MONITORING TIME-LAPSE CHANGES IN SOIL MOISTURE DURING ARTIFICIAL INFILTRATION WITH GEOPHYSICAL METHODS

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 using time-lapse electrical resistivity tomography (ERT) and ground-penetrating radar (GPR). In addition to acquiring standard 2D radargrams, two GPR techniques known to respond directly to water content were tested: mapping changes in the direct transmitter to receiver ground wave velocity, and changes amplitude of the reflection off of the surface measured by elevating the antennae. 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 plot on a forested hillslope in Weikert series soil. 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 plot, promoting subsurface flow. Hilbert-transformed time lapse GPR images revealed significant increases in signal amplitude due to increased water content. In addition, small-scale heterogeneity of moisture distribution showed consistent patterns in pre-infiltration radargrams, surface reflection amplitude maps, and ERT profiles. The calculation of volumetric water contents from the GPR data was problematic owing to difficulty calibrating measurements. Qualitatively, the time-lapse images showed rapid infiltration, with moisture increases observed immediately after infiltration as far as 80 cm downslope. Within 15 minutes the geophysical signatures of infiltration decreased, with a majority of measurements within 20-40% of pre-infiltration values after 1 hour. Additionally, the infiltration of a second pulse of water suggested interflow pathways corresponding to already wetted flow paths and microtopography. Conductivity (ERT), ground wave velocity, and surface reflection amplitude all indicated changes in soil moisture that were useful in characterizing the soil’s heterogeneity, infiltration rate, and flow path variability.