Reviewing on Factors and Critical Conditions of Rill Erosion
Rill erosion plays an important role in soil erosion process in loess hillslopes. It is a quantum leap during the process of soil erosion on slopes, and also is the beginning of a qualitative change in the process of soil erosion. On the basis of studying many papers, the authors review the study of factors and critical conditions of rill erosion. Rainfall and runoff, soil, topography, soil surface characteristics and land management all have their impact on rill erosion. On the one hand, researches use a variety of parameters to represent the impact of rainfall and runoff, soil, topography and land management. On the other hand, there are two forms of soil surface characteristics, namely, surface roughness and surface soil crust. The studies on the impacts of both of them are still in a qualitative research stage, and some studies have shown they can weaken rill erosion, the other studies concluded that they would exacerbate the occurrence of rill erosion. The generation of rill on the slopes needs certain conditions, and the critical runoff, soil and terrain conditions have their corresponding research results. The critical conditions for the generation of rill are not numbers, but are related to many factors. Critical conditions for rill erosion are not fixed values, but all of them should be a comprehensive index of other factors. At the same time, the future possible research direction and contents of the influencing factors and the critical conditions of rill erosion are pointed out. More experiments and field observation data are needed. In rill erosion factors research, the following points need further works: (1) the quantitative analysis of the impact of rainfall and runoff on the joint action of rill erosion; (2) screening key indicators to represent the effect of soil, topography and land management factors; (3) an in-depth study on how the soil surface characteristics affect rill erosion, and a quantitative analysis of the impact of surface roughness and soil crust on rill erosion; (4) the interaction of various factors with the impact of rill erosion. Meanwhile, the studies of critical conditions of rill erosion need more investments, which include: (1) exploring the critical rainfall conditions for rill erosion, and finding out its relationship with critical runoff conditions, (2) a systematic study on the critical conditions in different areas, to obtain a judgment with strong applicability about whether rill erosion will occur.
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