Landscape position,surface hydraulic gradients and erosion processes

Different hydraulic gradients, especially due to seepage or drainage, at different locations on a hillslope profile may have a profound effect on the dominant erosion processes. A laboratory study was designed to simulate hillslope processes and quantify effects of surface hydraulic gradients on erosion for a Glynwood clay loam soil (fine, illitic, mesic Aquic Hapludalf). A 5 m long, 1·2 m wide soil pan was used at 5 and 10 per cent slopes with an external watering tube to vary the soil bed's hydrological conditions. Different combinations of slope steepness with seepage or drainage gradients were used to simulate the hydrologic conditions on a 5 m segment of a hillslope profile. Runoff samples were taken during rainfall-only and rainfall with added inflow. Results showed that, under drainage conditions, interrill processes dominated and rilling was limited. The surface contained scattered crescent-shaped pits after the run. Under seepage conditions, rilling processes dominated and the inflow introduced at the top of the soil pan further accelerated the headward erosion of the rills. Erosion rates increased by as much as 60 times under seepage conditions representative of the lower backslope when compared to drainage conditions that generally occur at the upper backslope. This indicated that rills and gullies on backslopes and footslopes may be catalysed or enhanced by seepage conditions rather than form from flow hydraulic shear stress alone. An understanding of spatial and temporal changes that affect both hillslope hydrology and erosional processes is needed to develop accurate process-based erosion prediction models. This knowledge may lead to different management practices on landscape positions where seepage occurs. © 1998 John Wiley & Sons, Ltd.