A physically-constrained inversion method for rapid and robust estimation of soil hydraulic parameters from one-dimensional upward infiltration

Abstract

Low-cost, simple, efficient, and robust methods for measuring soil hydraulic properties are essential for land management and hydrological modeling across scales. The recently developed Approximate Solutions under Constant Pressure (ASCP) method provides a promising approach. Here, we further developed a new inversion approach, the Piston-type Approximate Solutions under Constant Pressure (PASCP) method, which greatly enhances the robustness of the estimated results by reducing the sensitivity of the ASCP method to measurement errors through two key innovations: (1) an original explicit formula linking soil saturation time to hydraulic parameters, used as a physical constraint to improve inversion accuracy; and (2) an interval constraint that explicitly accounts for measurement errors and enhances robustness, tolerating up to 99.9 % of observed errors. The PASCP method allows simultaneous estimation of all Brooks-Corey hydraulic parameters (n, 1/h_d, and K_s) from only an upward infiltration test conducted on a standard 5-cm soil core, with easily obtainable input data. The new method was evaluated through numerical verification and laboratory validation, covering 12 soil samples and four initial moisture conditions. Numerical verifications demonstrated excellent agreement with theoretical parameter values (R² ≥ 0.998). Laboratory tests confirmed PASCP’s robust performance even in the presence of measurement errors (R² ≥ 0.804), showing 31–60 % improvement in R² compared to the original ASCP method. Overall, PASCP provides a practical, reliable technique for estimating hydraulic parameters across a wide range of soil textures under low initial moisture conditions (S_ei ≤ 0.3).