The elements of the water balance, namely precipitation, runoff, evapo-transpiration, and storage change, their interaction and special attributes in the mountains are presented using the example of the European Alps, with particular reference to Switzerland. Strong differentiation in the alpine climate over time and space exerts a significant influence on the water cycle. This chapter therefore discusses each of the elements of the water balance with particular reference to the influence of mountains and their measurement, as well as the spatial differentiation characteristics. The analysis of the water balance is accompanied by a discussion on the attributes and differences at different altitudes and in different climatic regions. Finally, the importance of alpine water resources for water supplies in the adjacent lowlands is examined.

In mountainous regions water often constitutes the dominant - and even in many cases the only - useful natural resource. Therefore, water is used for various purposes, especially for the production of hydropower, for water supplies, irrigation, and artificial snow making. The use of water resources and other human interventions, for example structural control and the correction of water courses, may result in fundamental quantitative changes in the natural water cycle or in the natural runoff characteristics on varying time scales. These interventions also significantly influence a wide range of other physical, chemical, and morphological parameters of rivers and streams.In the future changes in runoff patterns forced by climate change as well as a growing demand for water for various applications in mountainous regions must be assumed. As a consequence, the strain on natural water supplies in alpine regions will be much stronger, especially at the local level. As mountains play an important role as water towers for the lowlands, this may also impact the water supply in the surrounding regions.