A New GRACE Downscaling Approach for Deriving High-Resolution Groundwater Storage Changes Using Ground-Based Scaling Factors

To compensate for the coarse resolution of groundwater storage (GWS) estimation by the Gravity Recovery and Climate Experiment (GRACE) satellites and make better use of available observed groundwater-level (GWL) data in some aquifers, a ground-based scaling factor (SF) method is proposed here to derive high-resolution GRACE GWS estimates. Improvement is achieved by using the gridded SF derived from assimilating ground-based GWL observations. The proposed SF method is tested on the North China Plain (NCP, ∼140,000 km²), where a dense network of observation wells and a consistently estimated specific yield (SY) data set are available, to demonstrate its effectiveness and practical applications. The sensitivities of SF-estimated GWS accuracy to the specification of SY and the assimilation of GWL observation data are explored through four designed numerical experiments. Results show that this novel ground-based method can reduce the impact of SY uncertainty on GWS estimates, particularly in regions with more pronounced regional GWS trends. The accuracy of SF-estimated GWS is primarily determined by whether the assimilated wells can reflect the regionally averaged GWS trend. GWS accuracy is less dependent on the number of available wells assimilated. The estimated GWS trend (2004–2015) in NCP is −32.6 ± 1.3 mm/yr (−4.6 ± 0.2 km³/yr), with contrasting GWS trends found in the west Piedmont Plain (∼54,000 km², with a loss of −66.8 mm/yr) and the coastal Eastern Plain (∼20,000 km², and a gain of +7.2 mm/yr). Despite the limitations of regional and time scale dependence inherent in SF method, this study highlights the benefits of assimilating in situ observed GWL data instead of using model simulations in estimating SF to downscale GRACE GWS to the higher-resolution that is desired by local water resources management.