Temporal variability of extreme precipitation in Cotonou: stable climate signal and worsening urban flood impacts

Urban flooding has worsened significantly in Cotonou (major flood events in 2010, 2018, and 2020), yet this deterioration occurs against a backdrop of largely stable extreme precipitation. This study analyses the temporal variability of extreme precipitation indices in Cotonou over 40 years (1984–2024) in order to explain this paradox. Based on daily rainfall data from the Cotonou synoptic station, eleven standardised ETCCDI climate indices were calculated at four temporal scales (annual, monthly, seasonal, and decadal). Monotonic trends were detected using the Mann-Kendall test and Sen's slope estimator, while structural breakpoints were identified through a combined application of the Pettitt and Buishand tests. Contrary to widespread expectations of intensifying extremes, the results reveal that only the Simple Daily Intensity Index (SDII) presents a significant downward trend (Sen's slope = −0.084 mm d −1 yr −1 , p =0.034). Three structural breakpoints were detected during the 2008–2012 period based on the Buishand criterion ( Q >1.45): SDII in 2011, CWD in 2008, and R50mm in 2012. The frequency and intensity of extreme events (R95p, R99p, R95pTOT, R99pTOT) show no significant trend at any of the temporal scales considered. These findings demonstrate that the worsening of urban flood impacts in Cotonou results primarily from accelerated urbanisation – population growth of +140 % and soil impermeability of 70 %–80 % – rather than from climate change intensification, a pattern consistent with observations in other rapidly urbanising West African coastal cities.