How can large scale integrated surface - subsurface hydrological model be used to evaluate long term climate change impact on groundwater reserves

Estimating climate change impacts on groundwater represents one of the most difficult challenges faced by water resources specialists. One difficulty is that simplifying the representation of the hydrological system often leads to discrepancies in projections. Simplified functions can still be used for specific conditions if they are verified with calibration, but their use becomes uncertain if applied stresses go beyond the calibration conditions, which is typical for climate change scenarios. Another key element is a reliable estimate of groundwater recharge, which is crucial in the context of climate change impact on groundwater, because it represents the connection between atmospheric and surface-subsurface processes. In this study, we present an improved methodology for the estimation of climate change impacts on groundwater reserves, with the development of a spatially-distributed, physically-based, surface – subsurface flow model, implemented and calibrated for the Geer basin (465 km², Belgium), with the finite element model HydroGeoSphere. The simultaneous solutions of surface and subsurface flow equations, as well as the internal calculation of the actual evapotranspiration as a function of the soil moisture in the defined evaporative zone, improve the representation of interdependent processes like recharge, which typically depends on various terms of the whole water cycle (precipitation and evapotranspiration in the surface domain, evapotranspiration in the vadose zone, evapotranspiration in the saturated zone when water levels are close to the ground surface, and surface water – groundwater interactions). More simple or externally coupled models do not provide the same level of accuracy. The use of fully integrated surface – subsurface models has recently gained attention, but simulations require substantial computer resources and most simulations published have been limited to small catchments or short time periods (several hours or days). The integrated model of the Geer basin has been calibrated using groundwater levels and surface water flow rates between 1967 and 2003. Transient simulations were run between 2010 and 2100, which is a challenging test of the modelling methodology compared to the short time-scale transient simulations more usually performed. Climate change simulations from 6 RCM were applied to the model and show that significant decreases are expected in groundwater levels (up to 8 meters) and in surface water flow rates (up to 33%) by 2080. Comparisons between two temporal discretisations are made to try optimizing the model between performances and computing demand, with the goal of further applying stochastic climate change scenarios (180 scenarios from 2010 to 2100).

ISBN:978-7-5625-2417-5

Publication year:2009