Global Validation of SWOT Water Surface Elevation with Gauge Data and Analysis of River Width

The Surface Water and Ocean Topography (SWOT) mission, equipped with a Ka-band radar interferometer, is designed to provide high-precision measurements of inland water surface elevation (WSE), width, slope, and estimated discharge，, enabling global investigations of hydrological processes, ecosystem dynamics, and climate-driven changes in surface water resources. To fully exploit the advanced global observation capabilities of SWOT for hydrological applications, a robust and systematic validation of its inland water products is essential.

 In this study, we conduct a global-scale validation of SWOT-derived water surface elevation (WSE) by comparing SWOT measurements  (River and Lake Single-Pass Vector Product) against in situ observations from 4,676 gauges for rivers and 514 stations for Lakes and reservoirs worldwide. To this end, SWOT observations are first coarsely matched to gauge locations using spatial distance thresholds between satellite ground tracks and hydrological stations. This step is followed by station-by-station manual inspection to ensure accurate spatiotemporal matching. The accuracy of SWOT lake and river WSE products is then evaluated at the global scale using least-squares fitting. In addition, we assess the effects of different quality control flags and uncertainty parameters on data accuracy by testing individual and combined filtering strategies under multiple threshold settings. To evaluate the accuracy of SWOT river width estimates, we analyze the relationship between gauge-based WSE and river widths derived from different satellite passes. Gauge WSE is used as an intermediate variable to quantify systematic width offsets among passes and to examine the influence of cross-track distance on river width bias.

In addition, we establish empirical relationships between gauge-based WSE and SWOT-derived river width to assess the accuracy and uncertainty of SWOT width estimates. The analysis focuses on a wide range of inland water bodies, including rivers of varying widths and lakes and reservoirs of different sizes, allowing an assessment of SWOT performance across diverse hydrological conditions. To examine observational consistency across processing stages and to quantify the impact of algorithm updates, we compare SWOT Version C and Version D data, with a focus on changes in WSE and river width accuracy.

This study provides a comprehensive assessment of the accuracy and consistency of SWOT inland water products in global scale and offers practical guidance on data version selection and quality control strategies for long-term hydrological applications of SWOT observations.