Assessment of the long term change in the extreme hydrological events across the West African basins

Understanding the variability in extreme hydrological events is the first step toward the reduction of its impacts on livelihoods. This work aims to identify possible changes in the hydrological regime of the West African basins which could explain the recent flooding observed across the region. For that purpose, 56 discharge stations were selected with the data ranging between 1922 and 2017. Multiple trend patterns on moving time slots of 20 years for stations having at least 40 years of data were performed using the modified Mann Kendall test and the strength of the trend computed using the Sen’s slope. Statistically significant trends in annual maximal discharge (AMD) have been found for many stations and distinct periods with different change directions. While a decreasing tendency was found for the AMD between the 1950 and 1970s, an increasing frequency was depicted from the 1980s to recently (2017) for most of the stations. The magnitude of change per year in AMD as depicted from Sen’s slope is substantially variable ranging from -14 to 12% relative to the mean discharge of the considered period. The highest increases in AMD were found for the stations located toward the Gulf of Guinea. The temporal variation in AMD is in line with the interdecadal rainfall variability in West Africa.