# Detection of Surface Water and Floods with Multispectral Satellites
## 5 Conclusions  
Optical satellites represent a straightforward instrument for flooded area and water body mapping. In fact, the multi-band sensors allow the spectral signatures of different objects to be exploited and information to be derived through a direct visual interpretation of scenes of specific bands or color composites. In addition, multispectral imagery allows information about river morphology dynamics and land cover changes to be integrated in flood risk modelling and provides good spatial resolution for flood management applications, although with some limitations. The proposed analyses had the more general aims of identifying sensors and methods most used and best suited for monitoring water-related processes, highlighting the potentials of spectral indices, and providing some general practical guidance for targeted applications in different contexts. We believe that this could be beneficial not only for satellite-based remote sensing but also for UAS-based environmental monitoring, which in addition allows the cloud cover issue that affects optical remote sensing to be overcome. The analyses presented here can, indeed, be transferred to airborne-based applications to identify the best methodology that can ensure a reliable flood-prone area delineation using multispectral sensors. The opportunities offered by UAS technology are not only related to the possibility of flying below cloud cover layers and vegetation canopies, as it also enables submeter-level spatial resolution acquisition necessary for a detailed understanding of flood processes (see [9]). On the other hand, UASs have the limitation of not being able to survey vast areas. Although this issue could be overcome with multiple UASs flying simultaneously over the same area, proper regulations that allow multi-UAS flight to be performed are still lacking. Nevertheless, the potentiality offered by such a system represents a versatile support to satellite imagery. Promising applications of passive remote sensing under cloud conditions are also those from the CYGNSS constellation, which presents the advantages of similar microwave sensors (such as Synthetic Aperture Radar (SAR)) of seeing through clouds but at higher temporal resolutions. New-generation passive satellites are, in fact, characterized by a higher revisit time over land and ocean, which increases the chance to have closer post-flood observations. The aforementioned CYGNSS is one example, but valuable applications, still at their dawn, are also those offered, for example, by the family of small satellites of CubeSat. Although suffering from cloud cover problems, as with optical data, it provides both high-temporal (nearly daily)- and -spatial-resolution (~3 m) multispectral imagery on the global scale [126; 127; 128]. Future research directions should expand the unprecedent opportunities offered by the new generation of passive satellites, which could be enhanced by merging data from multiple sources, thus combining the potential of multispectral imagery at high temporal resolution (e.g., CubeSat data) with the capabilities of radar data (e.g., CYGNSS). Moreover, hybrid approaches, also including NOAA AVHRR data, which increase the possibility of obtaining cloud-free data, could also be explored. Finally, the advent of emerging technologies such as hyperspectral imagery (e.g., the recently launched Environmental Mapping and Analysis Program (EnMAP) German satellite) offers the possibility to better discriminate the spectral signature of different surfaces (thanks to the many different spectral bands) and to be used in conjunction with multispectral data, allowing detailed information to be retrieved, which is not achievable with multispectral data alone. Regarding multispectral indices, future opportunities can be represented by the application of the WAVI in flood mapping studies. In conclusion, both optical and passive microwave satellite sensors will likely continue to represent a reference for Earth imaging applications, enabling a better understanding of surface water dynamics.