## 6 Conclusions  
The objective of this study was to develop a methodology that can estimate water levels in small water bodies (similar size to the along-track resolution) and/or complex topography environments where many waveforms are expected to be contaminated by land. The study compared the performances of the threshold, OCOG and two-step physical-based retrackers, but no significant differences were found in the results of the three retrackers. However, the main improvements found show that selecting the portion of the waveform to be fitted significantly affected the retrievals.  
The novel DEM-oriented waveform portion selection method could isolate the nadir peak from land-based contamination relatively well, and greatly improved the results with a reduction of RMSE by more than 0.5 m over Ebro Reservoir. Retracking the water levels from Level-1 waveforms using the combination of a retracker and the waveform portion selection method was more robust to land contamination than using Sentinel-3 Level-2 data directly. It also resulted in a better accuracy.  
The water levels were compared with in situ measurements and the Sentinel-3 Level-2 ocean retracker from ESA. The results show good agreement with the in situ measurements. The ubRMSE over the Ribarroja Reservoir with a width of about 400 m could reach 16 cm, while that over the Ebro Reservoir whose width is about 1.8 km, it could reach 28 cm. Waveform portion selection using DEM information showed its ability to retrieve water levels over small- to medium-sized water bodies. In contrast, the accuracy over water bodies located in mountainous areas still needs to be improved, but the results demonstrate the possibility of retrieving the water levels over a very small water body with a width of approximately 130 m.  
Overall, the Sentinel-3 SRAL has been proven to work over inland water bodies, even those with widths as small as 130 m. Retracking using the waveform portion selection method with an SRTM DEM as a pre-processing to the two-step physical-based retracker, the OCOG retracker and threshold retracker improved the accuracy with an optimal ubRMSE of 16 cm. Further steps need to be taken to explore the possibilities for both tracking water bodies that are not currently being tracked, and for improving the accuracies for very small water bodies with Sentinel-3 altimeter data.  
**Author Contributions:** [PERSON], [PERSON] and [PERSON] conceived, designed and implemented the research. [PERSON] and [PERSON] coded the program. [PERSON] performed the analysis of the data and drafted the manuscript. [PERSON] assisted in the data analysis and interpretation. All authors reviewed and improved the manuscript. The study was supervised by [PERSON].  
**Funding:** [PERSON] received grant DI-15-08105 from the Spanish Education Ministry (MICINN) and DI-2016-078 from the Catalan Agency of Research (AGAUR). This work was partially funded by the Spanish Ministry of Science, Innovation and Universities and the European Regional Development Fund through grant CGL2017-85687-R.  
**Acknowledgments:** The authors wish to thank the technical teams for their support, the Copernicus Programme, which provided open access to the Sentinel data, and the SAIH Ebro team, who provided the ground information.  
**Conflicts of Interest:** The authors declare no conflict of interest.