Heejun Yang, Tomo Shibata
Received 2020/05/08, Accepted 2020/07/16, Published 2020/09/03

Heejun Yang¹, Tomo Shibata¹

1) Institute for Geothermal Sciences, Graduate School of Science, Kyoto University, Japan

To classify the aquifer type and to estimate the pneumatic diffusivity using harmonic analysis and analytical expressions, we measured groundwater levels and barometric pressure in an unconfined aquifer with a thick unsaturated zone at a southern part of the Beppu volcanic fan, Japan. The groundwater level was inversely related to the barometric pressure, with little or no time lag behind barometric pressure. The groundwater level and barometric pressure exhibit periodic changes with prominent spectra at K₁ (lunar-solar diurnal) and S₂ (main solar semidiurnal) tidal constituents but not at O₁ (main lunar diurnal) and M₂ (main lunar semidiurnal) tidal constituents, which are the motions of solar and lunar relative to the earth. It indicates that the aquifer is an unconfined aquifer type with no earth tide effect. Pneumatic diffusivity in the unsaturated zone was estimated as 7.6 × 10^–2 to 5.7 × 10^–3 m^–2/s using changes between the groundwater level and the barometric pressure. The pneumatic diffusivity and the unsaturated zone thickness strongly affect amplitude ratios and phase lags between the groundwater level and the barometric pressure in the unconfined aquifer.

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