Uncertain climate change impact on hydropower production in the Swiss Alps

This study addresses two major challenges in the field of climate change impact analysis on water resources systems: i) incorporation of the largest possible range of potential climate change scenarios and ii) quantification of related modelling uncertainties. The developed methodology of climate change impact modelling is presented and illustrated through an application to a hydropower plant in the Swiss Alps that uses the discharge of a highly glacierized catchment. This case study has been chosen because it represents an area of water resources management highly vulnerable to climate change and because in Switzerland, hydropower represents about 75 % of consumed electricity. The potential climate change impacts are analysed in terms of system performance for the control period (1960 – 1989) and for the future period (2070 – 2099) under a range of climate change scenarios. The system performance is simulated through a set of 4 model types including the production of regional climate change scenarios based on global warming scenarios, the corresponding discharge model, the model of glacier surface evolution and the hydropower management model. The modelling uncertainties inherent to each model type are characterized and quantified. The obtained results show a statistically significant climate change impact on the hydrological regime: It shifts from the current so-called a-glacier regime (maximum monthly discharge in July and August) to a so-called nival type (maximum monthly discharge in June). Additionally, the total annual runoff undergoes a significant reduction due to the increase of temperature and evapotranspiration, decrease of precipitation and decrease of the glaciated catchment area. As a result, the system performance of the hydropower production plant decreases significantly.

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.

EPFL2005

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

Abdelkader Mezghani, André Musy, Bettina Schaefli

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.

2006

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

Bettina Schaefli

EPFL2005

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

Abdelkader Mezghani, André Musy, Bettina Schaefli

2006