# Detection of Surface Water and Floods with Multispectral Satellites
## 1 Introduction
Over the past few years, different methods for water segmentation and flooded area mapping using multispectral satellite images have been proposed in the literature. These are generally divided into single-band and multi-band approaches. While the former discriminates water features from other surfaces by thresholding a single band, usually the near infrared (NIR), which is highly absorbed by water, the latter uses a combination of two or more bands for deriving spectral indices. The multi-band method, also called water index method, is a promising approach for surface water mapping both in data-rich and data-poor catchments [25]. It performs better than the single-band method in detecting land surface water, since it exploits differences in the reflectivity of the selected bands [26]. Multispectral remote sensing-derived indices allow flooded areas and water bodies to be quickly and effectively recognized. However, this ability can be highly compromised by the presence of scattering noise, built-up areas, clouds, and shadows. In addition, spectral index performances can be different according to the geomorphological conformation of the territory, as well as the land cover conditions, i.e., crops, forests, artificial surfaces, or open water. In this context, this paper aimed to gain insights into the state of the art of multispectral satellite remote sensing and approaches to water extent delineation and flood mapping. In detail, the review aimed to achieve the following:  
1. Providing an overview of the current optical satellites used in flooded area and wetland inundation mapping, with a focus on some of the medium-high-spatial resolution sensors that offer free-of-charge data. 2. Highlighting the potential and limitations of the use of spectral indices for flood mapping and water segmentation, with particular attention to the land cover setting. A systematic review to address the first objective was initially carried out. The analysis of the literature allowed us to identify the most frequently used sensors in flood detection studies, as well as the main approaches and advances. Then, to address the second objective, a further investigation of the literature was conducted to identify relevant studies that used spectral indices for water extent delineation and flooded area detection. Several authors comparedthe use of different water index methods for both surface water body extraction [5; 27; 28; 29] and flood mapping [30; 31; 32]. Therefore, we summarized the main outcomes from the literature, identifying the most reliable indices to detect the presence of water (i.e., open water or flooded areas). In addition, a further analysis of the selected literature was conducted to classify the performances of the most used multispectral indices according to the land cover conditions, with the aim to provide guidance for targeted applications of flood mapping using multispectral remote sensing. The spectral indices considered included, among others, the Normalized Difference Vegetation Index (NDVI [33]), the Normalized Difference Water Index (NDWI [34]), and the Modified Normalized Difference Water Index (MNDWI [26]). The manuscript is organized into five sections, including the introduction. Section 2 describes the main characteristics of the most common optical satellites that offer free-of-charge data to provide a summary of their characteristics and data accessibility. The methodology adopted to conduct the proposed systematic review in the context of flood detection and hazard mapping from selected optical remote sensors is also illustrated, and an analysis of the main trends emerging from this review is presented. Section 3 introduces the most used spectral indices for the extraction of surface water extent and describes the main characteristics of a performance assessment for image classification and the corresponding error metrics.