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