Advancing River Discharge Monitoring in Ungauged Basins Using Satellite Altimetry and SWOT Observations

Monitoring inland water bodies is essential for understanding the hydrological cycle, environmental balance, and atmospheric processes within the Earth system. Effective water resource management, ecosystem sustainability, and insights into hydrological processes rely heavily on accurate river discharge monitoring. Traditionally, in-situ gauging stations have been used to measure river discharge, but the global network of these stations is limited due to high costs, accessibility issues, and political and economic challenges. Over recent decades, the number of in-situ stations has declined, leading to a growing reliance on remote sensing techniques for river discharge estimation. For the past 30 years, satellite radar altimetry has proven to be an invaluable tool for measuring water surface elevation. Efforts to convert altimetry-derived water levels into river discharge have employed various algorithms. The recently launched Surface Water and Ocean Topography (SWOT) mission, on December 15, 2022, offers global measurements of water surface elevation, river width, and slope, providing significant advantages over previous missions, including enhanced spatial-temporal coverage of continental water bodies. This study evaluates hydraulic parameters derived from satellite altimetry over the past three decades, focusing on their application in estimating river discharge at ungauged locations. Data from radar and laser altimeters, including Jason-2/3, SARAL/AltiKa, Sentinel-3A/3B, ICESat-1, and ICESat-2, were used to analyze water level variations over the Mahanadi and Ganga Rivers. Altimetry-derived water levels were validated against in-situ observations at virtual stations, revealing improvements in data quality over time. Lidar-based altimeters, with their small footprint, proved particularly effective in capturing water levels in narrow river reaches. Early SWOT performance evaluations show promising results for Water Surface Slope (WSS) estimation, demonstrating moderate agreement with GNSS-based measurements. The strong KaRIn backscatter from river channels facilitates river width delineation through thresholding. Additionally, laser altimeters offer a promising approach for approximating river bathymetry efficiently and non-invasively. This study also harnesses ICESat-2 data to approximate wet bathymetry within the Ganga River. For discharge monitoring at ungauged locations, altimetry data from Jason-2, Jason-3, SARAL/AltiKa, Sentinel-3A, and Sentinel-3B were used to evaluate hydrodynamic model-based rating curves along the Mahanadi River. Using the HEC-RAS hydrodynamic model, seven virtual stations were identified between Boudh and Mundali Barrage. These rating curves provide a cost-effective method for monitoring river discharge at ungauged sites. This work offers a comprehensive evaluation of altimetry and SWOT datasets, highlighting their accuracy, advantages, limitations, and implications for river discharge estimation.