Soil And Water Conservation Benefit And Mechanism Of Different Underlying Surface

Gully region of Loess Plateau, Nanxiao Hegou Basin has been the key areas of the study.Taking Nanxiao Hegou Basin as the research area, the paper studys the influences of different underlying surfaces like plant canopy and roots, and then quantitatively analyzes the Soil and water conservation benefit and mechanism with the mathematical model and experiments.The main conclusions can be drawn as follows:(1) Under the different vegetation structure the runoff and sediment yield have their own changes with the slope and rainfall intensity. But generally speaking,regardless of the runoff yield and sediment transport rate, amount of water and sediment parameters showed the rule as belows: the grass < the root < the bare.The results show that the vegetation canopy and root system can inhibit the erosion, but their effects are different.(2) The erosion rules of 4 different underlying surface flow fields in Nanxiaohegou Basin during 1975-1986 are: with the increase of vegetation degree, the erosion condition was further alleviated; with regard to the underlying surface, the erosion intensity will increase with the increase of rainfall when the vegetation condition is constant; there is obvious difference between Yangjiagou and Dongzhuanggou in vegetation degree and the erosion situation,compared with the non control basin (Dongzhuanggou) after decades of treatment, the amount of erosion was significantly smaller in Yangjiagou.(3) In general, canopy's benefit of reducing the flow of is higher than that of the root system,and under different experimental conditions, the contribution ratio of canopy and root system was more than 1.Compared with the benefit of reducing runoff the root part shows more obvious effect of reducing sediment than that of canopy. And under different experimental conditions, the ratio of contribution rate of root and canopy decreased by 1. The results indicate that ryegrass mainly inhibited soil erosion through its roots. The results of path analysis show that rainfall intensity, slope gradient and runoff yield can be regarded as the decision variables affecting the sediment yield,and the three have a positive effect on sediment yield. Generally speaking, the three factors influence the magnitude of sediment yield as belows:runoff yield >rainfall intensity > slope gradient. The result of SPSS output is consistent with the theory of affecting the sediment yield,and the three have a positive effect on sediment yield. Generally speaking, the three factors influence the magnitude of sediment yield as belows:runoff yield >rainfall intensity > slope gradient. The result of SPSS output is consistent with the theory of path analysis, and the method of path analysis by SPSS linear regression is feasible.(4) As a typical governed small watershed, the soil and water conservation benefits begins to gradually appear during 1975-1986 in Yangjiagou,the relative closeness value showed an increasing trend. However,because of the lack of control of the original watershed, the soil and water conservation benefits are not as obvious as that of the small watershed in Dongzhuanggou.And the simulation results are in accordance with the situation of watershed investigation. And the results show that the relative closeness degree calculated by the improved Topsis model can reflect the dynamic change process of soil and water conservation and benefit objectively and scientifically.(5) Under different slope and rainfall intensity, the size of flow velocity showed: bare slope > root > grass slope,and the reduction effect of the ryegrass root system is significantly greater than that of the canopy. At the same time, it is found that there is significant difference between different rainfall intensity treatments,the results show that the change of rainfall intensity has a significant effect on the reduction of root and canopy. The water depth in different underlying surface showed: the grass > the root > the bare. The Reynolds number Re under different underlying surface conditions is not very large, and it is less than the critical value of 500, show that the flow pattern of overland flow under different vegetation cover is laminar. The Froude number Fr of bare slope is much larger than that of grass slope and the slope of Fr which is more than the critical value of 1, belonging to the rapids, and Fr of vegetation cover and root less than 1, which belongs to the slow flow. And the resistance of the canopy was stronger than that of the root system.(6) The flow shear stress, flow power, unit flow power and cross-sectional unit energy increase with the increase of rainfall intensity and slope, and after cutting off the part of the canopy, the hydrodynamic factors showed some increase. The influence of flow shear stress on soil erosion is greater than that of water flow power and unit flow power. The results of the evaluation of soil anti - erosion by means of the model show that both the root and canopy can improve the ability of the soil particles to resist the erosion of runoff, and ryegrass improves the ability of soil erosion mainly by characteristics of root system.