The accuracy of the Sentinel-3A altimetry over Polish rivers
Michał Halicki, Tomasz Niedzielski
Abstract
This study is the first attempt to assess the accuracy of water levels measured by the Sentinel-3A altimetry at virtual stations (VS) located along Polish rivers. The studied rivers (Vistula, Odra, Warta, Bug, Narew, San) drain predominantly lowlands, and – based on width – can be classified as small and medium rivers (40–610 m in width). The accuracy assessment is based on a comparison between water level anomalies from 34 VS and water level data from the nearest adjacent gauges, acquired from April 2016 to August 2019. The time lags between water level time series (VS vs. gauge) are estimated so that pairs of hydrographs agree in phase, which enables the comparison. It is found that the root mean square error ranges from 0.12 to 0.44 m, with mean of 0.22 m. The Nash–Sutcliffe efficiency (NSE) varies between 0.40 and 0.98 (with mean of 0.84) for 67 pairs of time series, out of 68 considered. It is also argued that river width does not reveal any impact on the performance of the altimetric measurements over the study area. Likewise, land cover has not been identified as an environmental factor to constrain the data accuracy. However, it is found that complex river channel morphology and unfavourable geographical setting occur more frequently at VS with lower NSE (≤ 0.8). The study confirms the usability of the Sentinel-3A altimetry along small and medium rivers, identifies factors to constrain the accuracy and – most importantly – provides yet another regional-scale investigation into quality of satellite radar measurements over inland waters.