| Soil erosion has become a major environmental problem in the process of human survival and development.With the rapid development of society and economy since the reform and opening up,soil erosion is becoming more and more serious in China.Many environmental problems caused by it have been widely concerned in the society.By the end of June 2016,the total area of soil erosion in China reached 295 million hectares,accounting for 31.1%of the total land area.Soil erosion will not only cause serious soil erosion,but also cause a large loss of organic matter in the basin,which leads to the decline of soil fertility.How to study the dynamic mechanism of soil erosion caused by human activities in the basin and to explore the relationship between land use/cover,soil,rainfall,hydrological change and soil erosion has become an important basis and means to solve regional resource problems,environmental problems and ecological problems.The Yangtze River Basin is the largest river basin in China.Its sediment and POC flux has attracted more and more attention from scholars.It is of great significance to explore the sediment erosion and POC output flux in the Yangtze River Basin for soil and water conservation and ecological environment control in the basin.Based on collecting soil data,meteorological data and land use data of the study area,a SWAT model was used to analysis spatial and temporal distribution characteristics of sediment erosion.In addition,we achieved the result of POC flux by modifying the source program of the SWAT model.The main research contents and results are as follows:(1)Collect soil data,meteorological data and land use data to construct the SWAT model of the Yangtze River Basin.We use monthly data of water discharge and sediment among 2001 an 2006 to calibrate and monthly data of water discharge and sediment among 2007 and 2010 to validate.In general,the performance of the model is within acceptable limits,although the value of R2 and NSE is low for some hydrological stations during calibration and validation.The model can be applied to the simulate runoff and sediment in the Yangtze River Basin.(2)The sediment output data from 1991 to 2010 in the study area were achieved by inputting different land use data and daily meteorological data.From the time distribution,rainfall in the Yangtze River Basin is mainly concentrated in the year April to September.Sediment output is mainly concentrated in the annual summer(May to September),and the output of sediment in winter is relatively low.Before the impoundment of the Three Gorges reservoir,average annual sediment flux in the Yichang hydrologic station is 329.63Tg and average annual sediment flux in the Datong hydrologic station is 299.42Tg.After the impoundment of the Three Gorges reservoir,average annual sediment flux in the Yichang hydrologic station is 93.86Tg and average annual sediment flux in the Datong hydrologic station is 177.2Tg.From the spatial distribution,sediment erosion load in the upper reaches of the Yangtze River basin is much higher than that of the lower Yangtze River Basin.In addition,these subbasins with highest sediment load are mainly located in the border of the Chinese terrain steps.One part of them is located in the junction of the first terrain step and the second terrain step,and the other part of them lie in the junction of the second terrain step and the third terrain step.(3)Based on the simulation of sediment erosion,sediment erosion is the only source of POC in the watershed.The temporal and spatial variation characteristics of POC output in the Yangtze River Basin are obtained by coupling the sediment erosion module and the POC flux estimation module in the SWAT model.According to the POC flux simulation results,the POC output flux in the Yangtze River Basin is affected by the Three Gorges water conservancy project.The annual average POC flux is reduced from 4.09 Tg·yr-1 before Three Gorges Reservoir's water storage and power generation to 2.36 Tg·yr-1.There is a significant linear correlation between sediment and POC output fluxes in the Yangtze River Basin.The correlation between monthly mean sediment and POC output fluxes is 0.98,and the correlation between annual average sediment and POC output fluxes is 0.92.The TOC flux of TOC in the Yangtze River Basin is 3.48 Tg·yr-1,and the annual average POC output load is 1.8 t·km-2 yr-1.(4)The influence factors of sediment erosion in the basin were analyzed according to the model simulation results.The results show that sediment output in the Yangtze River Basin is mainly affected by two factors,natural factor and human factor.Natural factor mainly include meteorology and topography.Human factor include land use change and major human engineering.There is a wonderful correlation between the runoff and the sediment flux in the Yangtze River Basin,and the higher the runoff is,the greater the sediment flux is.Before Three Gorges Reservoir's water storage and power generation,the annual average sediment flux in Yichang hydrologic station is 329.63Tg,and the annual average sediment flux of Datong station is 299.42Tg.After Three Gorges Reservoir's water storage and power generation,the annual average sediment flux is 93.86Tg,and the annual average sediment flux of Datong station is 177.2Tg.The geographically weighted regression model was used to explore the correlation between precipitation,slope,land use and sediment load on the spatial distribution.The geographically weighted regression model had a correlation coefficient R2 of 0.71 and a corrected correlation coefficient R2 of 0.7.The results show that 70.3%of the sediment load in the basin can be explained by these three factors.The basin POC load distribution and sediment load and spatial distribution of the content of TOC,according to the simulation results of geographical space weighted model,POC basin erosion and sediment load correlation between the spatial distribution of the load space is strong,the content of TOC spatial distribution of POC spatial load distribution has little effect on erosion. |
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