2008 Volume 30 Issue 2 Pages 107-120
Morphometric observations of the crater lake at Aso Volcano, Japan, was carried out and the change of its water level, surface area and water volume between Aug 2000 and Aug 2003 were estimated. The lake water was sampled three times during this period and the concentrations of chloride in the water were measured. Mass balance was evaluated by solving the water and chloride budgets simultaneously.Results showed that the lake water was mainly supplied by volcanic input (3500-6200 ton/day) which occupied at least 60% of the water influx.Meteoric water flux (420-2500 ton/day) was not dominant, which occupied less than 20% of the water influx between Aug 2000 and Apr 2003. Lake water was mainly dissipated by evaporation from the lake surface (3800-5900 ton/day). Seepage from the lake bottom (880-2200 ton/day) occupied about 13-37% of the water outflux. Heat balance indicated that thermal energy of volcanic input was about 150-200MW, which occupied more than 95% of energy influx.Most heat loss occurred at the lake surface through evaporation (100-150MW). The specific enthalpy of volcanic input was estimated to be about 2500-4200kJ/kg, which corresponds to the enthalpy of superheated steam at several hundred degrees C.It was suggested that mass and heat are input into the lake by the addition of superheated steam at several hundred degrees C.Water flux and enthalpy of volcanic input between Apr 2003 and Aug 2003 were estimated to be larger than those between Aug 2000 and Apr 2003.This is consistent with the observation that the water temperature became higher, the water level was decreased and small mud eruption was occurred.Our results indicate that the crater lake at Aso ranks among the peak-activity volcanic lakes in the world.
