Bayesian Trend Analysis in Annual Precipitation, Number of Rainy Days and Extreme Events in the Kara River Basin (Togo and Benin)

In this study, trend is investigated in annual rainfall, rainy days and annual maximum rainfall in the Kara River Basin (Togo and Benin) between 1950 and 2010 using Bayesian inference. To this end, a time covariate is introduced in the location parameter of the Generalized Extreme Value (GEV) distribution to fit annual rainfall maxima (MAXAN) while Normal and Lognormal (LN) distributions were fitted to annual rainfall, and Poisson distribution to rainy days with time varying mean parameters. The Metropolis-Hasting algorithm, a Markov-Chain Monte Carlo (MCMC) method was used for posterior sampling. Bayesian Kriging was applied for spatial analysis. The results show that the GEV and Poison distribution parameters converged well for respectively MAXAN and annual rainy days while Normal distribution showed better results for annual rainfall than LN distribution. In general, no significant trend has been found in MAXAN except at Niamtougou and Kara where they are decreasing. The annual rainfall is in general decreasing while rainy days are increasing at some stations and static at others. The results suggest that the spatio-temporal distribution of MAXAN and annual rainfall which depend on West African monsoon dynamics seems to be also influenced by local factors such as relief. The decrease in annual precipitation is associated by the decrease in MAXAN and rainfall amount during some months of the rainy season but also rainy events distribution. For most of the stations, the annual rainfall is decreasing but there is a shift in the pic of the rainy season from Spetember to August. The spatial analysis shows a significant decrease in rainfall isohyets over the basin in 1970s and 1980s and a slight increase from 1990s. The results of this study which are those of a first direct assessment in the basin constitute good baseline indicators for the integrated river basin analysis, vulnerability assessment and the sustainable water resources management of the Kara River Basin.