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Groundwater is the most important drinking water source in Switzerland with 40% originating from pumping wells in alluvial aquifers and another 40% from springs. Since water resources planning is carried out on a long time horizon, it is important to know how aquifers might respond to climate change and what the implications for groundwater availability are. Climate change could affect both direct recharge by rainwater/melt water infiltration and indirect recharge via surface water bodies. The period of low or no direct recharge may become longer due to drier and warmer summers and indirect recharge may diminish during summer due to lower flow rates in rivers caused by earlier snowmelt and disappearing glaciers. To what extent such changes diminishes the groundwater availability strongly depends on the characteristics of groundwater flow systems and their ability to “bridge” the period of low recharge due to their storage capacity. Alluvial aquifers are often tightly linked to rivers and hence groundwater levels may follow the altered hydrologic regime with little delay. The response of aquifers with predominant direct recharge may be very variable depending on the storage capacity of the aquifer. While larger aquifers with extensive cover layers may be affected little by summer drought periods, small and shallow aquifer may show a rapid decrease in spring discharge.The main objective of the project is twofold. On the one hand it aims at developing and testing methodologies to evaluate the response of aquifers to climate change with a special emphasis on the possible diminished groundwater availability during summer months and make them available to endusers. On the other hand, it aims at applying the methods to typical hydrogeological situations in Switzerland and identifying scenarios where groundwater flow systems are particularly vulnerable to drought conditions. The study locations were chosen such that typical aquifer types used for water supply in Switzerland are covered while taking into account other studies on the effect of climate change of aquifers that are underway in Switzerland to avoid duplication. Based on these criteria the project focuses on alluvial aquifers and springs in fissured environments (crystalline and clastic sedimentary rock) with an emphasis on alluvial aquifers due their high importance for water supply. However, results from related studies on karst aquifers in limestone regions will integrate in the project as well (Project GLACIKARST and AQUA-MONTANA). Since the effect of climate change not only depends on the aquifer type but also on climatic conditions (e.g. role of meltwater), two study areas at different altitudes were selected. The sites include the upper Emmental valley with large pumping wells of the city of Berne in an alluvial aquifer and springs used for local water supply, and the Zinal Valley which includes fissured springs in a crystalline environment as well as an alluvial aquifer both influenced by snow and glacier melt water.Regarding methods, the response of alluvial aquifers to climate change cannot be investigated by considering them in isolation because groundwater (gw)-surface water (sw) interaction often plays a crucial role in their functioning and hence the behavior of the upgradient catchment area has to be considered as well. Although, it would technically be feasible to simulate the soil water (sow)/surface water (sw)/groundwater (gw) dynamics with a fully distributed model, a hybrid modeling approach was chosen in order to avoid overparameterization and to propose a methodology that is also applicable to other sites with a reasonable effort. The methodology consists of the coupling of a semi-distributed hydrological model (PREVAH or GERM) to simulate the response of the non-glaciated/glaciated upland catchment upgradient of the alluvial plain to climate change with an integrated sow/sw/gw model (HYDROGEOSPHERE) to simulate the alluvial plain and adjacent zones including selected springs. The modeling tool will be applied to evaluate how the seasonal trends of groundwater levels and spring discharges are altered due to climate change and to evaluate the implications for water supply. Specifically, at the upper Emmental site, it will be evaluated to what extent the pumping rates have to be reduced during dry periods (as was already necessary during the 2003 heat wave!) in order to ensure minimal instream flow requirements. At the Zinal site, it will be evaluated if the springs continue to supply sufficient water for the mountain resort despite earlier not melt or if not, to what extent the alluvial aquifers in the valley could serve as an alternative water source during summer. Finally conclusions will be drawn regarding the effect of climate change on aquifer behavior and groundwater availability by integrating results from other studies in the same field including other PNR61 projects (Weingartner, Livingstone, von Gunten, Seneviratne/Zappa), several other national research projects (CCHydro, SNF Glacikarst) and European projects (GENESIS, ACQWA). In addition to scientific publications, a methodology to assess the drought vulnerability of springs to climate change will be developed. Endusers will be involved on different levels. Endusers have already been involved in the selection of the study areas and the definition of the water management issues to be addressed. They will participate in an enduser group to provide immediate input on the planned activities and will be part of the working group developing the guideline. Further interaction with enduser will be established via the joint advisory group of the EAWAG projects and workshops and seminars organized for practitioners in this context.