Investigating Long-Term Terrestrial Water Storage Change in the Volga River Basin Based on the Extended Triple Collocation Weighted Fusion Model

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.