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

Abstract

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.