Impact of rainfall variability and land-use changes on river discharge in Sanaga catchment (forest–savannah transition zone in Central Africa)

Abstract Climate change/variability and land-use changes are the main forcings of river discharges variability. However, an understanding of their simultaneous impacts on river discharges remains limited in some parts of the world like in Central Africa. To shed light on this issue, the objective of this article was to investigate the effects of rainfall variability and land-use changes on river discharges in the Sanaga watershed (at Song Mbengue and Nachtigal gauging stations) and some of its sub-watersheds (Mbakaou, Lom Pangar, Magba and Bamendjing) over the 5 or 7 recent decades (depending on the data availability). To achieve this goal, hydrometeorological data of the Sanaga watershed and sub-watersheds were analyzed using the Pettitt and Mann-Kendall tests. Likewise, land-use changes in the watershed and sub-watersheds were also analyzed using supervised classifications of Landsat satellite images of the watersheds at two periods (1984 and 2020). The results show that annual rainfall decreased throughout the Sanaga watershed. This decrease is only statistically significant for the Sanaga watershed at Nachtigal (−5%), for which the study focused on relatively longer hydropluviometric series, including the 1950 and 1960s (wet decades). However, although the rainfall decreased in this watershed, the flows increased insignificantly according to the tests used in most cases. The 2010s seems particularly concerned by this increase, including in the Sanaga watershed at Nachtigal, where the general trend is downward. The flows increase in the Sanaga watershed would be the consequence of the increase in impervious areas in the latter (between +181.3 and +1,300% for built and roads and between +4.1 and +11.9% for bare soils), which would compensate for the drop in precipitation by increasing runoff. These results could be used for long-term planning of water demand and use in this watershed, as well as for improving future simulations of flows.