Characteristics And Control Scheme Of Non-Point Source Pollution In The Hanjiang River Basin

The water quality of the Hanjiang River directly affects the residents’ life and the ecological environment in the water source area of the middle route of the South-to-North Water Diversion Project.It is the ultimate goal of regional ecological environment improvement to study the complex non-point source(NPS)pollution in water environment.In this paper,the characteristics of rainfall,runoff and NPS pollution are studied from the three scales of "runoff plots-small watershed-watershed".The characteristics of nitrogen and phosphorus loss of 7 runoff plots under different underlying surface conditions are discussed.The variation characteristics and loss fluxes of NPS pollution in six typical floods and flood season(May to October)in small watershed are defined.The SWAT model is established for the Hanjiang River Basin above Ankang section to study the temporal and spatial distribution characteristics of NPS pollution,eveluate the reduction effects and comprehensive benefits of best management practices(BMPs)in the critical source areas(CSA),and propose the control sheme of NPS pollution.The main conclusions are as follows:(1)According to the monitoring analysis of rainfall,runoff,sediment,nitrogen and phosphorus loss,it shows that the correlation between rainfall,runoff,and sediment is obvious.The variation trends of runoff and sediment yield in all runoff plots are basically the same,which increase first and then trend to be stable with the increase of rainfall duration.The No.6(pepper monoculture)plot has the largest runoff and sediment yield,indicating that the main factor affecting the process of runoff and sediment yield is vegetation coverage.The concentrations of total nitrogen(TN)and nitrate nitrogen(NO3-N)in each plot show a certain difference,while the concentration of NH4-N decreases with the increase of rainfall duration.The concentration of phosphorus shows a certain difference,while the variation trends of total phosphorus(TP)and dissolved phosphorus(SRP)are basically the same.(2)There is a certain correlation between rainfall,runoff and pollutant loss in small watershed.The correlation coefficient(R2)of rainfall and runoff in typical floods is above 0.9,and that in flood season is 0.78.The correlation coefficient R2 of nitrogen and phosphorus concentration with runoff is 0.66 and 0.57,respectively.The results of pollutant loss flux showed that the main nitrogen loss is NO3-N,which accounted for about 50%of TN load.The loss of phosphorus is mainly SRP,accounting for about 75%of the TP load.(3)A SWAT model is established to analyze the spatial and temporal distribution characteristics of NPS pollution and identify the CSA in the study area.In terms of temporal distribution,the load in the foold season is significantly higher than that in non-liood season.In terms of spatial distribution,the runoff is corrected with the loss intensity of nitrogen and phosphorus load in each sub-basin,and the loss intensities of organophosphorus and particulate phosphorus load are significantly correlated with the sediment loss intensity.It is finally determined that the CSA of the basin are the 6,15,16,19,21,26,27 and 28 sub-basins(i.e.,Nanzheng County,Hantai District,Ningqiang County,Zhenba County,Hanyin County and Chenggu County).The total area of the 8 sub-basins accounts for 27.76%of the whole basin,and the TN load and TP load accounted for 48.62%and 53.22%in the study basin,respectively.(4)Based on the analysis of three spatial scales of NPS pollution yield intensity,it is found that the TN and TP loss intensity of runoff plot and small watershed are 0.120 kg/ha and 0.014 kg/ha,0.158kg/ha and 0.015kg/ha respectively under different rainfall events in flood season,and there is no significant difference.In the same period,the TN and TP loss intensity of small watershed and watershed are 1.835 kg/ha and 0.153 kg/ha,0.109 kg/ha and 0.006 kg/ha.The pollution loss intensity of small watershed was larger than that of watershed.The results show that the NPS pollution yield intensity decreases with the increase of the scale of the study area.(5)Among individual BMPs,the reduction rates of ’stubble mulch,grassed waterway and returning farmland to forest land’are relatively high.The best NPS pollution reduction effect of combined BMPs is ’stubble coverage+vegetation buffer zone+returning farmland to forest land’.The evaluation results of multi-attribute decision-making method show that the the comprehensive attribute value(Z value)of ’stubble coverage’ in individual BMPs is higher,which can be generalized in a large-scale.The comprehensive benefit attribute of combined BMPs is relatively good,and the comprehensive attribute value is about 0.8.In the CSA,the NPS pollution control schemes that can be adopted are that in Nanzheng County,Chenggu County and Hantai District,we can take the measures of ’stubble coverage+grassed waterway+returning farmland to forest land’.In Yangxian County,Ningqiang County,Mianxian County,Zhenba County,Xixiang County,Hanyin County,Hanbin District,Shiquan County and Ziyang County,the measures of ’stubble coverage’ and ’grassed waterway’ can be adopted.