Research On Non-point Source Pollution Source Analysis And Self-purification Capacity Of Watershed Based On SWAT

The Yiluo River Basin plays a crucial role in the sustainable utilization of water resources and the protection of ecological environment in the downstream of the Yellow River.Strengthening research on non-point source pollution in the Iluo River Basin is of great significance to the ecological protection of the downstream of the Yellow River.Taking the Iluo River Basin as an example,this paper reveals the sources and spatial distribution of nitrogen pollution in the study area based on the simulation and estimation of non-point source pollution,as well as the self-purification capacity and variation of nutrients in major rivers of the basin.By using the SWAT model and multiple scenarios,this study conducts quantitative analysis and identification of nitrogen pollution sources in the study area,and analyzes the impacts of different pollution sources and land use types on nitrogen pollution and its variation reasons.Based on the SWAT model and different self-purification capacity indicators,this study analyzes the self-purification capacity and its variation of nutrient pollution from non-point sources in major river sections of the study area,and systematically explores the physical meanings,influencing factors,and application ranges of each indicator.This study provides an important reference for the control of non-point source pollution in the hilly regions of central China.Research results show that according to the analysis of pollution source types,TN non-point source pollution in the Yiluo River Basin is mainly caused by nitrogen fertilizer application,atmospheric deposition,and soil nitrogen storage,while pollution from livestock and poultry breeding and rural living is relatively low.From the perspective of unit area TN input to the river from different land use types,the Yiluo River Basin is characterized as dry land > orchard > residential area > paddy field > forest land.In terms of time,the key period for controlling soil nitrogen storage and atmospheric deposition is the rainy season,and the TN input into the river caused by these two major sources accounts for 74% and77% of the total annual input into the river during the rainy season(June to August).For controlling pollution caused by rural living,livestock and poultry breeding,and fertilizer application,the key is to control the pollution discharge period.From the perspective of land use,nitrogen fertilizer application and domestic sewage discharge are the key pollution sources for orchard and residential areas,while the three major pollution sources for paddy fields and dry lands are nitrogen fertilizer application,atmospheric deposition,and soil nitrogen storage.From 2018 to 2021,the TN self-purification capacity of each river in the Yiluo River Basin was between 962.58 and 1186.23 tons/year,with a self-purification rate of 38.68% to 46.35%.Among them,the TN self-purification capacity of the Yiluo River accounted for more than 50% of the entire water system,but the self-purification rate was lower than that of the Yiluo River.The TN self-purification capacity was highest during the rainy season and lowest during the dry season.The self-purification capacity of each self-purification index showed different characteristics.Among them,the self-purification indexes RL and RA were positively correlated with the river morphological characteristics parameters and pollutant load,while the self-purification capacity of each river in the Yiluo River Basin was between 962.58 and 1186.23 tons/year,with a selfpurification rate of 38.68% to 46.35%.Among them,the TN self-purification capacity of the Yiluo River accounted for more than 50% of the entire water system,but the self-purification rate was lower than that of the Yiluo River.The TN self-purification capacity was highest during the rainy season and lowest during the dry season.The self-purification capacity of each self-purification index showed different characteristics.Among them,the self-purification indexes RL and RA were positively correlated with the river morphological characteristics parameters and pollutant load,while the self-purification indexes Kx,SL,Re,RLw,and Kt showed the opposite trend.In addition,the self-purification index Vf was only positively correlated with water temperature and was not related to other factors.Compared with other self-purification indexes,the self-purification indexes Vf and RA are suitable for spatial scale comparisons,while the other indexes are suitable for time scale comparisons.For differences in spatial comparisons,the self-purification rate Vf and self-purification RA should be used,and the remaining six indexes should be used for comparing the same river at different times.The self-processing length SL can be used to reflect the influence of hydrological conditions on the self-processing process,as it is sensitive to changes in hydrological conditions.Different water quality prediction results under different scenarios show that if the flow is artificially reduced by 30% to below the current TN input into and out of the Yiluo River,the TN concentration at the outlet of the basin will be reduced by 30%.