Contribution of rain, snow- and icemelt in the upper Danube discharge today and in the future

Abstract

The hydrological model of the Danubia decision support system allows the sources of runoff to be determined according to glaciermelt, snowmelt and rain at any location of the Upper Danube river network. The analysis shows for the past decade 1991-2000 that in glacierized head watersheds (e.g., Vent gauging station, 35% glacier area) there is about an equal amount of runoff originating from icemelt, snowmelt and rain (about 33% each). Further downstream the portion of icemelt decreases sharply even under present-day conditions with the effect that ultimately 2% of annual runoff is of glacial origin in Passau/Achleiten (basin area of about 77,000 km2 , current glaciation 0.5%), while about 6 originates from rain and 4 from snowmelt. This latter fraction is about twice as high as found for subbasins lying exclusively in the lowlands with no connection to the alpine region. Using the regionally adapted Remo scenario data based on the A1B emission scenario of IPCC, the future development of runoff sources is calculated, taking into account the dynamics of glacierized area reduction using the SURGES glacier model (Subscale Regional Glacier Extension Simulator). In about 30 years, the mean icemelt fraction in the glaciated head watersheds will be less than half of the one observed in the decade from 1991 to 2000. The proportion of snowmelt will be about the same, and rain contribution will increase by 50% to about half of annual specific runoff. After the confluence of the Inn River with the Danube at Passau, the portion from icemelt will be negligible, and 80% of runoff will be from rain and 20% from snowmelt after the year 2030. With the anticipated warming over the whole year and the drying out of the summer season the Alps’ capacity to export water will diminish, and water availability will be reduced, mainly through the loss of summer precipitation and increased evaporation, and not so much due to the loss of glaciermelt.