Diffusivity and One‐Dimensional Absorption Experiments

Profiles of normalized volumetric soil water content, Θ, during one‐dimensional absorption are often found to provide a unique profile in terms of the Boltzmann similarity variable, λ. Traditionally, λ(Θ) data have been used to obtain the diffusivity‐water content function D (Θ) by the method of Bruce and Klute. This paper highlights the well‐known inadequacy of this method, and outlines a procedure for deriving D (Θ) by the fitting of a mathematical function to the primary experimental data set, namely λ(Θ). An analytical expression for D (Θ) scaled by sorptivity is provided. A function of the form (1‐Θ) p is shown to fit the λ(Θ) of a fine sand adequately and provide a D (Θ) function that accommodates the soil water diffusivity measured using other methods at near‐saturation.