Study On Agricultural Non-point Source Pollution Characteristics Based On AnnAGNPS Model In Bahe River Of China

Bahe River is located in the middle of Xi'an,which originates from the Qinling Mountains.After the intersection with the Chanhe River,it flowed into the mainstream of the Weihe River.The water environment in the Bahe River Basin directly affects the quality of the Weihe River.The self-purification capacity of the water body in the Weihe River Basin is far lower than the non-point source pollutants it contains.The problem of water pollution is becoming more and more serious,and it has a greater impact on the economic development and social construction of Xi'an.Therefore,the treatment of non-point source pollution in the Bahe River Basin needs to be resolved.(1)This paper takes the Bahe River Basin as the research area and the Ba-Wei River estuary as the export of the basin.The runoff simulation accuracy of the AnnAGNPS model in the Bahe River Basin was verified by the 2010-2017abundance runoff daily data published by the Shaanxi Provincial Department of Water Resources.The Re of the runoff depth at the annual scale of the study area was between-7 and 15%,and both Ens and R~2 were greater than 0.90.On the monthly and daily scales,Re was stable within 10%,and the monthly scale was R~2 and the verification period.The Ens was greater than 0.85,and both R~2 and Ens were greater than 0.65 on the same daily scale.AnnAGNPS model has a good accuracy in the simulation of runoff in the Bahe River Basin.(2)The rain events from May to July in 2018 were used to determine and verify the simulation accuracy of the non-point source pollution load in the Weihe River Basin.Except for the rainfall on 20180704,the relative error Re between the measured date and the simulated value of total nitrogen was controlled within±14%.The relative error Re between the simulated and measured date of total phosphorus was maintained between 10%and 25%.The simulation effect of total nitrogen is better than that of total phosphorus.(3)The simulation results of Cell optimization showed that the CSA value of the Bahe River Basin was 4.00 km~2,the MSCL value was 400 m,the number of Cell was 873,and Reach was 350.The total nitrogen load per unit area of cultivated land decreased from 39.06 kg/ha to 36.76 kg/ha with the increase of CSA,and the total nitrogen load per unit area of construction land decreased from 24.49 kg/ha to 23.76 kg/ha.The change of CSA has little effect on the total phosphorus load per unit area.The Bahe River Basin changed significantly from1996 to 2016,and the water area increased from 17.47 km~2 to 68.10 km~2,and the runoff increased.(4)Different scenarios were set up in the study area to carry out simulation and analysis,and countermeasures and suggestions for non-point source pollution management are proposed.Under extreme meteorological conditions,the regional yield per unit area under the 5%guarantee rate>50%guarantee rate>95%guarantee rate,the trend of total nitrogen load and total phosphorus load per unit area is synchronized;the conversion of cultivated land in some sub-basins to construction land use(The upper reaches of the Bahe River and the Jingou River;the middle reaches of the Bahe River and the upper reaches of the Bahe River;the upper reaches of the Bahe River,the Jinggou River,the middle reaches of the Bahe River and the upper reaches of the Bahe River),total nitrogen load decreased by 3.48%,9.17%,and 12.65%,respectively,and total phosphorus load decreased by 12.01%,12.23%,and 24.24%,respectively;When the CN value was lowered by 10%,30%and 50%,total nitrogen load was reduced by 18.15%,26.89%and 38.4%,respectively.The total phosphorus load reduction rate was 10.09%,14.94%and 23.24%respectively.After the pollution load optimization,the reduction effect is good.