Evaluating Climate Change Impacts on Soil Moisture and Groundwater Resources Within a Lake‐Affected Region.

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]