River Water Level and Water Surface Slope Measurement From Spaceborne Radar and LiDAR Altimetry: Evaluation and Implications for Hydrological Studies in the Ganga River

Abstract

Satellite altimetry has revolutionized river monitoring, particularly for hydrologists working on river flow monitoring in sparsely or ungauged areas. Despite this, there is a lack of a comprehensive evaluation of radar and LiDAR altimeters with varying sensor specifications for river water level retrieval, seasonal change characterization, and water surface slope (WSS) using gauged long-term water level and global navigation satellite system data. This study addresses this gap by combined evaluation of radar (ENVISAT to Sentinel-3) and LiDAR (ICESat-1, ICESat-2) altimeters along the Ganga River, from Prayagraj to Varanasi. We found that all the radar altimetry missions showed better accuracy for water level retrievals (R2 > = 0.8; RMSE 0.11 to 1.16 m) and water level change quantification (RMSE 0.59 m). However, Sentinel-3 with synthetic aperture radar (SAR) acquisition mode outperformed (RMSE 0.11 to 0.14 m) all the radar missions having low resolution mode. Despite LiDAR missions' high vertical accuracy, they show relatively lower accuracy in water level time series generation due to nonrepeating characteristics. In contrary, ICESat-2 demonstrates potential in capturing spatial and seasonal variability of WSS, enhancing the accuracy of surface water and ocean topography (SWOT) discharge products when combined with SWOT River database. This study provides a comprehensive baseline for end-users interested in utilizing radar and LiDAR missions for various hydrological applications, including river discharge estimation. Moreover, the studied river reach shares the SWOT calibration orbit, allowing the utilization of generated satellite and in-situ databases for the effective evaluation of SWOT measurements.

Keywords

• Ganga river
• ICESat-2
• LiDAR altimetry
• radar altimetry
• remote sensing
• surface water and ocean topography (SWOT) mission