Research Of The Impact Of Gravel In Purple Soil Around The Three Gorges Reservoir On The Soil Properties And Preferential Flow

Purple soil are widely distributed in Three Gorges Reservoir, high content of gravel is an important feature in purple soil on the mountain. People are more and land is less in the reservoir, so the rate of land utilization is extremely high, and the land is mostly slope cropland, coupled with heavy and frequent rainfall, thus it’s very easy to cause serious soil erosion. With high-intensity farming, a large number of pesticides and fertilizers are put into use, excessive pesticides and fertilizers are run off together with the soil and water which cause serious pollution.Based on the difference of gravel content, this paper selected three typical land use types from small watershed of paddy field in WangJiaQiao in Zigui County as experimental subjects, analyzed and discussed the impact of gravel in purple soils on the other basic physical properties of the soil and the preferential flow through the methods of combining outdoor staining experiments and indoor basic experiments, thus it could provide a theoretical basis for the prevention of soil erosion of purple soil with gravel and the groundwater contamination. The main conclusions are as follows:(1) Gravel has an important impact on other basic properties of purple soil. The gravel concentration in purple slope soil increases with the depth, and it has a significant spatial heterogeneity. There is a positive correlation relationship between the gravel concentration and soil bulk density and large pore characteristics, and with soil depth increasing, the correlation becomes significant. There is a negative correlation between gravel concentration and total porosity and field capacity.(2) Gravel concentration and spatial distribution characteristics have a significant impact on soil water infiltration characteristics. Gravel concentration in this experiment is between0and24.34%. The maximum and the minimum depth of soil moisture migration all show a positive correlation with gravel concentration. When the soil gravel concentration increases with soil depth, gravel concentration of layer II reaches a certain content, water infiltration can migrate to the soil bedrock in general.(3) The total number of independent staining paths generally decreases with soil depth increasing. But different types of land use and soil level will affect the rule of the total number of independent staining paths with gravel concentration changing. Gravel concentration in layer I of woodland has a positive correlation relationship with the max number of independent staining paths and the relationship with the average is not obvious, orchard is in contrast to the cultivated land; In the soil of layer Ⅱ、Ⅲ, the specific value of independent staining paths in all sample plots all show a significant positive relationship with gravel concentration, however, special circumstances exist, the study finds that gravel concentration in the layer Ⅱ、Ⅲ of three typical land use types of soils effect the specific value of staining paths differently, but for overall when gravel concentration is less than10%, the specific value of staining paths is less than15%, and when gravel concentration is more than20%, the specific value of staining paths is more than60%. There are different effects on the changes of the number of independent staining paths of different SPW for gravel. In this experiment, the effects on the changes of the number of independent staining paths of SPW>200mm for gravel are not obvious in all the samples, but the effect on the number of independent staining paths of SPW<20mm is similar with the effect on the total number of independent staining paths. Only in the layer Ⅱ、Ⅲ of the soil, the average ratio of SPW<20mm is high, the three typical land use types follow as a whole that when gravel concentration of layer Ⅱ、Ⅲ is less than10%, the specific value of independent staining paths of SPW<20mm is less than20%in the study, when gravel concentration is at10%～20%, the specific value of independent staining paths of SPW <20mm is20%to70%, and when gravel concentration is more than20%, the specific value of independent staining paths of the SPW<20mm reaches more than70%.(4)Very rare uniform matrix flow appears in soil profile of purple soil sloping land with gravel in this experiment, the process of subsurface flow are mainly large pore preferential flow. Gravel concentration in soil in this study only affects the occurrence and development of the preferential flow, the impact on the type of preferential flow is not obvious. So the experiment uses different land-use types of land to classify for the trends of subsurface flow process in samples. The main types of interflow trends in woodland are:mixing macropore flow-low interaction macropore flow; The types of interflow trends in farmland are mainly high interaction macropore flow/mixing macropore flow/low interaction macropore flow-low interaction macropore flow; The main types of interflow trend in orchard main types is similar with farmland, but the high interaction macropore flow and mixing macropore flow occurs at a larger extent. It can clearly see that the phenomenon of lateral preferential flow in every sample occurs usually by interflow cross-sectional view in soils.