| Soil erosion is the one of worldwide environmental problems, especially in abundant developing country. Soil erosion has becoming the main reason of restricting the local economic development. For improving environment of human being and economic development, it has been the urgent affair of the research on the soil erosion law and controlling the soil-water loss. In our country, the native so much as worldwide people have concerned the soil erosion and soil-water loss on the Huanghe valley. Rill erosion and its sediment yield above Loess area are universal phenomenon of soil erosion and sediment yield. The sediment yield of the field between the gully is about 30%-70% that of the whole valley due to different cultivate and various soil-water conservancy means. It cannot be ignored in the soil erosion study of the soil erosion and its sediment yield on the field between the gully on Loess area. At present, though so much study has been done on rill erosion, it also has many problems not clearly. Such as mechanism of rill forming, rills flow pattern and so on. This till disputed, and the prediction of erosion on rill erodes zone is also difficulty. With the theories and methods of Geomorphology, Hydrology, Hydraulics and Sediment Movement, to study the runoff generation, erosion mechanism, and the parameter of erosion processes, is one of the current frontiers of rill erosion, its flow and hillslope processes both at home and abroad. This paper based on predecessor research, with theory analysis and indoor dewatering simulation experimentation, mainly studied the hydraulics mechanism of rill erosion processes over Loess slope. First, studied the law of runoff generation over rill erodes zone slope. Secondly analyzed the flow pattern of rill flow and the mechanism of rill forming processes, discuss the rill hydraulics geometry characteristics, such as the width of rill flow section, average flow depth, average velocity and flow volume. Third, induced the flow resistance law of the rill. Then, summarized the characteristics of rill erosion and its sediment yield on Loess. Eventually, studied the correlative parameters to the estimate prediction of rill erosion. The main results are following: (1) The rill flow pattern In the experimental range of slop grade and flux, the Flow Reynolds Number is changed from 500 to 4600, the infection of sediment capacity to viscosity is considered. Meanwhile, the Froude Number is bigger than 1. If Re=500 is the critical judgement between Laminar flow and turbulent flow, Fr=1 is the critical judgement between subcritical flow and supercritical flow, then the experimental results show that the rill flow pattern is supercritical turbulent flow. (2) Mechanism of rill forming Field observation and mould experiments show that, during the slop erosion evolvement processes from sheet erosion to gully erosion, pit-land formed on the slope flow way take a important role on place a premium on rill. The forming of pit-land come down to 3 aspects. ①. The low-lying on nature slope directly formed pit-land.②. The slope resistance is not equality, slope sheet flow will influx by the Theory of Minimum Energy Dissipation, after the sheet flow influx to stream, the power of flow erosion will increase more times. So the pit-land will appear in the way of stream.③. Asymmetry of slope soil erosion resistance will lead to local strong erosion. The pot, where the surface soil erosion resistance is weak, will firstly be eroded. Therefore, the pit-land formed here. After the pit-land formed, the headwater erosion will take force backward, then the rill will be evolved. (3) The resistance characteristics of rill In the range of experimental slope and flow, the formula of Darcy-Weisbach resistance coefficient of the rill over Loess slope, is 151.168Reλ=?4. 09+51.635J.The average value of Manning coefficient is 0.0468, and the calculation formula is n = 0. 177Q1.04J0.06. Moreover, both of them related with the rate of flow and gradient of the slope. These study of resistance characteristics is the basement of hydraulic calculation on rill stream. (4) The geometry characteristics of the rill Researching the geometry characteristics of the rill, is the basement of deeply studying the rill hydraulic characteristics. The indoor experiment show that 0.28, 0.47 and 0.25 is the indexes of stream section width, depth and average velocity with the relationship of stream volume. Andthe formula of rill width is B = 0 .112Q0.722J?0.378, the formula of rill depth is H = 1 .58Q0.724J1.8. These will helpful to farther researching on the mechanism of rill erosion and on rill erosion prediction. (5) Sediment transport capacity of the rill flow and its erode ratio The analysis of experimental data indicate that, the calculation formula of critical shear stress is τe = 771. 5????3 .2(γs?γ) d cosθ+????γγbb0????10dk????, and the estimating formula of rill flow erode ratio is D r = α(T c?S). There is line relationship between both sediment yield rate of rill erosion and rill steam stress, rill stream power and its sand sediment capacity. 0.25(kg/m2) is the critical shear stress of rill erosion beginning. (6) Integrative model over slope runoff generation on rill erodes zone. Both over infiltration model and filled runoff generation model are two abbreviated and special runoff generation mechanisms. It's set up of integrative model of slope runoff generation on rill erodes zone, which based on runoff generation mechanism on slope land, and on Horton infiltration curve. The model is controlled by both rainfall intensity and soil moisture storage, and can be applied in different watershed. It compromised both over infiltration and filled runoff generation, also adapt itself to either the small watershed or the unit slop which underlying surface is more even. Mechanism of integrative model is a general law, which will be equal to over infiltration model when infiltration index (Wm) is choose smaller, and will be equal to filled runoff generation model just when infiltration index (Wm) is choose bigger. Therefore, the integrative model has a wide range of application. (7) Prediction Model Sediment Yield of Rill Erosion over Loess Slope Cultivated land erosion styles over Loess slope is mainly rill erosion, so rill erode prediction is especially important. In this paper, the rill erode prediction model is a integrated rill erosion and sediment yield model, which not only consider 3 process of runoff generation over slope, flow influx and rill flow erosion, but the model conception clear, parameters less, process and mechanism specific. The model is validated by a serial data of a small flow zone in Xiji county Ningxia province, and the validating show the model is more coincidental to the natural rillerosion.
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