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Bilan et dynamique des interactions rivières-lac(s)-aquifères dans le bassin hydrologique du lac Tchad : Approche couplée géochimie et modélisation des transferts

Abstract : Lake Chad Basin (LCB) is a 2.5 billion km2 closed drainage basin, located in the center of Africa. Surface waters (rivers and lake) are unequally distributed and highly variable. The lake Chad, which shows strong surface variations, enhances the climatic variations over the LCB. The multi-layer aquifer system of the LCB is composed of two geologic units (the quaternary (Q) and the Continental Terminal (CT)) containing large water resources. They could imbalance the increasing freshwater scarcity in the area but their recharge needs to be constrained. The understanding of surface water variability and ground water recharge is essential to support water management in the area. In this work, we tried to quantify the hydro(geo)logical response of the LCB to climatic variability by combining geochemical and modeling approaches. Sampling of water reservoirs (surface, Q and CT aquifers) have been carried out between 2011 and 2013. Lake Chad levels modeling between 1955 and 2011 was calibrated from hydrological, chemical and isotopic data. It allows to disentangle evaporation from transpiration and from infiltration. Transpiration, neglected in previous studies, accounts for 15 % of the total evapotranspiration and is particularly high in the Archipelagos. Based on the chlorine dynamic in the LCB, a transit time of 10 yr from the upper basin to the lower rivers, a short renewal rate of lake waters and infiltration of lake waters toward the aquifer are estimated. The detection of thermonuclear 36Cl together with the stable isotopic composition of groundwaters allow to draw the present-day recharge distribution in the Quaternary aquifer. We show that present-day recharge occurs in 60 % of the groundwater sampled but is confined to the vicinity of surface waters. An evaporation between 85 and 98 % of lake waters in the surrounding of Lake Chad is estimated by the comparison of model-calibrated infiltration rates with groundwater geochemical data. Groundwaters with a contrasted geochemical signature are stored in the center of the piezometric depressions, suggesting no present-day recharge in these enigmatic groundwater features. Although it would require further investigation, the recharge of these waters could be related to the African Humid Period. The deep waters of the CT, with water ages of 300 000 yr, show geochemical patterns indicative of an old and humid recharge period. These original data on the multi-layer aquifer of the LCB give new insights into paleorecharge in the Sahelian band. GCM paleoclimatic simulations over the last millennium have been introduced in a rainfallrunoff model (GR2M), coupled with our lake level model. This preliminary approach shows the difficulty to reconcile lake level records from paleohydrological modeling.
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https://hal.archives-ouvertes.fr/tel-01298153
Contributor : Camille Bouchez <>
Submitted on : Wednesday, April 6, 2016 - 10:54:18 AM
Last modification on : Tuesday, September 7, 2021 - 3:36:31 PM

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  • HAL Id : tel-01298153, version 1

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Camille Bouchez. Bilan et dynamique des interactions rivières-lac(s)-aquifères dans le bassin hydrologique du lac Tchad : Approche couplée géochimie et modélisation des transferts. Hydrologie. Aix Marseille Université, 2015. Français. ⟨tel-01298153⟩

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