Can the Onset of Macropore Flow be Detected using Electrical Resistivity Measurements?

This study evaluated whether electrical resistivity monitoring could be used as a tool to detect the activation of macropores in soils. Experiments were performed where soil resistivity was measured using a Wenner array while individual soil macropores were sequentially saturated to simulate changes in macropore activation in a hydrologically active soil. The measured apparent resistivity was found to be highly dependent on the activated macropore porosity. An increase in macropore porosity from 0 to 4% decreased the apparent resistivity of the soil by up to 30%. A simple two-domain model was developed to evaluate the controls on apparent resistivity in a macroporous soil by conceptualizing the macropores as fluid-filled tubes embedded within a homogeneous soil matrix. The model qualitatively reproduced the trends of the observed data and provided reasonable bounds on the expected response of soils, suggesting that it captures first-order influences of macropores on apparent resistivity measurements. The model indicates that the magnitude of the reduction in apparent resistivity is strongly dependent on the contrast in resistivity between the soil (r(o)) and macropores (r(m)), which can be readily estimated in field monitoring scenarios. The r(o)/r(m) ratio is therefore suggested as a useful diagnostic for evaluating conditions where changes recorded in resistivity monitoring data might be used to detect the onset of macropore flow in field-based studies.