Soil erosion processes. I. The relative importance of rainfall detachment and dunoff entrainment

Abstract

Experiments carried out in a simulated-rainfall tilting-flume facility are reported in which sediment concentrations (c) in runoff water resulting from overland flow only, or from a combination of rainfall and overland flow, were measured under controlled conditions using a series of slopes (0.1, 05, 1, 3 and 5%). The mixture of rainfall (of rate 100 mm h-1) and runon of water at the top of the flume were arranged to provide a constant volumetric flux (1.0x10-3 m3 m-l s-1) at exit from the 5.8 m long flume. Two contrasting soil types were studied: a cracking clay (black earth or vertisol), and a slightly dispersive sandy clay loam (solonchak or aridisol). Two major processes which can contribute to soil erosion under rainfall are rainfall detachment and runoff entrainment. For both soil types, c was generally highest for the steepest slope and decreased with slope. For constant rainfall and/or runoff conditions, c generally decreased with time until an equilibrium concentration was reached. At this equilibrium, the relative importance of rainfall detachment and entrainment in terms of soil loss was dependent on soil type and streampower which incorporates effects of slope and water flux. For streampowers <0.1 W m-2 for the black earth, and <0.3 W m-2 for the solonchak, the greatest contribution to c was by rainfall detachment, whilst at greater streampowers entrainment was the dominant contributor to c. At any streampower, the contribution by rainfall detachment was greater for the weakly structured solonchak than for the well aggregated black earth. At lower strearnpowers, the interaction between erosion processes was found to give higher c than the sum of both sediment concentrations resulting from the separately occurring processes. At streampowers greater than approximately 0.5 W m-2, rainfall reduced eroded sediment concentration by suppressing rill development. The findings in this study suggest that both runoff entrainment and rainfall detachment can contribute to sediment concentration from 'interrill' areas.