Evaluation of SWOT HR PIXC version D water level time series of small lakes

Satellite altimetry has successfully monitored inland waters for more than 30 years and is increasingly important as the demand for freshwater grows and climate change accelerates. Launched in December 2022, the Surface Water and Ocean Topography (SWOT) satellite is the first to provide 2D spatially distributed elevation measurements, with a 21-day revisit time and better coverage depending on latitude and a nominal requirement to detect lakes as small as 0.06 km2. Here, we evaluate the SWOT L2 HR PIXC version D data product (if available at the time of writing) to construct time series of water surface elevation (WSE) and capture their relative WSE change in 37 Danish lakes with a surface area between 0.25 km2 and 40 km2 via the summary measures RMSE and Pearson’s correlation coefficient (PCC). We tested six selection criteria to aggregate one WSE value per lake and timestamp. The median unbiased RMSE of SWOT vs gauge is 5.34 cm, and the median PCC is 0.91. We find indications that SWOT’s PIXC data contains time–varying residual roll–errors over Danish lakes. We demonstrate that our approach performs slightly better than filtered SWOT L2 HR LakeSP prior data from Hydrocron in terms of RMSE and PCC (6.05 cm and 0.89), while retaining roughly double the number of valid timestamps over the overlapping period.