Understanding erosion processes using rare earth element tracers in a preformed interrill-rill system.

Tracking sediment source and movement is essential to fully understanding soil erosion processes. The objectives of this study were to identify dominant erosion process and to characterize the effects of upslope interrill erosion on downslope interrill and rill erosion in a preformed interrill-rill system. A coarse textured soil with 2% clay and 20% silt was packed into a physical model of a scaled small watershed, which was divided into eight topographic units and was tagged with eight rare earth element (REE) oxides. Three 30-min rains were made at the sequential intensities of 60, 90, and 120mmh, and runoff and sediment were collected every 2min at the outlet. REE concentration in sediment was measured and used to estimate source contributions after fine-enrichment correction. Results showed that interrill erosion rate and sediment concentration increased with downslope distance, indicating that sediment transport might have controlled interrill erosion rates. In contrast, rill erosion rate was limited by rill detachment and development process. Rill erosion contributed most soil loss; however, the proportion decreased from 78 to 61% as rainfall intensity increased and rill network matured over three rains. Interrill erosion was more sensitive than rill erosion to rainfall intensity increases. The former was mostly affected by rainfall intensity in this experimental setup, while the latter was controlled by flow discharge, gradient, and rill evolution stage. The greatest sediment concentration and delivery rate occurred in the stage of the fastest rill development. The increased sediment delivery from interrill areas appeared to suppress rill detachment by concentrated flow. This study enhanced our understanding of interrill and rill erosion processes and provided the scientific insights for improving soil erosion models.