Snow is an important temporary storehouse for winter precipitation. When it melts, after days, weeks or months, the water fills rivers and lakes, where it may be used to generate electricity. It also replenishes groundwater reserves, which are essential for drinking water supply and agriculture.
With over half its surface area located above 1000 m, Switzerland's topography is dominated by the Alps. Consequently, around a third of its precipitation falls as snow. Depending on the altitude and weather, fresh snow melts as soon as it comes into contact with the warm ground, the next time the sun shines or the following spring. If it falls in a glaciated region, it may not melt for decades. In other words, a large proportion is initially stored where it falls.
Snow as a temporary storehouse
Snow plays an important role as a temporary storehouse. In catchment zones in which a lot of snow falls, runoff quantities peak in May and June, when they reach more than twice the annual average. The seasonal differences are even greater in glacial catchment zones because of the additional ice melt, whereas the runoff cycle in catchment zones at lower altitudes is more evenly balanced throughout the year.
Critical flood situations can arise when high melt rates in the catchment zone combine with heavy rain. For this reason, the SLF runs an operational snow-hydrological service (OSHD), which predicts meltwater volumes. This contributes to the foresighted regulation of water resources. At the same time, snow melt quantities are important for the management of lakes and reservoirs harnessed to generate electricity. Snow melt makes up 70-80% of the total water used at large alpine power plants.
Rain and snowpack
When rain falls on an existing snowpack, the snowpack is often able to absorb some of the water. In case of prolonged and heavy rainfall, however, or if the snowpack is already saturated when the rainfall begins, additional melting can augment runoff. It is difficult to predict precisely what effect the snowpack will have. We are therefore investigating the processes that determine whether the snowpack has an intensifying or a weakening influence on runoff.
Snow in the forest
Snowpack accumulates and melts differently in forests compared with open areas. Tree canopies, for example, can intercept precipitation and absorb radiation. Since forests cover about 30% of Switzerland, they have a big influence on melt water runoff. To better understand snow melt in forests, we are investigating snow accumulation and thermal radiation inside a forest field area.