[A-HW19] Hydrology & Water Environment

[AHW19-05] A study on water cycles and geothermal processes in a crater lake: Okama in Zao Volcano

*Kazuhisa A. Chikita¹, Akio Goto², Jun Okada³, Hideo Oyagi⁴, Takashi Yamaguchi⁵ (1.Arctic Research Center, Hokkaido University, 2.Center for Northeast Asian Studies, Tohoku University , 3.Volcanology Research Department, Meteorological Research Institute, JMA , 4.Faculty of Policy Studies, Nanzan University, 5.Hokkaido Research Organization)

Keywords:Volcanic activity, Crater lake, Geothermal heat, Water cycle system

A water cycle system and geothermal processes in a crater lake, Okama (pH=3.1-3.4), in active Zao Volcano, Japan, were explored by estimating the hydrological and chemical budgets of the lake, and analyzing the time series of lake water temperature, respectively (Figure 1). In 2021, the lake level consistently increased by snowmelt plus rainfall in May–June, and then stayed nearly constant in the rainfall season of July–September. The hydrological budget estimated during the increasing lake level gave the net groundwater inflow at positive values. This suggests that the groundwater inflow to the lake is controlled by the water percolation into volcanic debris from the melting of snow remained in the catchment. Solving the simultaneous equation from the hydrological and chemical budgets evaluated the groundwater inflow G_in at 0.012 – 0.040 m³/s and the groundwater outflow G_out at 0.012 – 0.027 m³/s in May – September 2021. By adding the 2020 values of G_in and G_out, it was found out that the G_in and G_out exhibit the highly negative and positive correlations (R²=0.661 and 0.848; p<0.01), respectively, with the lake level or the lake volume. In the completely ice-covered season of 15 December 2021 – 28 February 2022, the lake water temperature increased at between the bottom and 15 m above the bottom at the deepest point, which reflects the geothermal heat input at the bottom. The heat storage change during the increasing water temperature was evaluated at a range of -0.4 – 5.5 W/m² as the 10-day moving average heat flux. By accumulating the daily heat storage change for the calculated period, the water temperature averaged over the heated layer increased from 1.08 to 1.56 °C. The small temperature increase reflects a stagnant state of the volcanic activity in Zao Volcano.