# Flood Monitoring Based on the Study of Sentinel-1 SAR Images: The Ebro River Case Study
## 5 Conclusions  
In this study, a functional methodology was proposed for flood monitoring at a necessary spatial resolution derived from SAR images. Specifically, it aimed to highlight the potential use of the Sentinel-1 SAR data sets of the European Earth Observation mission for flood monitoring occurring in the middle course of the Ebro River (Spain) in April 2018. In SAR images, continental waters have strong contrast in the backscatter values due to their low or null roughness in the absence of waves, behaving like a specular surface that reflects the radar return signal in a direction opposed to the sensor position. However, they need a correction and filtering process (orbital parameters, radiometric, noise reduction speckle, and geometric geometric). Evaluating the different Sentinel-1 parameters, our analysis showed that the best results were obtained using VH polarization configuration. Concerning the need to apply numerous filters to reduce speckle, which also reduces the resolution quality of the images and has an effect on the final flood mapping. Lee filtering with a \(7\times 7\) window was the most appropriate option and it was proven that it favors the elimination of \"false positives\". Two methods were applied to detect and delimitate non-permanent water surfaces from Sentinel-1 SAR images. Method 1 RGB composition is based on differences between the pre- and post-event images, in which a combination of bands is set for identifying these differences as visually remarkable. The calibration threshold technique or binarization is based on a simple and quick process that differentiates flooded areas from those that are not from the characteristic polarization histogram using backscatter values in permanent waters and in flooded areas. The advantages of using the RGB composition methodology is that it guarantees clear differentiation between the areas of permanent flooding and the temporarily flooded ones, while achieving distinction through the technique of calibration thresholds requires other processes. Another advantage of the RGB methodology is that it detects the presence of massive riparian vegetation and vertical growth that can hide the presence of water, and it is essential to get a complete map of the flooded areas. In the calibration threshold technique, the most representative values of the presence of water were mainly between \(-21\) and \(-24\) dB; therefore, a central value of \(-21\) dB was considered. When comparing our flood maps with the flood areas digitalized from vertical aerial photographs planned by the Hydrological Planning Office of the Ebro Hydrographic Confederation, the results were consistent, even considering that the dates of both sources differed by a day. This confirmed and validated the use of Sentinel-1 SAR images to obtain flood maps with the required accuracy. However, our flood maps derived from Sentinel-1 SAR images were consistent with flood risk areas provided by the SNCZI, and correspond to a flood between 10 and 50 years of return. In view of the results obtained, the usefulness of Sentinel-1 images as baseline data for the improvement of the methodological guide can be appreciated, and should be used as a new source of input, calibration, and validation for hydrological models to improve the accuracy of flood risk maps. Finally, the development of methodologies that allow the synergy of different SAR missions (Sentinel-1, PAZ, Radarsat, TerraSAR-X, TanDEM-X, etc.) and multispectral optical systems will increase both the precision and the temporal resolution of flood monitoring, and should also be considered as powerful tools in the management and mitigation of the risks and defense against the adverse effects associated with floods. All authors have a significant contribution to final version of the paper. Conceptualization, F.C.C., and M.D.M.M. ; Methodology, F.C.C., and M.D.M.M. ; Supervision, F.C.C. ; Processing SAR Sentinel-1 images, M.D.M.M. ; Flood maps analysis, F.C.C., and M.D.M.M. ; Investigation, F.C.C., and M.D.M.M. ; Validation, F.C.C.