| Loess Plateau is known for its extreme soil erosion because the wide distribution of gullies.There has been a lot of research about the sediment yield of gully and slope erosion on the Loess Plateau.Early studies on the Loess Plateau usually inclined to focus on the slope erosion,and few studies directly reveal the impacts of vegetation restoration on change of sediment sources in the small catchment of the Loess Plateau,considering the background of the“Grain for Green”project.Check dam is an effective soil conservation engineering measure and is in widespread use on the Loess Plateau.Sediment sequences in check dams provide deep insights into the long-term sediment deposition process of catchment ecosystems.Therefore,check dams can be used as a carrier to invert the historical erosion change of the source area,which is of great significance to the study of soil erosion in ungauged catchments.In this study,the sediment sources and particle size distribution characteristics of the small catchment are discussed.The specific intentions of this study were:First of all,taking the Washuta,Shayangou,Nianzigou,Hu Jiawan and Sijiaya catchment as the research target,carrying out field sampling and indoor analysis in slope,gully and dam area of the study areas.The particle size distribution and characteristics of sediment in small catchment were analyzed.The results are of great significance to the understanding of the mechanism of slope-gully-dam erosion transport in small catchment.Then,taking the Washuta catchment as the research object,sediment profiling samples were collected from the head of check dam to a vertical depth of 7.16 m.A total of 23 couplets were identified based on the thickness of deposition layers and extreme rainfall event records.The boundary of each flood couplet was easily distinguished based on a fine clay layer in the top and a coarse sand layer in the bottom.The silt volume and sediment yield of the dam were calculated.Using the sediment source tracking techniques to quantify the relative allocation of sediment provenance in a small catchment of the Loess Plateau and reveal the impacts of vegetation restoration on change of sediment sources in the small catchment of the Loess Plateau,considering the background of the“Grain for Green”project.The results are important to the promotion of the ecological restoration,comprehensive management of soil and water conservation and prediction of soil and water loss.The main conclusions are as follows:(1)During the process of sediment transport and erosion in small catchment,the particle size composition keeps changing and shows difference in each catchment.For example,the distribution of silt content in the slope-gully-dam area increased in the Washuta catchment,but it decreased in the Hujiawan catchment.Sediment selectivity occurs during the process of transport and deposition.In addition,because the silt particles are easily dispersed in the process of soil erosion,the soil with higher silt content is more likely to be eroded than the soil with higher clay content,and silt content also accounts for a relatively higher proportion in the eroded soil.(2)The sediment/source PSD ratios of the silt-sized particles for most study catchments were>1,which indicated that the silt were easily transported when the erosion events commenced in most study areas.Eroded materials are enriched in clay and silt-sized particles relative to the source soil during the erosion process.Compared with that of clay and silt soil,coarse particles was not inclined to be transported into dam.The sediment/source PSD ratios were<1 in most catchment.The sediment/source PSD ratios of the Hujiawan soil were the farthest from 1 among the five soils,which indicates that in hujiawan,sand are more easily transported and eroded.The values of median diameter of sediment particles were not significantly different among the five catchments.The sand particle has a thinning tendency from north to south areas.The soil particle size in the middle reaches of the Yellow River has no significant difference in the five small basins,and is mainly concentrated in the silt(0.002-0.05 mm),which indicates that the soil of the Loess Plateau in China has a high degree of uniformity.(3)The annual sediment yield of each flood couplet was calculated according to the bulk density of soil and the thickness of the sampling profile.The control area is 4.17 km~2in study area.The thickness of the sampling profile is 7.16 m and the accumulated sediment volume is 203,000 m~3.The cumulative sedimentation is 213,800 t in Washta catchment.The annual erosion modulus is about 3195 t/km~2/yr,1515 t/km~2/yr,2200t/km~2/yr,6139 t/km~2/yr,14,196 t/km~2/yr,14623 t/km~2/yr and 3235 t/km~2/yr.The average annual sediment yield is about 16,900 t,6,500 t,12,100 t,28,900 t,50,300 t,81,000 t and15,200 t.The average annual erosion modulus and the total sediment yield are about 7517t/km~2/yr and 213,800 t.The erosion and sediment yield in each year is closely related to the rainfall erosion and showed a trend of increasement from 2010-2016.(4)A total of 23 elements were identified and measured.A three-stage statistical procedure was applied to confirm source apportionment.The results indicate that the final composite fingerprinting comprised 4 individual constituents(Mg,Al,Co and Bi),and93.12%of the samples were successfully distinguished.A total of 23 couplets were identified based on the thickness of deposition layers and extreme rainfall event records along the sediment profile.The model GOF averages 0.86,which indicates that the model results are meaningful.This study presents reliable information on the importance of gullies as spatial source materials and verifies the applicability of tracing procedures for collecting information on sediment efflux from both slope and gully.In addition,geochemical properties can be used to discriminate different source materials due to differences in geology.(5)The results suggest that overall sediment contribution from gullies account for 65%and increased during 2010-2016 under the“Grain for Green”project.The sediment inputs from slope were predicted to be 35%.Sediment yield of gully and slope were 140,000 t and74,000 t,respectively.Vegetation coverage and its spatial-temporal changes significantly affect the hydrological process,sediment transport and the soil erosion process.The widely distributed fragment in the gully produced an unstable situation that made gully more erodible than the slope within the background of the“Grain for Green”project.The results show the importance of vegetation restoration in reducing slope erosion.Additionally,the high contribution of gully erosion also indicates that only vegetation measures cannot effectively control soil erosion on the Loess Plateau.Thus,additional engineering measures,such as check dams and sediment trapping through reservoirs,are required.(6)Soil erosion is significantly related to land use.The farmland in the slope area was approximated to have dominated the sediment efflux,with the relative importance of 22%,which indicated that a vulnerable slope without adaptive strategies for protecting soil and water is still the leading land use erosion type.Fallow grassland and fallow forest contributed 12%and 1%,respectively.Vegetation played a significant role in helping to conserve the soils in hilly regions by reducing slope soil erosion.Land use pattern is an important factor affecting sediment source.Therefore,in the comprehensive treatment of small catchments on the Loess Plateau,it is a feasible to implement the construction of terraced farmland instead of sloping farmland.In addition,natural fallow is also an effective strategy for soil erosion control on the Loess Plateau.(7)Comparing the variation of sediment particles with different diameters in slope-gully-dam area,it is found that the distribution trend of soil particle sizes in gully and dam sediments is very similar in Washuta catchment.This is mainly because gully is the main source of sediment in the study area.The results show that the change of grain size distribution can indicate the contribution of sediment source from different geomorphic units in small catchment.In addition,there is a turning point of particle size variation in the sedimentary profile of check dam(4.60m),which indicates that the dynamic conditions of the water and sediment have changed. |
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