Study On Non-point Source Pollution Simulation And BMPs Evaluation In Chenghai Basin Based On SWAT Model

In this study,SWAT model was established based on the full investigation of the natural and economic conditions of Chenghai basin,and the spatial-temporal distribution of non-point source pollution was analyzed.A typical sub-basin was selected to evaluate the reduction efficiency of different BMPs on nitrogen and phosphorus loads based on SWAT model,and the cost-benefit analysis was conducted.The main conclusions were as follows:(1)The river network was extracted based on the DEM,and the Chenghai basin was divided into 250 sub-basins.Based on the land use,soil data and slope,the basin was further divided into 1,727 hydrological response units(HRU).With the help of SWAT-CUP software,20 sensitive parameters were screened out and calibrated.The results showed that the simulated values were highly consistent with the measured ones,and the SWAT model could be well applied to the Chenghai lake basin.(2)The year 2007(839.1mm),2013(747mm)and 2006(683.7mm)were identified as wet,normal and dry year,respectively.The nitrogen(N)and phosphorus(P)loads were highest in wet year,second in normal year and lowest in dry year.N and P loss mainly occurred in summer(June to September),and Pearson correlation analysis indicated that rainfall was the main driving force of non-point source pollution.Nitrate nitrogen was the main form of N loss,while inorganic phosphorus for P loss.The N load in the west of Chenghai basin was significantly higher than that in the east.The N loss in the west was mainly in the form of nitrate nitrogen,while it was organic nitrogen in the east especially the northeast.The P loss in 1,3 and 7 subbasins were much larger than the others,and mainly in the form of particle inorganic phosphorus.The form of P loss may be related to the spatial scale of sub-basins.(3)In Guandi river watershed,the high spatial resolution land use map capable of distinguishing specific crop types was prepared using unmanned aerial vehicle(UAV)photography technology.Forestland,woodland and cultivated land were the main land use types,accounting for 52.52%,15.01% and 10.6% respectively.The main crops in cultivated land were corn,rice and ginger,with an area of 108.76 ha,12.85 ha and 5.2ha respectively.The precise spatial data of land use could make the simulation process closer to reality and improve the reliability of BMPs evaluation results.(4)On watershed scale,the reduction efficiency of N and P load ranked as follows: Buffer strip(BS)> Returning farmland with slopes above 15° to orchards(RF1)> Replanting under canopy(RF2)> Fertilization reduction(FR).The average reduction efficiency of BS was about 35%,and about 30% for RF1,but the reduction efficiency of RF2 and FR were less than 5%.The reduction effect of combined BMPs was better than that of single BMP.In combination of BS5+RF1+FR50,the reduction efficiency was close to 60%,while in combination of BS5+RF1,the reduction efficiency was about 55%.All of the BMPs combinations containing BS had high reduction efficiency.(5)The BS and RF1 had the lower CE value and higher CB value,which represented high return and good reduction effect.The cost of RF2 was high,the ability of controlling non-point source pollution was limited.Although the cost of fertilizer reduction was low,the reduction efficiency was the lowest.And the CB value was also low,which meant farmers may be less willing to cooperate.