The Volga River Basin is one of the most important water systems on the Eurasian continent. Variations in its terrestrial water storage (TWS) not only exert a profound influence on regional hydrological processes but also dominate the fluctuation of the Caspian Sea level (Correlation coefficient: 0.90). However, due to the limited temporal coverage of Gravity Recovery and Climate Experiment (GRACE) observations, understanding of the long-term evolution of TWS in this region remains insufficient. This study introduces a multi-source data fusion approach that does not rely on any a priori information from GRACE. By integrating ERA5-Land, FLDAS, and GLDAS products with the terrestrial water balance equation and the Extended Triple Collocation (ETC) algorithm, we reconstructed a long-term TWS time series for this region spanning more than four decades (1982–2023). Benefiting from the higher spatial resolution (0.1°×0.1°) of fusion model, a pronounced declines trend (approximately -15 mm/yr) in TWS within certain rivers and lakes was successfully identified. The decomposition results of terrestrial water storage anomalies reveal a progressively increasing amplitude of its seasonal component, while the long-term trend is primarily modulated by climatic wetness and dryness conditions. Snow water equivalent (SWE) and Soil moisture (SM) are the dominant contributors to TWS change, accounting for 48.3% and 36.1%, respectively. The contribution of SWE is largely concentrated in high-latitude regions with an increasing trend. This study makes up for the limitation of the insufficient temporal span in GRACE, providing an objective basis for understanding long-term TWS variations under climate change. ● A long-term, high-resolution hydrological model of the Volga River Basin was constructed by the Extended Triple Collocation (ETC) method. ● Analyzed the long-term trend changes of terrestrial water storage from a spatiotemporal perspective across the Volga River Basin. ● Investigated the relationship between the significant decline in TWSA within river and lake grid cells and changes in the Caspian Sea level ● Assessed the changes of individual water storage components and their contributions to terrestrial water storage over the Volga River Basin.
Li et al. (Sun,) studied this question.