Impact of near-surface hydraulic gradient on the interrill erosion process

The impact of near-surface hydraulic gradients on interrill erosion is still obscure. The objective of this study is to distinguish the dominant interrill erosion process in areas impacted by near-surface hydraulic gradients. A series of rainfall simulations were conducted on a clay loam soil subjected to near-surface hydraulic gradients that shifted from drainage/saturation conditions to seepage conditions under three rainfall intensities (30, 60 and 90 mm hr(-1)) and two slope gradients (5 degrees and 10 degrees). The results showed significant differences in soil loss between all the treatments. The sediment concentrations for seepage conditions were 0.57 to 7.02 times greater than those for drainage conditions. The correlation analysis indicated that the near-surface hydraulic gradient was a governing factor affecting interrill erosion. The critical flow rate was larger than 90 mm hr(-1), suggesting that thin sheet flow does not have sufficient power to detach soil particles without raindrop impact. Furthermore, the detachment rates by raindrop impact were 1.12 to 4.60 times greater for seepage conditions than for drainage conditions. As the near-surface hydraulic gradient shifted from drainage conditions to seepage conditions, it transitioned from transport-limited to detachment-limited, and the contribution of interrill erosion to overall erosion increased from 20.19 to 75.30%. The critical point of dominant interrill erosion process transition existed between saturation (SA) and artesian seepage in 20 cm of hydrostatic pressure head (SP20). The results emphasize the importance of the near-surface hydraulic gradients' impact on the interrill erosion process. Further investigations need to be verified in different soil types, steeper slopes and natural storms. Highlights The impact of near-surface hydraulic gradients on interrill erosion is only partially understood. Dominant interrill erosion processes are identified under near-surface hydraulic gradients. The critical point of dominant interrill erosion process transition exists between SA and SP20. The dominant erosion process varied as a function of near-surface hydraulic gradients.