Study On The Characteristics Of Non-point Source Pollution And Control Scheme In The Small Watershed Of Yingwugou In Danjiang River Basin

The prevention and control of non-point source pollution in the water source area of Danjiang River can effectively improve the utilization value of water resources,regional ecological environment function and water resources management in the middle reaches of the basin.It provides a reliable guarantee for the construction and comprehensive management of the water source area.It has gradually become a mainstream trend to carry out multi-factor comprehensive research on "small watershed" with relatively independent geographical unit of water cycle.Therefore,it is a great practical significance to study the characteristics of non-point source pollution in the Yingwugou small watershed of Danjiang River for the systematic treatment of water environmental pollution and water source protection.It lays a solid theoretical foundation for the perfect conclusion of the middle route of the national "South-to-North Water Diversion Project".This paper takes the Yingwugou small watershed of Danjiang River Basin as the research object.Based on the meteorological,hydrological and water quality data from 2017 to 2020,the response relationship between precipitation,runoff,sediment and pollution load is studied in five runoff plots and small watershed scales.The migration law and output mechanism of pollutants are revealed and the characteristics of non-point source pollution are summarized.SWAT model is established to analyze the spatiotemporal variation characteristics of sediment and various forms of pollution load and identify the key source areas of pollution.An optimal control scheme of non-point source pollution is proposed for the Yingwugou small watershed of Danjiang River Basin.According to the research,the conclusions are as follows:(1)Based on the daily precipitation data of 164 events in 2017 to 2020,the precipitation erosivity of each month in the year is estimated and the distribution characteristics of erosivity precipitation are analyzed.According to the analysis of precipitation in 8 fields,the runoff and sediment of each runoff plot increase synchronously with precipitation.The order of runoff and sediment per unit area of runoff plot is as follows: 9 plot > 11 plot > 13 plot > 20 plot > 19 plot.The relationship between precipitation and runoff(sediment yield)in different runoff plots is well fitted.The SCS-CN model has a good simulation effect on runoff plots and small watershed scales.The variation trend is basically the same and the error is within 25%.(2)Compared with the changes of nitrogen and phosphorus content in the soil before and after flood season,the loss degree under different land use is roughly the cultivated land >grassland > forest land.There is a positive correlation between the loss of nitrogen and phosphorus with precipitation and precipitation intensity.According to SPSS analysis,the correlation between runoff and various forms of nitrogen and phosphorus is obviously higher than other factors.The monthly variation of nitrogen is obvious,while the regularity of phosphorus is poor.The average concentration method is used to estimate the non-point source pollution load in different years.All kinds of non-point source pollution load accounts for more than 85% of the annual load.The proportion of non-point source pollution load of total nitrogen and total phosphorus is higher than that of sediment.(3)The SWAT model of Yingwugou small watershed in Danjiang River is constructed.The simulate area is divided into five sub-basins and 55 HRUs.Based on the measured hydrological and water quality data of the small watershed,the deterministic coefficient and Nash coefficient are selected as the evaluation indexes of calibration and verification.The results both meet the standard requirements.It is considered that the simulation results of the SWAT model for the Yingwugou small watershed are more reliable and reasonable.(4)SWAT simulation results show that runoff,sediment,nitrogen and phosphorus pollution loads are unevenly distributed throughout the year.It is mainly concentrate in June to September.According to the output of sediment and pollution load,the key source areas are identified by unit area load index method.The spatial distribution of organic nitrogen and organic phosphorus is similar to the distribution of key sources of sediment.The larger load output area is located in the intersection area of downstream and tributaries.The yield of unit area load of sediment,nitrogen and phosphorus pollution in different land use types is significantly different.The pollution degree of phosphorus is greater than that of nitrogen by means of equal standard evaluation.(5)A total of 7 scenarios are set up in the study.Different optimal control measures have great differences in reducing nitrogen and phosphorus non-point source pollution load,with a huge span from 5.18% to 45.78%.The reduction rate of the pollution load of the engineering measures is 7.14% to 45.77%.It is higher than the non-engineering measures(5.18% to28.69%).The measures with better control effect are height hedgerows in single measure and vegetation buffer zone + height hedgerows in combined measures.The comprehensive attribute value of height hedgerows is the highest based on the multi-attribute method of information entropy.The stubble cover is the most practical based on the cost-benefit evaluation.According to the comprehensive analysis,height hedgerows measures can be applied for key prevention and control in severely polluted areas,while stubble cover measures can be adopted for long-term and stable mitigation of non-point source pollution in areas with large areas and low pollution levels.