Investigating Subsurface Stormflow: 2D-ERT and Artificial Rain Simulations for Identifying Vertical and Lateral Flow Components

Subsurface storm flow (SSF), also referred to as interflow, plays an important role in runoff generation and flood events at the watershed scale. Its transient and spatially variable nature, however, presents significant challenges in investigating and measuring this subsurface process. Framed within the SSF Forcing research unit, supported by the German Research Foundation (DFG) and Austrian Science Fund (FWF), this study explores several methodologies for detecting and characterizing SSF at the plot scale with a focus on understanding its vertical and lateral flow components. The research unit goal is to enhance the accuracy of SSF representation in hydrological models, enabling better flood predictions and water management practices.

This research employs multiple techniques, including Electrical Resistivity Tomography (ERT), Time-Domain Reflectometry (TDR), and artificial rain simulations (ARS). These methods allow for the detailed examination of hydrological processes under controlled conditions, facilitating a comprehensive understanding of the dynamics of subsurface flow. TDR is used to quantify vertical water movement, providing baseline data for interpreting ERT profiles. Simultaneously, the use of two parallel ERT profiles within the irrigation plot enables continuous monitoring of subsurface flow pathways. These profiles capture both vertical infiltration and lateral interflow, which are key components of SSF.

While ERT and ARS are well-established techniques for tracing infiltrating water, distinguishing between vertical and lateral flow remains a challenge. The strong changes in the surface resistivity created by the artificial rainfall complicates the differentiation between vertical infiltration and lateral flow, due to its intrinsic limitations and the inversion artefacts that are exacerbated especially close to the surface. To address this, an 2D ERT reference line is positioned below the primary plot to isolate lateral flow from the influence of vertical infiltration. This reference line serves as a control, allowing for the validation of vertical and lateral interflow dynamics and ensuring the detection of deeper subsurface flows that are not influenced by direct rainfall input.

In addition, the significant change in the surface resistivity occurring within the ARS plot influences the resistivity measurements outside the irrigation plot in the reference ERT line, due to the tridimensionality of the physical phenomena. Therefore, this study primarily evaluates the effect of the irrigated waterfront on the 2D ERT resistivity measurements using forward modelling to subsequently focus on the lateral component of the subsurface runoff. Hence, it assesses the feasibility of using 2D-ERT data to identify subsurface stormflow in combination with ARS, addressing the challenge of differentiating flow components and mitigating inversion artifacts in the resistivity profiles.  Overcoming these challenges is necessary for improving the reliability of subsurface flow detection using ERT in hydrological research.