Estimation du champ de transmissivité d’un aquifère alluvial fortement hétérogène à partir de la résistance transversale. Application à la nappe du Haouz de Marrakech (Maroc)

Cet article a pour objectif l’estimation indirecte de la transmissivité (T, m2/s) de l’aquifère du Haouz (6 000 km2) au Maroc, à partir de la résistance transversale (R, Ωm2). L’aquifère du Haouz est constitué par une succession complexe de séries lenticulaires, argilo-marneuses ou formées d’éléments grossiers, d’âge plio-quaternaire reposant sur un substratum marneux d’âge miocène. Une importante base de données des valeurs de transmissivité (≈500) et de résistance transversale (≈2 500) a été compilée. Une recherche a ensuite été effectuée pour retenir les couples (Ti, Ri) caractérisant le même volume d’aquifère. Deux cas de résistance transversale sont considérés : 1) résistance transversale de l’ensemble mio-plio-quaternaire (RA); 2) résistance transversale des lentilles perméables uniquement (RB). La meilleure régression, de forme géométrique, est obtenue entre la transmissivité et la résistance transversale des lentilles grossières perméables. Cette régression est sensiblement améliorée lorsque les valeurs (T, RB) d’un même couple sont ramenées à un même état piézométrique de la nappe, après correction de la transmissivité en fonction des variations piézométriques de la nappe. On procède ensuite à l’estimation indirecte du champ de transmissivité de l’aquifère à l’échelle régionale à l’aide de l’équation de régression précédemment établie. Le champ de transmissivité ainsi estimé est validé par comparaison de ses propriétés statistiques (tendance centrale, dispersion, loi de distribution) à celles de l’échantillon des valeurs de transmissivité mesurées par pompages d’essais.This article aims at the indirect estimation of the transmissivity (T, m2/s) of the aquifer of Haouz (6,000 km2) in Morocco, using the transverse resistance (R, Ωm2). This aquifer consists of a complex succession of lenticular layers, argilo-marly or formed of coarse elements, of plio-quaternary age, lying on a marly substratum of miocene age. A significant database of the values of transmissivity (≈500) and transverse resistance (≈2,500) was compiled. A research was then carried out to select the pairs (Ti, Ri) characterizing the same volume of aquifer. Two cases of transverse resistance are considered: 1) transverse resistance of the mio-plio-quaternary unit (RA); 2) transverse resistance of the permeable coarse lenses only (RB). The best regression, of geometrical form, is obtained between the transmissivity and the transverse resistance of the permeable coarse lenses. This regression is significantly improved when the values (T; RB) of a given pair are brought back to the same level of the water table, after correction of the transmissivity according to the variations of the water table. The indirect estimate of the field of transmissivity of the aquifer is then carried out at a regional scale using the regression equation previously established. The estimated field of transmissivity is validated through the comparison of its statistical properties (central tendency, dispersion, distribution law) to those of the sample of the values of transmissivity measured by pumping tests

Modeling of Transient Two Dimensional Flow in Saturated-Unsaturated Porous Media

We study the transient two-dimensional flow problems related to the recharge of groundwater using an approach based on the physics of water transfer in the whole domain, including both the saturated and the unsaturated zones. Such a domain is limited by the soil surface and by the lower impervious boundary of the aquifer. The flow problem is solved using a numerical iterative procedure based on the alternate directions implicit finite differences method. To simulate the water transfer in an unsaturatedsaturated porous medium, we developed a mathematical model based on a single flow equation used for both zones. To this end, the unsaturated and saturated zones are regarded as only one continuum and the Richards equation is used for both zones. The simulated results were validated by the calculation of the mass balance, then on values obtained by an experimental model of 160 cm length and 50 cm in height. The numerical model reproduces in a satisfactory way the experimental results giving the spacetime evolution of the level of water table

ETUDE EXPERIMENTALE ET MODELISATION NUMÉRIQUE DES TRANSFERTS HYDRIQUES ET DE SOLUTE DANS UN MILIEU POREUX NON SATURE-SATURE

In this work, a mathematical modeling tool was developped to simulate in a simple and fast way, and with an acceptable precision, the water and solute transfers in unsaturated-saturated porous medium. The resolution of this problem in unstationary mode, is based on the use of an mathematical formulation which consists in considering as a single continuum the saturated and the unsaturated zones. The modeling tool is based on two numerical methods: the alternate directions implicit method (ADI) and the finite elements method (finite elements code Freefem++). This code was adapted to the equations used in this study, namely: Richards’ equation to study the water flow in the unsaturated zone, the diffusivity equation expressed in a linear and nonlinear form for the groundwater flow and the transport equation of advection-dispersion type to study the solute transfer. The advantage of the Freefem++ code is that it generates selfadapting grids. This is interesting to analyse the transfers at singular zones, in particular at the capillary fringe. Experiments were carried out on a laboratory physical model, consisting in a sand filled tank of 160 cm length and 50 cm in height. This experimental model enabled to follow the spacetime evolution of the water-table and the propagation of a tracer (Fluorescein) from the infiltration strip to the discharge system of the model. The modeling tool was validated using experimental data measured on the small-scale physical model and some results taken from the literature. It allowed to predict with an excellent precision the water-table position and the propagation of the non-reactive solute in both unsaturated and saturated compartments. A comparative study between the two numerical methods was carried out and showed that even if the finite differences method provides satisfactory results, the use of the finite elements Freefem++ code presents a light advantage and provides slightly more accurate result

Applications of remote sensing and GIS for groundwater modelling of large semiarid areas: example of the Lake Chad Basin, Africa

Abstract Because of its large extent and the extremes of its climatic and environmental conditions, the Lake Chad Basin is an example of a region where it is extremely difficult to collect hydro(geo)logical field observations. So far, the scale and the scope to which remote sensing and GIS can assist groundwater modelling in such regions has not been fully exploited. We detail applications of remote sensing and GIS to improve groundwater modelling of the large superficial Quaternary aquifer, which covers 500 000 1cm 2 and forms the main water resource of the basin. Satellite imagery and GIS enabled us to refine the location of recharge and discharge areas. In a GIS framework, relevant maps and pertinent satellite images were analysed together with hydrogeological data. The rationale was to search and map key characteristics in the terrain that indicate groundwater discharge and recharge areas. In addition, maps and low cost satellite data, such as AVHRR and Meteosat, were used for a thorough mapping of the fluctuations of Lake Chad extent over the last three decades. Using GIS, this valuable information was implemented in a transient groundwater model, with the MODFLOW program. The model calibration was greatly improved by the use of remote sensing data

Eléments en faveur d'une karstification anté-oligocène supérieur dans le Narbonnais

A drowned karst located in limestones of superior Jurassic, under a covering of superior Oligocene, was revealed by hydrogeological researches carried out by one of us (M.R.). Elements from the filling of the karst were brough up during the cleaning by air-lift of the exploitation well. Their analysis together with observations made in a natural cave situated in the neighbourhood, lead us to attribute an oid age (ante-superior Oligocene) to the karstiflcation period of the Jurassic limestones.Des recherches hydrogéologiques effectuées par l'un de nous (M.R.) en 1976, au Nord-Ouest de Narbonne, ont abouti à la mise en évidence d'un karst noyé dans des calcaires du Jurassique supérieur sous un recouvrement datant de l'Oligocène supérieur. L'analyse d'éléments de remplissage du karst, remontés au cours du nettoyage du forage d'exploitation par emulsion, conjointement avec des observations effectuées dans une cavité pénétrable située à proximité, nous ont permis d'attribuer un âge ancien (anté-Oligocène supérieur) à la période de karstiflcation des calcaires jurassiques.Razack Moumtaz, Salvayre Henri. Eléments en faveur d'une karstification anté-oligocène supérieur dans le Narbonnais. In: Géologie Méditerranéenne. Tome 7, numéro 2, 1980. Géologie générale. pp. 187-191

Modeling daily discharge responses of a large karstic aquifer using soft computing methods: Artificial neural network and neuro-fuzzy

This paper compares two methods for modeling karst aquifers, which are heterogeneous, highly non-linear, and hierarchical systems. There is a clear need to model these systems given the crucial role they play in water supply in many countries. In recent years, the main components of soft computing (fuzzy logic (FL), and Artificial Neural Networks, (ANNs)) have come to prevail in the modeling of complex non-linear systems in different scientific and technologic disciplines. In this study, Artificial Neural Networks and Adaptive Neuro-Fuzzy Interface System (ANFIS) methods were used for the prediction of daily discharge of karstic aquifers and their capability was compared. The approach was applied to 7 years of daily data of La Rochefoucauld karst system in south-western France. In order to predict the karst daily discharges, single-input (rainfall, piezometric level) vs. multiple-input (rainfall and piezometric level) series were used. In addition to these inputs, all models used measured or simulated discharges from the previous days with a specified delay. The models were designed in a Matlab™ environment. An automatic procedure was used to select the best calibrated models. Daily discharge predictions were then performed using the calibrated models. Comparing predicted and observed hydrographs indicates that both models (ANN and ANFIS) provide close predictions of the karst daily discharges. The summary statistics of both series (observed and predicted daily discharges) are comparable. The performance of both models is improved when the number of inputs is increased from one to two. The root mean square error between the observed and predicted series reaches a minimum for two-input models. However, the ANFIS model demonstrates a better performance than the ANN model to predict peak flow. The ANFIS approach demonstrates a better generalization capability and slightly higher performance than the ANN, especially for peak discharges

Hydrogéologie des réservoirs fracturés de socle (analyses statistique et géostatistique de la fracturation et des propriétés hydrauliques ; application à la région des montagnes de Côte d'Ivoire (domaine archéen))

POITIERS-BU Sciences (861942102) / SudocSudocFranceF

Caractérisation in situ des réservoirs en roches massives fracturées (écoulement, transport et acidification des eaux)

POITIERS-BU Sciences (861942102) / SudocPARIS-BIUSJ-Sci.Terre recherche (751052114) / SudocSudocFranceF

Approches quantitatives des flux d'eau dans un système sol-nappe-rivière (application au bassin de la Charente, à l'amont de la ville d'Angoulême (16))

LA GESTION DES RESSOURCES EN EAU DOIT ETRE CONSIDEREE A L'ECHELLE DU BASSIN VERSANT. DANS CE TRAVAIL, LA MODELISATION DES FLUX D'EAU EST ETUDIEE DANS UN SYSTEME GLOBAL SOL-NAPPE-RIVIERE. LES OBJECTIFS SONT D'OBTENIR DES FLUX SPATIALISES ET PRECIS, NOTAMMENT PRES DU FLEUVE : LE DOMAINE D'APPLICATION SE SITUE DANS LE BASSIN DE LA CHARENTE A L'AMONT DE LA VILLE D'ANGOULEME (CHARENTE, FRANCE). UNE SYNTHESE DES CARACTERISTIQUES DES DIFFERENTS ETATS DU SYSTEME SOL-NAPPE-RIVIERE EST ABORDEE. APRES AVOIR INVENTORIE ET COMPLETE LES INFORMATIONS RELATIVES AU BASSIN, UNE MODELISATION NUMERIQUE EN DEUX TEMPS EST REALISEE. LA MODELISATION DE LA ZONE NON-SATUREE EST ELABOREE A PLUSIEURS ECHELLES : PARCELLAIRE, COMMUNALE ET BASSIN ENTIER. CELLES-CI ONT INDUIT PLUSIEURS APPROCHES ET CONDUIT A L'UTILISATION D'OUTILS DIFFERENTS. AINSI, L'ACCES A L'OCCUPATION DU SOL A L'ECHELLE DU BASSIN N'A PU ETRE OBTENUE QU'AVEC LE RECOURS A LA TELEDETECTION. UNE MODELISATION HYDRODYNAMIQUE DU BASSIN A PERMIS DE VISUALISER LES VARIATIONS SPATIALES DES FLUX ECHANGES ENTRE LES COURS D'EAU ET LES NAPPES. L'IMPORTANCE DES ECHANGES EST QUANTIFIEE SUR LA PERIODE D'UNE ANNEE. LES PROPRIETES RESERVOIRS DE L'AQUIFERE DU JURASSIQUE MOYEN ET SUPERIEUR SONT MISES EN EVIDENCE. ENFIN, L'IMPORTANCE DES PRELEVEMENTS AGRICOLES ET LEUR INFLUENCE SUR LES VARIATIONS DES NIVEAUX DANS LES NAPPES ET LES RIVIERES SONT EVALUEES ET INTERPRETEES SPATIALEMENT ET TEMPORELLEMENT. LA MODELISATION DEMONTRE LA PRESENCE DE LIENS PLUS FORT ENTRE LA NAPPE ET LA RIVIERE EN CERTAINS ENDROITS DU BASSIN ET LA VULNERABILITE DES NAPPES DEVANT L'IMPORTANCE DES PRELEVEMENTS.POITIERS-BU Sciences (861942102) / SudocPARIS-BIUSJ-Sci.Terre recherche (751052114) / SudocSudocFranceF

Assessment of Groundwater Quality in the Dogger Aquifer of Poitiers, Poitou-Charentes Region, France

International audienceThe Poitou-Charentes, located in the Center-West of France, is a region where economy is based mainly on agriculture. This region, made up of 4 Departments (Vienne, Charente, Charente Maritime, Deux-Sèvres), is supplied largely by groundwater exploitation, both for consumption and for irrigation. This resource is thus vital to the region and its preservation is a major issue. The objective of this study is the determination of the groundwater quality in the Dogger aquifer of Poitiers (Vienne Department), which is the main water resource for this area and to achieve a better understanding of the factors influencing groundwater mineralization. Sixty-six wells, [...