Statistical and numerical analyses of the influence of climate variability on aquifer water levels and groundwater temperatures: The impacts of climate change on aquifer thermal regimes

In this study, we aimed to assess the magnitude of hydrogeological alternations, specifically those changes in the aquifer thermal regime, that are associated with climate change in the Sendai Plain, Japan. Five General Circulation Models (GCMs), HADCM3, MIROC, ECHAM5, CSIRO and CCSM3, and three greenhouse gas emission scenarios, A2, A1B and B1, were linked with the mesoscale climate using a statistical downscaling technique to produce a range of plausible future scenarios. Time series analyses of water level records at different aquifer depths at different observation points suggested that the vertical groundwater recharge was more predominant than the horizontal water flow. To simulate the heat transport in the subsurface layers, a model was constructed using the U.S. Geological Survey's numerical code for energy transport in variably saturated porous media (VS2DH). Water levels in shallow and deep aquifers were simulated using the Predefined Impulse Response Function in Continuous Time (PIRFICT) modeling method. The accuracy of the model parameters was assessed by comparing the simulated water levels to observations at a middle depth in the aquifer.All water level simulations from the PIRFICT method and the VS2DH numerical model agreed with the observed water level records, with R ² _adj=62-91% and RMSE=0.031-0.188m. Based on our results, the Sendai area may be warmed by a range of 1.3-4.7°C (2.66°C, 2.87°C and 3.89°C for 25%, 50% and 75% percentiles, respectively) during the 2060-2099 time period, compared to the observed averages between 1967-2006. The future annual precipitation projections averaged over the same time period ranged from -1% to 30% (85, 164 and 219mm/year for 25%, 50% and 75% percentiles, respectively) compared to observations from 1967-2006. When the effects of all model scenarios were considered, aquifers in the Sendai Plain may be warmed by a range of 1.0-4.28°C (0.61-4.59°C with various uncertainties) at 8m of depth from the ground's surface compared to 2007 observations. Additionally, climate change effects will expand into deeper aquifer depths, but the impacts will vary according to the transient change of ground surface temperature and changes in ground water recharge rates. The use of 15 GCM scenarios in our study projected the impacts of a changing climate on the aquifer thermal regime in a reliable range, which may have a critical impact on groundwater-dominated ecosystems.