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The change process of cultivated layer quality for slope farmland at plot scale is influenced by the interactive effects between erosive rainfall and artificial farming activities at different temporal and spatial scales. For a slope surface, there exists a successive erosion process of rainfall-runoff-infiltration and the consequence is first reflected in the quality of cultivated layer. Among the factors induced the erosion process at plot scale, terrain is an important factor because it could redistribute the ratio of rainfall--infiltration, which directly cause a different erosion process for an erosive rainfall and the different degradation degree during a rainy season and a growth season. Taking purple soil slope farmland as an example, the paper firstly analyzed the varying characteristics of cultivated layer quality of slope farmland, then quantitatively evaluated the effects of four terrain factors, altitude, slope angle, effective soil thickness and slope position on cultivated layer quality and their sensitivity of impact, adopted principal component analysis and evaluation model. The results indicated: 1) Such soil properties as cultivated layer thickness, soil organic matter and soil bulk density of cultivated layer were all within the suitable scope and soil nutrient properties were generally at middle & low levels. The variation coefficients of soil bulk density and soil total porosity showed weak variability (CV<10%), presenting 7.97% and 8.36% respectively and the other soil properties showed moderate variability. 2)Minimum data set for cultivated layer quality evaluation of purple soil slope farmland consisted of eight soil parameters, bulk density, penetration resistance, soil organic matter, clay content, cultivated layer thickness, soil shear strength, saturated water conductivity and bioavailable phosphorus. Soil physical properties are more important than chemical properties except soil organic matter for erosion control and crop growth of slope farmland. Evaluation based on the MDS revealed that the cultivated layer quality varied spatially over the slope, being 0.458 in the low part of the slope, 0.443 in the middle part of the slope and 0.417 in the upper part of the slope and the quality value of the lower slope was 3.39% and 9.83% higher than that of the middle slope and upper slope positions, respectively. 3) The factors affecting cultivated layer quality divided into four types: slope-angle impacted type, slope location impacted type, effective soil layer thickness impacted type and altitude impacted type and they accounted for 38.89%, 22.22%, 14.81% and 24.08% of the total samples taken from slope farmland respectively. The slope angle was the main determinant of cultivated layer quality because it could induce directly many ecological processes at plot scale. The slope angle and effective soil layer thickness could be modified by such soil and water conservation measures as terrace, biological ridge and soil improvement methods. But the slope location and altitude could not be modified easily only by selecting suitable crop system. 4) The quality of cultivated layer had positive correlation with its thickness and location in the slope, and negative correlation with altitude and slope angle. Sensitivity analysis revealed that the determinants were ranked in the order of altitude (-0.399) > location in the slope (0.192) > slope angle (-0.112) > effective soil layer thickness (0.110) based on their impact on cultivated lay quality. These results could provide some useful evaluation parameters and its regulating direction for cultivated layer quality, which contributed to the sustainable utilization of purple soil slope farmland. Furthermore, other samples of slope farmland should be studied in order to find out the critical values of MDS according to 2 functions of cultivated layer, erosion control and soil productivity. [ABSTRACT FROM AUTHOR]

Cultivated layer soil quality is highly sensitive to both soil erosion and artificial farming tillage in Purple Hilly Area. Soil erosion is a key factor that causes soil degradation and soil productivity instability of cultivated layer for slope farmland. Taking slope farmland in purple hilly area as an example, this paper has conducted cluster analysis on soil quality, permeability and cultivated layer type of slope farmland with different land productivity grades, and further discussed the effect of soil erosion on thickness of cultivated layer and the soil body configuration with reasonable cultivated layers of slope farmland. Results indicated that: (1) The soil quality of cultivated layer for purple soil slope farmland presented a moderate variability, the variation coefficients were 12.18% and 37.26% for the effective soil layer thickness (15-80 cm) and the cultivated layer thickness (15-25 cm), respectively, and the variation coefficient was 94.51% for the soil available phosphorus, indicating that the variability of soil physical properties was weaker than that of chemical properties. The cultivated layer configuration below the third grade of land productivity was almost full virtual or full real profile structures, and meanwhile, the production of slope farmland with the fifth grade of land productivity fell by about 50%. (2) There were significant (P<0.05) differences in soil physical properties, water-retention properties and tillage performances of cultivated layer at different slope position and different vertical depth of purple soil slope farmland, in which the soil bulk density showed upper slope > lower slope > middle slope, and soil shear strength showed upper slope > middle slope > lower slope, the soil stable infiltration rate showed the contrary order, and the soil penetration resistance was middle slope > upper slope > lower slope. The soil bulk density, shear strength and penetration resistance all presented bottom soil layer > subsoil layer > cultivated layer except soil infiltration; about 70% of the rain stored in 0-40 cm soil layer of slope farmland could be used by crops. (3) The average annual thinning of cultivated layer thickness varied between 1.04 and 3.04 mm for the moderate erosion degree of slope farmland. The reasonable cultivated layer of slope farmland could be established effectively by regulating such means as the minimum data set (MDS), which included soil layer thickness, cultivated layer thickness, soil bulk density, soil shear strength, soil organic matter and soil infiltration capacity for evaluating the cultivated layer soil quality of slope farmland, and moreover, the reasonable cultivated layer configuration overall should maintain the type of upper part loose and lower part tight, the cultivated layer thickness of 20-25 cm and the effective soil layer thickness of 50-60 cm. (4) Purple soil slope farmland could be divided into 4 kinds of cultivated layer types, whose obstacle cultivated layer was mainly manifested as the effective soil layer thickness restricted type (Class II), the cultivated layer thickness restricted type (Class III) and the soil nutrients restricted type (Class IV), accounting for 30%, 10% and 3% of total cultivated layer, respectively; so the regulating of reasonable cultivated layer of purple soil slope farmland should focus on effective soil layer thickness and cultivated layer thickness. These results can not only deepen the objective understanding of relationship between soil erosion and cultivated layer degradation but provide some essential parameters and technical supports for constructing reasonable cultivated layer of slope farmland in purple hilly area. [ABSTRACT FROM AUTHOR]

Various engineering accumulations, caused by all man-made activities during mining process, have resulted in serious artificial soil and water loss in the coal mine area because of great difference in material composition, slope structure and runoff and sediment characteristics from original landforms, which would make great potential risk for the mining safety production and the surrounding areas. Recently, researches begin to focus on artificial soil and water loss in project areas, however, mechanism of runoff erosion on engineering accumulation slopes is still unclear, especially hydrodynamic characteristics in the process of runoff erosion needs further study. Taking typical coal mine engineering accumulation as example, soil test method and field scouring experiment were conducted to study the runoff erosion characteristics and critical hydrodynamic conditions of engineering accumulation slopes in the coal mine area. In this paper, some physical properties, hydraulic properties and runoff erosion process on slopes of engineering accumulations with different producing sources were all comparatively studied. Field scouring experiments were carried out from July to August 2013 at the Soil Erosion Experiment Site for production and construction projects in Southwest University, Chongqing. The field scouring experiments contained 5 flow discharges (i.e. 10, 15, 20, 25 and 30 L/min) and 4 slope gradients (i.e. 25°, 30°, 35° and 40°). The paper mainly studied the hydrodynamic characteristics of engineering accumulation slopes in order to reveal the characteristics of runoff and sediment yield and soil erosion rate, and to analyze the relationship between soil erosion and critical hydrodynamic conditions and the critical slope gradient under different flow discharges. The results indicated that: 1) During runoff erosion process, both flow velocity, flow shear stress and runoff power of engineering accumulation slopes exhibited different degrees of fluctuation, showing a range of 0.187-0.526 m/s, 24.336-126.542 Pa and 2.763-46.861 N/(m·s), respectively, however, resistance coefficient existed a fluctuation from weak to strong in the range of 2.236-19.337. 2) Both runoff yield rate and sediment production rate of engineering accumulation slopes exhibited a trend of first increase and then stability during runoff erosion process except for the 10 L/min flow discharge. The runoff yield rate tended to 0.5, 3.0, 3.8, 6.3 and 9.0 L/min, respectively, under different flow discharges (10 to 30 L/min), however the sediment yiled rate varied between 0 and 27.51 kg/min and the soil detachment rate changed between 9.570 and 4616.064 g/(m2·min). 3) There were great difference in critical flow shear stress and critical runoff power among engineering accumulation slopes with different slope gradients. At sheet erosion stage, both the critical flow shear stress and critical runoff power of 30° engineering accumulation were the minimum, whose values were 23.95 Pa and 1.76 N/(m·s), respectively; at rill erosion stage, however, the critical flow shear stress of 25° engineering accumulation was the minimum and the critical runoff power of 40° engineering accumulation was the least; and the soil erosion rate had a significant linear relationship with the flow shear stress and runoff power. 4) When flow discharge was 10-30 L/min, the critical slope gradients of engineering accumulations were 34.8°, 35°, 33.7°, 34° and 35.2°, respectively. These results not only could provide essential technical-parameters for predicting soil and water loss caused by mining production, but also would help to arrange some vegetation measures for soil and water ecological restoration of these engineering accumulations. [ABSTRACT FROM AUTHOR]

Engineering accumulation, a kind of loose soil-sock mixture consisted of soil, parent rock and stone caused by various artificial engineering during producing and constructing process, is the most serious erosion-landform in production and construction areas because it can greatly change the terrain, soil and vegetation conditions of original landform in a very short term, thereby causing great soil and water loss under a certain rainfall-runoff condition, and making more difficult for ecological restoration in these project areas. In order to revel the runoff erosion mechanism and its process for engineering accumulations with different slopes and soil-rock ratios, we carried out field scouring experiments from August to November 2012 at the Soil Erosion Experiment Site for production and construction projects in Southwestern University, Chongqing (29° 48' 50.2" N, 106° 24' 48.8 "E). The field scouring experiments were conducted under the simulated runoff conditions according to the regional rainstorm occurrence frequency with the 5, 10, 15, 20, 25 L/min for purple soil deposits and 5, 7.5, 10, 12.5, 15 L/min for yellow sand deposits, respectively. In the experiment, some physical properties, hydraulic properties and runoff erosion process on underlying surfaces of engineering accumulations with different producing sources and soil-rock ratios were all comparatively studied. The results indicated that material composition and infiltration capacity of the two loose engineering accumulations were remarkably different from original landform. The yellow sand deposit was dominated by ≤ 0.25 mm particles with particles variation coefficient as high as 1.2-2.0 times than its original landform and its stable infiltration rate was 1.70-4.07 times than original landforms, respectively; while purple soil deposit was well-distributed, its' variation coefficient of particles was 2.2 times than original landform and the stable infiltration rate was 7.02-11.59 times than original landforms. The experiments also showed that soil-rock ratio change could influence the slope infiltration performance for the two engineering accumulations and the effects of engineering construction on the purple soil deposits were more powerful than that of the yellow sand deposits. Some dynamic parameters on underlying surfaces of engineering accumulations increased with increasing flow discharge, in which flow velocity of yellow sand deposits ranged from 0.155 to 0.318 m/s, flow shear stress varied between 27.632 and 57.154 N/m2, and soil detachment rate changed from 0.337 to 77.071 g/(m2·s); while the flow velocity, flow shear stress and soil detachment rate of purple soil deposits were 0.184-0.281 m/s，35.525-53.600 N/m2 and 1.445-61.910 g/(m2·s), respectively. For rocky underlying surfaces, runoff yield rate had continuously waving variations within 9 min after runoff yielded. When flow discharge increased gradually from 5 to 25 L/min, the average runoff yield rate of rock underlying surface from yellow sand deposit increased by 510.38%, 531.96 % and that of purple soil deposit increased by 73.20% and 57.61%, respectively; for same conditions, the cumulative runoff of underlying surfaces showed the similar trend of partial soil deposits larger than rocky deposits, and that of yellow sand deposit higher than purple soil deposit. For rocky underlying surfaces, sediment yield rate presented continuously multi-peak multi-valleys. When flow discharge increased from 5 to 25 L/min gradually, the average sediment production rate of rock underlying surface from yellow sand deposit were 0.001, 0.03, 0.542 kg/(m2·min), respectively, while that of purple soil deposit were 0.0003, 0.012, 0.034 kg/(m2·min), respectively. The collapse on gully wall caused by gravity from underlying surfaces was an important cause for the fluctuating variations of the sediment yield rate. These results provide essential parameters and technical supports for predicting soil water loss caused by production and construction projects and for selecting vegetation measures for soil and water conservation in similar areas. [ABSTRACT FROM AUTHOR]