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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 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]

The interaction between surface water and groundwater during flood events is a complex process that has traditionally been described using simplified analytical solutions, or abstracted numerical models. To make the problem tractable, it is common to idealize the flood event, simplify river channel geometry, and ignore bank soil heterogeneity, often resulting in a model that only loosely represents the site, thus limiting its applicability to any specific river cross‐section. In this study, we calibrate a site‐specific fully‐integrated surface and subsurface HydroGeoSphere model using flood events for a cross‐section along the South River near Waynesboro, VA. The calibration approach presented in this study demonstrates the incorporation of fining direction regularization with a highly parameterized inversion driven by natural stimuli, to develop several realistic realizations of hydraulic conductivity fields that reflect the depositional history of the system. Specifically, we calibrate a model with 365 unique material zones to multiple flood events recorded in a dense well network while incorporating possible fining sequences consistent with the depositional history of the riverbank. Over 25,000 individual simulations were completed using calibration software and a cloud platform specifically designed for highly parallelized computing environments. The results of this study demonstrate the use of fining direction regularization during model calibration to generate multiple calibrated model realizations that account for the depositional environment of the system. Article impact statement: Incorporating fining direction regularization when calibrating a fully‐integrated surface and subsurface flow model to multiple flood events. [ABSTRACT FROM AUTHOR]

This study evaluated the effects of CO2 leakage on the geochemical composition of groundwater in various geological media through long-term column experiments. Four columns were set up with soil representing a silicate aquifer; clean sand; a sand and limestone mixture; and alluvium soil, respectively. The experiments were conducted under the same experimental conditions for approximately one year. As the CO2-saturated synthetic groundwater was introduced into the columns, a decrease in pH and increases in electrical conductivity (EC), alkalinity, and concentrations of cations and trace elements were observed in all geological media. However, different patterns of changes were also observed depending on the mineralogical and physico-chemical characteristics of each material. As the column operation continued, while the pH decreased and low alkalinity values were more evident in the silicate soil and clean sand columns, the carbonate column continued to show high alkalinity and EC values in addition to high concentrations of most cations. The alluvium soil showed distinctive cation-exchange behaviors during the initial introduction of CO2. The results indicate that changes in the geochemical composition of groundwater will depend on the characteristic of the geological medium such as pH buffering capacity and cation exchange capacity. This study can be useful for monitoring and managing the impacts of CO2 leakage in various aquifer environments. [ABSTRACT FROM AUTHOR]