  Self-potential (SP) data are of interest to vadose zone hydrology because of
their direct sensitivity to water flow and ionic transport. There is
unfortunately little consensus in the literature about how to best model SP
data under partially saturated conditions and different approaches (often
supported by one laboratory data set alone) have been proposed. We argue herein
that this lack of agreement can largely be traced to electrode effects that
have not been properly taken into account. A series of drainage and imbibition
experiments are considered, in which we find that previously proposed
approaches to remove electrode effects are unlikely to provide adequate
corrections. Instead, we explicitly model the electrode effects together with
classical SP contributions using a flow and transport model. The simulated data
agree overall with the observed SP signals and allow decomposing the different
signal contributions to analyze them separately. By reviewing other published
experimental data, we suggest that most of them include electrode effects that
have not been properly taken into account. Our results suggest that previously
presented SP theory works well when considering the modeling uncertainties
presently associated with electrode effects. Additional work is warranted to
not only develop suitable electrodes for laboratory experiments, but also to
assure that associated electrode effects that appear inevitable in longer-term
experiments are predictable, such that they can be incorporated in the modeling
framework.
