# Detection of Surface Water and Floods with Multispectral Satellites
## 2 Multispectral Satellite Remote Sensing for Flooded Area and Wetland
### Trends in Using Multispectral Imagery for Flood Mapping  
The international Scopus academic database was used to acquire the relevant literature in the field of flood mapping extracted from free-of-charge optical satellite data. The multispectral sensors considered for the review were the Landsat, Sentinel-2, and MODIS satellites. An initial exploration was carried out using a list of keywords for each satellite. An example of search instructions for the Landsat program is the following: (flood AND mapping AND Landsat) OR (floodplain AND mapping AND Landsat) OR (flood AND hazard AND mapping AND Landsat) OR (flood AND detection AND Landsat). A total of 1153 documents were initially found in the database. A series of exclusions criteria were used to limit the search results only to journal articles, conference papers, and book chapters in the English language published in the last twenty years. All the relevant literature on flood mapping using multispectral imagery accessible to the authors was included. A further screening of the titles and abstracts of the selected papers was carried out to exclude articles that were not a topic of interest for the research study. Finally, multi-sensor approaches (i.e., those including a combined use of, for example, optical and radar data; optical imagery acquired with different sensors, such as Landsat and Sentinel-2, Landsat and MODIS, etc.; optical images and other sensors or data, such as National Polar-orbiting Partnership Visible Infrared Imager Radiometer Suite (NPP-VIIRS) and Soil Moisture Active Passive (SMAP); or satellite data integrated with hydrological modelling) were not considered, since the focus of this analysis was to only address single-sensor applications. The above criteria led to a selection of about 146 articles from 2002 to 2021 published in high-impact journals in the field of hydrological processes and remote sensing (e.g., MDPI's _Remote Sensing_, Elsevier's _Remote Sensing of Environment_ and _Journal of Hydrology_, and Springer's _Natural Hazards_).  
Figure 1 describes the number of studies exploiting specific satellite products for flooded area and wetland inundation mapping. Relevant documents that met the search criteria were found starting from 2002, 2008, and 2018 onwards for Landsat, MODIS, and Sentinel-2 data, respectively. Therefore, it is worth highlighting that the dates reported in the figure do not refer to the years of the launch of each. It is interesting to remark that the total number of studies (dashed black line) increased significantly over time thanks to the increased availability of free-of-charge data. Considering that Landsat is the longest-running remote sensing system, it also represents the most used dataset. MODIS and Sentinel-2 are more recent missions, and their trends are not clearly detectable. Nevertheless, it seems that MODIS initially received a good level of attention, while the number of studies exploiting this product started to slightly decrease after 2012. On the other hand, the history of Sentinel-2 has still to be written, since the 2B instrument started to operate only in 2017, and few documents on the topic of interest were found from 2018 onwards.