Recharge and baseflow constrained by surface-water and groundwater chemistry: case study of the Chari River, Chad basin
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Recharge and baseflow constrained by surface-water and groundwater chemistry: case study of the Chari River, Chad basin Julio Gonçalvès 1 & Abdallah Mahamat Nour 2 & Camille Bouchez 3 & Pierre Deschamps 1 & Christine Vallet-Coulomb 1 Received: 24 January 2020 / Accepted: 21 October 2020 # Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract 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 4years’ 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. Keywords Groundwater/surface-water relations . Groundwater recharge/water budget . Hydrochemistry . Conceptual models . Chad
Introduction At small, mostly hourly, time scales the components contributing to river discharge, i.e. rapid runoff, lateral hypodermic (or surface) flow, and baseflow, can be identified using various methods of hydrograph separation such asrecession curve analysis based on a “Maillet type” (exponential) reservoir decay, a recursive digital filter method and geochemical tracers (Nathan and McMahon 1990; Tallaksen 1995; Klaus and
* Julio Gonçalvès [email protected] 1
Aix Marseille Univ, CNRS, IRD, INRAE, Coll France, CEREGE, Aix-en-Provence, France
2
Université de N’Djamena- Laboratoire HydroGéosciences et Réservoir, N’Djamena, Chad
3
Univ Rennes, CNRS, Géosciences Rennes, UMR 6118, Rennes, France
McDonnell 2013). Geochemical hydrograph separation using three end-members and thus three reservoirs is now widely used (Klaus and McDonnell 2013), but it needs large geochemical and hydrological databases. When data are scarce, the analysis has to be limited to two reservoirs. This can apply to rap
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