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An accurate estimation of water resources dynamics is crucial for propermanagement of both agriculture and the local ecology, particularly insemi-arid regions. Imperfections in model physics, uncertainties in modelland parameters and meteorological data, as well as the human impact on landchanges often limit the accuracy of hydrological models in estimating waterstorages. To mitigate this problem, this study investigated the assimilationof terrestrial water storage variation (TWSV) estimates derived from theGravity Recovery And Climate Experiment (GRACE) data using an ensemble Kalmanfilter (EnKF) approach. The regionconsidered was the Hexi Corridor in northern China. The hydrological modelused for the analysis was PCR-GLOBWB, driven by satellite-based forcing datafrom April 2002 to December 2010. The impact of the GRACE data assimilation(DA) scheme was evaluated in terms of the TWSV, as well as the variation ofindividual hydrological storage estimates. The capability of GRACE DA toadjust the storage level was apparent not only for the entire TWSV but alsofor the groundwater component. In this study, spatially correlated errors inGRACE data were taken into account, utilizing the full errorvariance–covariance matrices provided as a part of the GRACE data product.The benefits of this approach were demonstrated by comparing the EnKF resultsobtained with and without taking into account error correlations. The resultswere validated against in situ groundwater data from five well sites. Onaverage, the experiments showed that GRACE DA improved the accuracy ofgroundwater storage estimates by as much as 25 %. The inclusion of errorcorrelations provided an equal or greater improvement in the estimates. Incontrast, a validation against in situ streamflow data from two river gaugesshowed no significant benefits of GRACE DA. This is likely due to the limitedspatial and temporal resolution of GRACE observations. Finally, results ofthe GRACE DA study were used to assess the status of water resources over theHexi Corridor over the considered 9-year time interval. Areally averagedvalues revealed that TWS, soil moisture, and groundwater storages over theregion decreased with an average rate of approximately 0.2, 0.1, and0.1 cm yr<sup>−1</sup> in terms of equivalent water heights, respectively. Aparticularly rapid decline in TWS (approximately −0.4 cm yr<sup>−1</sup>) wasseen over the Shiyang River basin located in the southeastern part of HexiCorridor. The reduction mostly occurred in the groundwater layer. Aninvestigation of the relationship between water resources and agriculturalactivities suggested that groundwater consumption required to maintain cropyield in the growing season for this specific basin was likely the cause ofthe groundwater depletion

Improving estimates of water resources in a semi-arid region by assimilating GRACE data into the PCR-GLOBWB hydrological model