EVALUATION OF THE MULTISTEP OUTFLOW METHOD FOR THE DETERMINATION OF UNSATURATED HYDRAULIC PROPERTIES OF SOILS

Abstract

The moisture retention characteristic h(θ) and the hydraulic conductivity curve K(θ) of 57 soil samples were determined using the Multistep Outflow method. Using the Mualem-van Genuchten h(θ) and K(θ) models, the effect of four different parameter vectors on the estimation of the hydraulic functions was analyzed. These parameter vectors differed in the number of fixed parameters between two and four, corresponding to different assumptions and/or available information concerning the value of the parameters. To compare estimated saturated hydraulic conductivity values (Ks) with measured values from the same sample, a modified Multistep Outflow method was developed. The effect on the estimation of parameters using additional moisture content values, which were included in the objective function, was analyzed. A geostatistical analysis of the data was performed to identify the source of variation in the observed parameter distributions. Analysis of the four different parameter vectors showed that Ks and l, a parameter characterizing pore size interaction, should be regarded as fitting parameters. Additional data points of the moisture retention characteristic, which were obtained directly from the outflow experiment, provided better estimates of the moisture retention characteristic. Parameter estimates of the moisture retention characteristic obtained from the four-parameter case did not provide a good description of h(θ).