Combining time-lapse electrical resistivity and self-potential methods to assess soil moisture dynamics in a forested catchment under the rainfall event

Abstract. Infiltration process and soil moisture dynamics in response to rainfall are crucial for subsurface flow generation, landslide hazard control, and hydro-ecosystem management. Geophysical tools such as the electrical resistivity tomography (ERT) and self-potential (SP) method have proven useful in providing greater temporal and spatial subsurface information and making connections to water content or water flux. In this study, the combination of ERT and SP was used to enhance our understanding of water infiltration processes caused by natural rainfall, subsurface structures, and plant root distribution. We installed 100 ERT electrodes with an electrode spacing of 0.5 m on a tree-covered hillslope to conduct ERT surveys every two weeks from June to October 2022, and increase the frequency of measurements during rainfall events. Non-polarized electrodes were used to measure SP along the hillslope and at various depths. Time-domain reflectometry (TDR) sensors were used to obtain the soil water content as an accurate reference to establish the petrophysical relationship between soil moisture and resistivity. Based on the robustness assessment of these relationships, the results suggest that the Dipole-Dipole configuration may help to obtain a more reasonable interpretation of resistivity at the forest site than the Wenner configuration. The joint interpretation of the SP and ERT results showed that water flow in the study area is dominated by vertical direction, and two preferential flow paths due to the fractured and permeable soil layer interface provide important vertical hydrological connections between deep and shallow soil layers. In addition, the subsurface infiltration processes were strongly influenced by the trees with different root characteristics. Trees with shallow roots tend to retain more water content at the surface. Deep-rooted trees absorb and store water in deeper layers, resulting in significant abrupt changes in the water content of the deeper soil layers. The results of this study provide a basis for investigating the hydrological connectivity and climate sensitivity of soil water distribution by linking rainfall with subsurface information provided by the ERT and SP.