Altitude precipitations, groundwaters and climate change of the North-Chilean altiplano

The study focusses on water resources and their origins in an Andean region of north-east Chile, located between 21 and 23 deg. S., 67 and 69 deg. W. and 3000 and 6000 m in altitude (Atacama Desert region). Isotopic and geochemical analysis of rainfall, surface and groundwaters are used to study the climatology and hydrology of these tropical Andean mountain slopes and Altiplano, based on the monitoring of a climatologic station installed on the Altiplano at 4260 m height, and on spatialized sampling of precipitations between 1994 and 1997. Isotopic study shows that the sources are spatially organised in two distinct groups: western slopes sources present isotopic rates and deuterium excesses higher than those of springs and salars located on eastern slopes and Altiplano. Western slopes rainfalls are equally enriched both in isotopic rates and deuterium excesses in comparison with eastern slopes and Altiplano rainfalls. The isotopic spatial variations of rainfalls and sources are in coherence, which seems to indicate a generic and time-stable relation between rainfalls and underground waters. Nevertheless, the hydrologic study reveals that sources and rivers flows are largely independent from rainfalls, except for exceptionally high intensity events. Dating using radiocarbon techniques indicates age values of 20000 to 100 years for sources water. Very low (even undetectable) values of Tritium rates in underground waters confirm that contribution of recent waters to surface flow is low, generally less than 20%. The current recharge of underground waters is considered as being very low and slow. West-East isotopic variations tend to confirm the alternate influences of Pacific and Atlantic air masses. South-North variations, mainly observed on Altiplano sources, tend to indicate, against expectation, the predominance of Pacific air masses in tropical and subtropical sectors of the Altiplano during long periods in the past. (author)