Study On The Generation Condition Of Slope Runoff And The Effect Of Grass And Forest On Soil In Loess Area Of Shaanxi Province

This research aimed the mechanism of control soil erosion at grass and forest land in loess plateau and took slope erosion and soil improvement by trees and grass under different vegetation types as the research object. Combining macro with micro study, experiment research with numerical simulation, positioning with half positioning observation, short-term observation with long-term monitoring was used to determine the main influencing factors of slope erosion and the effect of forest and grassland on soil improvement. Based on the analysis of the main influencing factors of slope erosion and the effect of forest and grassland on soil improvement, this research illuminated the rainfall-soil moisture threshold value for runoff producing and analyzed the impact of land disturbance/restoration on runoff and sediment yield. And this research provided the basic theory for analysis of soil and water conservation benefit and soil erosion prediction model during the process of vegetation restoration and reconstruction in loess area and also provide technology support for forestry ecological restoration project in this areas. This research also provided the basic theory for construction efficient vegetation of soil and water conservation on the semiarid Loess Plateau. The main results are as follows:(1) According to the research of the influencing factors of slope soil erosion, we found that rainfall was the greatest factor influencing runoff, then followed by soil, topography and vegetation conditions; specifically, rainfall amount, rainfall intensity, soil steady infiltration rate and vegetation coverage had the greatest influence on runoff. Soil condition was the greatest factor influencing on sediment yield, followed by runoff, rainfall, topography and vegetation conditions; specifically, runoff, rainfall amount, rainfall intensity, soil bulk density, slope gradient and vegetation coverage had the greatest influence on sediment yield. Under the condition of heavy rain, rain and topography were the most important influential factors on runoff, and followed by soil and vegetation; specifically, rainfall amount, slope gradient, rainfall, slope aspect, soil bulk density and vegetation types had the greatest influence on runoff. Under the condition of heavy rain, runoff had the largest effect on sediment yield; however, the respective effect of rainfall, topography, soil and vegetation on sediment yield was not significantly stronger than others, the comprehensive influence of rainfall, topography, soil and vegetation combined and had more significant influence on sediment yield.(2) Based on the basic theory of excess infiltration, we got the rainfall-soil moisture threshold model:I= [(Ms-Mw) H D+ζ]/T, I stands for rainfall intensity, T stands for rainfall duration, Ms stands for saturated soil moisture, Mw stands for antecedent soil moisture, H stands for soil infiltration depth, D stands for soil bulk density, and E, stands for rainfall loss. Based on this model, we got the rainfall-runoff thresholds of the drought year, normal year and rainy year during the rainy season in Wuqi County. We also mapped the critical rainfall erosion I-T related diagrams.(3) During PPS, topography was the most critical factor for the runoff, followed by soil, vegetation and rainfall, while soil was the most important factor for sediment yield, followed by rainfall, vegetation, topography and runoff; during PLR, runoff was primarily affected by vegetation, followed by rainfall, topography and soil, while the major factor for sediment yield was rainfall, followed by soil, runoff, topography and vegetation. Our findings revealed that the vegetation types Hippophae rhamnoides+Pinus tabulaeformis and Hippophae rhamnoides were the better plant selections for reforestation during the land disturbance period in this area, especially if the slope gradient was less than 20°.(4) In the study area, the variability and sensitivity were different in these 22 soil index, however, there was no strength variation and strength sensitivity index. According to research, plant community diversity was more closely related to soil chemical properties, then followed by soil biological index and soil physical index. After the coupling model of slope species diversity and soil factors was set up, and the descending order of coupling degree between species diversity and soil was Hippophae rhamnoides+Pinus tabulaeformis (gentle slope), Hippophae rhamnoides, Hippophae rhamnoides+Pinus tabulaeformis (steep slope), Pinus tabulaeformis and Lespedeza davurica+Leymussecalinus. Further study, we concluded that the descending order of ranking soil quality at different vegetation types in study area were:Hippophae rhamnoides+Pinus tabulaeformis (gentle slope), Hippophae rhamnoides, Hippophae rhamnoides+Pinus tabulaeformis (steep slope), Pinus tabulaeformis and Lespedeza davurica+Leymussecalinus.