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The impact of climate change on surface water resources is reasonably well studied. However, the impact on groundwater resources has only been considered by a few studies worldwide. Here we present an analysis of climate change impacts on groundwater resources in a well‐instrumented 6,800‐km2 watershed in the Laurentian Great Lakes Basin. We employ a physics‐based modeling pipeline consisting of an ensemble of high‐resolution regional climate model projections based on the Weather Research and Forecasting model and the fully integrated three‐dimensional hydrologic model HydroGeoSphere. The Weather Research and Forecasting model is run at a resolution as fine as 10 km using two different physics configurations, while HydroGeoSphere simulates the terrestrial hydrosphere at subkilometer scale, from deep groundwater to surface water, including surface water‐groundwater interactions. The two Weather Research and Forecasting model physics configurations exhibit opposite climate change responses in summer precipitation. The hydrologic simulations follow the climate forcing, but due to the memory of the subsurface, differences in summer affect the entire seasonal cycle. In the drier climate scenario groundwater levels and recharge decline, while in the wetter scenario groundwater levels rise (recharge remains unchanged). Soil moisture changes accordingly, but primarily in late summer. It is also shown that the magnitude of climate change impacts on groundwater is strongly modulated by local physiographic features. In particular, regions where the groundwater table is deep (below 2 m; 15% of the area) show a high sensitivity to changes in climate forcing. Furthermore, changes in groundwater levels, recharge, and soil moisture typically occur in the same regions, suggesting potentially compounding impacts. Plain Language Summary: In many watersheds groundwater maintains minimum flows and soil moisture during dry periods. In this study we investigate the possible impacts of different climate change scenarios on groundwater and soil moisture in a major watershed in southern Ontario, Canada. We use a state‐of‐the‐art hydrologic model that simulates groundwater and surface water, together with climate scenarios that are derived from an ensemble of high‐resolution regional climate projections. The main result is that changes in groundwater and soil moisture generally follow the direction of changes seen in net precipitation from the climate models, but the spatial pattern and magnitude of changes are strongly modulated by local topography and geology. In general, regions that have a deeper groundwater table today are more sensitive and may experience larger changes in the future. As expected, our simulations do not match observations perfectly, but we believe that we are able to identify uncertainties that are likely to affect the conclusions. The primary uncertainty here lies in the change in summer precipitation, which dominates the climate change response: it is large enough to either cause a drier or a wetter future, and the interaction between surface water and groundwater appears to spread changes in summer across the entire year. Key Points: Climate change impact modeling using a regional climate model and a fully integrated surface water‐groundwater modelPrimary uncertainty in summer precipitation controls soil moisture and has delayed influence over seasonal groundwater fluctuationsMagnitude of climate change impacts on soil moisture and groundwater are strongly modulated by local physiography [ABSTRACT FROM AUTHOR]

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 10 km, 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 correction—much less than for global climate models—is 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. Key Points: The effective use of an RCM ensemble for integrated hydrologic modeling using a representative average seasonal cycle is exploredWithin the study area, the largest uncertainty in hydrological modeling of climate change impacts is the change in summer precipitationChanges in summer precipitation affect groundwater storage, which can significantly impact baseflow throughout the year [ABSTRACT FROM AUTHOR]