The Sentinel-1 Reference Water Mask identifies the pixels classified as open and calm water - both permanent and seasonal - by applying the GFM ensemble water mapping algorithm to a two-year “data cube“ (or time series) of Sentinel-1 SAR backscatter intensity.
Whereas the mapping of permanent water extent uses as input the median backscatter of all Sentinel-1 data from a time period of two years, the mapping of seasonal water extent uses as input the median backscatter of all Sentinel-1 data from a given month over a two-year period. As a result, twelve masks are available, one per month, which includes information on the permanent and seasonal reference water extent. This parameter database is updated once a year. For example, the NRT system running in 2021 will rely on Sentinel-1 Reference Water Mask extracted from the Sentinel-1 pre-processed data cube from 2019 and 2020.
Radar shadow and low sensitivity exclusion layers as well as HAND index are applied to the reference water mask (and associated uncertainty layer) to correct pixels that were possibly misclassified. Finally, the Copernicus Global Surface Water Maximum Water Extent layer (Pekel et al. 2016) is used to remove possible false positive classifications, while the Copernicus Water Body Mask is used to correct false negatives (e.g., large lakes with roughened surface that has falsely been classified as land) and to enforce a consistent land-sea border.
In order to identify the observed water extent, the Sentinel-1 observed flood extent (derived by the GFM ensemble flood mapping algorithm) is joined with the Sentinel-1 reference water mask (derived by the GFM ensemble flood mapping algorithm), which represents the extent of open water bodies under normal conditions. In the literature, reference masks of the permanent water extent are often used for this purpose (Wieland and Martinis, 2019).
A truly permanent water area would mean that there was observed water coverage in every single observation of the considered time-period; i.e. the Water Occurrence (WO), which is the ratio between the number of water detections during a certain time-period and the number of valid observations of the same period, would be 100%. In order to consider uncertainties in the single water segmentations and the occurrence of hydrological extreme events the WO threshold is usually relaxed to a value of 85-90 % (e.g., Pekel et al., 2016).
For further information on how the product is generated please check also: https://extwiki.eodc.eu/GFM/PDD/GFMoutputLayers