Simulation Study On Migration Of Typical Herbicides In Sihe River Basin Based On SWAT Model

The Sihe river is an important source of water and sediment for the Nansihu Lake as main rivers into the lake.The ecological environment of the Sihe river watershed directly affects the ecological health of the lake,especially the aquatic environment.Since the policy of reform and opening up,the increase in the population,industry and agriculture in the watershed has led to the increasing demand for water resources.Furthermore,the application of pesticides and fertilizers has caused serious aquatic pollution in the watershed.The paper has studied the simulation of output from water yield,sediment yield,nitrogen,phosphorus and herbicide(atrazine and its transformed products)in Sihe watershed by SWAT model.The research builds the aquatic environment database through field survey,lab analysis and data statistics.Temporal and spatial changes of herbicide were performed under different scenarios,such as vegetated filter strip.At last,some suggestions have been proposed to control the output of nonpoint source pollutants.The main conclusions of this research are as follows:(1)The residues of atrazine and its metabolites DEA,DIA in the river water was obtained through field sampling and indoor laboratory analysis.Residue is concentrated in time from May to July.Among residual concentration,the DEA is the highest,ranging from 0 to 4.33?g/L,and then is DIA from 0 to 0.083?g/L.The atrazine concentration is the lowest,from 0 to 0.033?g/L.(2)Constructing the spatial database and the attribute database of nonpoint source pollutants through collect data,the river network was generated from the DEM and river data,and then the watershed were divided into 31 sub catchments and 781 hydrological response units.Then relevant measured data were used to calibrate and validate the simulation results of runoff and nonpoint source pollutants.R~2,NS and RE were 0.70,0.66,17%and 0.71,0.61,r1.5%for calibration and validation of runoff,respectively.R~2,NS and RE were 0.81,0.75,12.02%and 0.72,0.35,25%and 0.57,0.46,22%for later of atrazine,DEA and DIA,respectively.(3)Annual water yield in Sihe watershed is 245.074-316.857 mm,and which is higher in eastern and western area.Annual sediment yield is 0.053-5.892 t/ha,which is higher in southern area than northern area and higher in eastern area.Output of N and P in western and northeastern of the watershed are more serious,and annual N and P load is 0.242-5.95 kg/ha and 0.05-2.364 kg/ha,respectively.Water yield,sediment yield,N and P load were all associated with slope in Sihe watershed.Water yield had a negative correlation with slope,as well as sediment yield,N and P load correlated with slope positively.In addition,the spatial distribution of water yield,sediment yield,N and P loads were consistent with the temporal variation of precipitation,and there was a high positive correlation between precipitation and water yield,sediment yield,N and P loads.(4)There is a high correlation between the herbicides output and runoff output.The transport of herbicides from land to river is strongly related to meteorological and hydrological factors such as precipitation and runoff.The precipitation process is the main driver for the transport of herbicides in the foliage and soil,accompanied by the runoff process to the river and causing the accumulation of herbicides in the river.The output is affected by precipitation and application time,and the output time is concentrated mainly during July to August.The output accounts for more than 69%of the whole year.The output is affected by the length of the river channel and output of sediment.The output of atrazine is mainly located in upstream of eastern and central region,and output of catchment outlet is located in middle of stream.The spatial pattern of DEA and DIA output is similar,mainly located at the outlet of the downstream basin and in the central region.(5)Compared to the unset scenario,the output of herbicides is greatly reduced(DEA>Atrazine>DIA)under scenario of vegetated filter strip,and the output of herbicides during the wet season is reduced significantly.The output of atrazine was most obvious in the midstream area,the output of DEA was most obvious in the downstream area,and the output of DIA was most obvious in the midstream and downstream areas.In both scenarios,the output of herbicides from the main channel was greater than that of the tributary channel.Under unset scenarios,the output of herbicides in the middle-lower reaches is greater than upstream.Under scenario of vegetated filter strip,the total output of herbicides was mainly located in upstream and middle stream.The output of herbicides in the upper-middle and downstream reaches were significantly reduced.(6)In order to control the transport of herbicides and their metabolites in the Sihe watershed,it is necessary to take measures in the aspect of herbicide application,interception and herbicide degradation of the environment.For example,to protect the length of the river,build vegetated filter strip,reduce the application amount of pesticides and so on.