Simulation Study Of Non-point Source Pollution In The Liuyang River Basin Based On The SWAT Model

The Liuyang River is a first-class tributary of the Xiangjiang River,and its main flow area includes Liuyang City,Changsha City and some towns in Changsha County.It mainly supplies water for domestic,industrial and agricultural use in Changsha City and Liuyang City,and has a profound impact on regional economic development.While point source pollution has been controlled,surface source pollution has become an increasingly prominent problem and an important constraint on regional economic development.This study analyses the changes in land use in the watershed by interpreting remote sensing image data from the watershed in 2008 and 2018 in ENVI software to obtain land use data for these two periods and calculate the land use types for these two periods.By collecting soil,meteorological,hydrological and nutrient data,the SWAT model of the Liuyang River basin was established,and the model was applied to simulate and study the runoff and surface source pollution in the basin,assess the applicability of the model in the basin,and analyse the surface source pollution load and spatial distribution characteristics of the basin.Finally,the influence of slope and precipitation factors on the output of surface source ammonia nitrogen and total phosphorus was studied,and the main conclusions obtained were as follows.(1)Forest land and cropland are the main land types in the watershed,and the area of these two types of land remains stable;the annual rate of change of construction land is large;the inter-annual variation of land use in the watershed is flat;the greatest intensity of transfer is the conversion of construction land to cropland,and the increment of forest land is mainly converted from cropland and watershed.(2)The results of the runoff simulation were R2=0.90 and Ens=0.81,which showed a good agreement between the simulated and measured values and a high reliability of the parameters.The results of the validation period simulation were R2=0.87 and Ens=0.75,which were slightly better than the validation period,and the overall simulation results of runoff were better.The Ens in NH3-N and TP simulation results are over 0.67,the results meet the The accuracy requirements were met.The peak values in the simulated results are smaller than the measured values,which is due to the fact that during the peak period,a large flood occurred and the runoff carried various types of surface debris into the water body,which affected the simulation of runoff and various pollution factors.(3)The largest proportion of the contribution of both ammonia nitrogen and TP in the Liuyang River Basin is in Sub-basin No.17,the sub-basin controlled by the basin outlet control hydrological station(Hameli Station),and the largest pollution load of ammonia nitrogen in the Liuyang River Basin is in Sub-basin No.17,and the largest pollution load of TP in the basin is in Sub-basin No.20.Overall,the pollution loads and contribution rates are higher in sub-basins No.17 and No.20.Because these two sub-basins are located on both sides of the main river channel of the basin,with dense rural settlements,frequent agricultural activities and a high concentration of arable land,and the limited self-purification capacity of river waters,nutrients entering the river in the upper and middle reaches are not fully decomposed and collect in the lower reaches,resulting in higher pollution in sub-basins 17 and 20.(4)By analysing the effect of slope on the pollution load from surface sources,it can be seen that the increase in slope makes it easier for precipitation to form runoff,which strengthens the scouring effect of runoff and intensifies soil erosion,resulting in a higher surface source pollution load of total phosphorus.The correlation between total phosphorus load and precipitation is stronger,reaching 0.85,and the surface source pollution load increases with the increase of precipitation.(5)By analysing the overall process of surface source pollution,corresponding control measures are proposed in a targeted manner.In terms of source control,measures such as soil testing and fertiliser distribution methods and biological fertiliser substitution can be adopted to reduce nutrient production;in the process of transport,terraces can be developed in medium slope areas and high slope areas can be returned to forests;in terms of terminal treatment,artificial wetlands and protective forest belts can be established around farmland.