Annual Water Recurrence (1984-2019)

The Water Recurrence map documents the inter-annual variability of water availability. This map shows how frequently water returns from one year to another (expressed as a percentage). One can discriminate between places that are flooded on an episodic basis (shown in orange) and those that are regularly (and therefore predictably) inundated (shown in light blue).

As for the occurrence, the recurrence is expressed as a percentage, but unlike the occurrence map the recurrence map is not necessarily computed on the full archive (1984 to 2019). The recurrence map's mapping period starts in the year in which water was first observed at any given location (not when the first valid observation was obtained), and ends with the year in which the water was last observed. For a 'new' reservoir (i.e. one appearing when a dam is built sometime after 1984) the period we consider is between the year the reservoir fills and 2014. Recurrence maps are available both on a monthly basis (i.e. 12 maps showing the water recurrence for each single month of the year, allowing us to account for seasonality), and on an annual basis (i.e. one map showing the annual water recurrence, without seasonality considerations). A 100% water recurrence means that water is present every year from the beginning of the archive or since the first water observation year. Conversely low percentages characterize places where inundation is far from systematic, such that the lower the figure the greater the variability in inundation pattern.

For more detail see: Jean-Francois Pekel, Andrew Cottam, Noel Gorelick, Alan S. Belward, High-resolution mapping of global surface water and its long-term changes. Nature 540, 418-422 (2016) [https://www.nature.com/articles/nature20584].