ARS | Publication request: SOIL EROSION BY SURFACE WATER FLOW ON A STONY, SEMIARID HILLSLOPE

Submitted to: Catena
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 3, 1998
Publication Date: N/A

Interpretive Summary: Generally where water erosion rates on upland areas are greatest, soil detachment by surface water flow is active. Flow depths in small concentrated flow areas on hillslopes, often termed rills, are typically on the order of a centimeter or less and slopes may be quite steep. It has generally been thought that velocities of surface water runoff are influenced mostly by the depth of flow and the steepness of the slope. This study suggests otherwise. It corroborates several other recent laboratory and field studies on various types of soil types which show a lack of dependence between rill flow velocity and slope gradient. This result is important, because in practice hydrologic and erosion modelers and model users use a type of runoff equation which assumes a dependence of flow velocity on slope. Often, too, experimentalists design experiments with the assumption of such a dependence, without often taking the data requisite to testing the assumption. According to the results of this study, our focus as scientists at this point should be on determining other realistic and useful velocity and depth relationships for rills, and determining how the rill hydraulics functions within the system of surface runoff routing on hillslopes. This study also corroborates recent experiments on cropland soil which indicate that soil erosion on these hillslopes was a function of the depth, slope steepness, and velocity of flow.

Technical Abstract: Soil erosion on hillslopes occurs by processes of soil splash from raindrop impacts and sediment entrainment by surface water flows. This study investigates the process of soil erosion by surface water flow on a stony soil in a semiarid environment. A field experimental method was developed whereby erosion by concentrated flow could be measured in pre- defined flow areas without disturbing the soil surface. The method allowed for measurements in this study of flow erosion at a much wider range of slopes (2.6 to 30.1 percent) and unit discharge rates (0.0007 to 0.007 m2 s-1) than have been previously feasible. Flow velocities were correlated to discharge, but not to slope. The lack of correlation between velocity and slope might have been due to the greater rock cover on the steeper slopes which caused the surface to be hydraulically rougher and thus counteract the expected effect of slope on flow velocity. The detachment data illustrated limitations in applying a linear hydraulic shear stress model over the entire range of the data collected. Flow detachment rates were better correlated to a power function of either shear stress (r2=0.51) or stream power (r2=0.59).