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Over the past decades, spaceborne Global Navigation Satellite System Reflectometry (GNSS-R) has attracted increasing attention from the remote sensing community. Its short revisit time and moderate spatial resolution make it a powerful tool for monitoring large-scale surface water changes, with an unprecedented capability to penetrate even dense forests. Numerous studies have confirmed the potential of spaceborne GNSS-R to monitor surface water extent and water level. Building on our previous work, this study introduces a novel approach to efficiently retrieve surface water storage (SWS) through the combination of surface water extent and depth. Here, the surface water extent is derived from spaceborne GNSS-R data (monthly temporal resolution, 0.01 spatial resolution), while the water depth is estimated by integrating the surface water extent with topographic information. The Amazon Basin was selected as the study area, and the SWS variations from July 2019 to December 2024 were computed, showing significant variability at both seasonal and interannual scales. Comparisons to multiple satellite-based hydrological datasets—including Gravity Recovery and Climate Experiment (GRACE) total water storage anomaly (TWSA), river discharge, and precipitation—show strong correlations with the estimated SWS (R = 0.90, 0.80, and 0.85, respectively), with time lags of one month for TWSA, two months for discharge, and precipitation peaks occurring approximately two months earlier than the SWS peaks. These results collectively demonstrate the effectiveness of our proposed method. Finally, this study indicates a potentially “slow drying trend” in the Amazon, characterized by a statistically significant decline in surface water extent across all sub-basins, along with a significant decrease in SWS in the Amazonas and Solimoes sub-basins, although this finding should be considered preliminary in light of the relatively short spaceborne GNSS-R observation period. Consistent with this pattern, multiple hydrologically relevant variables derived from satellite remote sensing, including sea surface salinity in the Amazon River estuary, exhibit coherent changes that support this drying signal. Given that most existing studies report only notable isolated drought events in the Amazon, this emerging pattern of slow drying shows that more attention and further investigation are needed, especially in light of the relatively short, five-year period covered by the current GNSS-R record.
Ma et al. (Fri,) studied this question.