Climate variations and hydrologic regime of a glacierized basin in the tropical Andes

Due to the high sensibility and the short lag time response, glaciers are considered as relevant indicators of the climate change in the Tropics. Climatic signals lead to variation of the glacier mass balance, ie the difference between the accumulation terms (precipitation) and the ablation terms (melting and sublimation). The discharge of the proglacial stream integrates most of the reaction of the glacier in response to climate variability. This paper presents the main results of a research conducted since 1991 in the Central Andes. It is based on the results obtained on the Zongo Glacier, Cordillera Real of Bolivia (16°S, 68°W). The 2.1 km2 glacier is included in a 3 km2 basin. The hydrologic regime is analyzed at different time scales and compared to the mass balance measurements and to the energy fluxes measured by an automatic weather station installed at 5150 m asl, closed to the Equilibrium Line Altitude.

The runoff shows an appreciable seasonal variability, with low discharges in the dry season (May to August) and high values in the humid season (October to March). Net all-wave radiation is the main source of energy at the glacier surface and does not show any well-marked seasonality along the year. Sensible heat flux remains continuously positive along the year and is of minor importance compared to net radiation or latent heat flux. The latent heat flux is reduced during the wet season because of a lower vertical gradient of humidity and energy supplied by radiation is directly consumed by melting : this explains why discharge is high. During the dry period, a large part of the energy supplied by radiation is used to sublimate snow or ice: therefore, energy available for melting is reduced and melt rates are low.

The El Niño-Southern Oscillation (ENSO) events have a great influence on tropical glaciers, as demonstrated by the last 1997-98 ENSO very strong event. The main factor responsible for the dramatic melting of Zongo Glacier during this cycle has been the scarce precipitation during the wet season which has reduced the accumulation and caused a low albedo on a large part of the glacier. The two successive hydrologic cycles (Sept.96-Aug.97 and Sept.97-Aug.98) were particularly contrasted: 1996-97 was marked by a weak La Niña event although the following year experienced one of the strongest El Niño of the century. During these two years, the detailed energy balance on the glacier shows that small differences of the meteorological variables (precipitation, air-temperature, humidity, wind speed, incident short-wave radiation) from one year to the other lead to large variations of the surface energy fluxes.

Since 1973, monthly discharges from Zongo Glacier have been calculated, on the base of two daily readings at a staff gauge downstream the hydrometric station. The established discharges from Zongo Glacier are compared to monthly discharges from an other, less glacierized, basin. During ENSO warm events, basins with a small influence of glaciers exhibit low discharges as a consequence of a deficit of precipitation.