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Central and Southern Europe is undergoing a drying trend driven by increased evapotranspiration and rising air temperatures, even though precipitation levels remain stable. In the Bug River Basin, GRACE observations indicate that total water storage (TWS) declined at a rate of 8.8 ± 5.2 mm/year between 2012 and 2023. To validate this trend, we analysed spatial and temporal discrepancies between TWS-GRACE and water budget-based estimates (TWS-WB). Using ensemble data assimilation techniques, we integrated hydrometeorological data with TWS-GRACE. Regression models developed for TWS simulation were employed to adjust TWS-GRACE estimates. The results demonstrate that TWS fusion effectively mitigates uncertainties in TWS-GRACE caused by its low spatial and temporal resolution. Correlation analysis between TWS-fusion and TWS-GRACE identified errors in GRACE solutions and commonly used autoregressive methods for filling data gaps. Our findings show that model developed in this study significantly improved alignment between TWS-GRACE and TWS-WB, reducing RMSE from 34.7 to 14.9 mm/month. The proposed data fusion approach based on combining GRACE observations with precipitation, evapotranspiration, and runoff data, offers a viable alternative for extending TWS-GRACE time series beyond the GRACE observational period. Additionally, our research provides valuable insights for downscaling GRACE data and addressing challenges in spatial and temporal interpolation, which remain critical in water resource studies.