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This study aims to assess the impact of climate change on water resources in a large watershed within the Laurentian Great Lakes region, using the fully integrated surface-subsurface model HydroGeoSphere. The hydrologic model is forced with an ensemble of high-resolution climate projections from the Weather Research and Forecasting (WRF) model. The latter has been extended with an interactive lake model (FLake) to capture the effect of the Great Lakes on the regional climate. The WRF ensemble encompasses two different moist physics configurations at resolutions of 90, 30, and 10km, as well as four different initial and boundary conditions, so as to control for natural climate variability. The integrated hydrologic model is run with a representative seasonal cycle, which effectively controls natural climate variability, while remaining computationally tractable with a large integrated model. However, the range of natural variability is also investigated, as are the impacts of climate model resolution and bias correction. The two WRF configurations show opposite climate change responses in summer precipitation, but similar responses otherwise. The hydrologic simulations generally follow the climate forcing; however, due to the memory of the subsurface, the differences in summer propagate throughout the entire seasonal cycle. This results in a set of dry scenarios with reduced streamflow and water availability year-round and a set of wet scenarios with increased streamflow for all times excluding the spring peak, which does not increase. Most of the analysis focuses on streamflow, but changes in the seasonal cycle of baseflow and groundwater recharge are also analyzed.

Plain Language Summary In this study we investigate the impact of climate change on water resources using state-of-the-art computer simulations. The simulations were conducted using physically based models, which simulate the circulation of the atmosphere, rainfall, and the flow of water above and below the surface. The region of interest here is the Grand River Watershed, located in the Great Lakes region (southern Ontario, Canada). We show that with high resolution and a physical representation of the Great Lakes, only a simple correctionmuch less than for global climate modelsis necessary, in order to simulate a realistic climate. We find that predicted climate change impacts on water resources depend strongly on some approximations commonly made to represent thunderstorms and precipitation in climate models. The most likely scenario, based on our analysis, is an increase in precipitation and streamflow in all seasons except spring, but some scenarios also show less precipitation in summer, which results in lower streamflow year-round. A major result is that differences in summer precipitation can affect streamflow in all seasons, but only if the interaction with groundwater is properly accounted for. At the moment uncertainty in future summer precipitation changes limits our ability to predict impacts on water resources.