Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/17/2001
Publication Date: N/A
Interpretive Summary: A relatively new geophysical tool called ground-penetrating radar was used to identify and map clay horizons below the soil surface. The size and shape of these clay horizons are important as they can influence chemical transport as well as soil water availability that governs crop yield. The location, size, and orientation of these subsurface clay horizons were accurately quantified using ground-penetrating radar. In addition, a soil moisture system was linked to the ground-penetrating radar data in an attempt to monitor water dynamics throughout on a small 7.5 ha watershed. Soil moisture data successfully captured water movement on top of these clay lenses as well as water flow through the bulk of the soil matrix. As a result, a methodology was developed which allows the subsurface structure to be accurately quantified. This method will eventually allow scientists and industry to accurately quantify, for the first time, water and chemical lloadings from agricultural land to the surrounding environment and can als be used to help farmers understand the spatial variability associated with crop yields.
Technical Abstract: Currently, no methodology exists to accurately estimate watershed-scale chemical and groundwater fluxes. A protocol is presented for identification of subsurface convergent flow pathways which are required for the determination of subsurface fluxes. More than 15.8 km of georeferenced ground-penetrating radar (GPR) data were collected on both coarse and fine resolution sample grids. The coarse sample grid consisted of 25 m x 25 m transects acquired on a 7.5 ha watershed, while a finer spatial resolution grid (2 m x 2 m) was used to intensively sample 22 blocks (0.06 ha each) located within the watershed. The GPR data were brought into a geographic information system (GIS) in order to map extent, orientation, and depth of subsurface layers governing groundwater movement. GPR image profiles of the soil stratigraphy were used to create 3-D maps of the depth to the first continuous subsurface restricting layer. Spherical models generally provided the best fit to experimental semivariograms of the restricting layer depth at a variety of spatial scales. However, the distance over which these data showed any spatial dependency, i.e., as reflected by the ranges of the semivariograms, was highly dependent upon the scale of observation. Hydrologic models were used in a GIS to determine potential convergent flow pathways from topographic maps of the subsurface restricting layers. A network of soil moisture probes, which collect more than 3,600 volumetric water content measurements each day, allowed for the GPR-identified subsurface flow pathways to be verified and quantified in real-time. This suggests that a methodology incorporating both GPR soil profile data and real-time soil moisture sensors may be used to identify subsurface flow pathways and to monitor large-scale groundwater fluxes.