Prescribed burning effects on the hydrologic behavior of gullies in the South Carolina Piedmont
Gullies found in the Piedmont of South Carolina are legacies of past land use and erosion. Although the majority of these gullies are now under forest vegetation and perceived as geomorphologically stable, the question of gully contribution to nonpoint source pollution remains undetermined, especially when these gullies are subjected to prescribed burning or other forest disturbance. Six prescribed burned and two reference gullies draining mature pine stands grown on former cotton fields were instrumented at the Long Cane Ranger District, Sumter National Forest, South Carolina to characterize the hydrologic behavior of these gullies and to investigate response to prescribed burning. Flow in the gullies was observed for one year of pre-burn and one year of post-burn conditions. Hydrologic behavior of these eight gullies varied greatly in the pre-treatment period. During the study, 48 rainfall events exceeding 12.7mm were recorded, but one reference gully never produced flow and three treatment gullies flowed during three events or less and only in the pre-burn period. Although significant storm events occurred throughout the year, all gully flow events occurred between December and March. Double-mass and graphical analysis of relative stormflow production of the reference and treatment gullies in the pre- and post-treatment periods did not indicate an effect of the controlled burn on flow behavior, but the post-burn year was characterized by drought. The observed inter-annual variation in gully behavior was large. Observations of local groundwater conditions with piezometers and electrical resistivity surveys indicated that gully flows were controlled by the presence of a flow-restricting layer below the gully bed.