From a Enhanced SWOT River Product (L3) to L4 Global Bathymetry Estimation

The Surface Water and Ocean Topography (SWOT) mission, operational since December 2022, provides unprecedented global observations of rivers, but raw data—water surface elevation (WSE) and river width—often suffer from gaps due to instrumental artifacts (e.g., dark water, specular ringing). To improve the usability of these observations, we propose an enhanced Level-3 (L3) product that integrates quality-based filtering and densification techniques. The process begins by filtering observations using SWOT’s quality flags, followed by outlier removal and constrained smoothing. For WSE, smoothing enforces spatial continuity and a monotonically decreasing elevation profile, while river width undergoes lighter smoothing due to its natural variability. Gaps are filled by interpolating across overlapping SWOT swaths, leveraging spatial-temporal correlations. The resulting L3 product—densified WSE and river width—has been validated over more than 5,000 spatio-temporal validation points from SWOT’s RiverSP CalVal datasets, demonstrating significant improvements in data coverage and consistency.

Building on this L3 foundation, we derive a Level-4 (L4) global bathymetry product. For each river location, densified (WSE, river width) pairs are reorganized in the (elevation, width) plane, and a constrained smoothing algorithm generates a continuously increasing width-with-elevation relationship, reflecting natural cross-section geometry. This L4 product standardizes "dry" (visible) and "wet" (submerged) bathymetry representations, offering a robust framework for diverse hydrological regimes—from lowland rivers to estuaries.

Designed to advance hydrodynamic modeling, flood risk assessment, and water resource management, this L4 product transforms SWOT’s enhanced L3 observations into actionable, high-level insights for regional and global applications.