Analysis of surrogate indicators for evidence of subsurface preferential flow pathways: impact of subsurface preferential flow on variability of NDVI

Watershed-scale processes governing chemical fluxes to adjacent ecosystems are so poorly understood that effective strategies for mitigating chemical contamination cannot be formulated. Characterization of hydrologic processes and chemical behavior at the watershed scale is critical to the development of sustainable agricultural practices. Identifying locations for monitoring hydrologic processes like subsurface preferential flow is difficult because conventional sampling methods are inadequate for measuring this highly variable, yet critical process. A method for detecting and mapping subsurface preferential flow pathways based primarily on ground-penetrating radar (GPR) data and digital elevation maps (DEM) was developed. This procedure was confirmed for a Maryland cornfield using real-time soil moisture data, maps of within-field grain yield, and remotely sensed imagery. Unfortunately, it is economically unfeasible and logistically impractical for producers to use the GPR-DEM procedures to map subsurface preferential flow pathways for all crop fields. It may, however, be possible to use remotely sensed imagery, grain yield maps, and a DEM as surrogate indicators of subsurface preferential flow pathways occurring at or near crop rooting depth. The normalized difference vegetation index (NDVI) shows an increase with distance from primary, secondary and tertiary preferential flow pathways during above-normal rainfall growing seasons. There appears to be a decrease of NDVI with distance from preferential flow pathways for a dry/drought year. Imagery collected during drought conditions appears especially useful as only within-field locations with subsurface irrigation from preferential flow pathways maintain vigor. Maps of vegetative productivity derived from remotely sensed imagery may be more useful than within-field grain yield maps for detecting and delineating locations of subsurface preferential flow pathways. The ability to delineate field locations with a high probability of subsurface preferential flow pathways will allow producers to better manage crop production, and mitigate losses of agricultural chemical inputs to neighboring ecosystems and waterways.