Fig. 3
Hydrological gravity change at NAOJ Mizusawa from 2018 to 2020. a Cyan bars and blue lines indicate the daily and total annual precipitation (\(R(t)\) and \({R}_{tot}(t)\)), respectively, observed by a hyetometer at NAOJ Mizusawa. Orange lines indicate the total annual evapotranspiration (\({E}_{tot}(t)\)), calculated from the Penman (1948)’s model and meteorological data observed at NAOJ Mizusawa. b A black line indicates the water level of the unconfined aquifer (\(h(t)\)) under the ground of NAOJ Mizusawa. In this panel, \(h(t)\) is shown with respect to the ground surface altitude (\({z}_{s}\)). c Cyan, blue and black lines indicate the calculated soil moisture variations (\(\theta (z,t)\)) at the depths of 15–20, 45–50 and 95–100 cm from the ground surface, respectively. \({\theta }_{max}\) and \({\theta }_{min}\) are the maximum and minimum soil moisture values for NAOJ Mizusawa (= 0.52 and 0.28 m3/m3), respectively (see Kazama et al. 2012). d Gray and red solid lines indicate the hydrological gravity changes of \({g}_{w}\left(t\right)\) and \({g}_{w}^{out}\left(t\right)\) (Eqs. (1) and (2)), respectively, calculated from the spatial integrals of \(\theta (z,t)\). Gray and red dashed lines indicate the averages of \({g}_{w}\left(t\right)\) and \({g}_{w}^{out}\left(t\right)\), and they are oriented at 0 and 10 μGal, respectively, so as not to overlap \({g}_{w}\left(t\right)\) with \({g}_{w}^{out}\left(t\right)\) in this panel. e Black and gray lines indicate the superconducting gravity data, obtained by the SG016 (\({g}_{obs}(t)\); Fig. 2c). Yellow triangles indicate the observed hydrological gravity change, which was identified by comparing the time variation in \({g}_{obs}(t)\) with those in \(R(t)\), \(h(t)\) and \(\theta (z,t)\). A red line indicates the superconducting gravimeter data after the hydrological effect of \({g}_{w}^{out}\left(t\right)\) was subtracted from \({g}_{obs}(t)\)