Just as one balances the household budget with income, savings and expenditures, the hydrological budget is a balance of gains and losses: precipitation minus evaporation minus changes in storage (snow, glaciers, lakes, groundwater) = discharge.
The water flowing in the rivers and streams of Switzerland is a renewable, available resource essential for human life and well being. Because so many of the rivers that flow through other European countries originate in Switzerland, Switzerland has a great responsibility as the “water tower of Europe”. Thanks to extensive measurements of precipitation and runoff, as well as of the stored snow and ice masses, the water balance of Switzerland is well documented. We know, for example, that 40% of Swiss discharge comes from snowmelt, and barely 2% from the melting of glaciers.
In contrast to runoff, evaporation of water is usually not measured directly but is derived from the water balance (evaporation=precipitation minus runoff minus reservoir changes; Spreafico & Weingartner 2005). Evaporation takes place at different locations: from water surfaces, from soil and from plant pores (transpiration). The total evaporation is called “evapotranspiration”. Evapotranspiration depends on the air temperature and the amount of water present in the soil. Higher air temperatures result in an increase in the maximum possible (potential) evaporation. This means that for an increase of actual (real) evaporation, there must first be enough water present in the soil.
Evapotranspiration is not the same throughout Switzerland: because evaporation through plant pores contributes significantly to the total evapotranspiration, vegetation density is important. As one moves from the agricultural fields and forests of the Central Plateau up through the meadows and forests of the pre-Alps to the talus material and glaciers of the Alps, vegetation density decreases with elevation. This is due to both decreasing temperature and decreasing land use intensity. Thus, the potential evaporation from plants also decreases with elevation.
Especially in summer, evaporation in the Alps leads to a “recycling” of precipitation. Up to two thirds of the evaporated water forms new thunderstorm clouds during its ascent, which then rains down again regionally (van der Ent et al. 2010).