Study On Nutrients Output Change Law From Non-point Source In Xiangxi Basin Of The Three Gorges Reservoir

With the effective controlling of point source pollution in China, the proportion of non-point source caused by fertilizers application and soil erosion is increasing, and nitrogen and phosphorus load from nonpoint source is the main source which results in water eutrophication. Correspondingly, management of water environment have changed rapidly, and the total load control based on the watershed unit is important techniques after the concentration control. However, it is aimless to control non-point source pollution owing to the lack of knowledge on the sources, migration process and load spatial distribution of pollutants. After the impounding of the Three Gorges Reservoir, the water environment and water of them main river is relativly stable and good, while branchs water is eutrophicating and deteriorating, which is inevitable result owing to the too much nutrient input and changes of the water hydrodynamic situation. Xiangxi River basin is a typical agricultural and forestry basin in the Three Gorges basin, and the key problem of its water environment is eutrophication, however reported on its non-point nutrient load is quite limited.Xiangxi Basin is selected as a demonstration area in the Three Gorges, field monitoring and model simulating are applied to study non-point nutrient erosion and load distribution in different temporal and spatial scales, such as typical plots, typical tributary and whole basin, and establish distributed hydrological model to simulate non-point source nutrients distribution. The related study contents and results are as follows:1) Monitoring nitrogen and phosphorus concentration of Xiangxi Bay and main tributaries, investigating water functional plan objectives, and diagnosing the facing water environment pressure.2)Dividing into different non-point sources classification, building precipitation runoff plots, measuring the soil organic matter, cation exchange capacity(CEC), mechanical composition and occurrence characteristics of nitrogen and phosphorus. soil organic matter is directly proportional to median diameter and inversely proportional to CEC, the correlation coefficients are0.623and0.555respectively, the variation coefficient of nitrogen and phosphorus is NO3-N(0.43)>PO4-P(0.32)> TP(0.22)>TN(0.17)>NH4-N(0.16). The correlation coefficients of TN and ORG-N, TP and particle P are0.939and0.884.3) Monitoring precipitation runoff, soil erosion and nutrient loss in established runoff plots, studying the pollution intensity and its influence mechanism. As a whole, surface runoff yield is yellow brown soil>yellow soil>purple soil>limestone soil, and cultivated land>orchard land>forest land>tea land. Soil and nutrient erosion modulus is cultivated land>orchard land>tea land>forest land.4) According to water quality and quantity monitoringsimultaneously, analyze point and non-point pollutant concentration, baseflow and direct runoff, point and non-point source flux. the result of point and non-point concentration of TN, TP, NO3-N, NH4-N, PO4-P is0.704mg/L and1.746mg/L、0.142mg/L and0.199mg/L、0.531mg/L and0.876mg/L、0.096mg/L and0.231mg/L、0.092mg/L and0.068mg/L; BFI (baseflow index) is0.312, precipitation runoff is68.8%; the contribution ratio of non-point TN and TP is84.6%and75.6%.5) Based on distributed hydrological model SWAT, establishing attributive and spatial database, classifying different spatial scale simulation unit, constructing SWAT model and parameters suitable for Xiangxi Basin, and simulating non-point source nutrient load, the coefficients of Ens and R2are0.86and0.91.6) According to simulated results, analyzing the spatial and temporal distribution law of nitrogen and phosphorus load. In space, TN load intensity is cultivated land>rice land>orchard land>grassland>forest land, TP load intensity is orchard land>cultivated land> grassland>rice land> forest land, TN and TP load intensity is increasing with the decreasing of forest land, and Gaolan River>Gufu River> Nanyang River. In time, TN and TP load achieve the maximum (TN3475.96t and TP399.20t) in2007and the minimum (TN2036.72t and TP226.44t) in2005, the majority is distributed in flood season, its contribution ratio is84.1%and89.4%.7) TN、TP and soil erosion intensity have the similar spatial distribution, the relevance coefficients of TN and TP loss load and soil erosion are0.804and0.817, and precipitation runoff are0.537and0.471rspectively. The research results provide with theoretical basis for controlling non-point pollution and eutrophication, and realizing water ecosystem health and sustainable development.