# A Tool for Pre-Operational Daily Mapping of Floods and Permanent Water Using Sentinel-1 Data
## 1 Introduction  
Timely and continuous monitoring of the extent of surface water bodies is very important in the management of the living environment. On the one hand, during periods when precipitation is significantly lower than the average one, the consequent shortage of water propagates through the hydrological system and may cause drought in the different segments of the hydrological system itself, including surface water [1], whose spatial distribution represents therefore a fundamental piece of information for drought management. On the other hand, when heavy precipitation causes floods, near-real time data about floodwater extent and duration are essential to support rescue and damage recovery decisions and to facilitate rapid assessment of property loss and damage [2]. Several past studies (e.g., [3; 4; 5]) demonstrated that synthetic aperture radar (SAR) systems are suitable tools for surface water mapping. They combine a high spatial resolution (from hundreds of meters to approximately 1 m, depending on the SAR acquisition mode) with the capability, typical of microwaves, to provide data during day and night and even in the presence of cloud cover. Moreover, there is a clear physical mechanism for radar scattering from water, that is, specular reflection [6]. Unless strong wind is blowing abovethe water surface, the latter has a very low roughness, and most of the incident radar energy is scattered towards the specular direction. Therefore, a water surface is generally characterized by a low backscattering and appears dark in a SAR image. Conversely, the rough non-flooded terrain exhibits a higher backscattering and a brighter image tone [7], because it scatters the radar signal in many different directions, included the backscattering one. Specular reflection is not always the prevailing effect of the radar scattering from water. In urban and agricultural flooded areas (if plants emerge from water), a dihedral effect involving water surface and vertical structures such as stems, shrubs, or walls can take place, producing an increase of the backscattering. However, several unknowns (e.g., structure and geometry for vegetation, or height and orientation of buildings for urban areas) influence the radar response from these targets [8]. Consequently, although new insights were provided on detecting inundations in agricultural areas [9; 10; 11; 12; 13] and urban settlements [8; 14; 15; 16; 17], this task is still very challenging. Not only missed detections (i.e., omission errors), but also commission errors may occur when mapping surface water. In areas with complex topography, the shadowing effect strongly influences the radiometry of SAR data and produces a low backscatter that can be confused with the backscatter from water bodies. Low backscatter values are measured by SAR even in the presence of heavy precipitation (especially when dealing with X-band data) and wet snow [18; 19; 20; 21]. Furthermore, very smooth surfaces like asphalt (roads, airports, parking lots) and/or very dry areas like deserts appear dark in SAR imagery and may be misclassified as water. Despite the challenges previously discussed, many processing algorithms have been proposed for mapping permanent water and floodwater using SAR data in a fully automated way (e.g., [11; 17; 22; 23; 24; 25; 26]). Therefore, this application can be considered mature for operational/pre-operational implementation, at least for what concerns open water. Currently, the Copernicus Emergency Management Service (CEMS) uses satellite imagery to rapidly provide geospatial information about natural disasters. Regarding floods, SAR data are very often used by CEMS for mapping purposes to overcome the limits of optical data related to the presence of clouds and the impossibility to acquire data during nighttime (e.g., [27]). CEMS is available 24 h a day, 365 days-a-year, but is an on-demand service, which is triggered on request by authorized users.