| Micro-topography is one of the important factors affecting soil erosion process, which also seriously affects the surface runoff and sediment. Study on the regulation evolution of micro-topography on micro scale(2cm×2cm) as well as its influence on the sediment yield and runoff generation during the soil erosion process can provide a theoretical reference for the further understanding of the soil erosion process and for the establishing of a soil erosion model. Thus, four common tillage treatments(artificial hoeing tillage(AB), artificial digging(AD), contour tillage(CT) and rake wear leveling(CK)) were chosen, which were commonly used in loess slope farming, and then piecewise rainfall experiments were conducted under two kinds of rainfall intensity(60mm/h and 90 mm/h) and three kinds of t slope(5°, 10° and 20°). Laser scanning was used to obtain the micro-topography relative elevation data during the soil erosion process and geographic statistics, anisotropy and fractal dimension theory were adopted to investigate the evolution and the characteristics of spatial distribution of the surface micro-topography during the soil erosion process(splash, sheet erosion, rill erosion) and the influence on the sediment yield and runoff generation. The main conclusions showed as follows:(1) The results of classical statistical analysis showed that: The classical statistical analysis of cultivation measures both obey the normal distribution,thus, the semi variance function model could be used to simulate the real surface micro topography. The tillage slope micro-topography variation coefficient(Cv) under two different rainfall intensity were less than 0.1. In general, the Cv decreased with the slope, in addition, the Cv under different tillage treatments followed in the order of CvCT>CvAD> CvAB>Cv CK during the soil erosion process. In summary, the tillage slope micro-topography under CT treatment had stronger spatial heterogeneity than other tillage treatments, thus, it had a stronger inhibition on the soil erosion.(2) The results of the semivariogram analysis showed that: The semi variance function model could be used to fit the relative height of the micro-topography, and the parameters of the semi variance function model could be used as point cloud data for the Kriging interpolation in ArcGIS. The spatial autocorrelation of micro-topography was mainly affected by the structural factors, such as tillage treatments, slope and so on. Our results indicated that the texture ratios(C/(C+Co)) were all higher than 25% across all tillage treatments, which all exhibited a moderate or high degree of spatial correlation.In addition, the range of micro-topography increased with increasing rainfall intensity and decreasing tillage slope. The greater the variation of the range was, the worse the continuity and the stronger the spatial variability of micro-topography showed. Thus, the tillage slope with the biggest initial roughness had the worst spatial continuity and the strongest spatial variability.(3) The anisotropy analysis results showed that: During the development process of water erosion, the micro-topography of slope farmland had obvious anisotropy during the soil erosion process, which was increased with increasing rainfall intensity as well as slope. In general, the anisotropy under different tillage treatments followed in the order of CT>AD>AB>CK,suggesting that the tillage slope with a higher initial roughness had a stronger anisotropy.(4) The fractal dimension analysis results showed that: The micro-topography of different tillage treatments had good fractal characteristics, the fractal dimension showed an order of DCT>DAD>DAB>DCK. In general, the tillage slope with a higher value of fractal dimension had a complicated space structure as well as a stronger spatial variability. Thus, the spatial variability under different tillage treatments followed in the order of CT>AD>AB>CK. Meanwhile, the fractal properties were also affected by rainfall intensity and tillage slopes(5) The result of micro-topographic factors showed that: The effect of rainfall on micro slope is obvious. In general, the overall shape of slope tended to be gentler under the action of rainfall. The changes of micro slope following the trend of CT >AD >AB, and increased with increasing the rainfall intensity and initial tillage slope compared with CK. The micro-aspect direction of AB, AD and CT treatments mainly concentrated in the southeast and the north-aspect, especially for the CT tillage treatment. The effect of rainfall on micro-aspect under different tillage was in the order of CK >CT >AD >AB, and the variation of micro-aspect increased with increasing rainfall intensity.The other terrain factors, such as the degree of relief, the cutting depth of soil surface and the confluence cumulant decided on the state of space distribution during the soil erosion process, the greater the corresponding values were, the stronger the spatial variability showed. In general, CT treatment had a stronger spatial variability, thus, its inhibitory effect on water erosion development was stronger than other treatments.(6) The elevation changes of the surface micro-topography during the soil erosion process: This result indicated that the tillage treatments had inhibiting effect on splash erosion expect for the CK, and the most significant inhibitory effect was found under CT due to the maximum reduction of the elevation value. In addition, the reduction of the elevation value increased with increasing the rainfall intensity as well as tillage slope.(7) The effect of surface micro-topography on the amount of splash erosion and runoff and sediment yield showed that: In the stage of splash erosion, the net splash erosion followed the trend of TspCK> 0>TspCT >TspAD>TspAB, while the total splash erosion followed the order of CT>AD>AB>CK, indicating that the tillage slope with initial roughness has a good inhibit effect on splash erosion compared with CK. In the stage of sheet erosion, the runoff and sediment yield of different tillage treatments increased with increasing rainfall intensity and slope, which also depended logarithmically on the tillage slope. In the stage of rill erosion, the runoff and sediment yield of different tillage treatments increased with increasing rainfall intensity and slope, which also depended positive and negative logarithmically on rain times(y=±aln(t),R2>0.5). In the sheet erosion and rill erosion stage, the runoff and sediment yield followed the trend of CK>AB>AD>CT, suggesting that the runoff and sediment yield increased with increasing the initial roughness. The change trend of runoff yield and sediment yield under different tillage treatments showed that CT was the ideal soil conservation tillage treatment.Laser scanning was used to obtain the micro-topography relative elevation data before and after the artificial rainfall experiments, and geographic statistics were adopt to generate high precision M-△DEM through the Kriging interpolation. Then, semi variance function, anisotropy, fractal dimension and other theoretical methods were utilized to analysis the evolution and the characteristics of spatial distribution of the surface micro-topography during the soil erosion process. Our result indicated that CT had strongest spatial heterogeneity and the most complex of its spatial structure and the splash erosion as well as sediment yield were minimum compared with the other tillage treatments. This all suggest that CT tillage treatment is the most ideal soil and water conservation tillage treatment in loess slope farming. |
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