USING AN INVERSE METHOD TO ESTIMATE THE HYDRAULIC-PROPERTIES OF CRUSTED SOILS FROM TENSION-DISC INFILTROMETER DATA

An inverse procedure was used to estimate the soil hydraulic character istics of a two-layered soil system-soil surface crust and subsoil-fro m data obtained during a tension-disc infiltration experiment. The inv erse procedure combined the Levenberg-Marquardt nonlinear parameter op timization method with a numerical solution of the axisymmetric variab ly-saturated flow equation. The objective function was defined in term s of the cumulative infiltration curve and the final water content mea sured directly below the tension-disc infiltrometer at the end of the experiment; this final water content was assumed to correspond to the final supply pressure head. We analyzed two infiltration experiments c arried out with a 25-cm diameter tension-disc infiltrometer. One exper iment was carried out on a two-layered system, and a second after remo val of the surface crust covering the sandy subsoil. Both experiments were performed with six consecutive supply tensions. We first analyzed the infiltration experiment for the subsoil only, thus yielding its h ydraulic characteristics. Subsequent analysis of the infiltration expe riment for the two-layered system with known hydraulic properties of t he subsoil provided estimates of the hydraulic properties of the surfa ce crust. We further compared the estimated hydraulic parameters of th e subsoil with those obtained using Wooding's analytical method [Woodi ng, R.A., 1968. Steady infiltration from a shallow circular pond. Wate r Resour. Res. 4, 1259-1273] and predictions based on a neural network model requiring textural input information. All three methods generat ed roughly the same results. The numerical inversion technique proved to be a convenient tool for estimating the soil hydraulic properties o f both the surface crust and the subsoil. (C) 1998 Elsevier Science B. V. All rights reserved.