| The soil erosion is one of the most serious problems in the Loess Plateau, where erosion situation and erosion driving forces are complex and changeable, and has become the key area for soil and water loss research. Influenced by the comprehensive effect of geologica、 climate and vegetation status, a unique water-wind erosion crisscross region formed in this area, where wind and water works separately or interact mutually. At the same time, the ecological environment in the Loess Plateau is so harsh that vegetation landscape diversity is limited by moisture conditions, and patterns of vegetation landscape is complex and disparate. In our study, with considering the mechanism between them and their influence degree, we make use of methods in soil erosion and vegetation landscape patterns research, and we hope we can provide theoretical support and the scientific basis for comprehensive control of soil erosion.This articl takes the Liudaogou watershed in water-wind erosion crisscross region as research object. This study researches the soil erosion situation during a historical period, quantitatively calculates and divises the annual average water and wind erosion rate respectively, with major methods containing field sampling and laboratory analysis、137Cs nuclide tracer technology and Universal Soil Loss Equation. Meanwhile, the content of soil137Cs and its relationship to slope gradient factor, the content and relationship among soil137Cs and soil organic matter、total nitrogen、soil clay partical are reseached. And through the analysis of the typical slope vegetation patch pattern index factor, the comprehensive effect of Vegetation patches spacial pattern to soil erosion in water-wind erosion crisscross region is discussed at the same time.The main results of this work are as follows:1. Through the analysis of the content of137Cs in background point and different slope aspects, we draw a conclusion that the loss of137Cs is critical, soil erosion in the watershed is serious, with an average annual rate of9100t km-2a-1, reached a very intensive erosion degree. For the driving force of water and wind erosion、geography、 vegetation cover factors are various, there exists an obviously difference among slope aspects in this region. The soil erosion rate of all aspects is in sequence of northwest slope >southwest slope>northeast slope>north slope>west slope>east slope>south slope>southeast slope, with their average annual rates are11000t km-2a-1、10472t km-2a-1、10140tkm-2a-1、9400tkm-2a-1、9300t km-2a-1、7700tkm-2a-1、7500t km-2a-1、7200tkm-2a-1respectively.2. We discuss the correlations between the content of soil137Cs and slope gradient factor and among the content of soil Cs and soil organic matter、total nitrogen、soil clay partical. The content of soil137Cs and slope gradient factor are are significant negative correlation at P=0.01level, with their correlation coefficient is0.726. The content of soil137Cs and soil organic matter、total nitrogen are significant positive correlation at P=0.05level, with their correlation coefficient are0.336、0.414respectively. Making regression analysis between slope gradient and the content of Cs of the same slope position on different slope aspects, it was found that slope gradient Y can be represented by function of the content of137Cs X, they are:upslope, Y=-0.0072X2+5.6412X-810.56(R2=0.7401); midslope, Y=0.0078X2-6.7434X+1714.9(R2=0.2905); downslope, Y=-0.0074X2-6.074X+1522.7(R2=0.9422). The relevance of the137Cs content and slope gradient is higher on upslope and downslope, while it’s lower on midslope. The degree of correlation is weak between the content of137Cs and soil clay partical.3. We research the correlation between soil erosion rate and vagetation patterns indexes. The soil erosion rate and Patch Cohesion Index have significant negative correlation, with correlation coefficient0.726at0.05level; certain negative correlations are found between soil erosion rate with vegetation coverage、Patch Area、Largest Patch Index、Aggregation Index, with their correlation coefficients are-0.550、-0.464、-0.553、-0.651respectively. On the contrary, significant positive correlations are found between soil erosion rate and Patch Density、Landscape Division Index at P=0.05level, with their correlation coefficients are0.691、0.686; some certain positive correlation exists between soil erosion rate and Perimeter-Area Fractal Dimension, their correlation coefficient is0.484. Most vegetation pattern indexes have a very significant and significant correlation at P=0.05and P=0.01level. Our study suggests that vagetation spacial patterns with grassland and shrubland et al. have a remarkable influence on soil erosion in water-wind erosion crisscross region.4. The ratio between wind and water erosion amount was studied quantificationally in water-wind erosion crisscross region. This ratio in nouthwestern slope is the biggest in all eight aspects, reached to41:59; the minimum is the ratio in southern slope in eight aspects, only7:93. The average rate of water erosion and wind erosion of eight aspects in this watershed were6800t km-2a-1and2100t km-2a-1, respectively. And the average ratio between water and wind erosion amount eight aspects in this watershed was about3.34:1. The wind erosion rate occupied about21.6%in total erosion rate annual average. The ratio of wind and water erosion amount varied in eight aspect of slopes, could be ranged in the order of northwest slope>west slope>north slope>southwest slope> northeast slope>southeast slope>east slope>south slope. Water force is the main external force of slope erosion in water-wind erosion crisscross region. Furthermore, the hazard caused by wind erosion cannot be ignored. |
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