Improved groundwater simulations by multi-criteria calibration of a global hydrological model in the river basins of France

Groundwater is the most important freshwater source considering its abundance, demand, quality, and accessibility. However, estimation of groundwater storage variations is notoriously difficult due to the lack of information on physical aquifer properties, the connectivity among other water storages, the scarcity of observations at regional scales and high uncertainty associated with the observations if available. In the current study, we tried to improve the simulations of groundwater storage variations of a global hydrological model with a simple bucket-type groundwater module – the WaterGAP Global Hydrological Model (WGHM) – in four French river basins. Sensitive model parameters were calibrated against in-situ streamflow observations and GRACE-based observations of total water storage anomalies in a multi-criterial calibration framework. A regional data set of in-situ observations of groundwater levels and deduced basin-average groundwater storage variations (see Hsu et al., this meeting) was used for validation of the calibration results. In a second setup, the in-situ groundwater observations were introduced as additional observations into the calibration framework.  We found significant improvement in the simulated groundwater dynamics in terms of their seasonal signals and amplitudes after calibration. Overestimated negative groundwater trends in a few river basins caused by overestimated human groundwater use in the original model could also be corrected by the calibration approach. In an additional calibration experiment, specific yield was introduced into WGHM as a new calibration parameter and its calibrated values were compared to those obtained from regionalizing the hydrogeological information.