Inverse Modeling of Mass-conservative Numerical Model for Variably Saturated Seepage Flow

概要

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An inverse method for identifying the unsaturated hydraulic conductivity in variably saturated seepage flow model is proposed. As the basic equation which governs the forward problem, the mixed-form Richards equation is considered which is conservative in mass balance describing the flow in terms of pressure head and moisture content. For functional representation of the relative hydraulic conductivity (RHC) which is a major unknown parameter to be identified, the free-form parameterized function (or sequential piecewise cubic spline function) which provides high flexibility in identifying the functional form of the parameter is employed. To determine values of the coefficients of the function in the manner that errors between the observed and computed pressure heads are minimized, a simulation-optimization algorithm with the aid of the Levenberg-Marqurdt method is constructed. The method proposed is applied to in-situ soil column in an upland crop field, and its validity is examined in terms of reproducibiliry of desorption process in the soil. The results show that the simulation-optimization algorithm is successful in finding RHC of functional form, and that the forward solution model with the RHC parameter so identified could well reproduce actual desorption process when air-temperature dependency of the observed pressure head variation is appropriately eliminated.