Rare earth elements tracing interrill erosion processes as affected by near-surface hydraulic gradients

Authors: WANG, CHENFENG - Beijing Forestry University; WANG, YUJIE - Beijing Forestry University; WANG, BIN - Beijing Forestry University; WANG, YUNQI - Beijing Forestry University; ZHANG, WENLONG - Beijing Forestry University; Zhang, Xunchang

Submitted to: Soil & Tillage Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/26/2020
Publication Date: 4/26/2020
Citation: Wang, C., Wang, Y., Wang, B., Wang, Y., Zhang, W., Zhang, X.J. 2020. Rare earth elements tracing interrill erosion processes as affected by near-surface hydraulic gradients. Soil & Tillage Research. 202. https://doi.org/10.1016/j.still.2020.104673.
DOI: https://doi.org/10.1016/j.still.2020.104673

Interpretive Summary: Upland soil erosion generally consists of rill (small channel of < 10 cm in depth) and interrill (sheet erosion between rills). Understanding interrill erosion processes is important for developing an interrill erosion prediction model. Six rare earth elements (REEs) were applied as sediment tracers to different slope segments and soil layers to track soil erosion as affected by near-surface hydraulic gradient (drainage, saturation, and seepage conditions) under three rainfall intensities of 30, 60, and 90 mm/h. The results showed that the contributions of interrill soil loss from each tracer segment to the total soil loss first increased and then decreased along the slope under drainage-saturation conditions, while it continuously increased downslope under seepage conditions. Transports by raindrop-induced and flow-driven rolling, creeping, and/or sliding were the dominant transport modes. The interrill erosion rate was limited by sheet flow transport capacity under drainage-saturation conditions, while it was limited by raindrop detachment rate under seepage conditions. The relationships developed in this study to estimate sediment transport capacity and soil detachment rate as a function of rainfall intensity, slope gradient, and slope length were better described by power equations. These new equations fitted the experimental data better than the existing interrill erosion empirical equations, reducing absolute errors by 38% to 87%. In addition, interrill erodibility should be further divided into interrill sediment transportability under transport-limited conditions and interrill detachability under detachment-limited conditions.

Technical Abstract: Understanding interrill erosion processes is important for the development of a process-based interrill erosion model. Six rare earth elements (REEs) were applied in different slope segments and soil layers to track sediment movement and deposition in order to gain insights into the near-surface hydraulic gradient-affected (drainage, saturation, and seepage conditions) interrill erosion processes under three rainfall intensities of 30, 60, and 90 mm/h. The results showed that the contributions of interrill soil loss from each tracer segment to the total soil loss first increased and then decreased along the slope under drainage-saturation conditions, while continuously increased under seepage conditions. Transports by raindrop-induced and sheet flow-driven rolling, creeping, and/or sliding were the dominant transport modes. The dominant process of interrill erosion was transport-limiting under drainage-saturation conditions and detachment-limiting under seepage conditions. Under transport-limited conditions, raindrop-induced transport was more efficient than raindrop-impacted sheet flow-driven transport. However, the raindrop-impacted sheet flow-driven transport was more efficient than raindrop-induced transport under detachment-limited conditions. The response relationships of sediment transport capacity and soil detachment rate to the near-surface hydraulic gradient, rainfall intensity, slope gradient, and slope length could be better described via power equations (R2 = 0.81, NSE = 0.81). The R2 and Nash-Sutcliffe model efficiency (NSE) of the power equations were 2.53% to 1840.00% higher than those calculated with existing interrill erosion empirical equations, and the average absolute relative error (RME) derived in this study decreased by 38.03% to 87.21%. In addition, interrill erodibility should be further divided into interrill sediment transportability under transport-limited conditions and interrill detachability under detachment-limited conditions.