Field Evaluation of Preferential Flow In Agricultural Soils of the Mississippi Delta

In the Bogue Phalia basin in the Delta region of northwestern Mississippi, as in many farmed areas, intensive use of agricultural chemicals has led to their detection in the environment; however, contributing unsaturated zone processes are not well understood. The soils are fine textured and often exhibit surface ponding and runoff after irrigation and rainfall as well as extensive surface cracking during extended dry periods. Fields are typically land-formed to promote surface flow into irrigation ditches and streams that feed into larger river ecosystems. Pesticides detected in surface water within the Delta are presumed to derive from surface runoff. In this study we assessed transport mechanisms within and below the root zone of a fallow soybean field by performing a 2-m ring infiltration test with 67 hours of ponding using tracers and subsurface instruments for measuring soil-water content and matric potential. Water percolated rapidly below the pond to a depth of 60 cm, indicating that vertical preferential flow within the root zone is an important process even though observed surface cracking was minimal. Extensive lateral flow of water at shallow depths was apparent as the surface wetted up outward to several meters from the pond in all directions with some evidence of preferentiality along slope toward the drainage ditch. Deeper lateral flow was detected to 1.2 m depth; limited detection of water movement below that depth suggests a flow-impeding layer, possibly a result of agricultural practices, inhibiting downward percolation to the aquifer. Results from this study show that during periods of high infiltration, shallow lateral flow may be an important mechanism for chemical transport within the unsaturated zone that may contribute to discharge in nearby ditches and streams.