Abstract

Knowledge of the vadose zone and hydrological processes in near‐surface soils is crucial for making agricultural decisions, understanding contaminant propagation, and describing infiltration and recharge.  In July 2012, artificial infiltration experiments were conducted at the Shale Hills Critical Zone Observatory in central PA and monitored with various geophysical methods, including time‐lapse electrical resistivity tomography (ERT), ground wave ground‐penetrating radar (GPR), and off‐ground surface reflection GPR. These methods were assessed for their ability to observe changes in soil moisture content during infiltration at a small, 1 m by 3 m survey grid situated on a forested hillslope.  Artificial infiltration events consisted of the addition of 26.5 or 53 L (7 or 14 gal) of water at constant head to a 1 m long, ~10 cm deep trench situated 20 cm upslope of the survey grid, promoting subsurface flow. Hilbert‐transformed time lapse GPR images reveal significant increases in signal amplitude due to increased water content. In addition, pre‐infiltration radargrams, surface reflection amplitude maps, and ERT profiles indicate noteworthy small‐scale heterogeneity of moisture distribution in the Weikert series soil. The calculation of actual volumetric water contents from the GPR methods was problematic owing to inaccuracy and lack of reproducability in calibration to soil moisture probe data, but time‐lapse changes qualitatively reveal trends in the site’s hydropedology. In particular, time‐lapse images imply rapid infiltration, with moisture increases observed after the completion of infiltration as far as 80 cm down slope and fading with consecutive 15 minute surveys. Additionally, the infiltration of a second pulse of water suggests the possibly of variable interflow pathways activated by the already wet soil corresponding to microtopographical trends determined by off‐ground GPR travel times.

Citation

Lichtner, D., Nyquist, J., Toran, L., Guo, L., and Lin, H. (2012): Monitoring time‐lapse changes in soil moisture during artificial infiltration with geophysical methods: Field reports regarding surface reflection GPR, ground wave GPR, and time‐lapse ERT at Shale Hills CZO. Bachelor of Science, Temple University.