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

Abstract Understanding interrill erosion processes is important for the development of process-based interrill erosion models. The objectives of this study were to identify dominant interrill erosion processes and to improve interrill erosion predictive equations. 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−1. The results showed that the contributions of interrill soil losses from each tracer segment to the total soil loss first increased and then decreased along the slope under drainage/saturation conditions, while they continuously increased under seepage conditions. Transport by raindrop-induced and sheet flow-driven rolling, creeping, or sliding was the dominant transport mode. The dominant process of interrill erosion was transport-limited under drainage/saturation conditions and detachment-limited under seepage conditions. Under transport-limited conditions, raindrop-induced transport was more relevant than raindrop-impacted sheet flow-driven transport. However, the raindrop-impacted sheet flow-driven transport was more important 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 described well via power equations (R2 ≥ 0.81). The R2 values of the power equations were 2.5 %–1840.0 % higher than those calculated with existing interrill erosion empirical equations, and the average absolute relative error (RME) derived in this study decreased by 38.0 %–87.2 %. In addition, interrill erodibility should be further divided into interrill sediment transportability under transport-limited conditions and interrill detachability under detachment-limited conditions.