Recharge and baseflow constrained by surface-water and groundwater chemistry: case study of the Chari River, Chad basin

The Chari-Logone watershed is the only hydrologically active part of the Lake Chad Basin (Central Africa). The Chari-Logone River and Lake Chad exchange water with the surrounding unconfined aquifers of sedimentary and crystalline rock types. In this study, the groundwater contribution to stream flow was quantified by taking advantage of a comprehensive hydrological and chemical database from the 1970s for the upper catchment, which has no equivalent to date. The study area was limited to the larger Chari catchment where the exchanges are mainly oriented from the aquifers to the stream. Upon identification of the mixing poles using end-member mixing analysis (EMMA), and a Monte Carlo inversion of a monthly mixing mass-balance model, an annual averaged base flow of 59 ± 10% of the total stream flow was estimated for 1969–1973. Then, a hydrological model accounting for baseflow and surface flow was calibrated using a 47-year-long monthly discharge time series, and the 4-years’ monthly averaged chemical data available. The simulations yielded an average baseflow of 64% of the annual discharge. The time-constant of the reservoir model (τ = 0.2 years) used to simulate the baseflow was interpreted in terms of hydraulic diffusivity, yielding a value in the order of 10¹ m² s⁻¹. This large value corresponds to a highly productive bedrock and thick sedimentary aquifers. The main contribution to the Chari stream flows is restricted to a 140,000-km² subcatchment located in the southern tropical zone, where the recharge rate reaches 72 ± 6 mm year⁻¹.