Simultaneous measurement of water retention and electrical conductivity in soils: Testing the Mualem‐Friedman Tortuosity Model

A conceptual model that accounts for the influence of pore geometry is needed to obtain soil water conductivity from bulk soil electrical conductivity measurements in unsaturated soils. Mualem and Friedman (1991) proposed such a model, based on the hypothesis that the tortuosity factor affecting bulk soil electrical conductivity is identical to that defined for prediction of soil hydraulic conductivity. Soil water retention curves (model input) and bulk soil electrical conductivity (model output) of two soils were measured simultaneously over a wide range of soil water conductivity using a multistep outflow (MSO) experiment in combination with time domain reflectometry. The model with only one free parameter can describe the measured data. Predictions indicate a large dependence of the model outcome on the value of the fitting parameter. The fitted tortuosity parameters were used for inverse modeling of the water flow during the MSO experiments, obtaining unique estimates of the hydraulic conductivity.