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 effe ct on the dominant erosion processes. A laboratory study was designed to simulate hillslope processes and quantify effects of surface hydrau lic 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 soi l bed's hydrological conditions. Different combinations of slope steep ness with seepage or drainage gradients were used to simulate the hydr ologic conditions on a 5 m segment of a hillslope profile. Runoff samp les were taken during rainfall only and rainfall with added inflow. Re sults showed that, under drainage conditions, interrill processes domi nated and rilling was Limited. The surface contained scattered crescen t-shaped pits after the run. Under seepage conditions, rilling process es dominated and the inflow introduced at the top of the soil pan furt her accelerated the headward erosion of the rills. Erosion rates incre ased by as much as 60 times under seepage conditions representative of the lower backslope when compared to drainage conditions that general ly 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 hillsl ope hydrology and erosional processes is needed to develop accurate pr ocess-based erosion prediction models. This knowledge may lead to diff erent management practices on landscape positions where seepage occurs . (C) 1998 John Wiley & Sons, Ltd.