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 ma ss balance. The roughness parameters needed to calculate the turbulent flux es over the surface are deduced from direct sublimation measurements perfor med regularly on the field site and serve as calibration parameters. For th e 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 importan t 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 v ertical gradient of humidity. The sensible heat flux (6.0 W m(-2)), continu ously positive throughout the year, and the conductive heat flux in the sno w/ice (2.8 W m(-2)) also bring energy to the surface. There is a good agree ment between the monthly ablation calculated by the energy balance and the ablation evaluated from stake measurements. The seasonality of the proglaci al stream runoff is controlled by the specific humidity, responsible for th e sharing of the energy between sublimation and melting.