Simiation Study Of Non-Point Source Pollution In Fenhe River Basin Under Changing Environmrnt Based On MIKE SHE Model

The China Ecological Bulletin states that the combined share of emissions from agricultural and domestic sources in China exceeds 95%,and that both sources are dominated by non-point sources.Non-point source pollution as the main source of surface water quality in the Fenhe River,the scale of management and the degree of harm has become a key factor in hindering the improvement of water quality standards.In recent years,with the intensification of human activities,the water quality of the Fenhe River has been further deteriorated,seriously threatening human water security and ecological environment.In the context of global warming,future climate change will also significantly change the spatial and temporal distribution of water resources,which in turn will drive changes in non-point source pollution in the river basin.Therefore,the simulation of non-point source pollution in the Fenhe River Basin and exploration of the impacts of human activities and climate change can provide a scientific and reliable basis for the fine-grained prevention and control of non-point source pollution and water resources protection in the Fenhe River Basin under the changing environment.Based on the analysis of water resources and water environment in Fenhe River Basin,this study applied MIKE SHE and MIKE 11 to establish a coupled hydrodynamic-water quality model to reveal the characteristics and patterns of non-point source pollution in Fenhe River Basin under changing environment by predicting land use scenarios and climate change scenarios in the future period.The main findings are as follows:（1）By analyzing the characteristics of rainfall runoff trends in the study area,it was found that the rainfall series of Taiyuan,Jiexiu and Yuncheng meteorological stations in the basin showed an overall decreasing trend,and the main cycles of the three stations were 4a,33a and 8a,respectively.The runoff series of Lancun and Yitang hydrological stations showed an increasing trend,while Hejin hydrological station showed an overall decreasing trend.There is a 7a main cycle in the runoff series of Lancun and a 6a main cycle in the runoff series of Hejin.Yitang station did not show any obvious cycle change pattern.The Hurst index of rainfall and runoff series is greater than 0.5,which indicates that the existing trend of rainfall runoff will continue.The pollution source analysis shows that the point source contribution is mainly from sewage treatment plants;the COD non-point source load contribution is mainly from rural residents’ living,and the NH₃-N non-point source load contribution is mainly from livestock and poultry breeding.（2）The results of the MIKE SHE and MIKE 11 coupled models show that the NSE of runoff rates at Lancun,Yitang,and Hejin stations are above 0.70 for both regular and validation periods,R² reaches above 0.8,and P_bias is located below 25.00%.The regular NSE of COD and NH₃-N rate at Shanglan,Wangzhuangqiaonan and Hejin cross-sections are all located above 0.60,R² is above 0.75 and P_bias is below 20%,while the validation period NSE is above 0.65,R² is above 0.85 and P_bias is below 30%;the coupled model of MIKE SHE and MIKE 11 has high accuracy and can better simulate MIKE SHE and MIKE 11 coupled models have high accuracy and can better simulate the hydrodynamic water quality process in Fenhe River basin.The rainfall in the Fenhe River basin during the abundant period accounts for more than 70%of the annual rainfall,the runoff during the abundant period accounts for more than 60%of the annual runoff,the rainfall and runoff during the dry period both account for less than 12%of the annual,and the peak rainfall and runoff can correspond well,showing an obvious correlation.The pollutants COD and NH₃-N in the basin show a fluctuation pattern similar to the trend of rainfall and runoff,and the average percentages of COD and NH₃-N in the abundant water period are 77.87%and 71.51%.（3）The ranking of the area occupied by land use types in different periods is:Farmland>Forestland>Grassland>Aritificial surface>Water bodies>Wetland>Shrubs>Bare ground.In terms of area change,from 2000 to 2020,the area of farmland showed a continuous decreasing trend,the area of aritificial surface showed a continuous increasing trend,and the area of forestland and grassland showed an alternating increase and decrease;the land use quantity changed more strongly from 2010 to 2020.In terms of spatial distribution,the center of gravity of farmland and grassland moves to the northwest,forestland to the northeast,and aritificial surface to the southwest.land use changes in the watershed in 2030,2040,and 2050 indicate that the proportion of farmland and forestland area shows a decreasing trend,grassland area increases compared to 2020,and aritificial surface area is the land use type with the largest increase.Runoff changes in the future period decrease with the increase in the area of farmland,forestland and grassland,and increase with the increase in the area of aritificial surface.COD and NH₃-N concentrations in the future period showed a trend of flattening in the abundant water period and changing more in the flat water period with the change of land use structure;the COD and NH₃-N non-point source loads decreased and then increased in different future periods,and the aritificial surface had the greatest impact on non-point sources,followed by farmland and the least by grassland.（4）The monthly average rainfall in the future period（2021-2050）under the SSP1-2.6,SSP2-4.5 and SSP5-8.5 scenarios shows a trend that the abundance period is lower than the base period（1991-2020）,and the flat and dry periods are higher than the base period,and the monthly average potential evapotranspiration is lower than the base period in both the abundance and flat periods.In terms of distribution,rainfall and potential evapotranspiration are more dispersed in the abundant period and more concentrated in the dry period.From the perspective of outlier,the SSP5-8.5 scenario has the most extreme rainfall events,and the SSP2-4.5 scenario has the least extreme rainfall events.The overall variation of monthly runoff under the SSP5-8.5 scenario is the largest,followed by the SSP1-2.6 scenario,and the monthly runoff under the SSP2-4.5 scenario is more moderate due to the dual effect of land use and climate change.From the scenarios,the magnitude of COD concentration changes in the watershed under the dual effects are,SSP5-8.5>SSP2-4.5>SSP1-2.6;NH₃-N concentration changes are,SSP5-8.5>SSP1-2.6>SSP2-4.5;from the period,both COD and NH₃-N concentrations change more in 2050s,showing an overall increasing trend,and the changes were not obvious in 2040s and 2030s.The highest COD load in the future period appeared in the 2050s period under the SSP5-8.5 scenario,reaching 25,164.56 tons,an increase of 25.04%relative to the base period,and the highest NH₃-N load appeared in the 2040s period under the SSP5-8.5 scenario,reaching 391.70 tons,an increase of 36.32%relative to the base period.