Characterizing Soil-Pipe Networks with Pseudo-Three-Dimensional Resistivity Tomography on Forested Hillslopes with Restrictive Horizons

Soil pipes are important hillslope-hydrology features affecting mechanistic processes such as subsurface drainage, streamflow, erosion, hillslope stability, and discharge response to rain fall and snow melt. However, the limited capacity to characterize these pathways in situ remains a major limitation for process-based understanding and incorporation into coupled hydrologic models. The goal of this study was to investigate the use of pseudo-three-dimensional electrical resistivity tomography (ERT) as a field method for characterizing soil-pipe spatial distribution and connectivity on a forested hillslope with a specific interest in detecting the formation of soil pipes from root decay and combustion. We conducted detailed ERT surveys on three 6.12-by 6.12-m plots before and after a prescribed burn located on a 2-ha clearcut of coniferous forest underlain by a fragipan. Excavation following the ERT measurements showed that soil pipes were mostly empty with 86% containing <25% decaying root matter. The mean diameter of pipes was 4.3 cm with a mean length of 1.6 m. Such empty pipes would resolve as elongated high-electrical resistivity features. However, a high resistivity layer corresponding to the Bw and BE soil horizons overlaying a fragipan was observed on all plots with the majority of soil pipes larger than 1 cm diameter occurring within these horizons. The lack of observable contrast between pipes and the high resistivity layer as well as reduced data coverage within the vicinity of large soil pipes may have limited our ability to effectively characterize soil pipes occurring within these hillslopes. We conclude that the use of ERT surveys to detect large macropores pathways may be more suitable to settings where soil pipes are not occurring within high restrictive layers.