Shape of Time Domain Reflectometry Signals during the Passing of Wetting Fronts

Core Ideas Precise wetting front arrival times can be determined from local TDR readings. Wetting shock fronts due to preferential flow are sharp. This approach uses the control volume with two‐rod TDR probes as the longitudinal axis. Estimates of preferential flow rates follow from this approach. Viscous flow theory expects sharp wetting shock fronts during infiltration in permeable media, but time domain reflectometry (TDR) measurements, using horizontally installed two‐rod probes, reveal concave and convex increases at the early and late stages during the passing of the front, i.e., the TDR signals are S‐shaped. Wetting front dispersal was initially considered as the cause, due to variations in the flow path lengths at the profile scale. However, later studies favored processes that are closer to the scale of the TDR control volume. In this study, an approach was developed that quantifies the shape of TDR signals exclusively with local features. It improves the determination of the arrival time of a wetting shock front at the depth of a TDR probe, and it ultimately supports the notion of sharp wetting shock fronts at the scale of the probe's length that evolve during preferential infiltration.