Self-potential signals of soil columns experiencing evaporation and transpiration

The self-potential (SP) method has been used in hydrological sciences to monitor many hydrologic processes thanks to the electrokinetic coupling between water flow and streaming current in geological materials. Despite many useful applications, quantitative interpretations are still rare, in particular for unsaturated soils where the water fluxes are of orders lower than that in saturated conditions. In this study, we used laboratory soil column tests to simulate vadose zone hydrologic processes (drainage, evaporation, and transpiration) and to generate SP data in low water flow conditions. The measured water fluxes and SP signals in different hydrologic stages of the tests are used to study if electrokinetic coupling is still the dominant mechanism for the SP signals in unsaturated, low-flow conditions. Theoretical models of electrokinetic and electrodiffusion couplings are also used to guide the analysis. It is shown that the SP signals measured during soil evaporation and plant transpiration in this soil column test were not only caused by unsaturated water flows in the soil column through electrokinetic coupling. Instead, they are likely related to the ion concentration gradient in the soil column, which creates an electrical current of a diffusive nature. The ion concentration gradient is likely related to the different reaction rates of mineral–water interactions in saturated and unsaturated soils. This study, therefore, highlighted the importance of considering the electrodiffusion coupling in interpreting the measured SP signals in vadose zone hydrology.