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Rill erosion constitutes one of the mechanisms of soil loss by water on agricultural land. However, studies on hillslope rill erosion characteristics and its intrinsic mechanisms are still unclear. The objectives of this study were to investigate the impacts of rainfall intensity and slope gradient on hillslope rill erosion processes, rill flow hydraulic characteristics and dynamic mechanisms. A soil pan (10 m long, 1.5 m wide and 0.5 m deep and with an adjustable slope gradient of 0–30°) was subjected to rainfall simulation experiments under three rainfall intensities (50, 75 and 100 mm h −1 ) of representative erosive rainfall and three typical slope gradients (10, 15 and 20°) on the Loess Plateau of China. The results showed that rill erosion exhibited significant contributions to hillslope soil erosion, occupying 62.2–84.8% of hillslope soil loss. The equation between the rill erosion rate with rainfall intensity and slope gradient was generated, which indicated that the impacts of rainfall intensity on hillslope rill erosion were greater than those of slope gradient. For the experimental treatments, the mean headward erosion rates varied between 2.2 and 8.2 cm min −1 , and they increased with an increase in either rainfall intensity or slope gradient. Most rill flow belonged to turbulent and subcritical flow regimes. The critical shear stress, the critical stream power, and the critical unit stream power of rill occurrence were 0.986 Pa, 0.207 N m −1 s −1 , and 0.002 m s −1 , respectively. Additionally, hillslope rill erosion was sensitive to rill flow velocity and stream power. In a word, rainfall intensity and slope gradient exhibited important impacts on rill erosion processes and its hydrodynamic characteristics. Therefore, preventing rainfall erosion and weakening slope gradient effects through conservation tillage are useful for reduction of rill erosion at loessial hillslopes. [ABSTRACT FROM AUTHOR]

Rill erosion is recognized as an important process of water erosion on agricultural land. The objectives of this study are to examine the effects of rainfall intensity on rill network development and to present some indicators for a quantitative description of rill morphology. A soil pan (10 m long, 3 m wide and 0.5 m deep and with an adjustable slope gradient from 0 to 30°) was subjected to three successive rains under rainfall intensities of 50 and 100 mm h − 1 . The results showed that rainfall intensity significantly affected rill erosion, especially in the active period of rill network development. The magnitude of rill erosion was 28.5 and 33.1 kg m − 2 and contributed 78.6% and 76.2% to the soil loss under rainfall intensities of 50 and 100 mm h − 1 , respectively. The formation of rill network under the 50 mm h − 1 intensity was more complex than that under the 100 mm h − 1 intensity; for the latter rill networks developed fast and then varied slightly. The mean rill inclination angle ( δ mean ), rill density ( ρ ), degree of rill dissection ( μ ) and mean rill tortuosity complexity ( c mean ) increased with the increase of rains under the same rainfall intensity. The μ value was the optimal derivative morphological indicator to estimate rill erosion and morphology, which was followed in descending order by δ mean , c mean and ρ . [ABSTRACT FROM AUTHOR]