DAHITI – Monitoring Water Levels and Water Surface Slopes in Small Lakes and Rivers Using SWOT KaRIn Measurements

Classical satellite altimetry has been successfully used to monitor the water levels of inland waters, such as rivers, lakes and reservoirs, for more than three decades. In December 2022, a new generation of altimeter mission called Surface Water and Ocean Topography (SWOT) was successfully launched. SWOT is equipped with a classical nadir radar altimeter similar to that on Jason-3, as well as a new Ka-band Radar Interferometer (KaRIn). KaRIn uses the principle of SAR interferometry and, thanks to its 120 km wide swath and 21-day repeat science orbit, has the capability to monitor almost every inland water body worldwide.

In this contribution, we present two new methods for deriving hydrological parameters over inland waters. Water level time series are derived for around 2,700 lakes and reservoirs in Germany. For around 250 river reaches, however, we have derived not only water levels, but also time-varying water surface slopes from the high-resolution SWOT pixel cloud dataset. This dataset enables us to monitor the water levels of very small water bodies. We use SWOT data measured during the fast sampling orbit (03/2023 – 07/2023, 1-day repeat cycle) and the science orbit (since 07/2023, 21-day repeat cycle). This contribution also discusses the challenges due to measurement noise, data gaps, and dark water pixels when using SWOT KaRIn data and how the new approach addresses these challenges. In addition, a preliminary quality assessment is performed on the SWOT pixel cloud data of Version C and the new Version D, which has been available since May 2025.

To assess the quality, the resulting time series of water levels and water surface slopes are validated against in-situ data and compared with the official LakeSP and RiverSP SWOT products. The validation of 112 lakes and reservoirs results in a median RMSE of 6.3 cm. The validation of 276 river reaches results in a median RMSE 10.7 cm. Compared with the official LakeSP and RiverSP products, the results of the new approach show higher accuracy and more data points. All time series of water levels and surface water slopes are freely available on the web portal of the „Database of Hydrological Time Series of Inland Waters„ (DAHITI, https://dahiti.dgfi.tum.de).