Sediment Production By Erosion And Nutrient Loss In The Typical Watershed Of Headwaters Region Of The Middle Route Of South-to-North Water Transfer Project

Danjiang River watershed is an important water source culvert area of South-to-North Water transfer middle line project in our country.Due to the serious soil and water loss in the region,the problems of reservoir siltation and water resources security are caused.There are few studies on the contribution of land-based slope erosion to reservoir sediment deposition and non-point source pollution.Therefore,the Erlongshan Reservoir watershed in the upper reaches of the Danjiang River is taken as the research area.Based on the soil loss equation,measured sediment data and laboratory experiments in China,the rules of soil and water loss and soil nutrient loss in the basin,the characteristics of reservoir sediment deposition and Water quality,sediment are analyzed,and the influence of slope erosion and nutrient loss on the reservoir is clarified.The main conclusions are as follows:（1）From 1973 to 2020,the average annual soil erosion modulus of Erlongshan Reservoir basin was 6.95 t·ha^-1·a^-1,and the annual soil erosion amount was 180.72million tons.The soil erosion in the basin is mainly mild erosion,and the severe erosion is mainly concentrated in the middle and lower reaches of Banqiao Basin.The erosion area of cultivated land and grassland was relatively large,and mainly concentrated on mild erosion.（2）The dead storage capacity and effective storage capacity of Erlongshan Reservoir decreased gradually.In 2020,the dead storage capacity of the reservoir will be reduced by 57.27%and the total storage capacity by 21.58%compared with the original dead storage capacity.The siltation of dead storage capacity accounted for60.8%of the siltation volume.The precipitation,land use and soil erosion explained74.5%of the sediment yield,and 78.2%of the sediment yield.This indicated that sediment production is the main factor of reservoir sediment variation.（3）The mean values of organic carbon,total nitrogen and total phosphorus were12.85 g·kg^-1,0.59 g·kg^-1and 0.37 g·kg^-1,respectively.Organic carbon and total nitrogen were consistent,with the order of forest land>cultivated land>grassland.The loss of organic matter,total nitrogen and total phosphorus was 2362.98 million tons,108.49 million tons and 68.04 million tons on the slope.The spatial distribution of carbon and nitrogen in the sediments of Erlongshan Reservoir is consistent,and the Banqiao fracture face value is the highest,which is 14.27 g·kg^-1and 1.25 g·kg^-1.The highest phosphorus content was found in Majie section(0.73 g·kg^-1).The overall water quality is good,but the ammonia nitrogen content exceeded the standard.The enrichment rates of organic matter,total nitrogen and total phosphorus were all greater than 1,and the order was total nitrogen>total phosphorus>organic matter,with the highest enrichment rate of total nitrogen.（4）The nutrient loss of the basin had a positive and significant effect on the nutrient enrichment of the reservoir（p<0.05）,and the explanations of organic carbon,total nitrogen and total phosphorus loss were 83.30%,85.20%and 82.40%.Among them,the impact of the Majie basin on the reservoir is slightly higher than that of Banqiao basin,so more attention should be paid to the impact of the Majie basin in the future.