Spatial–Temporal Soil Water Dynamics beneath a Tree Monitored by Tensiometer-Time Domain Reflectometry Probes

Abstract

Tensiometer-coiled time domain reflectometry (T-TDR) probes have been developed in previous studies, but have not been applied in the field. In this study, we applied T-TDR probes to the simultaneous monitoring of soil water content (θ) and soil water potential (ψ) on a profile beneath a tree in a forest stand, and analyzed the temporal and spatial variations in soil water dynamics in a root-containing environment. The results showed different features in the relationships between the mean and standard deviation of spatial θ and ψ, which exhibited convex-upward shapes and negative curvilinear shapes, respectively. High spatial variability was observed at intermediate values of θ and small values of ψ. Matrix flow and preferential flow accounted for 75% and 25% of the area beneath the tree. Although the infiltration processes were dominated by matrix flow, preferential flow acting for a short time could cause an average θ or ψ to reach their maximum values at all of the locations. Preferential flow primarily occurred at a “hot spot” around a coarse root. Small changes in θ and ψ were generally observed at a “cold spot” beneath a lateral root. Integrated information from multiple sources of θ and ψ could help to evaluate soil water dynamics when one exhibited large spatial variation during the wetting or drying processes, and greatly help to improve the accuracy for detecting the presence of preferential flow in a short measurement period.