## 5 Discussion
The width of the Itoiz Reservoir with satellite data varies from 400 m to more than 2 km depending on the different satellite tracks, which drift daily. In addition, the Irabia Reservoir is a very small water body with a width of approximately 130 m, making the retrieval of its surface height more challenging. Besides the waveform quality, the size of the water body and the surrounding terrain, another reason for the differences with in-situ data from the gauging station is the distance between the gauging station and the Sentinel-3 tracks, varying from a few meters to several kilometers. Satellite tracks are always drifting (up to 1.5 km), making the footprints always changing in their locations for each water body. With the altimetry mission originally designed for ocean, the accuracy of geophysical corrections over inland water ranges from a few centimeters to tens of centimeters, depending on the size of the water body and wind conditions [79; 90; 91]. These effects will most definitely influence the final results, and need to be further considered quantitatively. The results of the three retrackers, in combination with the waveform portion selection method, always give better accuracies than the results of the Sentinel-3 Level-2 ocean retracker (the results of both sets of retrackers are shown in Figure 6 and Table 5). Therefore, given the lack of an ice retracker over inland water bodies, retrieving the water levels by retracking the Level-1 waveforms and using the waveform portion selection method constitutes an alternative approach to obtain more accurate water levels. The waveform portion selection method using DEM information greatly improved the results shown in both Figures 5 and 6. As mentioned above, the vertical accuracy of SRTM DEM is approximately \(\pm\)16 m (absolute) and \(\pm\)6 m (relative). In the case of a 6 m error, the difference of choosing the right peak corresponding to nadir would be about 3 m. Therefore, if two large peaks were very close to each other, the portion of the waveform chosen might not be accurate. However, for most cases, the DEM error is small compared to the waveform tracking window, which is more than 60 m, and thus optimistic results can be obtained. Besides, waveform portion selection using DEM information improved the water level results over water bodies where only a few or no clean waveforms are available, and retained more results to have a better complete time series. In the case of the water level changing enormously and the waveform being heavily contaminated at the same time, the waveform portion selection approach may lose its capability of precise retracking. The different retrackers obtained a good accuracy with in situ measurements. The ubRMSEs showed smaller values compared with RMSEs for all retrackers over different reservoirs, as listed in Table 5. The bias between the retrieved water levels and the in situ measurements existed in most cases, which may be related to the drift of the river course (the measurements are not co-located with the Sentinel-3 track). Other error sources include the waveform quality, which depends on the surrounding terrain. These include the geometry of the lake shore and the characteristics of the terrain, all of which contribute to the shape of the waveform. Nevertheless, Sentinel-3 has been proven to work for small water bodies. We detected some issues related to the Sentinel-3 SRAL tracking mode. The open loop tracking mode, which depends on the on-board DEM to locate the tracking window, loses its track over the Cavallers Reservoir, which is surrounded by mountains, but also in Mequinenza and San Salvador Reservoirs, which are in relatively flat areas. We provided the correct water level height information to make Sentinel-3 able to capture the correct water level in the open loop tracking mode using the service altimeter open loop tracking command for hydrology monitoring ([[https://www.altimetry-hydro.eu/](https://www.altimetry-hydro.eu/)]([https://www.altimetry-hydro.eu/](https://www.altimetry-hydro.eu/))).