| Glaciers are important storage components in the hydrological cycle. Their runoff yield is
often anticorrelated to water stress in the low-lands. With the strong recession of Alpine
glaciers expected for the 21st century, the question of water supply security in the summer
months both for small mountainous catchments and for macroscale transboundary watersheds
arises.
The contribution of glaciers to stream-flow runoff from large-scale drainage basins in
Europe is analyzed for the major streams originating in the Alps – Rhine, Rhone, Po and
Danube. Detailed information on glacier storage change is available from monthly mass
balance data for 50 Swiss glaciers for the period 1908-2008. Storage changes are extrapolated
to all glaciers in the European Alps. By comparison of monthly runoff yields from glacierized
surfaces in the summer months and measured runoff at gauges along the entire
length of the streams, the relative portion of glacier melt water for each month is
calculated.
The drainage basin of the Rhone with a size of 100’000 km2 (1% ice-covered)
shows a 26% contribution of glacier storage change to August runoff over the last
century. In the lower Danube (0.06% glacierization) glacier melt water accounted
for 8.5% of observed runoff in September of the extreme year 2003. The relative
importance of glacier contribution to runoff does not scale linearly with the percentage of
glacierization, as high glacier runoff in summer dominates low-land areas with little
precipitation and high evapotranspiration. Thus, glacial melt waters are relevant to the
hydrological regime of macroscale watersheds and do not only have a regional impacts. By
transiently modelling future glacier retreat until 2100 using climate scenarios, a
reduction of glacierized area in the Alps to 12% of the current value is found. In
consequence, summer runoff contribution from currently glacierized basins will decrease,
intensifying issues with water shortage in summer also in poorly glacierized catchments. |