Frequency analysis of the evolution of hydropluviometric maxima in the Volta watershed at the Oti-Pendjari outlets

In recent years, hydroclimatic disasters have become increasingly recurrent and weaken environmental and human systems. The objective of this study is to conduct a frequency analysis of rainfall and hydrometric maxima in the Volta watershed at the Oti and Pendjari outlets. The annual maximum values in 24 hours of rainfall and extracted flows have undergone a series of work including a global statistical assessment; statistical hypothesis tests followed by a graphical and numerical evaluation (AIC, BIC) and quantile estimates for the return periods of 2 years, 5 years, 10 years, 15 years, 20 years and 30 years. The statistical tests include independence and stationarity tests; the extreme adjustment laws used are those of GEV (three parameters), Gumbel (two parameters) and Weibull (two parameters). The results show that out of the 21 stations, the maximum sampling size of the maximum annual daily rainfall is 40 years (most stations) and minimum 28 years (only the Porga station). The maximum value of the extremes recorded so far is 226 mm and a minimum of 19.4 mm at the Bakoissi station. The average of the maximum annual rainfall in 24 hours of all the stations in the basin is 74.35 mm, thus indicating a well-watered area at the advent of extreme rainfall. It also shows high asymmetry and flattening coefficients across the data from the Natitingou station and low at the Tanguiéta station, demonstrating a variability of extremes in the Oti-Pendjari watershed. It is briefly apparent that 18 of the 21 stations have data that follow the Gumbel law, while stations such as Fada Ngourma and Natitingou follow that of GEV and Tanguieta that of Weibull. The estimation of the quantiles at the different return periods is 173 mm (maximum) for a return period of 50 years at Barkoissi and 61.5 mm (minimum) at Porga.