The European Commission's Joint Research Centre went through every Landsat image since 1984 and recorded, for each 30-metre square, in what percentage of the months with a usable image there was water in sight. A permanent lagoon scores close to 100%; a low spot that fills up in wet years, between 10 and 30%; a high paddock, zero.
It is the most direct way to see whether a property floods without waiting for rain. A visit in a dry year shows nothing, and thirty-seven years of images do: a low spot that flooded in 1987, 2001 and 2017 shows up even if it is cropped today.
The report sums it up in four classes: occasional water (10 to 25% of the time), floods often (25 to 50%), water most of the time (50 to 75%) and permanent water (75% or more), with the share of the property in each. Below 10% it is discarded: at that frequency it is usually cloud shadows or misclassified lagoon edges.