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 Ci of ionic solutes in soils is directly proportional to soil solution electrical conductivity σw. Time domain reflectometry (TDR) measures both soil water content θ and bulk soil electrical conductivity σa using the same probes. However, physical/conceptual models are required along with TDR measurements in order to use TDR for in situ estimates of Ci. 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 FG considering flow of electrical current through randomly distributed capillary soil pores. Two FG 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 FG. When fitted to the same measured water retention data, the four WRMs provided substantially different magnitudes for FG. The model was then compared in terms of σw estimates using the two new FG in combination with field-measured θ and σa. One of the new FG produced smaller estimated σw 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 σw in comparison with independent measurements.