The influence of snow cover on runoff regime and its change

Abstract

The seasonal snow cover has a special significance in shaping the local runoff regime in the Upper Danube basin, since over 8 % of the area is located at elevations above 2,000 m a.s.l., where the snow cover remains until early summer. It temporarily retains the solid portion of precipitation and successively releases it with snow melt. A causal allocation of total discharge into rain, snow melt and ice melt is needed to quantitatively analyse the influence of snow storage in subsections of the drainage network. The contribution of snow melt to discharge can only be approximated by defining an indicator variable. The maps illustrate the indicator for the amount of snow melt in discharge for the decade 1991-2000 based on measurement data and for the decade 2031-2040 calculated with the REMO regional Baseline climate scenario. The index is equivalent to the mean annual percentage of snow melt in discharge at a relative accuracy of around 10 %. In the lowlands, the runoff regime is primarily determined by rainfall; the snow storage has maximum significance in the inner alpine valleys. The maps were enhanced with pie charts for 18 gauges, which are separated into sections according to the amount of rain, snow or ice runoff. They reveal that in high elevation glaciated basins, the discharge source is just about equally represented by snow melt, ice melt and rain. In the future, discharge will be determined more by rain than by snow, especially at the northern Alpine fringe and in the large river valleys.