Browsing by Autor "Anne Coudrain"

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Données isotopiques (<sup>87</sup>86/Sr) et changements hydrologiques depuis 15 000 ans sur l'Altiplano andin
(Taylor & Francis, 2002) Anne Coudrain; M. Loubet; Thomas Condom; Amal Talbi; Pierre Ribstein; Bernard Pouyaud; Jorge Quintanilla; Claudine Dieulin; Bernard Dupré
Résumé Actuellement, le bassin sud (17°-22°S, 126 000 km2) de l'Altiplano reçoit environ 39.4 × 109 m3 an−1 d'eau, dont 4% proviennent du bassin nord (14°-17°S) par le Rio Desaguadero. Le rapport 87Sr/86Sr des écoulements de surface actuels présentent des valeurs distinctes sur les deux bassins permettant d'utiliser ce traceur pour reconstituer des conditions hydrologiques passées contrastées du bassin sud. Pour la période aride de 10 000 à 2000 ans BP, les données acquises et la modélisation du transport de strontium dans un aquifàre renforcent les Hypothàses d'une valeur des précipitations de 80% de l'actuel et de l'absence d'alimentation significative du bassin sud par le Rio Desaguadero. Pour les hauts niveaux d'eau de la phase lacustre Tauca, vers 15 000 ans BP, les données hydrologiques et isotopiques indiquent que pour une alimentation totale estimée à 110 109 m1 an1 les apports du bassin nord pouvaient être limités à 5%.
Geophysical surveys for identifying saline groundwater in the semi‐arid region of the central Altiplano, Bolivia
(Wiley, 2001) Roger Guérin; Marc Descloîtres; Anne Coudrain; Amal Talbi; Robert Gallaire
Abstract In the central part of the Bolivian Altiplano, the shallow groundwater presents electrical conductivities ranging from 0·1 to 20 mS/cm. In order to study the origin of this salinity pattern, a good knowledge is required of the geometry of the aquifer at depth. In this study, geophysics has been used to complement the sparse data available from drill holes. One hundred time‐domain electromagnetic (TDEM) soundings were carried out over an area of 1750 km 2 . About 20 geological logs were available close to some of the TDEM soundings. Three intermediate results were obtained from the combined data: (i) the relationship between the electrical conductivity of the groundwater and the formation resistivity, (ii) geoelectrical cross‐sections and (iii) geoelectrical maps at various depths. The limited data set shows a relationship between resistivity and the nature of the rock. From the cross‐sections, a conductive substratum with a resistivity of less than 1 Ω·m was identified at most of the sites at depths ranging from 50 to 350 m. This substratum could be a clay‐rich formation containing brines. Using derived relationships, maps of the nature of the formation (sandy, intermediate and clayey sediments) were established at depths of 10 and 50 m. Discrimination between sand and clays was impossible where groundwater conductivity is high (>3 mS/cm). In the central part of the area, where the groundwater conductivity is low, sandy sediments are likely to be present from the surface to a depth of more than 200 m. Clayey sediments are more likely to be present in the south‐east and probably constitute a hydraulic barrier to groundwater flow. In conclusion, the study demonstrates the efficiency of the TDEM sounding method to map conductive zones. Copyright © 2001 John Wiley & Sons, Ltd.
Glacier shrinkage and water resources in the Andes
(Wiley, 2005) Bernard Francou; Anne Coudrain
For more than a century glaciers around the world have been melting as air temperatures rise due to a combination of natural processes and human activity. The disappearance of these glaciers can have wide‐ranging effects, such as the creation of new natural hazards or changes in stream flow that could threaten water supplies Some of the most dramatic melting has occurred in the Andes mountain range in South America. To highlight the climatic and glacial change in the Andes and to encourage the scientific community to strengthen the glacier observation network that stretches from Colombia to the Patagonian ice fields, the Instituto Nacional de Recursos Naturales (INRENA), Perú, and the Institute of Research and Development (IRD), France, recently organized the second Symposium on Mass Balance of Andean Glaciers in Huaráz,Perú.
Subsurface Transfer of Chloride After a Lake Retreat in the Central Andes
(Wiley, 2001) Anne Coudrain; Amal Talbi; E. Ledoux; Michel Loubet; Jean Vacher; Edson Ramírez
The area under study covers 3500 km2 in the upstream part of the closed catchment basin of the salt crust of Uyuni. This crust is a remnant of the saline Lake Tauca, which covered the area about 15,000 years ago. In the downstream part of the aquifer, the Cl concentration of ground water and Cl content in the unsaturated zone exceed 20 meq/L and 18 kg/m2, respectively. With the present hydrological conditions under semiarid conditions, the ground water residence time in the study area exceeds 3000 years. Transient simulations over 11,000 years were made using initial conditions as the retreat of Lake Tauca and taking into account a low recharge during the arid mid-Holocene period. The modeling simulates ground water flow, Cl transport, and ground water residence time. It includes the evaporation from the aquifer that leads to the accumulation of chloride in the unsaturated zone. Results of the modeling are consistent with the observations if it is assumed that the Cl previously accumulated in the unsaturated zone was flushed back into the aquifer around 2000 years B.P., contemporaneously with the end of the arid period.