Soil hydraulic models selection based on in-situ measurements

Soil hydraulic properties are usually expressed as functional relationships between the soil hydraulic conductivity and water content or matric potential, and between the matric potential and water content. Both functions are highly nonlinear, and direct in-situ measurements are time consuming and involve considerable uncertainty. Therefore, methods based on soil hydraulic models, i.e., functions chosen a priori and containing parameters estimated from measurements, are of interest. The most widely used models are those of Gardner in 1958, Russo in 1988, Brooks and Corey in 1964, and van Genuchten in 1980. For such models, an equation is derived that relates the developed expression for the first moment, M, of the diffusivity function with a combination of measured parameters: Saturated hydraulic conductivity, capillary length, saturation and residual water content, and soil sorptivity. The M expressions are derived for the three models in the form of explicit functions of their parameters. Three overlapping M ranges are determined where the selection of a respective model is preferable.