Abstract:This study analyses the uncertainty induced by the use of different state-of-the-art climate models on the prediction of climate-change impacts on the runoff regimes of 11 mountainous catchments in the Swiss Alps having current proportions of glacier cover between 0 and 50%. The climate-change scenarios analysed are the result of 19 regional climate model (RCM) runs obtained for the period 2070-2099 based on two different greenhouse-gas emission scenarios (the A2 and B2 scenarios defined by the Intergovernmental Panel on Climate Change) and on three different coupled atmosphere-ocean general circulation models (AOGCMs), namely HadCM3, ECHAM4/OPYC3 and ARPEGE/OPA. The hydrological response of the study catchments to the climate scenarios is simulated through a conceptual reservoir-based precipitation-runoff transformation model called GSM-SOCONT. For the glacierized catchments, the glacier surface corresponding to these future scenarios is updated through a conceptual glacier surface evolution model.The results obtained show that all climate-change scenarios induce, in all catchments, an earlier start of the snowmelt period, leading to a shift of the hydrological regimes and of the maximum monthly discharges. The mean annual runoff decreases significantly in most cases. For the glacierized catchments, the simulated regime modifications are mainly due to an increase of the mean temperature and the corresponding impacts on the snow accumulation and melting processes. The hydrological regime of the catchments located at lower altitudes is more strongly affected by the changes of the seasonal precipitation. For a given emission scenario, the simulated regime modifications of all catchments are highly variable for the different RCM runs. This variability is induced by the driving AOGCM, but also in large part by the inter-RCM variability. The differences between the different RCM runs are so important that the predicted climate-change impacts for the two emission scenarios A2 and B2 are overlapping.

A multireplicate multimodel ensemble of hydrological simulations covering the 1860-2099 period has been produced for the Upper Durance River basin (French Alps). An original quasi-ergodic analysis of variance was applied to quantify uncertainties related to General Circulation Models (GCMs), Statistical Downscaling Models (SDMs) and the internal variability of each GCM/SDM simulation chain. For temperature, GCM uncertainty prevails and SDM uncertainty is nonnegligible. Significant warming and in turn significant changes are predicted for evaporation, snow cover and seasonality of discharges. For precipitation, GCM and SDM uncertainty components are of the same order. A high contribution of the large and smallscale components of internal variability is also obtained, inherited, respectively, from the GCMs and the different replicates of a given SDM. The same applies for annual discharge. The uncertainty in values that could be experienced for any given future period is therefore very high. For both discharge and precipitation, even the sign of future realizations is uncertain at a 90% confidence level. These findings have important implications. Similarly to GCM uncertainty, SDM uncertainty cannot be neglected. The same applies for both components of internal variability. Climate change impact studies based on a single SDM realization are likely to be no more relevant than those based on a single GCM run. They may lead to poor decisions for climate change adaptation.