| Loess Plateau is an important source of sediment in the Yellow River,and the source control of coarse sediment is the top priority of soil and water conservation work in the middle reaches of the Yellow River.As an important soil and water conservation measure in the Loess Plateau,check dam not only holds back the erosion sediment,but also records the sediment yield process and source information of small watershed.Through these information,the characteristics of sediment yield and sediment source of small watershed can be studied,which is of great significance to the study of soil erosion in areas where data are lacking.This paper takes the Huangfuchuan Hanjianggou watershed as an example.Through field siltation layer profile and sediment source sample collection and indoor analysis,relevant information such as erosive rainfall in the basin during the study period,as well as the physical and chemical properties of sediment source and profile sediments are analyzed.Based on the historical rainfall data,measured deposition thickness and particle size distribution,the time series of the sedimentary cycle layer of the dam land was established,and the characteristics of erosion and sediment yield and the variation of sediment yield intensity in the basin were studied.The contribution rate of sediment source in dam-controlled watershed was quantitatively calculated by using the composite fingerprint identification technology.It can provide important scientific basis for comprehensive control of soil and water loss and rational allocation of soil and water conservation measures.The main conclusions are as follows:(1)Rainfall,erosive rainfall and rainfall erosivity in Hanjianggou watershed are extremely unevenly distributed during the year,and the sedimentary sediment particle size at different depths of the profile varies significantly.From 2007 to 2016,the erosive rainfall showed an upward trend,and the average monthly rainfall erosivity for many years was the largest in July,at 486.19(MJ·mm)/(hm2·h).The rainfall,erosive rainfall,and rainfall erosivity from June to September accounted for 72.8%,80.1%and 85.5%of the whole year,respectively,and there were significant differences between rainy and dry seasons.Sand(>0.05mm)is the main particle size in both the source area and the siltation profile,and there was significant difference in the particle size of the sediment at different profile depths.With the passage of time,the grain size of the sediment in the dam-controlled watershed has a tendency to refine.(2)Based on the basic principle that large water and large sediment correspond to each other,the time coordinates of erosion and sediment yield of Hanjianggou check dam were established according to the correlation between rainfall data,measured silting thickness and silt particle size distribution.From 2007 to 2017,the check dam had a total sediment storage of 99.20×104t,and the average annual erosion modulus was 16889.65 t/(km2·a),which was a severe erosion area.With the passage of time,the annual erosion modulus of the basin showed a decreasing trend.There was a significant correlation between secondary erosion and sediment yield and secondary erosive rainfall,and between annual erosion and sediment yield and annual erosive rainfall(P<0.05).Under the similar rainfall conditions of moderate rain(20～25mm)and heavy rain(25～50mm),the erosion modulus decreases with the passage of time.(3)Based on the measurement and analysis of the physical and chemical indexes of sediment samples from grassland,sloping farmland,trench wall and sediment profile,the sediment sources in the basin were analyzed by using the composite fingerprint identification technology.The best fingerprint factor combination of sediment sources in this watershed is Cr+As+Mo+Cd,and the correct discrimination rate reached 90.7%.Trench wall is the main contribution source of sediment in dam land,with the contribution rate of 65.8%,followed by slope land,with the contribution rate of 29.0%,and grassland,with the contribution rate of 5.2%.(4)Using the obtained sediment contribution rate of each source area,the changes in the erosion and sediment yield of each sediment source area during the study period are analyzed.From 2007 to July 2017,the average annual erosion modulus of grassland,sloping farmland and trench wall was 1053.80 t/(km2·a),4904.66 t/(km2·a)and 10931.22 t/(km2·a),respectively.With the increase of the gyration layer depth,the secondary sediment yield in each source area showed an increasing trend.With the progress of siltation,under the similar rainfall conditions of moderate rain(10～25mm)and heavy rain(25～50mm),the sediment yield of the sediment source area showed a downward trend,and the decrease was more obvious under heavy rain(25-50mm). |
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