Water movement in unsaturated soils gives rise to measurable electricalpotential differences that are related to the flow direction and volumetricfluxes, as well as to the soil properties themselves. Laboratory and field datasuggest that these so-called streaming potentials may be several orders ofmagnitudes larger than theoretical predictions that only consider the influenceof the relative permeability and electrical conductivity on the self potential(SP) data. Recent work has partly improved predictions by considering how thevolumetric excess charge in the pore space scales with the inverse of watersaturation. We present a new theoretical approach that uses the flux-averagedexcess charge, not the volumetric excess charge, to predict streamingpotentials. We present relationships for how this effective excess chargevaries with water saturation for typical soil properties using either the waterretention or the relative permeability function. We find large differencesbetween soil types and the predictions based on the relative permeabilityfunction display the best agreement with field data. The new relationshipsbetter explain laboratory data than previous work and allow us to predict therecorded magnitudes of the streaming potentials following a rainfall event insandy loam, whereas previous models predict three orders of magnitude too smallvalues. We suggest that the strong signals in unsaturated media can be used togain information about fluxes (including very small ones related to film flow),but also to constrain the relative permeability function, the water retentioncurve, and the relative electrical conductivity function.
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