Impact of climatic variability on water resources in a humid tropical climate
This study aimed to show the appearance of a climatic change in the last decade in Bonguanou department, situated in eastern Ivory Coast (humid tropical zone) and to elucidate its impact on groundwater supply. To this end, different statistical and hydrologic methods have been used. The application of the Nicholson index and Pettitt test to the rainfall series (1920-2005) revealed climatic variability, characterized by two great different phases. The results show a rupture in 1968, which marks an alteration in the rainfall and hydrological regimes. This rupture is associated with a diminution of the rainfall by about 19% on average and an increase of 0.7°C in the temperature of the region. It suggests climatic warming in this tropical environment and is characterized by two great periods: a humid period, 1920 to 1968, and a dry or deficit period, 1969 to 2005. The hydrological balance method and the Maillet method were used to determine the impacts of climatic warming on water resources in this region. The results show that, before the rupture (1968), the annual average recharge was 217 mm; in dry period, it decreased to 145 mm, a drop of 33%. The annual average surface flow, which was 80 mm before the rupture, is reduced to 35 mm. The estimated recession coefficients show a more rapid draining of the regulating reserves in the dry period (21 d) compared with the humid period (27 d). The average annual mobilized water volumes in the aquifers have sustained a drop of 30%. In humid period, this annual average volume was estimated to have been 1.475 km3 in the Bongouanou region. The agreement of the estimates for the recharge volume and for the mobilized water volumes, by two independent approaches (hydrological balance and hydrogram analysis), before and after the 1968 rupture, shows the importance of regular and sustained rainfall for recharge of the hard rock aquifer.