Spatio-temporal assessing rainfall and dam impacts on surface runoff in the Sanaga river basin (transition tropical zone in central part of Cameroon)

The Sanaga River Basin (SRB) plays an important role in the national development of Cameroon, due to its hydroelectric, agricultural and mining potentials. The present study aims at analysing the spatiotemporal impact of rainfall and different dams on the streamflow time series over the SRB while evaluating them individually. To conduct this study, the methodology consisted to collect the rainfall and stream flow time data series from ten meteorological stations and ten national hydrometric stations. These data cover the period from 1940 to 2010 (71 years) for the former and from 1945 to 2012 (68 years) for the latter. All of these data, which cover the entire basin geographically, were processed using: mathematical calculations (specific flow, volumes and annual water flow), simple statistical methods (dispersion parameters, Thiessen mean, etc.) and non-parametric statistical methods (Mann Kendall, Pettitt and Hubert tests/models). The result reveals that the average rainfall varies between (1497.62 ± 154.43) mm and (1837.80 ± 158.18) mm for the Bétaré-Oya and Goura catchments, respectively. However, Bétaré-Oya and Nachtigal basins show a break in their annual with a decline of about − 5.64% and − 5.02% in 1969. The Sanaga s.s. at Goyoum and the Sanaga at Songmbengué recorded their lowest water levels in April (198.74 m³·s⁻¹ and 772.11 m³·s⁻¹ respectively). The maximum monthly flows for the vast majority of stations are observed in October with values varying between 151.65 m³·s⁻¹ (69.25 L·s⁻¹ km²) at the Noun station in Bamendjin and 5293.46 m³. s⁻¹ (40 L·s⁻¹ km²) at the Sanaga station in Edéa and exceptionally in September at the Djérem station in Mbakaou with 1027.22 m³·s⁻¹ (51 L·s⁻¹ km²). On an annual scale, the Sanaga drainage basin has been experiencing a period of deficient runoff, manifested by the decrease in water flow since the end of the 1970s, with the longest periods obtained generally in stations located upstream of the basin (Bamendjin (1964/65), Magba (1969/70) and Mbakaou (1970/71) stations). This modification reveals the decrease in average annual flows between − 5 and − 42% depending on the station, with an average of − 23%, which is twice to four times greater than precipitation (− 5 to − 13.50%). However, the impact of the pluviometric decline in the 1970s associated with the installation of the Mbakaou dam in 1969 was so strong that the reservoir dams set up after this date have had no effect on the evolution of the annual flow in downstream stations of Djérem at Mbakaou, Sanaga s.s. at Nachtigal, from Sanaga to Songloulou and Edea. Hence, of all the events that could have affected the water resources of the Sanaga drainage basin in recent years, the pluviometric decline of the 1970s and the installation of the Mbakaou dam in 1969 are considered to be major events. Yet, the rainfall impact is higher; in the order of − 18.17% while that of dams is only − 2.63%. In the basin as a whole, there is a decrease in frequencies and a loss of the highest flow classes, accompanied by an increase and creation of new classes of lowest flow rates after the break phase. This would partly explain the energy crisis that Cameroon is experiencing through its southern interconnected network. However, this basin still has significant hydraulic potential, hence the ongoing construction of the Nachtigal and future Songmbengué dams.