Seasonal and interannual variability of water-covered areas in the Ouémé delta (Bénin) from 2015 to 2023.

The Ouémé delta, located in southern Benin, serves as the main tributary system for the Nokoué lagoon, fed by the Sô and Ouémé rivers, that cross a vast plain of wetlands. This region experiences pronounced seasonality driven by the African monsoon, leading to significant river level fluctuations that cause major flooding and pose threats to the livelihoods and safety of riverine communities.

The aim of this study is to assess the spatio-temporal variability of water levels and inundated areas extent at seasonal and interannual scales from 2015 to 2023, using in-situ and satellite data in the Ouémé Delta.

First, water level data obtained from in-situ and Sentinel 3 altimetry in the rivers and the lagoon, show significant seasonal variability, with a difference of 8 meters between the dry season (December-April) and the wet season (September-November) in the Ouémé river. Strong interannual variability was also observed in Nokoué Lagoon, particularly between 2020, which experienced a minor flood (+0.75 m) and 2022, characterized by a major flood (+1.5m).

Then, the flood extent was analyzed using Sentinel-1A radar imagery. The average inundated area is estimated to 100 km² during wet season. Monthly flood probability maps indicate that the southern part of the Sô River is the most frequently flooded. These maps further highlight the southward propagation of flooding during the wet season. Since 2018, a marked increase in flood extent has been observed. From 2015 to 2017, the average inundated areas was around 40 km². However, post-2018, this figure has more than doubled. Between 2018 and 2023, significant year-to-year variations were observed, with a difference of 90 km² inundated areas between 2020 and 2022.

These obtained results, validated with independent data (Suomi-NPP/VIIRs flood data), provide a better understanding of the hydrological dynamics in the Ouémé delta. This work is conducted as part of the SCOast-DT and TOSCA projects, funded by the CNES (French space agency).