QUANTIFYING SUBSURFACE HYDROLOGY WITH GROUND PENETRATING RADAR AND AGRICULTURAL MONITORING DATA

Subsurface-water dynamics can influence crop growth within a production field, as well as the fate of surface-applied fertilizers and pesticides migrating thru agricultural land. Ground-penetrating radar (GPR) and digital elevation maps (DEM) were used to locate and quantify subsurface flow where water appears to have converged into discrete subsurface pathways. In this study, the GPR protocol was extended to a 3.2-ha corn-production field which drains into a first-order stream. Soil moisture and yield patterns within the production field as well as nitrate base-flow concentrations in the neighboring first-order stream were monitored to confirm the extent of the subsurface-flow pathways. Over 9,000 soil-water content readings were analyzed daily using moisture capacitance probes for three growing seasons while corn grain production was collected using a GPS-based crop-yield monitor. Soil-water contents and yield were averaged over three spatial zones (0-5 m, 5-10 m, and 10-15 m) representing increased distances from the GPR-identified flow pathways. Results show that the spatially-averaged yields during a drought year decreased with increasing distance from the GPR-identified subsurface-flow pathways. In addition, spatially-averaged, soil-water contents for three consecutive growing seasons demonstrated a consistent decrease in soil-water content with increasing distance from the GPR-identified subsurface-flow pathways. In addition, nitrate concentrations and nitrate fluxes in the first-order stream increased with increasing base-flows supporting the conceptual model of discrete pathways draining into the riparian area. This research suggests that subsurface flow pathways exist and can have a dramatic impact on production as well as the quality of water exiting land with this climate and soil morphology.