Study On The Migration Pattern Of Non-point Source Nitrogen And Phosphorus In The Yongjiang River Basin Under The Current Land Use Pattern

With the accelerating pace of agricultural science and modernization in China,agricultural land has been expanded in a disorderly manner,the natural system of the original watershed has been destroyed,and the nitrogen and phosphorus elements in the soil have migrated to the water bodies with precipitation and runoff,causing serious eutrophication of water bodies.The migration process of nitrogen and phosphorus elements in the river basin and the land on both sides can be divided into two categories:point source migration and non-point source migration.In this paper,based on the land use of the Yongjiang River basin under the Geographic Information System(GIS)platform,the SWAT(Soil and Water Assessment Tool)model was used to study the basic rules of non-point source nitrogen and phosphorus transport processes in the Yongjiang River basin,and the non-point source(NPS)pollution load was analyzed.The spatial and temporal distribution of non-point source(NPS)pollution load and its main influencing factors were analyzed,and the migration process of nitrogen and phosphorus in river water bodies in the basin was also analyzed.The main research contents and results are as follows:Chapter 1 discussed the current situation of eutrophication of domestic water bodies,introduced that non-point source pollution of nitrogen and phosphorus was the main cause of eutrophication of water bodies,analyzed the main sources,pollution characteristics and occurrence mechanisms of non-point source pollution of nitrogen and phosphorus in domestic and foreign river basins,and summarized the migration laws of non-point source pollution of nitrogen and phosphorus in river basins with respect to land use.The method of applying the SWAT model to study the migration laws of non-point source pollution of nitrogen and phosphorus in the Yongjiang River Basin was proposed,and the research contents and technical route of this article were described.Chapter.2 analyzed the geographical location,natural environment,social environment,water environment and sources of non-point source pollution in the Yongjiang River Basin through data collection and survey statistics,and constructed an attribute database of non-point source pollution in the research area of the Yongjiang River Basin.Corresponding trend analysis and reasonable evaluation of future expectations were carried out,providing a data basis for the construction of the SWAT model.Chapter 3,supported by the GIS platform,established the model data basis of the Yongjiang River Basin digital elevation model(DEM),soil use data,soil database,meteorological data,hydrological and water quality data,etc.,and calibrated and verified the runoff and water quality of the Yongjiang River Basin.Finally,a SWAT model for non-point source pollution of nitrogen and phosphorus in the Yongjiang River Basin was constructed.Chapter 4 analyzed the spatial and temporal distribution characteristics of nitrogen and phosphorus nonpoint source pollution in the Yongjiang River basin through SWAT model simulations,and discussed the nonpoint source pollution loads generated by different land uses and their influencing factors.(1)The temporal distribution characteristics showed that the interannual variation of nonpoint source pollution load was more consistent with the trend of rainfall,and rainfall and sediment,total nitrogen and total phosphorus all showed a certain degree of positive correlation;in terms of intraannual variation,rainfall,sediment loss,total nitrogen and total phosphorus load all reach their highest values in summer,while in winter,these indicators all reached their lowest values.This was due to the fact that the Yongjiang River basin belonged to the subtropical monsoon climate zone,with more rain in summer and less rain in winter,while there was a significant rise and fall in the downstream section of the river due to the tidal influence.(2)The spatial distribution characteristics showed that the annual average total nitrogen and total phosphorus loadings in the Yongjiang River basin from 2010 to 2021 were mainly concentrated in the middle and lower reaches of the Yao River and the upper reaches of the Yongjiang River,where the highest pollution basins were:sub-basins 4,28,34,53 and 74,all of which have TN loadings exceeding 800 tons/year,TP loadings exceeding 80 tons/year,and sediment output exceeding 500,000 tons/year.These areas had a concentrated population and extensive arable land,and the fertilization of the land provided the main material source of nitrogen and phosphorus.(3)By taking 855 HRUs in the Yongjiang River basin as evaluation units,key nutrient loss areas were identified based on the intensity of nitrogen and phosphorus pollution load from non-point sources in each HRU.86 HRUs were identified as key nitrogen and phosphorus nutrient loss areas in the Yongjiang River basin,and these loss areas were the key areas for surface source pollution control in the Yongjiang River basin.(4)Based on the hydrological response unit(HRU)output of the SWAT model simulation under the current land use pattern,the effects of different slopes and soil types in land use on the non-point source pollution load were summarized.Chapter 5 simulated the migration process of nitrogen and phosphorus nutrients in the river network system of the Yongjiang River Basin based on the calibrated SWAT model and the current land use scenario landscape pattern,and quantitatively studies’the migration process of nitrogen and phosphorus pollutants in the Yongjiang River Basin.(1)The TP and TN concentrations in each section of the Yongjiang River Basin showed a right-skewed distribution.Nitrogen nutrients in each section of the river were mainly in the form of nitrate and organic nitrogen,and the concentration of ammonia nitrogen were relatively low.Organic phosphorus accounts for a relatively high proportion of phosphorus nutrients.(2)The sediment concentration and sediment loss contributed by non-point sources to the river in the Yongjiang River Basin were positively correlated and significantly correlated,indicating that the sediment concentration of the water body was greatly affected by non-point source soil erosion.The total nitrogen and total phosphorus concentrations of the mainstream water body were positively correlated with the non-point source input nitrogen and phosphorus pollution load of the entire basin.This was because nitrogen,phosphorus,and other nutrients accumulated in the soil and on the ground were washed into the water body due to rainfall,irrigation,and other factors,resulting in their widespread distribution and being greatly affected by natural conditions,making them difficult to control.(3)The river sections of each sub-basin in the Yongjiang River Basin had a strong retention effect on sediment,TN,and TP.Chapter 6 summarized the main content of this article.This study used the SWAT model and measured data to simulate and study the output law of non-point source pollution loads and the migration law in the river system under the current land use pattern.By identifying the spatiotemporal distribution characteristics and key contribution source areas of non-point source nitrogen and phosphorus nutrients pollution in the Yongjiang River Basin,the impact of land use change on the output load of nitrogen and phosphorus nutrients was quantitatively revealed,which provided theoretical support for water environment pollution control and land planning management in the Yongjiang River Basin,and provided scientific reference for water environment problem control in other river basins.