Aquifer classification and pneumatic diffusivity estimation using periodic groundwater level changes induced by barometric pressure

: To classify the aquifer type and to estimate the pneu‐ matic diffusivity using harmonic analysis and analytical expressions, we measured groundwater levels and baro‐ metric pressure in an unconfined aquifer with a thick un‐ saturated 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 baro‐ metric pressure. The groundwater level and barometric pressure exhibit periodic changes with prominent spectra at K 1 (lunar-solar diurnal) and S 2 (main solar semidiurnal) tidal constituents but not at O 1 (main lunar diurnal) and M 2 (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 thick‐ ness strongly affect amplitude ratios and phase lags between the groundwater level and the barometric pressure in the unconfined aquifer.