Hydrological variability of large rivers in West Africa: gap filling with earth observations and daily rainfall-runoff modelling under climate change

In West Africa, hydrological variability remains poorly understood in many watersheds where observation networks are sparse. After gap-filling discharge time series through earth observation datasets and daily rainfall-runoff modeling, past and future hydro-climatic variability in the upper catchments of the Gambia, Koliba-Corubal, Kayanga-Geba, and Senegal rivers is investigated. CHIRPS rainfall and GLEAM evapotranspiration global datasets are used to simulate runoff in 38 sub-basins over 1981-2023 with the GR4J model. Seven RCM models from CORDEX-Africa are then employed to investigate climate change impacts on river flow under RCP4.5 and RCP8.5 scenarios over 2036-2065. Robust performance observed in 34 basins (KGE > 0.5) confirm the effectiveness of the approach including at a daily time step in poorly gauged basins. A dry period (1981- 1993), followed by two wet periods (1994-2007, 2008-2023) are identified using standardized precipitation index (SPI), standardized streamflow index (SSI) and the non-parametric Mann-Kendall test. Increased variability in extreme flows and a later flood are also observed in some basins. Climate change projections point towards an important decrease in flow in the Senegal river subbasins, reaching up to 70% under both scenarios, compared to the reference period (1986-2015). In the Gambia, Kayanga-Geba and Koliba Corubal subbasins, no homogenous trend is detected with some RCMs leading to a decrease while others, including GDFL and CSIRO circulation models, project an increase in runoff. Understanding non-stationarity in West African basins in the context of climate change is essential to support stakeholders in defining adequate river basin development strategies.

Keywords: Gap filling, Remote sensing datasets, Hydrological Modelling, Hydrological Variability, West Africa, Large Rivers; GR4J, CORDEX-Africa, Climate Change.