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Title: Gaining insights into interrill soil erosion processes using rare earth element tracers

Author
item Zhang, Xunchang
item Nearing, Mark
item Garbrecht, Jurgen

Submitted to: Geoderma
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/2/2017
Publication Date: 4/15/2017
Citation: Zhang, X.J., Nearing, M.A., Garbrecht, J.D. 2017. Gaining insights into interrill soil erosion processes using rare earth element tracers. Geoderma. 299:63–72.

Interpretive Summary: Increasing interest in developing process-based erosion models requires better understanding of the relationships among soil entrainment, sediment transportation, and sediment deposition. The objectives are to 1) identify the limiting process between soil entrainment and sediment transport for sheet erosion (erosion by thin sheet flow), 2) understand the relationships between transport and deposition by tracking sediment movement with sediment tracers, and 3) verify the effects of slope length on sheet erosion. Five Rare Earth Element (REE) tracers were applied in five segments of a 4-m uniform slope to track soil movement in each segment under man-made rainfall. Six rains were applied with 60 mm h-1 for the first four and 90 mm h-1 for the last two. Sediment in runoff and deposition along the slope were measured. Results confirmed that sheet erosion was controlled by the transport capacity of the sheet flow . As the slope length increased, soil erosion rates initially increased and then decreased, indicating that the upper section was dominated by erosion and the lower section by transport. Sediment influx from upslope clearly suppressed soil entrainment downslope. Raindrop-driven particle rolling prevailed in the upper section while flow-driven rolling predominated in the lower section. More importantly, the amount of sediment discharged from a segment was positively correlated to the amount of sediment deposited downslope from that segment, implying that re-entrainment, transport, and deposition occurred simultaneously in the system. The positive correlation indicated that sediment was transported in a form of bedload, mainly rolling. Given the ability of the REE tracers to track movement and fate of soil particles, the REE tracking technique will provide erosion modelers and hydrologists a useful tool generating much needed spatial erosion data for developing, improving, and validating process-based erosion models.

Technical Abstract: Increasing interest in developing process-based erosion models requires better understanding of the relationships among soil detachment, transportation, and deposition. The objectives are to 1) identify the limiting process between soil detachment and sediment transport for interrill erosion, 2) understand the dynamic relationships between transport and deposition by tracking sediment fate with multiple tracers, and 3) verify the effects of slope length on interrill soil erosion. Five Rare Earth Element (REE) tracers were applied in five bands or segments to track sediment dynamics in each band on a 10%, 4-m long uniform slope under simulated rainfall. Six rains were applied with 60 mm h-1 for the first four and 90 mm h-1 for the last two. Sediment in runoff and deposition along the slope were measured. Results confirmed that interrill soil erosion was controlled by the transport process. As the slope length increased, soil erosion rates initially increased and then decreased, indicating that the upper section was dominated by erosion and the lower section by transport. Sediment influx from upslope clearly suppressed soil detachment downslope. Raindrop-driven rolling prevailed in upper section while flow-driven rolling predominated in lower section. More importantly, the amount of sediment discharged from a segment was positively correlated to the amount of sediment deposited downslope from that segment, implying that re-detachment, transport, and deposition occurred simultaneously in the system. The positive correlation indicated that sediment was transported in a form of bedload, mainly rolling. Sediment delivery ratios estimated for each tagged segment tended to slightly increase downslope, suggesting the flow-driven transport was more efficient than the raindrop-driven transport. Given the ability of the REE tracers to track movement and fate of soil particles, the REE tracking technique will provide a useful means for generating much needed spatial erosion data for validating the transport-distance theory as well as other process-based erosion models.