Assessment of climate change impacts on Alpine discharge regimes with climate model uncertainty

The present study analyzes 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 glaciation rates between 0 and 50 %. The analyzed climate change scenarios are the result of 19 Regional Climate Model (RCM) runs obtained for the period 2070 – 2099 based on two different green house 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) called HadCM3, ECHAM4/OPYC3 and ARPEGE/OPA. The hydrological response of the studied 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 obtained results 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 undergoes in most cases a significant decrease. For the glacierized catchments, the simulated regime modifications are mainly due to the increase of the mean temperature and 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 to a large part by the inter-RCM variability. The differences between the different RCM runs is so important that the predicted climate change impacts for the two emission scenarios A2 and B2 are overlapping.

Prediction of climate change impacts on Alpine discharge regimes under A2 and B2 SRES emission scenarios for two future time periods (2020-2049, 2070-2099)

Abdelkader Mezghani, André Musy, Bettina Schaefli

The present work analyzes the climate change impacts on the runoff regimes of mountainous catchments in the Swiss Alps having current glaciation rates between 0 and 50 %. The hydrological response of 11 catchments to a given climate scenario is simulated through a conceptual, reservoir-based precipitation-runoff transformation model called GSM-SOCONT (Schaefli, 2005). For the glacierized catchments, the glacier surface corresponding to this future scenario is updated through a conceptual glacier surface evolution model. The analyzed climate change scenarios were derived from 19 climate experiments obtained within the EU research project PRUDENCE (Christensen et al. 2002). They are the results of 9 state-to-the-art Regional Climate Models (RCMs) driven by three coupled Atmosphere-Ocean General Circulation Models (AOGCMs), respectively HadCM3/HadAM3H, ECHAM4/OPYC3 and ARPEGE. The two first families of climate change scenarios correspond to changes in seasonal temperatures and precipitations simulated for the period 2070-2099 under the two green house gas emission scenarios A2 and B2 defined by the Intergovernmental Panel on Climate Change (12 experiments are available for A2 and 7 for B2). From the 19 PRUDENCE experiments 19 climate changes scenarios were additionally developed for a transient period (2020-2049) corresponding in first approximation to a global warming scenario of +1°C.

2005

Assessment of future snow pack in two Alpine regions

Mathias Thierry Pierre Bavay, Michael Lehning, Sebastian Schlögl

The snow cover in the Alps is heavily affected by climate change. In this study we present an assessment of the future snow water equivalent and snow depth for the Swiss Canton of Graubünden and the Aare catchment in the central Swiss Alps. The data of 48 automatic weather stations in the Aare catchment, respectively 34 automatic weather stations in Graubünden, have been used as the input for Alpine3D, a spatially distributed model for the highresolution simulation of alpine surface processes. These catchments have been modeled at 200 m horizontal resolution for a 13 years reference period (1999-2012). Three different emission scenarios for temperature and precipitation (A2, A1B and RCP3PD) and for three different time periods (2020-49, 2045-74, 2070-99) have been taken from the Swiss Climate Change scenarios CH2011 Plus. By applying simple daily shifts of temperature and precipitation to the measured time series, input for the smallscale climate scenarios has been generated. The model results show a decrease smaller than 20 % in the snow depth for the first time period 2020-49 for all three emission scenarios. For the time 2070-99 however, the relative decrease in snow depth is as high as 70 % for the A2 scenario. The most sensible elevation zone for climate change is located around 800–1200 m, where the simulations show almost no snow for 2070-99. The winter season starts, depending on the emission scenario and the elevation zone, 2-4 weeks later and ends 5-8 weeks earlier. Towards the end of the century the snow cover changes will roughly be equivalent to an elevation shift of 500-700 m or 600-900 m for A1B or A2, respectively. The number of snow days will be halved at an elevation of around 1500 m and is predicted to 0-2 snow days in the Swiss Plateau and the low Rhine valley in Graubünden. The A2 scenario projects a catchment-wide snow water equivalent reduction of up to two thirds towards the end of the century. These changes in snow duration and snow amount will affect changes in the melt water runoff, which has important consequences for winter tourism and hydropower generation.

2013

Quantification of modelling uncertainties in climate change impact studies on water resources

Bettina Schaefli

This PhD thesis presents the development of a methodological framework to analyse potential climate change impacts on a high mountainous water resources system and to quantify the associated modelling uncertainties. The main objective is to show whether state-of-the-art hydrological modelling techniques driven by currently available climate change scenarios enable a prediction of the long-term evolution of the analysed system. The case study is a highly glacierized catchment feeding a hydropower plant located in the Swiss Alps. The climate change impact analysis is based on a classical simulation approach: The system behaviour is modelled for an observed control period (1961 to 1990) and for a future period (2070 to 2099) characterised by a modified (predicted) climate. The climate change impact on the studied system is assessed through the comparison of some key characteristics of the system for the two periods (e.g. the mean annual discharge or the hydropower production). The system simulation is completed through a set of four models, a water management model, a hydrological discharge model, a glacier surface evolution model and a model for the production of local scale meteorological time series (precipitation and temperature) based on global and regional climate model outputs. The local scale models have been specifically developed for the purposes of this thesis. For each of them, an appropriate statistical method for the quantification of the inherent modelling uncertainties has been developed. A special emphasis is given to the modelling uncertainties induced by the conceptual hydrological model. A method has been developed to quantify the statistical and the multi-objective modelling uncertainty in a multi-model framework including several equivalent model structures. This method has been specially designed for the quantification of the prediction uncertainty in climate change impact studies but it is transposable to other hydrological modelling contexts. The overall prediction uncertainty and the contribution of each source of modelling uncertainty is quantified through Monte Carlo simulations of the system behaviour combining successively the different sources of modelling uncertainty. It is shown that the uncertainties induced by the prediction of the climate evolution are much higher than the ones induced by the local scale models of the system behaviour. The uncertainty related to the use of different regional climate models is however nearly as important as the one due to the choice of the underlying global climate model and the green house gas emission scenario. Using a fixed hydrological model structure, the predicted climate evolution induces a significant reduction of the hydropower production performance due to a considerable hydrological regime modification. The available data, the current discharge modelling techniques and the knowledge about the underlying processes are however not sufficient to chose an objectively best model structure. Considering the multi-model approach (including different hydrological model structures), an unambiguous prediction of the hydrological reaction to the analysed climate change reveals impossible at the given temporal horizon.