Abstract's details
Validation of Water Surface Elevation and Extent over French lakes using the Surface Water Ocean Topography (SWOT) mission
Event: 2025 SWOT Science Team Meeting
Session: Hydrology: HR SWOT Data (Data Validation & Enhancement)
Presentation type: Poster
Natural lakes and artificial reservoirs—collectively referred to as lakes—are a fundamental component of terrestrial hydrology and play a vital role in supporting ecological processes, sustaining terrestrial biodiversity, regulating biosphere functioning, and facilitating a wide range of human activities (Wang et al., 2025). Furthermore, lakes act as significant carbon sinks (Bar-On et al., 2025), contributing to the regulation of terrestrial carbon fluxes. In recognition of their importance, lakes have been designated as Essential Climate Variables (ECVs) by the Global Climate Observing System (GCOS) of the World Meteorological Organization (WMO). Despite their critical role, the monitoring of lakes at global scales remains challenging—particularly due to the scarcity and uneven spatial distribution of in situ observations. The deployment and maintenance of in situ monitoring infrastructure are often cost-prohibitive, and where such stations do exist, accessibility may be limited by logistical or environmental constraints.
Over the past few decades, satellite remote sensing has emerged as a valuable tool for observing lake dynamics at regional to global scales, offering temporal revisit compatible with hydrological processes. However, traditional satellite missions have typically provided either surface water extent from multispectral sensors or elevation data from nadir altimeters. The launch of the Surface Water and Ocean Topography (SWOT) mission in December 2022 represents a paradigm shift, as it is the first satellite mission dedicated specifically to continental hydrology. SWOT enables the simultaneous observation of surface water extent and elevation at high spatial resolution, allowing estimate of surface water volume, using its Ka-band Radar Interferometer (KaRIn) instrument. A recent study has demonstrated the potential of SWOT to accurately monitor surface water elevation over large lakes (Bazzi et al., 2025).
This study aims to evaluate and validate SWOT-derived surface water extents and elevations over a selection of lakes in France, encompassing a variety of sizes, geographic locations, and elevations. We utilize the Level-2 KaRIn High-Rate (HR) Lake Single-Pass (SP) vector product during both the calibration phase (April 2023) and the scientific phase (since July 2023). SWOT water level data were compared with both in situ gauge measurements and conventional radar altimetry. Comparison with in situ observations across 30 lakes revealed a weak correlation (R² = 0.0159) and a root mean square error (RMSE) of 0.025 m. This low correlation is likely influenced by lake size, which plays a significant role in determining measurement accuracy. In contrast, comparisons with conventional radar altimetry over 27 lakes showed strong agreement, with a correlation coefficient of R² = 0.899 and a RMSE of 0.023 m. Additionally, we assessed SWOT-derived surface water extents for 509 lakes using the Global Water Watch database, which is based on Landsat and Sentinel-2 optical imagery. This comparison shows a correlation of R² = 0.81 and a RMSE of 0.004 km². When applying the “good pixel” quality flag, the correlation improved significantly to R² = 0.906.
References :
Bar-On, Y. M., Li, X., O’sullivan, M., Wigneron, J. P., Sitch, S., Ciais, P., ... & Fischer, W. W. (2025). Recent gains in global terrestrial carbon stocks are mostly stored in nonliving pools. Science, 387(6740), 1291-1295.
Bazzi, H., Baghdadi, N., Ngo, Y. N., Normandin, C., Frappart, F., & Cazals, C. (2025). Assessing SWOT interferometric SAR altimetry for inland water monitoring: insights from Lake Léman. Frontiers in Remote Sensing, 6, 1572114.
Wang, J., Pottier, C., Cazals, C., Battude, M., Sheng, Y., Song, C., ... & Pavelsky, T. M. (2023). The Surface Water and Ocean Topography Mission (SWOT) Prior Lake Database (PLD): Lake mask and operational auxiliaries. Authorea Preprints.
Back to the list of abstractOver the past few decades, satellite remote sensing has emerged as a valuable tool for observing lake dynamics at regional to global scales, offering temporal revisit compatible with hydrological processes. However, traditional satellite missions have typically provided either surface water extent from multispectral sensors or elevation data from nadir altimeters. The launch of the Surface Water and Ocean Topography (SWOT) mission in December 2022 represents a paradigm shift, as it is the first satellite mission dedicated specifically to continental hydrology. SWOT enables the simultaneous observation of surface water extent and elevation at high spatial resolution, allowing estimate of surface water volume, using its Ka-band Radar Interferometer (KaRIn) instrument. A recent study has demonstrated the potential of SWOT to accurately monitor surface water elevation over large lakes (Bazzi et al., 2025).
This study aims to evaluate and validate SWOT-derived surface water extents and elevations over a selection of lakes in France, encompassing a variety of sizes, geographic locations, and elevations. We utilize the Level-2 KaRIn High-Rate (HR) Lake Single-Pass (SP) vector product during both the calibration phase (April 2023) and the scientific phase (since July 2023). SWOT water level data were compared with both in situ gauge measurements and conventional radar altimetry. Comparison with in situ observations across 30 lakes revealed a weak correlation (R² = 0.0159) and a root mean square error (RMSE) of 0.025 m. This low correlation is likely influenced by lake size, which plays a significant role in determining measurement accuracy. In contrast, comparisons with conventional radar altimetry over 27 lakes showed strong agreement, with a correlation coefficient of R² = 0.899 and a RMSE of 0.023 m. Additionally, we assessed SWOT-derived surface water extents for 509 lakes using the Global Water Watch database, which is based on Landsat and Sentinel-2 optical imagery. This comparison shows a correlation of R² = 0.81 and a RMSE of 0.004 km². When applying the “good pixel” quality flag, the correlation improved significantly to R² = 0.906.
References :
Bar-On, Y. M., Li, X., O’sullivan, M., Wigneron, J. P., Sitch, S., Ciais, P., ... & Fischer, W. W. (2025). Recent gains in global terrestrial carbon stocks are mostly stored in nonliving pools. Science, 387(6740), 1291-1295.
Bazzi, H., Baghdadi, N., Ngo, Y. N., Normandin, C., Frappart, F., & Cazals, C. (2025). Assessing SWOT interferometric SAR altimetry for inland water monitoring: insights from Lake Léman. Frontiers in Remote Sensing, 6, 1572114.
Wang, J., Pottier, C., Cazals, C., Battude, M., Sheng, Y., Song, C., ... & Pavelsky, T. M. (2023). The Surface Water and Ocean Topography Mission (SWOT) Prior Lake Database (PLD): Lake mask and operational auxiliaries. Authorea Preprints.