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Recursos hídricos en los Andes: Lago Titicaca
Abstract
RESSOURCES EN EAUX DANS LES ANDES : LAC TITICACA. Une analyse par régression a été menée entre les variations de niveau du Lac Titicaca et les entrées du système (apports des affluents et précipitations). Les coefficients de corrélation, quoique relativement faibles, montrent aux abords du lac un effet de persistance qui diminue lorsque les stations sont éloignées du plan deau. Linertie du lac se manifeste lannée suivante pour les affluents du lac et évidemment pour le lac lui-même. Ces résultats confirment la présence dun mécanisme de rétro alimentation (effet boomerang). Cela montre que le système du Lac Titicaca est fragile, face à une utilisation excessive de ses eaux. Las regresiones estudiadas de cambios de nivel en el lago Titicaca contra precipitaciones y aportes en su cuenca, aunque relativamente bajas, muestran un efecto de persistencia en las proximidades del lago que decrece conforme las estaciones se alejan de éste. Lo que evidencia que la inercia del lago se manifiesta en el período siguiente (lag uno) en los afluentes del lago y obviamente en el propio lago, fenómeno que parece confirmar la presencia de un mecanismo de retroalimentación (efecto boomerang). Este comportamiento muestra la fragilidad del lago frente a un aprovechamiento consuntivo excesivo. WATER RESOURCES IN THE ANDES: LAKE TITICACA. The studied regressions of the level changes in the Lake Titicaca versus the rainfall and the runoff in the basin, even being relatively low, show an effect of persistence in the proximity of the lake which decreases as we go away from it. This makes evident that the lake inertia manifests itself the following period (lag one) in the lake tributaries and obviously in the lake itself, phenomenon which seems to confirm the presence of a feedback mechanism (boomerang effect). This behaviour shows the fragility of the lake against the excessive consumptive uses of the waters in its basin.
... One further drawback to this model is that it assumes that changes of lake level did not produce significant changes in the ambient climate (e.g., humidity or rainfall in the lake basin). This is an unrealistic assumption for a lake as large as Lake Titicaca (Taborga and Campos, 1995) and, as the model results are fairly sensitive to humidity, may be a significant source of error. ...
A simple mass balance model provides insight into the hydrologic, isotopic, and chemical responses of Lake Titicaca to past climatic changes. Latest Pleistocene climate of the Altiplano is assumed to have been 20% wetter and 5°C colder than today, based on previous modeling. Our simulation of lacustrine change since 15,000 cal yr B.P. is forced by these modeled climate changes. The latest Pleistocene Lake Titicaca was deep, fresh, and overflowing. The latest Pleistocene riverine discharge from the lake was about 8 times greater than the modern average, sufficient to allow the expansion of the great paleolake Tauca on the central Altiplano. The lake δ18O value averaged about −13‰ SMOW (the modern value is about −4.2‰). The early Holocene decrease in precipitation caused Lake Titicaca to fall below its outlet and contributed to a rapid desiccation of paleolake Tauca. Continued evaporation caused the 100-m drop in lake level, but only a slight (1–2‰) increase (relative to modern) in δ18O of early Holocene lake waters. This Holocene lowstand level of nearly 100 m was most likely produced by a precipitation decrease, relative to modern, of about 40%. The lake was saline as recently as 2000 cal yr B.P. The timing of these hydrologic changes is in general agreement with calculated changes of insolation forcing of the South American summer monsoon.
The strong correlation between the stable isotope ratios of oxygen and hydrogen of precipitation and mean annual temperature observed in high latitudes is the basis for the traditional interpretation of stable isotope content of paleoprecipitation archives in terms of paleotemperatures. However, the precipitation delta18O, delta2H and d-excess are integrated climate parameters that reflect a combination of temperature and amount effects further influenced by seasonality, air-mass source and history. Here we use a 16-year dataset of monthly delta18O, delta2H and d-excess from six stations located in the Canadian Arctic from the Canadian Network for Isotopes in Precipitation (CNIP) to rigorously examine the modern isotope-climate relations in this area. The CNIP network provides unprecedented spatial and temporal coverage of the Canadian North, which can be used to inform interpretation of paleoprecipitation archives from elsewhere in the circumpolar Arctic. Time-series of isotope anomalies from the Arctic stations (1989- 2004), show some spatially coherent trends, but also include periods when regional differences in the direction and timing of isotope anomalies can be identified. The most striking pattern in the data is the regional grouping of isotope-climate responses with clear differences existing between stations located in the northeast and those in the southwest. The isotope anomaly time-series from the northeastern stations are very sensitive to the seasonality of precipitation, unlike the stations in the southwestern portion of the area. These two groups also emerge in spectral analyses of the isotope anomaly time-series and in temporal correlations between isotope anomalies and gridded climate anomalies. Our results illustrate different controls on the isotopic labelling of modern precipitation across the Canadian Arctic and give new insight into the potential for very different climate signal preservation from paleoprecipitation archives in this region.
Strontium concentrations and strontium isotopic ratios were measured in natural waters and carbonate sediments from throughout the Bolivian and Peruvian Altiplano in order to improve hydrologic and paleohydrologic mass balances with the ultimate goal of better understanding the paleoclimatic history of the central Andes. Rivers flowing into Lake Titicaca have a wide range of strontium isotopic ratios that exhibit spatial patterns consistent with the lithologies of the different drainage basins. Because of the limited exchange of water between the two main sub-basins of Lake Titicaca, Lago Grande and Lago Huiñaimarca, and between the sub-basins of Lago Huiñaimarca, there are significant differences in strontium isotopic ratios between the sub-basins. Calculated elemental balances of strontium in Lake Titicaca are in reasonable agreement with previously published budgets of water and major elements. However, the strontium isotopic budget indicates that the lake is not in isotopic steady state. This also implies that the major-element budgets are unlikely to be in steady state. Lake Titicaca had a higher-than-modern strontium isotopic ratio during the early and middle Holocene. Elevated values persisted in Lago Grande until at least 2000 cal yr BP, consistent with other evidence that modern hydrologic conditions (namely overflow) were not established until that time. An isotopic budget calculated for late-Pleistocene paleolake Tauca in the central Altiplano suggests that between 70% and 83% of its riverine inputs were derived from Lake Titicaca overflow. This calculated flow represents about a 30-fold increase over the average discharge of the modern Rı́o Desaguadero at Desaguadero or a seven-fold increase over its average discharge into Lago Poopó. The strontium isotopic budget (if complete) precludes the possibility that decreased evaporation alone could have accounted for the existence of paleolake Tauca.
A multidisciplinary approach has allowed the reconstruction of Late Pleistocene and Holocene lake fluctuations in the Lake Titicaca basin. Three high lake levels (prior to 25/28 kyr BP. between 14 and 10.5 kyr BP and from 0.4 kyr BP) and two severe droughts (25/28 and 14 kyr BP and 10.5 and 0.4 kyr BP) are the main paleohydrological characteristics of the Lake Titicaca evolution.
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