Applications of the inverse approach for estimating unsaturated hydraulic parameters from laboratory flow experiments

Abstract

Estimates of soil hydraulic parameters are essential for predicting and describing water movement in unsaturated soils. Inverse approaches to estimating soil hydraulic parameters have gained great favor. However, limited effort has been placed on obtaining estimates using observed experimental data. Moreover, little has been advanced in applying these approaches to larger scale, multidimensional systems. In this research, several transient laboratory experiments were conducted using both repacked and intact soil cores, and larger scale 1 and 3-dimensional repacked soil columns. Measurements of soil water matric potential and water content were used to obtain parameter estimates for the closed form van Genuchten soil water relations. For the soil cores, measurements were obtained using pressure outflow, upward infiltration and evaporation procedures. For the 1-dimensional soil columns, these same data were obtained using upward and downward infiltration procedures. For the 3-dimensional soil columns, a point source application of water was used. Optimizations were carried out With HYDRUS-1D and HYDRUS-2D using observed matric potential and water content data to define the objective function. For the repacked and intact soil cores, parameters were also estimated by nonlinear least squares fit to retention data obtained from the pressure outflow and evaporation procedures. Parameter estimates obtained by nonlinear least squares fits to the pressure outflow and evaporation retention data were seen to be similar to those obtained by optimization. However, parameter estimates based on data obtained from the upward infiltration experiments were considerably different than those obtained from the evaporation experiments. These differences are attributable to hysteresis. Parameter estimates obtained for the repacked soil cores were seen to be inadequate in terms of predicting matric potential responses in larger scale soil columns. This was also true when parameter estimates obtained from the larger scale repacked soil columns were used to predict matric responses during 3-dimensional flow in the largest scale soil columns. The results of this work show that inverse methods when used in conjunction with upward infiltration and evaporation procedures can provide accurate estimates of unsaturated hydraulic parameters. When used together, these experimental methods can be used to obtain data and hydraulic parameters describing both the imbibition and drying branches of the soil water retention response. These results further show that estimated parameter values based on measurements made at one scale tend to be inappropriate in terms of adequately describing systems at larger scales.