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

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