GPR Evidence of the Influence of Vegetation and Structure on Subsurface Flow in An Epikarst Limestone, Edwards Plateau, Texas

Three-dimensional multicomponent ground-penetrating radar (GPR) reflection data and horizontal GPR transmission profiles were acquired and analyzed to better understand the infiltration and subsurface flow at a hydrologic experimentation site. Previous researchers conducted a set of shallow (< 2.5 m) subsurface hydrology experiments during simulated rainfall events within a small plot (7 x 14 m) on the fractured and karsted limestone of the Edwards Aquifer region near San Antonio, Texas, USA, to better understand the influence of Juniperus ashei brush control on the local hydrology. At a trench located on the down slope side of the site, lateral subsurface flow was observed emanating from open joints, bedding planes, karst features, and root systems occupying these spaces. Tracer experiments showed a high degree of variability in tracer recovery, advection speed, and concentration depending on the location of the application of the tracer. Both 3-D multicomponent GPR reflection images and coherency, and inversion of GPR horizontal transmission profiles were utilized to identify the main conduits of flow within the experimentation site in order to explain the observations of the experiments and to show a correlation of brush to these conduits. Inversions of the horizontal transmission profiles indicate that some conduits are filled with soil while others have no fill. This information helps explain the high spatiotemporal variability in the tracer data. Additionally, the GPR and hydrologic experiments suggests that Juniperus ashei significantly impacts infiltration by redirecting flow towards its roots with hydrophobic litter and stemflow. This study demonstrates that GPR provides a noninvasive tool that can improve future subsurface hydrologic experimentation.