LINKING THE GARDNER AND CAMPBELL MODELS FOR WATER RETENTION AND HYDRAULIC CONDUCTIVITY IN NEAR-SATURATED SOIL

Abstract

Linking the classical Gardner and Campbell models for soil hydraulic properties yields a combined Gardner-Campbell (GC) relationship for predicting unsaturated hydraulic conductivity (K) from soil-water retention. The new GC water retention and hydraulic conductivity relationships are described by simple linearized expressions based on Campbell-b-scaled values of volumetric soil-water content (θ). The GC hydraulic properties models include four parameters, the pore connectivity parameter x (typically between 1 and 2), the Campbell pore size distribution parameter b, the Gardner macroscopic capillary length λ, and the air-entry soil-water matric potential (ψ e ). In the GC model for K(θ), these parameters are merged into a single dimensionless parameter, A. The GC models are applied to seven different undisturbed soils within a soil-water matric potential ψ with a range of 0 to -60 cm H 2 O. Results show that the simple GC ψ(θ) model adequately describes water retention data close to saturation, and that the GC K(θ) model performs well for predicting near-saturated hydraulic conductivity. The GC water retention and hydraulic conductivity parameters were incorporated into Wooding's equation for steady-state infiltration rate to examine the effects of variability in physical characteristics on infiltration. The steady-state infiltration rate was less sensitive to the GC water retention and hydraulic conductivity parameters than the saturated hydraulic conductivity. The GC models, which combine advantages of the Gardner model for linearization of the Richards' equation and the simple Campbell model for soil-water retention, seem useful for describing or predicting near-saturated soil hydraulic properties and for stochastic simulations of field-scale water infiltration.