Seth M. Dabney1, Daniel C. Yoder2, and Dalmo A. N. Vieira1. (1) USDA/ARS, National Sedimentation Laboratory, P. O. Box 1157, 598 McElroy Drive, Oxford, MS 38655-2900, (2) University of Tennessee, Biosystems Engineering and Soil Science, 2506 E J Chapman Drive, Knoxville, TN 37996
Ephemeral gully erosion is caused by concentrated surface runoff, often with the interacting influences of subsurface piping or seepage processes. Ephemeral gullies usually occur in topographic depressions within tilled fields, and are small enough to be filled in with farming equipment. Ephemeral gullies may be connected to edge-of-field overfalls, or they may terminate within fields where slope gradients become small. Historically, ephemeral gully modeling has been tackled in several ways, including: applying excess shear stress or stream power detachment and transport equations to an assumed uniform channel as though erosion occurred through uniform downcutting (or first downcutting to a resistant layer and later widening), modeling erosion as the rate of migration of a discrete head cut, or application of topographic indices. All of these approaches include some measure of soil strength or erodibility, which is usually parameterized with a critical minimum value of the driving force for erosion to occur and a proportionality parameter between increased driving force and increased erosion. Erodibility could be varied to reflect the indirect impact of the subsurface flow on the erosion, though this usually has not been done. This presentation will (1) briefly review some of these approaches, (2) discuss the approach being taken to add ephemeral gully estimation to the RUSLE2, (3) describe ongoing work to extend this work to 2-D field landscapes, and (4) conclude by highlighting some challenges to the state of our understanding of how subsurface-flow processes alter erodibility and transport capacity concepts and implications for future ephemeral gully modeling efforts.
