Elevational dependence of climate change impacts on water resources in an Alpine catchment

Abstract. An increasing interest is directed toward understanding impacts of climate change on water related sectors in a particularly vulnerable area such as the Alpine region. We present a distributed hydrological analysis at scale significant for water management for pristine, present-days, and projected future climate conditions. We used the upper Rhone basin (Switzerland) as a test case for understanding anthropogenic impacts on water resources and flood risk in the Alpine area. The upper Rhone basin includes reservoirs, river diversions and irrigated areas offering the opportunity to study the interaction between climate change effects and hydraulic infrastructures. We downscale climate model realizations using a methodology that partially account for the uncertainty in climate change projections explicitly simulating stochastic variability of precipitation and air temperature. We show how climate change effects on streamflow propagate from high elevation headwater catchments to the river in the major valley. Changes in the natural hydrological regime imposed by the existing hydraulic infrastructure are likely larger than climate change signals expected by the middle of the 21th century in most of the river network. Despite a strong uncertainty induced by stochastic climate variability, we identified an elevational dependence of climate change impacts on streamflow with a severe reduction due to the missing contribution of water from ice melt at high-elevation and a dampened effect downstream. The presence of reservoirs and river diversions tends to decrease the uncertainty in future streamflow predictions that are conversely very large for highly glacierized catchments. Despite uncertainty, reduced ice cover and ice melt are likely to have significant implication for aquatic biodiversity and hydropower production. The impacts can emerge without any additional climate warming. A decrease of August-September discharge and an increase of hourly-daily maximum flows appear as the most robust projected changes for the different parts of the catchment. However, it is unlikely that major changes in total runoff for the entire upper Rhone basin will occur in the next decades.