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

Various disputes exist regarding the critical shear stress (tau(c)) of rill erosion determined by linear regression. Alternatively, some researchers have attempted to measure critical shear stress (tau(o)) 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 tau(o). Thus, this study was conducted to identify the reliability of the method determining tau(o) by comparing the values of tau(c) and tau(o) and their relationships with rill erodibility (K-r) 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 tau(c) was 1.90 times greater than that of tau(o). No significant relationship was found between tau(c) and K-r. An inverse changing trend between K-r and tau(o) was observed from Yijun to Zizhou, while both K-r and tau(o) exhibited increasing trends from Zizhou to Yulin. The tau(o) of six sampling sites all demonstrated increasing trends with slope gradient (S), which contradicted the statement of the WEPP model that tau(c) was independent of S. The relationships between tau(c) and the measured soil properties were poor, whereas the tau(o) increased exponentially with soil organic matter. Generally, tau(o) did not present significantly better results than tau(c); the reliability of tau(o) 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.