New geometry factors for hydraulic property‐based soil solution electrical conductivity models

Improved methods are needed to intensively measure ionic solutes in soils. The concentration C i of ionic solutes in soils is directly proportional to soil solution electrical conductivity σ w . Time domain reflectometry (TDR) measures both soil water content θ and bulk soil electrical conductivity σ a using the same probes. However, physical/conceptual models are required along with TDR measurements in order to use TDR for in situ estimates of C i . We discuss a modeling approach [ Mualem and Friedman , 1991] based on assumed analogy between tortuosity of electrical and hydraulic flow paths in variably saturated soils. We review the model approach, then derive a general expression for a pore geometry factor F G considering flow of electrical current through randomly distributed capillary soil pores. Two F G are derived based on two conceptual considerations of tortuous capillary length. Four water retention models (WRM) are used to describe soil hydraulic properties in the F G . When fitted to the same measured water retention data, the four WRMs provided substantially different magnitudes for F G . The model was then compared in terms of σ w estimates using the two new F G in combination with field‐measured θ and σ a . One of the new F G produced smaller estimated σ w than did that proposed in the original model. This is desirable based on our own and several other published comparisons that indicated the original model may overestimate σ w in comparison with independent measurements.