Bimodal unsaturated hydraulic conductivity derived from water retention parameters by accounting for clay‒water interactions: deriving a plausible set of hydraulic parameters

Abstract

We developed a novel, lognormal, pore-scale, unsaturated hydraulic conductivity model, K(ψ)model, which does not require saturated hydraulic conductivity, Ks, as an input parameter. K(ψ)model is derived solely from hydraulic parameters describing a bimodal, lognormal, pore-scale, soil water retention curve θ(ψ). The K(ψ)model is based on the Hagen‒Poiseuille equation, which represents the soil as a bundle of parallel, non-intersecting capillary tubes. To improve the modelling of fine-textured soils we introduced a novel model to consider the clay‒water interaction. This model assumes that clay‒water interaction occurs for soils having more than 30% of clay and an effective matrix porosity greater than 35%. Compared to previously developed models, the K(ψ)model does not require the use of integrals and can be computed from a spreadsheet and distinguishes between macropore (non-equilibrium) and matrix (equilibrium) flows. The K(ψ)model gives improved results when the hydraulic parameters are dynamically constrained and when θ(ψ) describes a bimodal, lognormal distribution. Suggested Reviewers: