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Title: Interrill soil erosion processes on steep slopes

Author
item Zhang, Xunchang
item WANG, ZHANLI - Northwest Agricultural & Forestry University

Submitted to: Journal of Hydrology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/22/2017
Publication Date: 3/22/2017
Citation: Zhang, X.J., Wang, Z. 2017. Interrill soil erosion processes on steep slopes. Journal of Hydrology. 548 (2017) 652–664.

Interpretive Summary: Upland erosion is divided into rill (small channel) erosion and sheet erosion between rills (called inter-rill erosion). To date interrill erosion processes on slopes are not yet fully understood. The objectives are to 1) identify the limiting processes between soil detachment and sediment transport on steep slopes, 2) characterize the interactive effects between rainfall intensity and flow depth on sediment transport efficiency and mode, and 3) develop a lumped interrill erosion model. Three factors included rainfall intensity (0.8, 1.0, 1.7, 2.5, and 2.8 mm min-1), slope gradient (17.6, 26.8, 36.4, 46.6, and 57.7%), and slope length (0.4, 0.8, 1.2, 1.6, and 2 m) were selected. A loess loam soil with 39% sand and 45% silt was used. Rain splash, sediment discharge in runoff, and flow velocity were measured. Results showed that rainfall intensity played a dual role not only in dislodging soil materials but also in enhancing sediment transport. Sediment transport was the limiting process controlling interrill erosion rate on steep slopes. Two major sediment transport modes were identified: rainfall-driven rolling/creeping and flow-driven rolling/sliding. The relative importance of each mode was largely determined by flow depth. The competence of the flow in transporting sediment decreased downslope as flow depth increased due to raindrop energy dissipation by ponding water. The optimal mean flow depth for the maximal interrill erosion rates was < 0.1 mm. Slope length was negatively related to interrill erosion rate. The negative correlation seemed stronger for heavier rains, indicating the dampening effects of flow depth. Any lumped interrill erosion model, developed from short slopes, is likely to overestimate erosion rates. The results will help 1) soil conservationists better understand interrill erosion mechanics and 2) erosion scientists develop more accurate interrill erosion prediction tools.

Technical Abstract: To date interrill erosion processes and regimes are not fully understood. The objectives are to 1) identify the erosion regimes and limiting processes between detachment and transport on steep slopes, 2) characterize the interactive effects between rainfall intensity and flow depth on sediment transport efficiency and mode, and 3) develop a lumped interrill erosion model. A complete factorial design with three factors was used. Three factors included rainfall intensity (0.8, 1.0, 1.7, 2.5, and 2.8 mm min-1), slope gradient (17.6, 26.8, 36.4, 46.6, and 57.7%), and slope length (0.4, 0.8, 1.2, 1.6, and 2 m). A loess loam soil with 39% sand and 45% silt was used. Rain splash, sediment discharge in runoff, and flow velocity were measured. Results showed that rainfall intensity played a dual role not only in detaching soil materials but also in enhancing sediment transport. Sediment transport was the limiting process controlling interrill erosion rate on steep slopes under high intensity rains. Two major sediment transport modes were identified: rainfall-driven rolling/creeping and flow-driven rolling/sliding. The relative importance of each mode was largely determined by flow depth. The competence of the flow in transporting sediment decreased downslope as flow depth increased due to raindrop energy dissipation. The optimal mean flow depth for the maximal interrill erosion rates was < 0.1 mm, which is much shallower than the widely reported 2 mm. Slope length was negatively related to interrill erosion rate. The negative correlation seemed stronger for heavier rains, indicating the impeding effects of flow depth. Any lumped interrill erosion model, developed from short slopes, is likely to overestimate erosion rates. Given the transport limiting conditions, the so called erodibility parameter, estimated with those models, is indeed sediment transportability. The effects of slope length on interrill erosion regimes need to be studied further under wider ranges of conditions.