Extract the water extent before flood event

[kleok]

The pre-flood water extent will be extracted by Sentinel-1 imagery and Sentinel-2 images before the flood.

• For Sentinel-1 acquisitions (@kleok )

• We select only recent (last month) Sentinel-1 acquisitions that are not related to "big" precipitations events.
• We calculate the median backscatter coefficient over time.
• We download a third-party dataset for water extent (e.g. JRC)
• We create a buffer zone of the water extent
• We calculate threshold (e.g. Otsu) and apply at median backscatter coefficient

• For Sentinel-2 acquisitions (@alekfal)

• We select only recent (last month) Sentinel-2 acquisitions with low cloud coverage (< 10%)
• We calculated MNDWI and apply cloud masks
• We calculate the median MNDWI values over time.
• We calculate a suitable MNDWI threshold at the buffer zone. We can have a look here.
• We apply thresholding at median MNDWI values.

• Combination of Sentinel-1 and Sentinel-2 water masks (@kleok, @alekfal)

• The final map is calculated as the union of Sentinel-1 and Sentinel-2 water masks