Monitoring subsurface moisture redistribution during sprinkling experiments using time-lapse ERT

Precipitation-driven lateral subsurface flow, known as subsurface stormflow (SSF), is an important process for generating runoff and can considerably contribute to streamflow during storm events. However, its transient and spatially variable nature complicates its investigation and measurement, and the limited number of systematic studies across contrasting land uses, soil types, and hydrogeological settings limits its accurate representation in hydrological models.

To improve the understanding of SSF, controlled sprinkling experiments were conducted at multiple sites in Germany and Austria, covering different land uses, soils, and climatic conditions. At each site, trenches were excavated downslope to intercept lateral subsurface flow at depths of up to 3 m. Artificial sprinkling experiments were performed over a surface area of approximately 200 m² at a constant irrigation rate of ~16 mm h⁻¹. During the experiments, trenchflow discharge was continuously measured at two depths, complemented by soil moisture and groundwater level observations.

Additionally, 2D time-lapse Electrical Resistivity Tomography (ERT) profiles were carried out during eight of the eleven sprinkling experiments, including six forested and two grassland sites. Time-lapse ERT measurements were acquired during the 3-hour irrigation, until 6 hours after the beginning of irrigation, and 24 hours after irrigation to capture delayed subsurface responses.

This study evaluates the contribution of ERT to resolving subsurface features that may control SSF generation and flow pathways, such as vertical heterogeneity, structural interfaces, and potential preferential flow zones. Time-lapse resistivity variations are analysed in conjunction with observations of soil moisture, trenchflow discharge, and groundwater levels, to assess the consistency between geophysical responses and hydrological dynamics. The study highlights both the strengths and limitations of ERT for characterising SSF-related subsurface dynamics and contributes to the integration of geophysical observations into hydrological studies.