Mean runoff defines the amount of water discharged on average during a year or season. It naturally depends on the precipitation that has fallen in the catchment and how much is lost from it through evaporation or long-term storage (e.g. in glaciers). It can also be affected by human activity such as water management intervention (e.g. irrigation, diversions, retention in reservoirs or lake regulation). Runoff and its seasonal distribution are important parameters for water ecology and management (hydropower, drinking water use, irrigation, navigation). Long-term discharge time series indicate how the water balance is changing due to these influences.
Individual annual flows are subject to very wide variations. No clear trends are apparent from a study of the 20th century as a whole for either the Rhein-Basel or the Rhône-Porte du Scex. If the two halves of the year are compared, it is apparent that mean flow tends to rise in winter and fall in summer.
Runoff reacts sensitively to climate change: The rise in air temperatures observed since the 19th century promotes glacier melt and increases evaporation. Winter flows increase because in an increasingly warm climate more precipitation falls as rain in the winter and less is temporarily stored as snow or ice. This means that some of the water which fills the reservoirs in summer as the snow and ice melt is lost, and summer flows tend to decline. The impact of this on nature, water quality and water management can be both positive (e.g. higher hydropower production in winter) and negative (e.g. drought and water shortages in summer). This makes an evaluation difficult.
Projections of the regional flow and its seasonal variations for the period 2071–2100 are computed in the EEA European Indicator System. An international comparison is not appropriate because modelled values rather than measured data are used internationally for the indicator.
The flow for the two major river basins studied, the Rhine and the Rhone, is based on data measured at federal flow monitoring stations. The mean values for anual flows, summer flows (April to September) and winter flows (October to March) are shown.