Event-scale infiltration and subsurface moisture dynamics in a fractured hillslope monitored by time-lapse electrical resistivity tomography

Study region The Touchih right-bank slope adjacent to the Liyutan Reservoir in central Taiwan is a structurally heterogeneous fractured rock hillslope in a humid subtropical setting. Historical instability and drainage interventions indicate spatially variable groundwater responses under rainfall forcing. Study focus This study investigates event-scale infiltration, moisture redistribution, and drainage using 510 quality-controlled hourly time-lapse electrical resistivity tomography (ERT) profiles acquired during multiple rainfall events. Temporally constrained inversion results were used to derive normalized relative water saturation (RWS) indicators and observation-based metrics of response lag, spatial variability, and saturation persistence. Geological information and available hourly groundwater-level observations supported hydrological interpretation, while normalized depth-of-investigation assessment constrained the principal interpretation depth. New hydrological insights for the region Rainfall-related responses differ markedly among structural domains of the slope. Fracture-influenced zones exhibit rapid wetting followed by relatively rapid drainage and short moisture persistence, whereas relatively intact or storage-dominated zones show delayed responses and prolonged moisture retention. The continuously constrained model region extends predominantly to approximately 27–30 m depth, within which spatially coherent ERT/RWS responses provide field evidence of structurally influenced hydrological heterogeneity. These findings show that saturation persistence, together with response timing, provides useful process-oriented information for monitoring fractured hillslopes under humid subtropical rainfall conditions.