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

We developed a novel, lognormal, pore-scale, unsaturated hydraulic conductivity model, K(psi)Model, which does not require saturated hydraulic conductivity, Ks, as an input parameter. K(psi)Model is derived solely from hydraulic parameters describing a bimodal, lognormal, pore-scale, soil water retention curve theta(psi).The K(psi)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(psi)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(psi)Model gives improved results when the hydraulic parameters are dynamically constrained and when theta(psi) describes a bimodal, lognormal distribution.