Comparison of Critical Shear Stress of Rill Erosion Estimated from Two Methods

Abstract

Various disputes exist regarding the critical shear stress (τc) of rill erosion determined by linear regression. Alternatively, some researchers have attempted to measure critical shear stress (τo) of rill erosion by observing the start of soil particle detachment and showed promising results. However, few studies have been conducted to evaluate the reliability of the method determining τo. Thus, this study was conducted to identify the reliability of the method determining τo by comparing the values of τc and τo and their relationships with rill erodibility (Kr) and influencing factors, using 360 disturbed soil samples (standing for the freshly tilled condition) from six sampling sites along a 330 km transect in the Loess Plateau. Results indicated that the mean τc was 1.90 times greater than that of τo. No significant relationship was found between τc and Kr. An inverse changing trend between Kr and τo was observed from Yijun to Zizhou, while both Kr and τo exhibited increasing trends from Zizhou to Yulin. The τo of six sampling sites all demonstrated increasing trends with slope gradient (S), which contradicted the statement of the WEPP model that τc was independent of S. The relationships between τc and the measured soil properties were poor, whereas the τo increased exponentially with soil organic matter. Generally, τo did not present significantly better results than τc; the reliability of τo also showed some uncertainties, such as the subjective judgment of the beginning detachment of soil particles. This result has great importance for deepening our understanding of the rill erosion mechanism.