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Abstract

The GlobalCDA Project has the goal to improve hydrological models by combining remote sensing information, among them satellite gravity observations from the GRACE mission, using a calibration/data assimilation (C/DA) approach. To validate the model results and C/DA efforts, independent data sets are crucially needed, such as in-situ groundwater (GW) observations to assess the ability of the model to describe GW storage changes. The main challenge arising from such comparisons is to capture basin-scale GW storage from a set of in-situ observations settled in highly heterogeneous lithologies with irregular & non-homogeneous sampling. Furthermore, the conversion from GW level to storage variations requires specific yield values, ideally given site-specific for each monitoring well. However, this information is largely not available and difficult to estimate in heterogeneous areas. We obtained ~3000 GW monitoring wells in France. Based on a high-resolution hydro-geological information system provided by the French geological survey and water authorities, we assign the borehole data to individual hydro-geological units. For the upscaling to river basin averages, we (i) aggregate the measurements from the same unit and (ii) account for the areal fractions of the units within the river basin. To conver GW level variations to storage changes, we assign literature-based specific yield values for each unit. It is compared to global porosity information GHYLMPS.This presentation also shows the resulting GW storage time series, averaged for individual French river basins and for 0.5° grid cells. Additionally, comparisons to GRACE-calibrated GW storage variations of the WaterGAP Global Hydrology Model (WGHM) will be presented.