Estimating unsaturated soil hydraulic properties from laboratory tension disc infiltrometer experiments

Four tension disc infiltration experiments were carried out on a loamy soil in the laboratory for the purpose of estimating the unsaturated soil hydra ulic properties. Sixteen tensiometers were installed in pairs at the follow ing coordinate (r, z) positions: (10, 2.5), (10, 5), (10, 10), (15, 5), (15 , 10), (15, 15), (15, 20), and (15, 30), where r represents the distance fr om the axis of symmetry and z is the location below the soil surface. A tim e domain reflectometry (TDR) probe was used to measure water contents at a depth of 2 cm directly below the tension disc. The first three experiments involved supply pressure heads at the disc of -20, -10, -5, and -1 cm, with the experiment lasting for -5 hours. The same supply pressure heads were a lso used for the fourth experiment, which lasted 6.25 days so as to reach s teady state at each applied tension. The measured data were analyzed using Wooding's [1968] analytical solution and by numerical inversion. The parame ter estimation method combined a quasi three-dimensional numerical solution of the Richards equation with the Marquardt-Levenberg optimization scheme. The objective function for the parameter estimation analysis was defined u sing different combinations of the cumulative infiltrated volume, TDR readi ngs, and tensiometer measurements. The estimated hydraulic properties were compared against results obtained with an evaporation experiment as analyze d with Wind's [1968] method. Water contents in the retention curves were un derestimated when both transient and quasi steady state experiments were an alyzed by parameter estimation. Unsaturated hydraulic conductivities obtain ed by parameter estimation and using Wooding's [1968] analysis corresponded well. Drying branches of the hydraulic conductivity function determined by parameter estimation also corresponded well with those obtained with the e vaporation method.