| Non-point source pollution is the main source of regional water pollution and has a greater impact on the quality of regional water environment.Quantitative research on non-point source pollution load is of great significance to the improvement of regional water environment quality.Caohai is the largest plateau natural freshwater lake in Guizhou.In recent years,due to the intensification of human activities,a large amount of pollutants such as nitrogen and phosphorus have entered the lake,which has caused eutrophication in the upper reaches of Caohai.The estimation of agricultural non-point source nitrogen and phosphorus pollution load in the Caohai Watershed is necessary for pollution identification and non-point source control.This paper takes the Caohai Watershed as the research area and obtains 806valid questionnaires.Investigate and analyze the natural environment and farmers’breeding conditions in the Caohai Watershed.The nitrogen and phosphorus of the five main rivers entering the lake are monitored through fixed-point monitoring methods.Use the pollution discharge coefficient method and the equivalent pollution load method to analyze the characteristics of pollution discharge and identify the source of pollution.The SWAT model is used to estimate the non-point source nitrogen and phosphorus pollution load of the Caohai Watershed for the ecological environment protection and restoration of the Caohai Nature Reserve.The main research results are as follows:(1)Investigation and analysis results show that 83.87%of farmers plant corn with an average planting area of 2.37 mu/household.84.12%of farmers plant potatoes with an average planting area of 2.17 mu/household.85.86%of farmers plant vegetables with an average planting area of 3.17 mu/household.Urea,compound fertilizer and organic fertilizer are main fertilization types in Caohai Basin.The annual scalar amounts of nitrogen fertilizer and phosphorus in the basin are 2058.29 and 335.21 t/a。(2)The results of the farmer’s breeding survey showed the main livestock raised are chickens,ducks,geese,pigs,sheep in the Caohai Watershed.According to estimates,108202 chickens,8492 ducks,4254 geese,8185 pigs,3526 cows,and9528 sheep are raised in the basin each year.Dongshan Village,Tianlong Village,and Zhengjiaying Village have a higher amount of livestock and poultry breeding.Haibian and Yazitang communities have a lower amount of livestock and poultry breeding.The distribution of aquaculture is high in the west and low in the east and north.(3)The pollution discharge coefficient calculated that the total annual discharge of total nitrogen and total phosphorus in the Caohai Watershed are 698.49 t and67.69 t,respectively.Among the three major pollution sources,domestic sewage discharges the highest.The total nitrogen emissions from domestic sewage,livestock and poultry breeding,and chemical fertilizer losses were 312.6,179.24,and 206.65t/a,respectively.While the total phosphorus emissions were 24.35,23.06,and 20.28t/a,respectively.The equivalent standard load analysis shows that the standard load of domestic sewage,livestock breeding,and fertilizer loss are 2.781×10~9,2.049×10~9,2.047×10~9 m~3/a respectively.The equivalent standard load ratio of domestic sewage is the largest(40.43%),which is the main source of pollution the Caohai Watershed;followed by livestock and poultry breeding and fertilizer loss,with equivalent standard load ratios of 29.80%and 29.77%respectively.(4)Based on the analysis of land use patterns of typical rivers in the basin,Wanxia River,Lujia River,and Wangjia River are representative rivers in typical farmland planting areas,Dazhong River is a typical residential river,and Baima River is a typical river affected by domestic sewage pollution sources.The results of river nitrogen and phosphorus monitoring and analysis showed that total nitrogen and total phosphorus emissions were 163.25 and 6.82 t respectively within the study time limit.Among them,Dazhong River had the highest pollution emission,with total nitrogen and total phosphorus emissions being 67.56 and 2.81 t respectively,accounting for 41.39%and 41.28%of the total river emissions.(5)The spatial and temporal distribution characteristics of rivers showed that the highest output of nitrogen and total phosphorus was 68.63 and 2.38 t in September.The output of nitrogen and phosphorus in rivers showed different characteristics during the wet and dry periods.The output in wet season was larger than that in dry season.The output of total nitrogen,ammonium nitrogen,nitrate nitrogen and total phosphorus of the representative River in the farmland intensive area in the wet season was higher than that in the dry season.Rainfall affects the nitrogen and phosphorus content of the river,and the nitrate nitrogen content of the five rivers all increase after the rain high.(6)Construct a non-point source pollution model database in the Caohai Watershed,and divide the Caohai Watershed into 32 sub-basins.The SWAT model simulation shows that the annual total nitrogen and total phosphorus loads in the Caohai Watershed are 309.05 and 7.59 t,respectively.The 16th sub-basin has the largest total nitrogen and total phosphorus output load,and the 9th sub-basin has no nitrogen and phosphorus output.The spatial and temporal distribution results show that the output of nitrogen and phosphorus in the wet season is higher than that in the dry season,and the source of total nitrogen and total phosphorus in the eastern sub-watershed is higher than that in the west. |
