| Soil erosion is one of the major environmental problems globally.Sediment source study,as one of the key contents of soil erosion research,was significant for the implementation of soil erosion control measures.This study traced sediment source in a small watershed with multi-level stone check dam in the black soil area of Northeast China used composite fingerprinting approach.Aims of this study were to quantify sediment source contribution of sediment in each-level of stone check dam;to analyze the influence of particle size ranges on the accuracy of sediment source quantification based on reasonable source classification of sediment sources;to discuss the influence of deposited sediment in upstream stone check dam to sediment deposition in downstream stone check dam;and to assess the feasibility of the composite fingerprinting approach in sediment source study of multi-levels of stone check dam.The main conclusions were as follows:(1)The enrichment of fingerprints could be divided into four groups for both black soil of source areas and sediment in stone check dam,including enrich in very fine particles(clay and fine silt),fine particles,coarse particles and very coarse particles.According to the content variation of fingerprints in different particle size ranges campaed to bulk samples(<1000 μm)for both black soil from source areas and sediments,its obvius that most of the fingerprints(>75.6%)were enriched in very fine particles(<10 μm),and depleted in other particle size ranges(10-500 μm)at the same time.A few fingerprints were enriched in fine particles(Sr,Na and K)(10-63 and 63-125 μm)or coarse particles(Co,Mn and Pb)(250-500 μm),while Ba were enriched in very coarse particles(63-125 and125-250 μm).For the black soil and sediment in the source areas of stone check dam at all levels,the enrichment degree of most fingerprints(67.6%)in each particle size range remained unchanged,i.e.,they were mainly enriched in the range of <10 μm particle size,and a few(32.4%)fingerprints has a grade change in the degree of enrichment in each particle size range.Contents of most fingerprints were changed in coarse particles,while they keep consistent in relative fine particles(<10 and 10-63 μm)for fingerprints between source areas and sediment at all levels of stone check dam.It is obvious that relative fine particles(<63 μm)may be better for screening of optimal fingerprints group for black soil,and due to their conservative behavior in relative fine particles.This provided a basic foundation for the quantification of sediment sources using the composite fingerprinting approach.(2)Contribution of slope to the sediment deposition in the stone check dam gradually decreases from upstream to downstream areas,while the contribution of the gully gradually increases.The most reliable sediment contribution was estimated based on fingerprints screeded in the <10 μm particles.There were optimal fingerprinting factor groups with good source classification results(cumulative correct discrimination rate >89.3%,average GOF>90%)using the particles <10,<63 and <1000 μm as the fingerprint factor carriers when classifed sediment source areas into slope,gully and sediment of upstream stone check dam.However,contribution of sediment sources to depostited sediment in each level of stone check dam was greatly changed when using optimal fingerprints groups from different pariticle ranges.The sediment deposited at all levels of check dam based on the optimal fingerprinting factor group of <10 μm particles mainly comes from the slope(The average contribution of the first,second and third grade check dam slopes are 83.3%,52.1% and 40.0% respectively).The contribution of each source area estimated according to the optimal fingerprinting factor group of <63 μm particles to the sediment contribution ratio at all levels of check dam was basically similar.The sediment deposited at all levels of check dam based on the optimal fingerprinting factor group of <1000 μm particles mainly comes from the gully(88.9% on average).Sediment source discrimination obtained based on particles of <10 μm was more reliable considering the distribution characteristics of fingerprints in different particle size ranges.Sediment source determined based on particles of <10 μm showed that the average contribution of slope decreased from 83.3% to 50.1%and to 40.0% for sediment depoditied in the first,second and third grade check dam from upstream to downstream areas,while the average contribution of gully increased from16.7% to 18.4% and to 30.9%.The average contribution of the sediment of the first and second grade check dam to the sediment deposited in the second and third grade check dams were similar(averaged 30.4%).(3)The optimal scheme of composite fingerprinting for the estimation of sedment sources of multi-level stone check dams is affected by three aspects as follows: First,it is more reasonable to choose the sediment deposited in the upstream check dam as the sediment source of the downstream check dam based on theaccuracy of source classification.Second,there are more fingerprints could identify sediment sources,more fingerprints with conservative behavious,and the optimal fingerprtints group had better cumulative sediment source correct classification when particle grain size was fine and narrow.Finally,from the error of source contribution estimation,the uncertainty of sediment contribution was smaller when using optimal fingerprints selected from particles of <10 μm(GOF=0.95,MAE=12.2%).Thus,it is obvious that fingerprints selected from fine particles with narrow grain size range can reduce the uncertainty of sediment contribution estimation.Results of this study were obtained based on a case study in the Xinmin small watershedas.The simple erosion environment,small stone check dams close to each other,and weak sediment sorting effect resulted in weak ‘deposit coarse sediment and release fine sediment’ effect.Thus,the composite fingerprinting approach could discrinimant sediment source of multi-levels of check dam.However,for watershed controlled by multi-level check dam with complex enviromenatal condition,it is significant to consider the effect of ‘deposit coarse sediment and release fine sediment’ effect in sediment source study. |
