# Flood Monitoring Based on the Study of Sentinel-1 SAR Images: The Ebro River Case Study
## 2 Study Area and Data Source  
The Ebro river runs through seven autonomous communities and its hydrographic basin is one of the largest in the Iberian Peninsula. Its great extent favors geological, topographic, climatic, and water balance variability. The study area was focused in the middle basin of the Ebro, from the villages of Novillas to El Burgo de Ebro, and this entire section of the Zaragoza province (Figure 1). It is characterized by an average and very smooth slope; it has a high sinuous index with a meandering morphology and wide floodplains [18]. Irrigated fields, such as wheat, alfalfa, corn, barley, or rice, and permanent crops, such as vineyards, olive groves, or apple trees, are prevalent on the banks of the river [19].  
Figure 1: Location map and study areas.  
The study of the great historical floods of the Ebro river, such as those that occurred in 1643, 1775, 1871, or 1961, and more recent ones, such as those of 2003 or 2015, provides information on the changing dynamics of the river's course and how the Ebro has tried to consolidate in the flood zone, encountering man-made barriers to prevent the flooding from occurring [20]. The Ebro is one of the most regulated rivers in the Iberian Peninsula and suffers from frequent flooding [21]. The data supplied by the Hydrological Information Systems (SAIH, acronym in Spanish), in operation since the end of 1997 at the CHE, has facilitated the correct management of the reservoirs of the basin and has helped to control the floods since then. They provide, among other data, information on the levels and water flows in the main rivers and affluents, as well as the level of dammed water in the reservoirs, through sensors or stations located therein. Table 1 shows the maximum values in the last three large floods of the Ebro River in 2003, 2015, and 2018 with data provided by the gauging stations of Castejon and Zaragoza [22]. The floods of 2003 and 2015 left maximum flow values higher than those of 2018. This is due, in part, to the fact that after the flood of 2015, measures were taken to improve the safety of the populations closest to the channel, restore the riverbed, create new deviation channels, and construct new containment infrastructure [23].  
The winter of 2018 was one of the rainiest since records were first kept. In late February and in March, there were considerable episodes of rain and snowfall, accompanied by wind, throughout the Iberian Peninsula. This produced a high accumulation of water in the Ebro River Basin, especially in the upper and middle sections, and snow at the headwaters of the rivers. This was aggravated by the fact that extreme values of soil moisture had been reached, saturating it and increasing the runoff. Days before the event, some reservoirs in the basin had been preventively drained. Between 7 and 12 April, an extraordinary flood, which is the object of study of this article, took place. This was due to persistent, widespread, and very significant rainfall, and also snowfall at heights between 800 and 1600 m, with lower temperatures that were unusual for this time of the year. To this was added, in the previous days, snow thawing in the Pyrenees in the early weeks of spring. In the first half of April, it rained in the basin more than three times the normal amount [24].