Palaeohydrogeology of the Senegal sedimentary basin: A tentative explanation of the piezometric depressions

Dieng, Babacar; Ledoux, Emmanuel; Marsily, Ghislain de

doi:10.1016/0022-1694(90)90268-3

Cited by 17 publications

(12 citation statements)

References 4 publications

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“…ARCHAMbAULT (1960ARCHAMbAULT ( , 1987 définit les « nappes en creux » comme des anomalies hydrogéologiques majeures, très répandues en Afrique subsaharienne semi-aride, se traduisant par des courbes piézométriques fermées et concentriques, des creux très accentués pouvant atteindre plusieurs dizaines de mètres au-dessous du niveau normal de la nappe. Les connaissances sur ces types de nappe se sont enrichies de nombreux travaux dont DEGALLIER (1962), DEPAGnE (1967, DIEnG et al (1990), nGOUnOU nGATCHA (1993), ARAnYOSSY et nDIAYE (1993), FAVREAU et al (2002, LEbLAnC et al (2003), GAULTIER (2004).…”

Section: The Relationship Between ²H Andunclassified

Apport de la géologie, de l’hydrogéologie et des isotopes de l’environnement à la connaissance des «nappes en creux» du Grand Yaéré (Nord Cameroun)

Ngatcha

Mudry²,

Aranyossy

et al. 2007

rseau

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La carte piézométrique de la nappe du Logone-Chari-Tchad met en évidence des anomalies piézométriques interprétées comme des « nappes en creux ». Les informations de l’hydrogéologie et des isotopes de l’environnement conduisent à remettre en question les grandes profondeurs des niveaux statiques observées par certains auteurs dans ces dépressions piézométriques. Les données hydrogéologiques démontrent que dans la zone déprimée de la surface piézométrique, l’aquifère est de type bicouche. Par ailleurs, la distribution des teneurs en isotopes stables (oxygène-18 et deutérium) et en tritium confirme le cloisonnement des aquifères : La relation δ 2H vs. δ 18O montre que les effets d’enrichissement par évaporation lors de la recharge des nappes ne sont très marqués que dans les eaux des nappes superficielles dont les niveaux statiques ne dépassent pas 20 m de profondeur. Les dépressions fermées dont les points les plus bas atteignent 60 m sous la surface du sol s’interprètent difficilement dans l’hypothèse d’une reprise évaporatoire. Il résulte de cette étude que l’absence de dépendance nette entre les niveaux piézométriques superficiels et les niveaux profonds place le problème des anomalies piézométriques du Grand Yaéré dans un contexte totalement différent de celui des anciennes interprétations qui s’appuyaient sur l’hypothèse d’une nappe libre généralisée monocouche. À l’avenir, la construction de la carte piézométrique de la nappe du Logone-Chari-Tchad devra tenir compte de la structure des deux nappes superposées.

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Section: The Relationship Between ²H Andunclassified

Apport de la géologie, de l’hydrogéologie et des isotopes de l’environnement à la connaissance des «nappes en creux» du Grand Yaéré (Nord Cameroun)

Ngatcha

Mudry²,

Aranyossy

et al. 2007

rseau

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“…Conversely, in aquifers with higher hydraulic diffusivity, one would expect the transient behavior to occur over shorter time periods as they adjust more rapidly to any hydraulic perturbation [ Neuzil , ]. However, several studies, based on numerical models, have examined the effect of past climatic conditions on present‐day hydrodynamics [ Burdon , ; Lloyd and Farag , ; Dieng et al ., ; Love et al ., ; de Vries , ; Coudrain et al ., ; Houston and Hart , ; Jost et al ., ; Sy and Besbes , ] showing that aquifers may present long‐term transient behaviors due to past‐climatic variations.…”

Section: Introductionmentioning

confidence: 99%

Time to reach near-steady state in large aquifers

et al. 2013

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[1] A new analytical solution of the flow equation has been developed to estimate the time to reach a near-equilibrium state in mixed aquifers, i.e., having unconfined and confined portions, following a large hydraulic perturbation. Near-equilibrium is defined as the time for an initial aquifer perturbation to dissipate by an average 95% across the aquifer. The new solution has been obtained by solving the flow system of a simplified conceptual model of a mixed aquifer using Laplace transforms. The conceptual model is based on two assumptions: (1) the groundwater flow can be reduced to a horizontal 1-D problem and (2) the transmissivity, a function of the saturated thickness, is assumed constant on the unconfined portion. This new solution depends on the storativity of the unconfined portion, the lengths of the unconfined and confined portions and the transmissivity, assumed to be constant and equal in both portions of the mixed aquifer. This solution was then tested and validated against a numerical flow model, where the variations of the saturated thickness and therefore variations of the transmissivity were either ignored, or properly modeled. The agreement between the results from the new solution and those from the numerical model is good, validating the use of this new solution to estimate the time to reach near-equilibrium in mixed aquifers. This solution for mixed aquifers, as well as the solutions for a fully confined or fully unconfined aquifer, has been used to estimate the time to reach nearequilibrium in 13 large aquifers in the world. For those different aquifers, the time to reach near-equilibrium ranges between 0.7 kyr to 2.4 Â 10 7 kyr. These results suggest that the present hydraulic heads in these aquifers are typically a mixture of responses induced from current and past hydrologic conditions and thus climate conditions. For some aquifers, the modern hydraulic heads may in fact depend upon hydrologic conditions resulting from several past climate cycles.

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“…This aquifer extends over 500,000 km 2 and is characterized by five large and closed piezometric depressions, also referred to as “hollow aquifers” [ UNESCO , 1972; Eberschweiler , 1993]. Elsewhere in the Sahel band, other large piezometric depressions have been reported and various theories about their formation and dynamics have been proposed [ Aranyossy and Ndiaye , 1993; Dieng et al , 1990; Durand , 1982]. Yet to date, few data have been gathered to support any particular explanation.…”

Section: Introductionmentioning

confidence: 99%

Application of Meteosat thermal data to map soil infiltrability in the central part of the Lake Chad basin, Africa.

Leblanc

Razack

Dagorne³

et al. 2003

Geophysical Research Letters

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1] In the central part of the Lake Chad Basin, Africa, the superficial Quaternary aquifer (500,000 km 2 ) forms the main water resource. Little is known about the aquifer recharge processes. Large piezometric depressions affect the aquifer and are still unexplained. Meteosat thermal composite data were used to infer qualitative information about time-space variations of soil moisture. Over the aquifer, Meteosat data reveal that after heavy rainfall, the piezometric depressions (Kadzell, Chari-Baguirmi) appear cooler than the surrounding areas (Manga and Harr). The interpretation is that above the depressions, rainwater accumulates at the surface and does not infiltrate deep into the ground, leading to the observed cooler ground. Accordingly, the depressions are characterized by low rainwater infiltrability, which presumably results in a small rainfall recharge. As far as we know, this is the first time that an observed surface phenomenon is directly related to the origin of some piezometric depressions.

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Palaeohydrogeology of the Senegal sedimentary basin: A tentative explanation of the piezometric depressions

Cited by 17 publications

References 4 publications

Apport de la géologie, de l’hydrogéologie et des isotopes de l’environnement à la connaissance des «nappes en creux» du Grand Yaéré (Nord Cameroun)

Apport de la géologie, de l’hydrogéologie et des isotopes de l’environnement à la connaissance des «nappes en creux» du Grand Yaéré (Nord Cameroun)

Time to reach near-steady state in large aquifers

Application of Meteosat thermal data to map soil infiltrability in the central part of the Lake Chad basin, Africa.

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