Fusing GRACE data into terrestrial water budgets to improve their predictive performance: a case study of the Bug River Transboundary Catchment, Polish-Ukrainian-Belarusian borderland

Abstract

We analysed variations in terrestrial water storage (TWS) over the Bug River Transboundary Catchment for the period 2012–2023. To do so, we utilized satellite gravimetry data from the Gravity Recovery and Climate Experiment (GRACE) mission, along with satellite and in situ components of the water balance (WB) approach, including precipitation, evapotranspiration, and runoff. TWS anomalies and month-to-month changes in TWS (DTWS) derived from GRACE data (TWS-GRACE/ΔTWS-GRACE) were compared with those from the water balance approach (TWS-WB/ΔTWS-WB) for the study area. The results showed a good agreement between TWS-GRACE and TWS-WB, with an average coefficient of determination of 0.77. This agreement is slightly stronger in the lowland areas and weaker in the upland regions. We also found that this consistency decreases during the dry months (April to September). In the next step, we performed data assimilation of GRACE and hydrometeorological data by applying a regression-based model. Our results demonstrated that such data fusion effectively mitigates the uncertainties in TWS-GRACE related to its low spatial and temporal resolution, improving the consistency between ΔTWS-GRACE and ΔTWS-WB, as shown by a reduction in root mean square error from 34.7 to 14.9 mm/month. The results revealed that between 2012 and 2023 the Bug River Basin faced alternating extreme decreases and increases in TWS. Although the magnitude of the trends varied slightly over different periods, the overall region exhibited negative trends in TWS changes, with an average rate of ~9 mm/year.