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Climate models project large changes in rainfall, but disagree on their magnitude and sign. The consequences of this uncertainty on optimal dam dimensioning is assessed for a small mountainous catchment in Greece. Optimal dam design is estimated using a Cost-Benefit Analysis (CBA) based on trends in seasonal temperature and precipitations from 19 IPCC-AR4 climate models driven by the the SRES A2 emission scenario. Optimal reservoir volumes are modified by climate change, leading to up to 34% differences between optimal volumes. Contrary to widely-used target-based approaches, the CBA suggests that reduced rainfall should lead to smaller water reservoirs. The resulting change in the Net Present Value (NPV) of water supply is also substantial, ranging from no change to a large 25% loss, depending on the climate model, even assuming optimal adaptation and perfect foresight. In addition, climate change uncertainty can lead to design errors, with a cost ranging from 0.3 to 2.8% of the NPV, depending on site characteristics. This paper proposes to complement the CBA with a robust decision-making approach that focuses on reducing design-error costs. It also suggests that climate change impacts in the water sector may reveal large, that water reservoirs do not always provide a cost-efficient adaptation strategy, and that alternative adaptation strategies based on water conservation and non-conventional water production need to be considered. (Résumé d'auteur

Abstract.Global changes are expected to exacerbate water scarcity issues in the Mediterranean region in the next decades. In this work, we investigate the impacts of reservoirs operation rules based on an economic criterion. We examine whether can they help reduce the costs of water scarcity, and whether they become more relevant under future climatic and socioeconomic conditions.We develop an original hydroeconomic model able to compare water supply and demand on a large scale, while representing 5 river basin heterogeneity.On the supply side, we evaluate the impacts of climate change on water inflows to the reservoirs. On the demand side, we focus on the two main sectors of water use: irrigation and domestic sectors. Demands are projected in terms of both quantity and economic value. Coordinated operating rules of the reservoirs are set up, considering spatial and temporal trade-offs. The objective is the maximisation of water benefits. 10The methodology is applied to Algeria at the 2050 horizon. Our results show that the supply-demand imbalance and its costs will increase in most Algerian basins under future climatic and socioeconomic conditions. Our results suggest that the benefits of operating rules based on economic criteria are not unequivocally increased with global changes. In some basins the positive impact of economic prioritisation is higher in future conditions, but in other basins it is higher in historical conditions. Given its generic nature and low data requirements, the developed framework could be implemented in other regions con-15 cerned with water scarcity, or extended to a global coverage.