The occurrence of dry periods on the Wupper catchment has increased in the last decades in conjunction with the shifting of the precipitation regime. In the frame of the Horizon 2020 project BINGO (Bringing INnovation to onGOing water management), the effects of climate change scenarios on the water cycle in the Wupper catchment area were investigated. To quantify these effects, a set of hydrological models (NASIM and SWAT) has been set-up, calibrated, and validated for the upper part of the

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... nn River catchment area -Wupper River's main tributary. This sub-catchment corresponds to one of the 15 inflows to the Große Dhünn Reservoir (GDR), the second largest drinking water reservoir in Germany. Both models were driven with climate data from decadal predictions, which have been selected instead of IPCC-RCP scenarios, as they provide a more realistic assumption of climate variability for the next 10 years. Ten decadal members based on the MiKlip (Mittelfristige Klimaprognose -medium-term climate prediction) framework have been prepared for the time span of 2015 to 2024. Additionally, a simulation with TALSIM-NG (a reservoir-oriented hydrological model) was carried out to obtain future 20 reservoir storage. Special focus was given to identify historical trends and compare them to future trends. Standardized Precipitation Index (SPI), Standardized Precipitation-Evapotranspiration Index (SPEI), and Standardized Runoff Index (SRI) were estimated for different seasons based on observed data to determine if they were abnormally dry or wet. SPI, SPEI, and SRI were also calculated with decadal predictions to evaluate future extreme dry periods. Uncertainties in climate data predictions are one of the greatest challenges. Observed and forecast time series were compared by means of statistical tests 25 in order to assess uncertainties in climate data predictions. Also, the application of two hydrological models aims to determine potential uncertainties, so that predictions are more reliable. Results indicate that SRI might be more appropriate to estimate drought periods for the study area in the frame of reservoir management -where inflow rates are of crucial importance -as this index quantifies losses in runoff formation processes. In terms of inflow rates to GDR, future changes indicate a reduction in runoff for the spring season, while an increment during winter. On the other hand, a clear change in pattern for fall and 30 2 summer seasons remains uncertain. Simulations of GDR reservoir volume with different climate scenarios show that water stress by the end of 2024 is not unlikely, so sustainable adaptation measures should be further considered. Effectively managing the GDR will become consequently more complex. Introduction Climate change is substantially altering not only global but also local hydrological dynamics and poses new challenges for 35 water managers, decision, and policy-makers. In order to react adequately to these challenges, detailed knowledge about potential future hydrological changes is needed. Many climate scenarios exist, however, there is still limited practical use of such scenarios for local water managers, as predictions are often too coarse in space and time to give sufficient information on local hydrological dynamics. Main goal of the BINGO project is therefore to give practical knowledge and tools in sufficient spatial and temporal resolution, so that local water authorities can investigate climate change impacts for their specific and 40 sometime small-scale regions and develop adequate adaption strategies. The local water board (the Wupper Association) is facing such challenges and is now, for the first time, able to use the decadal climate predictions of the BINGO project for a regional-specific analysis of future hydrological changes. The following study shows an example of such a small-scale climate change analysis for a local water association, and how these results can be used to give important information for an improved regional water management. 45 The inflow runoff plays a central role to assess water availability in a reservoir. In this study, different climate change scenarios were evaluated in order to identify potential future dry periods on the Große Dhünn Reservoir (GDR), the second largest drinking water reservoir in Germany. In operation since 1988, GDR is located within the Dhünn River catchment area, the main tributary of the Wupper River. The Wupper catchment lies in the state of North-Rhine Westphalia, Germany, with an area of 813 km² and a population of approximately 950,000 inhabitants (see Figure 1 ). The Wupper Association -responsible 50 for water quantity management and quality of all water bodies within the Wupper catchment -operates fourteen reservoirs with a total volume of 114 Mm³, fed by 21 rivers and creeks. Alone the Große Dhünn Reservoir (GDR), with a maximum storage volume of ca. 81 Mm³, supplies drinking water for ca. 500,000 people, serving also as emergency water supplier for the city of Düsseldorf, with a contingency volume of 10 Mm³ (BINGO D3.2, 2016). Due to GDR's regional significance, it is of crucial importance to assess future water security. 55 The occurrence of dry periods in the Wupper catchment has increased in the last decades together with the shifting of the rainy season (BINGO D3.1, 2016). This has important impacts on hydrological processes controlling the inflow to the reservoir and future water availability in the Wupper region. To be able to explain predicted changes in inflow rates, this study first analyzes past climate to detect historical trends. It then uses the hydrological models SWAT (Arnold et al., 2012) and NASIM (by Hydrotec) to predict future variations on inflow runoff. * Water treatment from GDR as well as its later distribution to the supply network is responsibility of the following water suppliers: Water Supply Association Rhine-Wupper (Wasserversorgungsverband-Rhein-Wupper -WVV, contractual raw water withdrawal = 5.7 Mm³) and Bergische Drinking Water Association (Bergische Trinkwasserverband -BTV, contractual raw water withdrawal = 36.3 Mm³) Data 110 Observed data Ground observations at Neumühle station are available since 1990. Time series of precipitation, temperature, reference evapotranspiration (ETo), and discharge were used to analyze historical climate in the study area as well as to evaluate decadal predictions skills. ETo was estimated with the Penman-Monteith method based on observed daily mean air humidity and temperature, wind velocity, and sunshine hours. Temperature time series of Buchenhofen station located in the city of 115 Wuppertal were used in order to identify past trends in the last decades. This station was selected since it provides long-term continuous temperature time series (since 1948). In addition, GDR storage time series are available since 1988 and used for comparison with simulated volume with different scenarios.