Bernard Pouyaud

Annual cycle of energy balance of Zongo Glacier, Cordillera Real, Bolivia

An 18-month meteorological data set recorded at 5150 m above sea level (asl) on Zongo Glacier, in the tropical Andes of Bolivia, is used to derive the annual cycle of the local energy balance and to compare it to the local mass balance. The roughness parameters needed to calculate the turbulent fluxes over the surface are deduced from direct sublimation measurements performed regularly on the field site and serve as calibration parameters. For the hydrological year September 1996 to August 1997, net all-wave radiation (16.5 W m−2) is the main source of energy at the glacier surface and shows strong fluctuations in relation to the highly variable albedo. An important peculiarity of tropical glaciers is the negative latent heat flux (−17.7 W m−2) indicating strong sublimation, particularly during the dry season. The latent heat flux is reduced during the wet season because of a lower vertical gradient of humidity. The sensible heat flux (6.0 W m−2), continuously positive throughout the year, and the conductive heat flux in the snow/ice (2.8 W m−2) also bring energy to the surface. There is a good agreement between the monthly ablation calculated by the energy balance and the ablation evaluated from stake measurements. The seasonality of the proglacial stream runoff is controlled by the specific humidity, responsible for the sharing of the energy between sublimation and melting.

Small glaciers disappearing in the tropical Andes: a case-study in Bolivia: Glaciar Chacaltaya (16o S)

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 km2). 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.

Influence of sublimation on stable isotope records recovered from high-altitude glaciers in the tropical Andes

Sublimation dominates the ablation process on cold, high-altitude glaciers in the tropical Andes. Transport of water vapor through the firn and exchange with ambient moisture alter the stable isotope composition of the surface layers. A sublimation experiment carried out during an ice core drilling campaign on Cerro Tapado (5536 m above sea level, 30°08′S, 69°55′W) revealed a strong enrichment in the 2H and 18O content in the surface layer. Concerning the deuterium excess, a decrease occurred at daytime, while during the night, the values remained comparatively constant. At daytime the sublimation is enhanced due to the higher moisture deficit of the ambient air accompanied by relatively high firn surface temperatures. Low surface temperatures at night cause condensation of water vapor in the firn pores near the surface and thus inhibit penetration of the isotopically enriched surface front into deeper firn layers. Measuring an isotope profile obtained through detailed sampling between the surface and 38 cm depth proved this mechanism. The observed modification of the isotopic composition at the surface was quantitatively described by a model, which also reproduced the mass loss measured with sublimation pans and calculated from relevant meteorological data. The results of this study suggest that the influence of sublimation on the preserved isotope record of ice cores under comparable environmental conditions is rather limited. In any case, simultaneous measurements of δ2H and δ18O help to identify layers in an ice core which might be effected by sublimation. However, since the mass loss due to sublimation was of the order of 2–4 mm per day during the experiment, important palaeoinformation from an isotope record could be eliminated during extended dry periods.

A new Andean deep ice core from Nevado Illimani (6350 m), Bolivia

Abstract A new ice core record from the Nevado Illimani (16°S), Bolivia, covers approximately the last 18 000 years BP. A comparison with two published ice records, from Sajama (18°S), Bolivia [Thompson et al., Science 282 (1998) 1858–1864] and Huascaran (9°S), Peru [Thompson et al., Science 269 (1996) 46–50], documents a regionally coherent transition from glacial to modern climate conditions in South America north of 20°S. The strong resemblance between the Illimani and Huascaran water isotope records and their differences from the Sajama record, in particular during the period from 9000 years BP to 14 000 years BP, suggest that local water recycling or local circulation changes played a major role for Sajama. We interpret the common Illimani/Huascaran water isotope history in terms of a common change from wetter/cooler conditions during glacial times to drier/warmer conditions in the Early Holocene.

Modelling the water balance in the glacierized Parón Lake basin (White Cordillera, Peru) / Modélisation du bilan hydrique du bassin versant englacé du Lac Parón (Cordillère Blanche, Pérou)

Abstract The White Cordillera (northern Peru), with a glacial surface of 631 km2, is the largest glacierized mountain range in the Tropics. Due to the lack of physical data from most of its sub-basins, it is difficult to build a physical model to estimate the water resource flowing from the glaciers at the present time and a fortiori for the future. The most recent GCM simulations indicate a significant increase in the temperature and an accelerated shrinking of the glaciers. Consequently, we sought a model that would be based on the data available within instrumented sub-basins. A theoretical/conceptual water model makes it possible to quantify the local glacier contribution, which could then be applied to the other non-instrumented sub-basins. A total of 43.6% of Parón Lakes instrumented sub-basin area (47.4 km2) corresponds to glacial surfaces. Within this sub-basin, a smaller watershed (8.8 km2), called Artesón, with 72.9% glacierized area, has been accurately observed over a 5-year hydrological period (September 2000–August 2005). This information allowed us to calibrate the model over the Artesón sub-basin. The parameters obtained were applied to the entire Parón basin using the same modelling approach.

Potential for climate variability reconstruction from Andean glaciochemical records

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 δ18O 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.

Calcul de la pluie sur le bassin versant du lac Titicaca pendant l'Holocène

The water levels of a lake situated in an endorheic catchment make it possible to calculate the associated rainfall rate on the basis of the water balance over the whole catchment. Evolution during the Holocene of water levels in Lake Titicaca (Bolivia), previously published, shows that in the most arid period, between 8 O00 yr and 4 O00 yr BP, the average level was 50 m lower than today. The calculated rainfall associated with this low level is 635 k 50 mmyr-I i.e. about 18 %lower than the present amount. (O Academie des sciences / Elsevier, Paris.) rain amount / endorheic catchment / Holocene / Titicaca / Bolivia