recharge during snowmelt periods, such as snow cover and soil frost evolution, snowmelt water runoff types, (c) the large-scale effect of soil frost on the aquifer recharge, and (d) winter situations that are critical with respect to flooding. In order to take into account spatial, altitudinal and climatic differences, the field study was run for two winter seasons at Gd St Bernard (2500 m), a site with very high winter precipitation and strong winds, and at Hannigalp above Grächen (2100 m), a ski resort with a rather dry winter climate. The different components of the soil water balance (lateral runoff, deep percolation, liquid soil water content) were measured on delimited plots of 5 m2. Additionally, a dye tracer experiment visualized the snowmelt infiltration patterns into the frozen, respectively non-frozen ground. At Gd St Bernard, investigations of the liquid water content and the soil temperature were additionally conducted at two secondary sites, differing in their orientation, so as to gain a better insight of the spatial soil frost expansion. Finally, an analysis of the water-table variation at the village of Grächen (some 500 m below Hannigalp) during the snowmelt was carried out for a 10-year time period, including our experimental seasons. We complemented our field investigations by using a numerical soil-snow-atmosphere transfer model that simulates the seasonal development of the soil frost and the snow cover, as well as lateral and vertical water flow in the top soil. This model was used as an upper boundary condition to a 2-D groundwater model, to calculate the outlet flow at the base of a simplified alpine aquifer. The two winters investigated had contrasting meteorological conditions, which resulted in intriguing differences and similarities with respect to the soil physical conditions: the first one having a thick snowpack and hardly any soil frost, and the second one with hardly any snow until January and a deep and persistent soil frost. In spite of a very thick snowpack, the water balance measurements from winter 2000/2001 showed for both sites zero to low surface and subsurface flow due to the fac