| Pollution of water is a worldwide issue.Because of its randomness,hysteresis,uncertainty,and concealment,non-point source pollution has surpassed point source pollution as the leading cause of water pollution.Because of the high proportion of sloping agriculture in northern China,which is dominated by overhanging hills and rivers,surface runoff is easily formed,allowing nitrogen,phosphorus,and organic pesticides in the soil to infiltrate water bodies.As a result,understanding the mechanism of agricultural non-point source contamination is critical for the protection of the basin’s water ecological environment.The Xiaoweihe River Basin,a typical agricultural area in Baoji City,Shaanxi Province,was selected as the study site in this paper,and the mechanistic characteristics of agricultural nonpoint source pollution were thoroughly investigated and addressed.The basic data on the water environment,socio-economy,and agricultural production in the Xiaowei River Basin from 2016 to 2020 were obtained through field survey and sampling analysis.The SWAT(Soil and Water Assessment Tool)analysis model was used to investigate the temporal and spatial distribution characteristics of nitrogen and phosphorus pollution load,the transfer direction of different land use types,and the contribution rate of pollution load in the study region.Based on the BMPs(Best Management Practices)analysis model,different scenarios of nitrogen and phosphorus reduction measures were set up to propose the best management measures for surface source pollution control in the study area.The main findings and conclusions of the research are as follows:(1)The SWAT model of the Xiaowei River basin was constructed,and the study area was divided into 35 sub-basins and 920 hydrological response units.Fifteen parameters that have a significant impact on the model simulation process were selected,and sensitivity analysis was conducted using SWAT-CUP,and the highest sensitivity to runoff was found to be CN2 and the highest sensitivity to total nitrogen and phosphorus was found to be FERT.2011-2020 measured monthly runoff data and 2016-2020 measured water quality data from Henglin section of Xiaowei River were used to rate and validate the model.The results all satisfy R2> 0.6 and Ens > 0.5,which meet the requirements of model accuracy,indicating that the SWAT model has high applicability in the Xiao Wei River basin.(2)Analysis of land use changes in the study area in 2010 and 2020 shows that,in terms of area,arable land accounts for the largest proportion,averaging 1307.21 km ~2,accounting for 67.14% of the total area of the Xiaowei River basin;followed by forest land,accounting for 19.48%,with an average area of 379.21 km~2;in terms of the magnitude of change,urban and rural,industrial and mining,and residential land In terms of the magnitude of change,urban and rural,industrial and mining,and residential land has increased more obviously,with an annual change rate of 5.03%;arable land has increased the most,with an increase of 47.46 km2 and an annual change rate of 0.63%;grassland has decreased the most,with a decrease of 110.54 km2 and an annual change rate of-5.18%.The most drastic changes were in cropland and forest land,with annual rates of 0.74% and 1.03%,respectively.(3)According to the results of the SWAT model’s simulation of agricultural nonpoint source load from 2016 to 2020,the annual output of total nitrogen is larger than that of total phosphorus in terms of time,and the changes in monthly average precipitation,total nitrogen,and total phosphorus pollution load are essentially the same,generally increasing first and then decreasing.From a spatial standpoint,the distribution of total nitrogen and total phosphorus output load is relatively similar,primarily concentrated in 26,28,32,33,34,and35 sub-basins,generally showing the characteristics of higher in the south and lower in the north,which is primarily related to the north of the study area being dominated by forest land,uninhabited,and with high surface coverage,and the south being dominated by cultivated land,which is greatly affected by human activities.(4)Four different pollution source scenarios were created.Surface runoff losses of total nitrogen and total phosphorus accounted for 5.6% and 6.1% of total pollution,respectively.Agricultural pollution was the most responsible for total nitrogen and total phosphorus pollution,accounting for 46.02% and 48.34% of total pollution,respectively,followed by livestock and poultry pollution,accounting for 32.58% and 26.79% of total pollution,respectively,and domestic pollution accounting for 15.32% and 18.56% of total pollution,respectively..(5)Four individual measures were simulated according to the actual situation to evaluate the reduction effect of BMPs on pollutants.The results showed that the reduction of total nitrogen and phosphorus was 37.53% and 36.82%,respectively,with the best reduction effect;the reduction of total nitrogen and phosphorus was 9.38% and 20.18%,respectively,with the second best reduction effect;the reduction of total nitrogen and phosphorus was 18.67% and20.18%,respectively,with 20% reduction of fertilizer application;and the reduction of total nitrogen and phosphorus was 12.46% and 13.49%,respectively,with 10% reduction of fertilizer application.The total nitrogen and phosphorus were reduced by 12.46% and 13.49%,respectively.Since substantial fallowing will lead to lower crop yield,no-till cultivation measures are considered to be the most suitable management measures for the area considering the economic benefits.Meanwhile,non-point source pollution in the watershed should be prevented at source,pay attention to the loss of nitrogen and phosphorus during crop cultivation from June to August and from September to October,apply chemical fertilizers rationally,and strengthen the indiscriminate discharge of domestic sewage as well as livestock and poultry farming sewage to ensure the development of land use in a sustainable direction... |
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