Study On The Slope Erosion Characteristics Of Purple Soil Under The Different Slope

The purple soil is characterized by soft texture and high erodibility. Among all the China soil types, the erosion area and erosion intensity of purple soil ranked the second just lower than the Loess. The soil erosion is very serious in the Three Gorges region as its main cultivated soil is purple soil. With the occurrence of severe soil erosion, a large number of topsoil nutrients of cultivated land in the Three Gorges Reservoir Area losses and results in a land productivity decline. Large amounts of erosion sediment and soil nutrient flow into the Yangtze River basin and the Three Gorges reservoir area, resulting in a large area eutrophication of water, posing a serious threat to river safety. The serious soil erosion of reservoir area calls for an urgent treatment. However, the state of nature slope surface soil erosion is an extremely complex process, where rainfall, landform, vegetation, soil properties, and other factors influences each other, interact with physical change and chemical reaction process. To study the slope soil erosion principle, the paper adjusts the slope to compare characteristics of runoff and sediment yield and surface soil nutrient loss on slopes of5°,15°and25°, by keeping erosion impact factors unchanged, such as rainfall intensity, soil properties. The tested soil was the purple soil and the artificial rainfall simulation experiment was carried out in ecological environmental monitoring station of Wanzhou district in Chongqing. The results are as follows:(1) Characteristics of slope runoff:During a rainfall, slope average infiltration rate is higher in the earlier rainfall, but with the increase of runoff time duration, soil is saturated gradually, the slope average infiltration rate will reduce, and ultimately reflected in the runoff intensity and the slope cumulative production flow increase. The rainfall intensity constant for1.737mm/min when slope gradient increasing, it shortened the residence time of the rainfall on the slope, reducing rainfall infiltration opportunities, which leads to the increase of slope runoff, the slope runoff intensity increased. Slope surface runoff characteristics such as slope average infiltration rate, slope runoff intensity and slope cumulative production flow, and runoff duration’ relationship can express by the power function available, including slope cumulative production flow and slope runoff duration has significant correlation, when the slope is25°, the correlation is the highest.(2) Features of slope sediment yield:during the same rainfall, the quantities of sediment yield in varied slopes all suddenly reach to their peaks at the early runoff yield; while gradually decline as runoff yield proceed on and tend to stay the same at later stage. In this process, changes in the curves of15°and25°are the most striking. The gradients impact a lot on slope sediment yield. When rainfall intensity is constant, the greater the gradient is, the more slope sediment yield is; the corresponding slope sediment yields to the gradients of5°,15°and25°respectively fluctuate among5.00～15.26g,23.05～95.49g,40.23～108.02g. Meanwhile, the peaks of sediment yields also appear in advance. The quantities of larger-grained sands in sediment yields increase first and then decrease (or decline) due to longer runoff yield, while silts increase all the time. Though the volatility of mechanical components of sediment yields in various slopes differ so much. The mechanical components of sediment yields do not change sharply in different slopes.(3) Features of sediment and nutrient loss:during a precipitation event, with extended runoff, erosion, sediment total nutrient, available nutrients and organic matter content obviously increase to their peaks at first, then gradually decrease and eventually tends to smoothly change. Erosions of sediment organic matter and total potassium content of total nitrogen, phosphorus have evident fluctuations. In the same process of rainfall, slope sediment total nutrient, available nutrients and organic matter content are of no significant changes. Sediment total nitrogen and total phosphorus content appear as15°>25°>5°, the performance of the sediment organic matter content is5°>25°>15°, while the maximum sediment total potassium occurs at a slope of15°,25°and5°. Sediment nutrient content reaches the peak for runoff time occurs in advance. Lasted strain stress relation between sediment nutrient before and after the peak and extended runoff is in line with changes of a power function.Under a certain slope gradient, the erosion sediment content of each nutrient in the sediment loads and erosion have varying degrees of positive correlation. And the correlations are better when the gradients are15°and25°. There are different degrees of correlation between the sand, silt, clay content in eroded sediment and the nutrient content in runoff and sediment:when Slope is5°, the nutrient content in eroded sediment and sediment of sand, silt and clay content were not significantly correlated; When the slope is15°, only total nitrogen of eroded sediment, available phosphorus of eroded sediment and sediment of sand, silt, clay content is significantly correlated; When the slope is of25°, in addition to organic matter, total nitrogen, potassium, other sediment nutrient and sediment sand, silt, clay content has significant correlation.(4) Runoff nutrient loss characteristics:the runoff total nitrogen, total phosphorus in the runoff process under3slopes (5°,25°,15°) showed more pronounced changes, that is,in early runoff, as the runoff gradually decreases, the runoff total nitrogen, total phosphorus concentration increased sharply to a peak, and the late runoff to maintain a stable trend. Runoff total nitrogen, total phosphorus concentration with runoff lasted has stress-strain relationship of power function change. With the increase of the slope, the runoff total nitrogen concentration and runoff total nitrogen loss will show a increasing trend on the whole, in the process of runoff yield runoff peak concentration of total nitrogen also increased with the increase of slope, peak concentration of time also in advance accordingly. But the slope runoff between size and total nitrogen total phosphorus from the runoff variation characteristics, under the3slope runoff curve of total phosphorus parts overlap, runoff total phosphorus concentration in general is15°>25°>5°, the runoff of total phosphorus loss performance for25°>15°>5°.When the slope gradients are5°,15°and25°, the runoff intensity has no obvious correlation with total nitrogen concentration and total phosphorus concentration of runoff but the correlation are obvious between it and the loss of total nitrogen and total phosphorus of runoff.