# Flood Monitoring Based on the Study of Sentinel-1 SAR Images: The Ebro River Case Study
## 2 Study Area and Data Source
### Sentinel-1 SAR Data  
Synthetic Aperture RADAR (Radio Detection and Ranging) is a powerful active remote sensing technology used for several applications, including flood monitoring. RADAR are active systems that illuminate the Earth's surface and therefore, images can be acquired by day during any lighting conditions, or at night, in the total absence of sunlight. In addition, these images are not affected by cloud cover, fog, or smoke because the RADAR signals have the ability to penetrate these covers [25; 26]. \begin{table}
\begin{tabular}{c c c c c c c} \hline \multirow{2}{*}{**Gauging Station**} & \multicolumn{2}{c}{**2018**} & \multirow{2}{*}{**Day**} & \multirow{2}{*}{**Hour**} & \multirow{2}{*}{**Annual**} & \multirow{2}{*}{**Annual**} \\ \cline{2-2} \cline{5-6}  & **Annual** & & & & & **Maximum** & **Maximum** \\ \multicolumn{1}{c}{**Average Level**} & **Average Flow** & & & & **Level of** & **Flow of** \\ \multicolumn{1}{c}{**of Ebro (m)**} & **of Ebro (m\({}^{3}\)/s)** & & & & **Ebro (m)** & **Ebro (m\({}^{3}\)/s)** \\ \hline
**Castejón** & 2.82 & 265 & 13 April 2018 & 9:30 & 7.77 & 2682 \\ \hline
**Tudela** & 1.22 & 270 & 14 April 2018 & 01:00 & 5.34 & 2413 \\ \hline
**Novillas** & 2.63 & - & 14 April 2018 & 11:45 & 8.24 & - \\ \hline
**Pradilla de Ebro** & 3.11 & - & 14 April 2018 & 20:30 & 8.51 & - \\ \hline
**Alagón** & 0.88 & - & 14 April 2018 & 07:30 & 2.26 & - \\ \hline
**Zaragoza–Ronda Note** & 2.61 & - & 15 April 2018 & 17:15 & 8.02 & 2041 \\ \hline
**Zaragoza** & 1.5 & 288.52 & 15 April 2018 & 19:45 & 5.36 & 2037 \\ \hline \end{tabular}
\end{table} 
Table 2: Representative values on the annual levels and flows of the Ebro river in different stations located in its course for the year 2018. Historical data taken from the SAIH of the CHE, 2019. Figure 2: Location map of the gauging stations considered in the study. Under the weather conditions in which floods occur, these types of images are the most suitable for study. Active radar systems incorporate an antenna that generates and transmits electromagnetic pulses (between 0.3 and 300 cm) to the Earth's surface, and after interacting with the objects, the echoes of the backscattered signal are collected and amplified by a receiver at certain frequencies and polarization modes. Currently, there is a wide variety of SAR missions that have improved spatial resolution, bands, revisiting times, and polarization modes that have increased the monitoring potential [27]. The backscattering coefficient, or sigma nought (\(\sigma_{0}\)), is the measure of the reflective strength of a radar target, defined as the per unit area on the ground, and is defined as the normalized measure of the radar return from a distributed target [28]. The amplitude and phase of the backscattered signal depends, among others, on the land surface properties, including the geometry, vegetation, roughness, and dielectric constant [29; 30]. Continental waters, due to their low or null roughness in the absence of waves, behave like a specular surface, reflecting the radar return signal in a direction opposed to the sensor position, and therefore show very low backscatter values (negative) with respect to other land covers. This strong contrast in backscatter values allows the precise delimitation of flooded areas with SAR images. When a rough terrain has slopes with an angle that is more steep than the sensor depression angle, it generates shadow regions that appear without signal, very similar to the flooded areas, which can easily be confused. As flood areas tend to be flat, this effect is minor and is usually correctly delimited with detailed topographic information [31]. In this study, it is been used Sentinel-1 SAR images because of their features, configuration, and the free data set available online from Earth Observation (EO) data resources. Sentinel-1 SAR datasets, coming before and after the event, were downloaded via the Copernicus Open Access Hub platform [32]. The Sentinel-1 sensor is part of an ESA (European Space Agency) satellite constellation.