# A Tool for Pre-Operational Daily Mapping of Floods and Permanent Water Using Sentinel-1 Data
## 5 Discussion
Nonetheless, by comparing the AUTOWADE-derived map with that derived from a S2 image acquired on the same day, it has been found that the image objects marked as flooded vegetation allowed us to fill some gaps present in the floodwater map that are not realistic from a hydrological point of view (Vercelli test case, Figure 13f), because the area affected by the flood has a quite uniform surface elevation. Several gaps are still present in the map and it is likely that flooded vegetation is underestimated. However,  
Figure 15: Areas classified as open floodwater by AUTOWADE and CEMS for the Springfield Clonara case study (EMSR 429). Blue: floodwater in both CEMS-derived and AUTOWADE-derived maps; red: floodwater only in the CEMS-derived map; yellow floodwater only in the AUTOWADE-derived map. it must be considered that, when dense vegetation is present, SAR is not sensitive to the presence of floodwater beneath the canopy (we remind again that pixels with \(NDVI\) > 0.7 are masked by AUTOWADE). Moreover, double bounce is the prevailing effect caused by flooded vegetation for stem-dominated vegetation, while, for leaf-dominated vegetation, volume scattering generally prevails [12] and surface water detection is complicated. Even in Figure 16, the presence of flooded vegetation is evident (red ellipse in panel d); while this target is not detected by the CEMS-derived map, in the AUTOWADE derived map the presence of gaps is reduced, and the area highlighted by the red ellipses is correctly labelled as flooded vegetation. Flooded vegetation mapping builds on the results of a previous study regarding the combined use of VV and VH polarizations [47] (confirmed in [11; 48]), based on the consideration that the dihedral effect, generated by an horizontal surface (water) and a vertical one (plants' stems), keeps the same polarization throughout the radar scattering path. Note that it has been demonstrated that in the presence of slanted vegetation, the double bounce scattering also generates a cross-polarized signal [55]. However, plants having a slanted root structure like saltwater mangroves are typical of tropical areas rather than temperate ones like Italy. The major limitation of AUTOWADE and of the BFE method implemented in AUTOWADE to map open water is related to the processing of a S1 frame where big water bodies are missing, and a reference water mask cannot be derived. In this case, the MCDWA cannot be created from the CDWA. This problem can be at least partially overcome using the edge between the original CDWA and the other areas to identify, through the buffer zones created around it, a subset of image pixels characterized by a bimodal histogram (as done in [35]). Although AUTOWADE has been conceived to work on a national (Italian) scale, the results of the validation (Section 4.3) demonstrate that the BFE can be applied in other European countries. In principle, nothing prevents a user from applying the BFE method outside Europe, but the reference mask derived from the WAW product is currently available only for Europe. Figure 16: Comparison, for the Rieti flood (EMSR496), among the CEMS-derived map (panel **b**), the AUTOWADE-derived map (panel **c**), and a couple of aerial photos taken on 26 January 2021, at 11:00 UTC (**d**,**e**). Panel (**a**) is the reference Google-Hybrid map. Same colors of the ellipses correspond to the same zone in the maps and the photos. In panels (**b**,**c**), cyan represents open floodwater; in panel (**c**), sea-green represents flooded vegetation. The other constraints are related to the SAR ability to detect water in some landscapes, like areas of high relief or dense vegetation and to the possibility to confuse some targets like melting snow, frozen soil, or desert areas with water surfaces. These constraints are common to any automatic algorithm that does not use any amount of supervision. However, the presence of water is very unlikely in steep slope areas.