| The land is the most important foundation for human’s living,but the gradually increasing problems of land degradation are becoming serious constraint in human development.As the most serious erosion of the Three Gorges Reservoir Area(TGRA),hinterland has a complex topography and fragile ecological environment.Thus it is always the focus of study on soil erosion in the TGRA.The Fengjie County and the Wushan County of the TGRA hinterland has been chosen as topics in the thesis.Based on the 3S techniques,five factors,i.e.,the Digital Elevation Model,precipitation data,soil,lithology,land use derived from thematic data were extracted.The temporal and spatial variation characteristics of soil erosion and sediment delivery of the study area from 1988 to 2015 has been calculated by applying the existing soil erosion model and sediment load model.Meanwhile,identifying the critical source areas(CSAs)of soil erosion and sediment yield in the Fengjie County and the Wushan County,this paper also analyzed the spatiotemporal evolvement of the CSAs and its internal mechanism,which provided a reliable theoretical basis and scientific guidance for the prevention and control of soil erosion in the TGRA.The main contents and achievements of this study are as follows:(1)Temporal change of soil erosion of the study areaFrom 1988 to 2015,the average soil erosion modulus of the study area showed437.88 t·km-2·a-1in 1988,1385.75 t·km-2·a-1in 2000,1276.83 t·km-2·a-1in 2010,and1093.94 t·km-2·a-1 in 2015.According to the Criteria of Soil Erosion Intensity Classification,soil erosion types in the study area were mainly for the light erosion.All cases were divided into two groups according to the year of changes of soil erosion intensity,namely before and after 2000.During 1988 to 2000,soil erosion wassignificantly increased,followed by the slight erosion and light erosion transferred to the higher grade of soil erosion intensity.Conversely,soil erosion has become improved,followed by the light erosion,moderate erosion,strong erosion and ultra strong erosion transferred to the lower grade of soil erosion intensity individually from 2000 to 2015.In recent 27 years,the slight erosion decreased firstly and then increased,reaching the valley value of 1565.67 km2 in 2000.On the contrary,the light erosion and moderate erosion increased firstly and then decreased,with the peak value of 4423.96 km2and977.99 km2 respectively in 2000.At the same time,the strong erosion and ultra strong erosion varied repeatedly with small amplitude of variation.Although the average soil erosion modulus of light erosion and moderate erosion is less than ultra strong erosion,the soil erosion area and soil erosion amount of the light erosion and moderate erosion increased from 1988 to 2000,which meant the contribution of the lower grade of soil erosion intensity to soil and water loss in the study area cannot be ignored.(2)Spatial distribution change of soil erosion of the study areaThe soil erosion intensities changes and relationship of the slight erosion,light erosion and moderate erosion played key roles in the whole network of soil erosion intensities change,and thus determined the characteristics of soil erosion intensities change in the study area.At the same time,strong erosion and ultra erosion increased annually.The spatial features of soil erosion intensities varied at different scales.During1988 to 2015,the high extent of soil erosion types change concentrated in areas along the northern and southern banks of the Yangtze River and its branches.The area of the slight erosion and light erosion turned to moderate erosion mainly distributed in the northern of the Yangtze River,while the area of slight erosion,light erosion and moderate erosion transferred to strong erosion and ultra erosion concentrated in the southern of the Yangtze River.At 27-year scale,the hot spot of soil erosion types change gradually concentrated in the towns near the Yangtze River,and then transferred to northwest of the study area.(3)Estimation of sediment delivery ratio(SDR)of the study areaThe motivation coefficient affecting the sediment delivery ranged from 0.2 to 0.3in the study area.And its mean value was 0.271.Moreover,the intervals of(0.104,0,3)widely distributed in the study area,and accounted for 84.51% of total area of the study area.However,the average value of resistance coefficient was 0.090,and its performance was complicated.The parallel ridge and valley zone,river alluvial gentle slope zone mesa zone had high resistance coefficient,due to low gradience of slope,while the alpine gorges zone possessed low resistance coefficient because of the steeper slopes.The SDR was in approximately normal distribution in the study area with a single peak,and its mean value was 0.245.High value areas of SDR concentrated on the two sides of the Yangtze River and its tributaries.Meanwhile,SDR had the tendency of the gradient decrease from the center of the river to the sides.(4)Correlation analysis of sediment transport and soil erosion of the study areaThe total sediment yield possessed on value year-on-year growth or reduction with soil erosion modulus.The annual variation trend of sediment yield was characterized as3 million 399 thousand and 100 tons in 2000,followed by 3 million 149 thousand in2010,2 million 703 thousand and 400 tons in 2015 and 1 million 101 thousand tons in1998.Moreover,the change trend of the sediment yields modulus and mean sediment yield was the same as the interannual variation of total sediment yield in the study area.The lower value areas of sediment yield were widely distributed in the study area,while the higher value areas were mainly located in the Yangtze River and its tributaries,and clustered in the medium high mountain in the southern of the study area.At 27-year scale,there contained a large number of low value areas of sediment load in the northern Yangtze River,with the reduction of high value area of sediment load.On the contrary,high value area of sediment load were further expanded and aggregated in the southern Yangtze River.There was a positive correlation between sediment transport intensity and soil erosion intensity.And the effects of moderate erosion and strong erosion on sediment load were gradually significant.(5)The spatiotemporal characteristic of the CSAs in the study areaDuring 1988 to 2015,the CSAs accounts for only about 5% of the total area,and it increased from 337.81km2 in 1988 to 382.11km2 in 2015 within a little scale.But the contribution rate of erosion and sediment yield was as high as 50%,which showed a trend of decreasing first and then increasing.Through the space,the CSAs gradually moved towards two sides of the Yangtze River and its branches,along the northeast-southwest direction.During the recent 27 years,the CSAs increased consistently in the southern towns of the study area,bu decreased in the central and northern towns in the study area.From 1988 to 2000,there was a large number of conversion between the CSAs and other risk levels.At 27-year scale,lower risk level turned to the CSAs gradually,and the risk levels changes in the north and south of Yangtze River of the study area were repeatedly.In the spatial distribution,18 towns existed generous CSAs in a long-term,such as Yongle Town,Baidi Town,HefengTownship,Miaoyu Town,Daxi Township,and so on.These towns need to improve the intensity of soil erosion control.(6)Influence mechanism of the CSAs of the study areaThe generation and development of the CSAs is driven by many factors.There existed gradient effect on the distribution of the CSAs and elevation,slope,lithology,channel in natural factors.The CSAs maintained a large mount at 500-1500 m elevation,25°-45° slope,limestone area and 0-1km channel buffer.Furthermore,human facors is an important reason to accelerate the change of the CSAs.In different types of land use,the CSAs decreased in the following order: slope farmland > grassland > shrub land >woodland > unutilized land > construction land > orchard > water body.Among them,the CSAs accounted for 90% of the total area in the slope farmland and grassland.The optimization of land use structure and orderly conversion of cropland to forest and grassland is an important way to effectively control the CSAs and soil and water loss control. |
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