Determination of Soil Parameters Utilizing Continuous Observation Data of Gravity and Soil Moisture: a Case Study at NAOJ Mizusawa

Abstract

　We determined soil parameters of the Van Genuchten’s model for the soil of Mizusawa VLBI Observatory, National Astronomical Observatory of Japan (NAOJ Mizusawa), utilizing the continuous data of superconducting gravity and soil moisture observed at the observatory. We calculated time variations in soil moisture and hydrological gravity change using the G-WATER [1D] software, and searched for optimal values of three soil parameters (θ_min, α, and n) by comparing the calculated time series with the observed ones. The optimal values of the parameters θ_min and n derived from the gravity data were 0.0 m³/m³ and 1.50, respectively, and they agreed with those derived from the soil moisture data independently. In contrast, the optimal value of the parameter α was inconsistent between the gravity and soil moisture data, mainly because the gravity change has a low sensitivity to the α’s variation. Instead, we determined the α’s optimal value so as to minimize an evaluation function, which was defined based on residuals for gravity and soil moisture. The α’s optimal value was obtained to be 0.45, and then the root-mean-square residuals for soil moisture and gravity were 0.0159 m³/m³ and 0.644 μGal, respectively. The optimal values of the three parameters correspond to typical values for silty soils, as did the actual value of the saturated permeability (K_s) which was measured in a previous study.