The surface energy balance of an active ice-covered volcano:: Villarrica Volcano, southern Chile

The energy balance of bare snow and tephra-covered ice near the glacier equilibrium line elevation on Villarrica Volcano, southern Chile, was investigated during 2004 and 2005, combining meteorological, surface temperature and ablation measurements with energy balance modelling. A tephra thermal conductivity of 0.35 W m(-1) K-1, and a critical tephra thickness of < 5 mm at which ablation is reduced compared to bare snow, were obtained from field data. These low values are attributable to the highly porous lapilli particles which make up most of the surface material. Modelled melt totals in the January to March period were 4.95 m and 3.96 m water equivalent (w.e.) in 2004 and 2005, respectively, compared with similar to 0.5 m w.e. melt for ice buried by > 0.1 m tephra. Windblown tephra impurities lowered snow albedo, but increased snowmelt by only an estimated 0.28 m w.e. over the same period. The net mass balance impact of supraglacial tephra at Villarrica Volcano is therefore positive, as thick ash and lapilli mantle most of the glacier ablation zones, probably reducing annual ablation by several metres w.e. In the accumulation seasons, frequent melting events were recorded with modelled daily snowmelt rates of up to 50 mm w.e.