Three-region Campbell Model for Unsaturated Hydraulic Conductivity in Undisturbed Soils

Abstract

A three-region Campbell (TRC) type model for predicting undisturbed soil unsaturated hydraulic conductivity from water retention is presented. The model assumes that hydraulic conductivity follows separate Campbell functions within the macropore (matric head ψ > −10 cm H2O), the mesopore (−10 ≥ ψ > −350 cm H2O), and the micropore (ψ ≤ −350 cm H2O) regions, and that soil water retention and two reference-point values of hydraulic conductivity are known. Conductivity and retention data for 100 undisturbed soils from the UNSODA database and 68 soils from a Danish database were used for model development. Conductivity for both highly structured (three-region) and weakly structured (two-region) soils mostly followed a piecewise linear function (with slope β) in a Log(conductivity) −Log(water content) plot, supporting the TRC model concept. A unique relationship between the Campbell soil-water retention parameter, b, and the unsaturated conductivity parameter, β, was found valid for both meso- and micropore regions. It was shown that the values of b in the mesopore and micropore regions are not correlated, making the use of single-region expressions (e.g., the Mualem–van Genuchten type models) questionable and suggests that a multiregion model with noncorrelated retention parameters between pore regions, such as the TRC model, may provide a conceptually more correct description of hydraulic conductivity. The TRC model yielded improved conductivity predictions in loamy and clayey soils whereas predictions for sandy soils were comparable to the single-region Campbell and van Genuchten models. TRC model predictions compared well with independent data for three differently textured soil profiles.