| More than 90% of the three gorges reservoir area is mountainous and hilly,more than 60% of the land is slope cultivated land,more than 70% of the soil is purple soil with poor corrosion resistance,and there is serious soil and water fertilizer loss.Agricultural non-point source pollutants are easy to migrate and transform with the rainfall process,and the runoff and sediment containing a large amount of N and P enter the three gorges reservoir,which has exceeded the absorption capacity of the reservoir itself,causing serious non-point source pollution,water quality decline and eutrophication of the three gorges reservoir.Since the impoundment operation of the three gorges reservoir in 2003,the problem of eutrophication has been prominent.The problem of eutrophication is prominent.A series of water environment effects caused by serious soil and water loss in the reservoir area have become the focus and focus of current soil erosion research.As a relatively independent catchment unit,small watershed is the basic unit of soil erosion occurrence and soil erosion control.It is the most effective technical way to prevent and control agricultural non-point source pollution in small watershed.Identification of sediment sources is a prerequisite for controlling sediment sources,soil erosion and non-point source pollution.This study selected the Wangjiagou catchment as the target.The catchment is naturally divided into two sub-basins(subcatchment A and B)by the landform.Soil samples from potential sediment sources in the Wangjiagou small catchment are collected,and suspended sediment samples at the outlet of the basin under typical extreme rainstorms in the reservoir area are collected.There are two subcatchments: 45 and 40 source samples were collected in subcatchments A and B,respectively,and 11 sediment samples were collected at the outlets of the two subcatchments from June to October.This research analyzed 48 geochemical elements,total phosphorus content and particle composition.By using the composite fingerprint identification technology,the best combination of fingerprint factors to distinguish sediment sources in the basin was selected,the relative contribution rate of each sediment source was calculated by using the multivariate mixing model,and the total phosphorus source of sediment in the basin was preliminarily estimated.The main results and conclusions of this paper are as follows:(1)In both subcatchments A and B,41 separate geochemical elements were available,and 7 elements(Ge,Hg,Mo,Pb,Sb,Se and Ho)were not detected in some source soils.Of these elements,all 41 elements passed the constraint that the mean sediment sample concentration was within the range of the source mean concentrations both in subcatchments A and B.In subcatchment A,four elements(Cd,Rb,Tl and Er)did not pass the second constraint,and in subcatchment B,10 elements(As,Ca,Cu,P,Sb,Tl,Ce,Er,Sm and Tm)did not pass the second constraint.Finally,in subcatchment A,27 properties returned a P value < 0.05 from the Kruskal-Wallis H test.In subcatchment B,among the elements that passed the threeconstraints,only 3 factors(Bi,Co,and Tb)returned P values > 0.05 in the Kruskal-Wallis H test.The optimum composite fingerprint consists of five individual properties(Zn,Ce,La,Li and Eu)in subcatchment A.The identification accuracy of single fingerprint factors was 60.0%,40.0%,57.8%,60.0% and 55.6%,respectively.these factors were followed by La(57.8%),Eu(55.6%)and Ce(40.0%).In subcatchment B,only 4 individual properties(Mn,La,Ni and Lu)were selected.The cumulative discriminant accuracy was 57.5%,77.5%,95.0% and 100.0%,respectively.The discriminant accuracy of single fingerprint factor was 57.5%,40%,47.5% and 32.5%,respectively.Both subcatchments have predictive powers of 100%.The correct rate of sediment source discrimination in the two sub-basins was >80%,which fully met the application requirements of composite fingerprint recognition technology.(2)The fingerprint method shows that the primary source in subcatchment A is slope land,which contributes 54.1%,with terraces with hedgerows and mulberry fields contributing 25.9% and 20.0%,respectively.Sediment contributions from slope land,terraces with hedgerow,mulberry field were 23.5-99.8%?0.0-85.8% and 0.0-66.9%,respectvly.In subcatchment B,mulberry fields contribute the most sediment at 38.0%,whereas slope land,terraces and terraces with hedgerows contribute 31.7%,22.0%,and 8.3%,respectively.Sediment contributions from slope land,terraces,terraces with hedgerow,mulberry field were 1.8-90.3%,0.0-34.6%,0.0-66.6% and 0.0-81.2%,respectvly.Fingerprint analysis of 11 rainfalls shows that the source of sediment varies at different times,mainly due to the changes in human activities and soil erosion,and these factors are uncontrollable.In subcatchment B,the advantages of reducing soil erosion are not fully reflected because.Comparing the two subcatchments,the greatest difference is that fewer mulberry fields contribute more sediment in subcatchment B.The periodic cultivation of silkworms is the main reason for the large amount of sediment contributed by mulberry fields.(3)Meanwhile,the temporal and spatial heterogeneity of soil erosion and the comprehensive influence of human activities in different growing seasons lead to different contribution rates of sediment in different periods.In the maize season,the yield of sand was mainly on the slope land.In the fallow season,local agricultural activities of farmers lead to the heterogeneity of erosion,such as straw mulching reduces sediment in sloping land,vegetable seedling breeding leads to significantly increased sediment in terraced fields,etc.,and the periodic sericulture in small watershed leads to the periodic change of mulberry sediment.Unreasonable human activities(i.e.,extensive channel excavation and highly intense machine cultivation)invalidate certain soil and water conservation measures.These reasons contribute to the large amount of sediment in the terraces.The contribution ratio of sediment total phosphorus in each erosion source area of Wangjiagou watershed was preliminarily estimated,and it was found that the contribution ratio of sediment total phosphorus was basically the same as that of sediment,except that the contribution ratio changed slightly.The contribution rate of sediment total phosphorus loss was the highest in slope land,followed by mulberry field and terrace land.Therefore,to control the sediment more reasonably and make the existing soil and water conservation measures play their due roles,farmers' production methods such as rational channel management and no tillage or less tillage should be further logically planned. |
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