Streaming potential (SPs) is the electric potential generated by fluid flow in a charged porous medium. The SPs signals are related to pore water velocity, bulk electrical conductivity, pore water charge excess, and soil porosity. While several studies have estimated hydraulic properties of the saturated zone from SPs, there have been fewer attempts to infer unsaturated hydraulic properties from SPs. From numerical and laboratory experiments in which infiltration and subsequent drainage was monitored with nonpolarizable Ag/AgCl electrodes and tensiometers, we showed that it is feasible to estimate three key Mualem–van Genuchten hydraulic parameters (fitting parameters α and n and saturated hydraulic conductivity Ks) and Archie's saturation exponent (na) using a coupled hydrogeophysical inversion approach. In addition to a reasonably good estimate of na, coupled hydrogeophysical inversion of actual SPs measurements during drainage provided estimates of α, n, and Ks that were comparable to those obtained from an independent inversion of the tensiometric data (matric heads). We concluded that coupled hydrogeophysical inversion of time-lapse SPs signals is a promising method for hydraulic characterization of the vadose zone. Accurate modeling of SPs signals is essential for reliable inversion results, but there is still debate about the appropriate model for the voltage coupling coefficient at partial saturation. Our experimental data showed a nonlinear and monotonic decrease in the absolute voltage coupling coefficient with decreasing saturation. A comparison of several available models with our experimental data showed that models that consider the relative permeability and the relative electrical conductivity in addition to the saturated coupling coefficient and water saturation were most appropriate.

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