# Flood Monitoring Based on the Study of Sentinel-1 SAR Images: The Ebro River Case Study
## 4 Results and Discussion
### Calibration Threshold Technique Results  
Before the application of this methodology for the extraction of several points from the radar images, specifically in zone 1 with VV and VH polarization, a comparison of how backscatter intensity values are modified in areas that have been flooded was considered (Figure 8). While the magnitude of intensity for the VV polarization, for both moments, is very variable and has a wide spectrum of values, the VH polarization is more defined within a certain range (Figure 9). The areas adjacent to the course of the river, mainly dedicated to cultivation, are characterized by partially rough surfaces and moderate vegetation. These move in values between \(-17\) and \(-19\) dB for the VH polarization, and between \(-11\) and \(-15\) dB for the VV polarization, with specific values that are out of this range. These same areas, subsequently flooded, reflect values between \(-21\) and \(-24\) dB approximately for VH polarization, and for VV, a wider range between \(-15\) and \(-21\) dB. The smooth surface generated by the sheet of water causes the backscatter intensity to decrease towards even more negative values. Figure 7: Aerial views of the study area on 14 April 2018, showing the flood of the Ebro and the characteristic riverbank vegetation as highly flooded (Source: CHE, 2018 [36]). Figure 8: Location map of the points considered in the comparison of backscatter intensity before and after the flooding event. Figure 9: Graphical representation of the values of backscatter intensity before and after the event, for the VV (**a**) and VH (**b**) polarization, for the indicated points. Through the graphics obtained by representation of characteristic points, we obtained an idea of the behavior of the radar on different surfaces. The calibration threshold methodology separated the dry zones from areas with the presence of a water sheet through the interpretation of histograms. In Figure 10, the relationship between polarizations and characteristic histograms for each of them is observed for the zone 1: Novillas-Pradilla de Ebro. The histogram of the VV polarization shows a very large curve representative of the intensity values ranging between \(-1.0\) and \(-22\) dB, a very wide spectrum characteristic of areas that range between very rough and moderately rough. While there is no clear projection of low backscatter values, the histogram of the VH polarization itself shows two clear populations where the specific intensity values of the image oscillate. This was chosen for further analysis. On the one hand, there is the curve that provides a greater number of pixels and that represents the areas without water, and on the other hand, a curve of smaller size and with smaller intensity values that represents the areas with water is shown. It is between both areas that the calibration limit can be set. Analyzing the histograms in greater depth, we observe that the cross polarization provides a wider range of backscatter values on surfaces with vegetation than the simple polarization, VV, which results in the latter omitting flooded areas [64]. Figure 10: Representation of the characteristic histograms of zone 1: Novillas–Pradilla de Ebro for VV polarization (up) and for the VH (down) in the radar image of 13 April 2018 filtered with the Lee \(7\times 7\) window size filter. To further specify the boundary point between the areas with water and without water, a water mask was generated on the VH polarization image. This layer was created by randomly capturing some flooded areas within zone 1 and subsequently generating a histogram. In this way, intensity values were obtained in which only areas with water or flooded areas are shown, independent of the frequency or number of pixels taken. It should be noted that this layer serves only to confirm and verify the suitability of the threshold taken with the total histogram of the zone.