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Impacts of climate change on the water reservoirs: less snow and ice
Compared with the 1980 to 2009 period, the expected temperature increase until the year 2085 is 3° C ± 1° C. This is bound to have an impact on seasonal hydrological reserves in Switzerland: The increase in temperature will be accompanied by a rise in the snow line. The average area covered by snow is being continually reduced, as is the thickness and duration of the snow cover. Finally, the reserves of snow available for melting are decreasing. Some 40% of the stream flow out of Switzerland during the 1980–2009 period consisted of snow melt. This percentage will fall to about 25% by 2085. This will lead to an increasing proportion of precipitation being free to drain away immediately, particularly in winter. Less than 2% of annual flow is currently derived from the summer glacier ice melt, but on watercourses near glaciers this represents a significant percentage in summer. Because the glaciers – which react only slowly to environmental changes – are too large for current and future climatic conditions they will continue to melt rapidly. This will lead to higher stream flows in the alpine catchments, if only for a relatively short time: up to about 2040 for the larger glaciers (in terms of volume); on smaller glaciers flows are already decreasing again. By 2100 it is likely that only 30% of the current volume of ice will remain, mainly in the Rhone basin.
Bild: NASAOutlook
The effect of climate change on the stream flows described above will have water management consequences. The existing flood protection measures must be reviewed in the Swiss Plateau and Jura. An increased risk of water shortages in the summer may mean a potential for conflict among the different users. As stream flow regimes and some water temperatures will change significantly, the legal provisions in various areas (introduction of cooling water, waste water, lake control regulations, residual water) must be reviewed. The need for additional (multipurpose) reservoirs must be clarified. More frequent and serious low water events and higher winter flows could increasingly affect navigation on the Rhine. David Volken, Federal Office for the Environment; Bruno Schädler and Pascal Blanc, Institute of Geography, University of Berne
Bild: NASAChanging flow regimes, increasing risks for high and low waters
In the near term (until 2035), annual Swiss available water resources will change very little, apart from temporary increases in the stream flows in heavily glaciated regions. In the long term (by 2085) the available water resources will fall slightly, particularly in the Lake Maggiore basin (Rivers Ticino and Toce, minus 10%). However, the seasonal distribution of stream flows (flow regime) will shift almost everywhere in Switzerland. This is to be explained by temperature-related reservoir changes (snow, ice) and by the predicted redistribution of precipitation over the course of the year: More precipitation in liquid form in the winter, much lower precipitation and reduced melt water volumes in summer. By the end of the century glacial and nival based catchments will only be found in isolated areas. The small catchments will increasingly be dominated by midland and meridional regimes. The Swiss Plateau will see the development of a new type of regime called pluvial de transition which will be characterised by a distinct minimum flow in August and two seasonal flow peaks in January and March. In many regions the stream flow is expected to be much higher in winter but lower in summer – except in the regions where glaciation remains. In most of the Swiss Plateau, therefore, the period of potential flooding will move from early summer to the winter season and will sometimes last longer. The frequency of flood events which are average (in the alpine foothills and the Alps) or high (in the Swiss Plateau and Jura) is also likely to increase in many regions. The major rivers, which are fed from numerous smaller catchments, will change accordingly. On the Rhine for example, a second seasonal maximum will develop over time in the winter in addition to the one in early summer. By the end of the century every second summer will be at least as warm as the summer of 2003 and summer precipitation will decrease very strongly (by 20 %). Droughts are likely to occur more often and last longer. In the watercourses of the alpine foothills and the Alps, low water events will move from winter to late summer and will be less pronounced. In the Swiss Plateau regions the flow during low water events will decrease considerably and these periods will be longer. For instance, the low stream flows of the Aare in late summer will gradually fall to levels lower than those currently found in winter.
Bild: NASA