Browsing by Autor "Robert Gallaire"

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Etude des précipitations et de la fonte sur un glacier tropical: le glacier du Zongo, Bolivie, 16°S
(Taylor & Francis, 2003) Jean Emmanuel Sicart; Pierre Ribstein; Bernard Francou; Robert Gallaire
Résumé Cet article présente les relations entre les précipitations et la fonte sur le glacier du Zongo en Bolivie où l'ablation et l'accumulation sont maximales en saison des pluies (été austral). Les nuages, provenant du bassin amazonien, donnent des précipitations à midi dans les vallées, puis dans l'après-midi en montagne. Lors de l'installation de la saison des pluies de septembre à décembre, les chutes de neige recouvrent temporairement la glace en fusion. Par effet d'albédo, l'énergie solaire absorbée et la fusion varient beaucoup et atteignent leur maximum lorsque le couvert neigeux est réduit. Au cœur de la saison des pluies de janvier à mars, la neige reste à basse altitude sur le glacier, l'albédo est fort et la fonte diminue. La saison sèche de mai à août est une période de faible ablation. La fonte sur les glaciers tropicaux dépend fortement de la répartition saisonnière des précipitations, par l'augmentation d'albédo due aux chutes de neige d'été.
Flow modelling in a high mountain valley equipped with hydropower plants: Rio Zongo Valley, Cordillera Real, Bolivia
(Wiley, 2004) Yvan Caballero; Pierre Chevallier; Robert Gallaire; Ramiro Pillco Zolá
Abstract Water management modelling of a hydroelectric system in a tropical high mountain context is presented. The study zone and the hydraulic network are described and the water management strategy analysed. Three different models are combined to describe the complexity of the specific hydrometeorological context: the spatial distribution of the climatic data over the river basin, the surface energy balance influence on the runoff production of a river basin and the surface flow transfer modelling through a hydraulic system. The atmospheric forcing spatial distribution is derived from the available climatic data records. The runoff production on the catchment's slopes is simulated using the land‐surface scheme ISBA. The system dynamics tool Vensim ® is used to simulate the hydraulic dynamics in the hydropower plants system. A short description of the three modelling methods is given, followed by the description of the coupled model construction. The simulation results of the ISBA land‐surface scheme on both a non‐glacial an a glacial sub‐basin during a 17 month period are presented. After pointing out the necessity of the water management model to simulate the river discharge at the outlet of the basin, the main reservoirs, simulated water level variations are shown. Copyright © 2004 John Wiley & Sons, Ltd.
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.
Glaciers and Climate in the Andes between the Equator and 30° S: What is Recorded under Extreme Environmental Conditions?
(Springer Nature (Netherlands), 2003) U. Schotterer; Martín Grosjean; Willibald Stichler; Patrick Ginot; C. Kull; H. Bonnaveira; Bernard Francou; H. W. Gäggeler; Robert Gallaire; Georg F. Hoffmann
Potential for climate variability reconstruction from Andean glaciochemical records
(Cambridge University Press, 2002) Patrick Ginot; Margit Schwikowski; U. Schotterer; Willibald Stichler; H. W. Gäggeler; Bernard Francou; Robert Gallaire; Bernard Pouyaud
Abstract In order to select glacier sites suitable for the reconstruction of short-term climate variability, chemical constituents of shallow cores from Chimborazo, Ecuador, Illimani, Bolivia, and Cerro Tapado, Chile, were analyzed to determine if they represent the climate and atmospheric conditions of the particular area. Pronounced variations were observed in the Chimborazo core, which were attributed to the seasonal occurrence of wet and dry periods. Using chemical tracers, the Pacific and the Amazon basin were identified as the two principal sources of atmospheric moisture. In the Illimani record, seasonality is obvious but is less regular than in the Chimborazo core. Chemical tracers point to the Amazon basin as a moisture source. Post-depositional sublimation masks a possible atmospheric signal in the glaciochemical records from Cerro Tapado. the arid conditions and a prevailing high condensation level also cause little variability in the stable-isotope content. Irregular, more negative δ 18 O values occur during the humid phases of El Niño events. Despite alterations due to sublimation, chemical tracers indicate the Pacific Ocean as a main moisture source for precipitation preserved on Glaciar CerroTapado.
Small glaciers disappearing in the tropical Andes: a case-study in Bolivia: Glaciar Chacaltaya (16<sup>o</sup> S)
(Cambridge University Press, 2001) Edson Ramírez; Bernard Francou; Pierre Ribstein; Marc Descloîtres; Roger Guérin; Javier Mendoza; Robert Gallaire; Bernard Pouyaud; Ekkehard Jordan
Abstract Glaciar Chacaltaya is an easily accessible glacier located close to La Paz, Bolivia. Since 1991, information has been collected about the evolution of this glacier since the Little Ice Age, with a focus on the last six decades. The data considered in this study are monthly mass-balance measurements, yearly mappings of the surface topography and a map of the glacier bed given by ground-penetrating radar survey. A drastic shrinkage of ice has been observed since the early 1980s, with a mean deficit about 1 m a −1 w.e. From 1992 to 1998, the glacier lost 40% of its average thickness and two-thirds of its total volume, and the surface area was reduced by >40%. With a mean estimated equilibrium-line altitude lying above its upper reach, the glacier has been continuously exposed to a dominant ablation on the whole surface area. If the recent climatic conditions continue, a complete extinction of this glacier in the next 15 years can be expected. Glaciar Chacaltaya is representative of the glaciers of the Bolivian eastern cordilleras, 80% of which are small glaciers (<0.5 km 2 ). A probable extinction of these glaciers in the near future could seriously affect the hydrological regime and the water resources of the high-elevation basins.