| The silt dam is one of the most important measures to block silt and reduce sand for the comprehensive treatment of the small watershed,it is also one of the most important soil and water conservation measures in the Loess Plateau.Many of the silt dams blocked the sediment channel into the Yellow River,trapped large quantity of sediment.The sedimentation in the silt dam recorded a series of erosive and sediment-producing processes and sediment source information in the basin,which can be used to date back to the characteristics of sediment yield and sediment sources in the basin,which is very important for the study of soil erosion in the data-scarce area.In this study,Huangjiagou watershed,a typical silt dam of the Huangfuchuan which located in the coarse sand area of northern Loess Plateau,was selected as the research area.Together with remote images,field sampling and indoor analysis to study the erosion-related information carried by sedimentation and explore the sedimentary characteristics in the dam,combining with the discipline that “the stronger the precipitation,the thicker the flood couplets” and hydrological station rainfall data to construct the corresponding erosion and sedimentation time information,to reflect the intensity of soil erosion and sediment yield in the small watershed,and further study the contribution of different source to the sediment in the dam,so as to determine the source of erosion in the study area.This study showed that soil erosion model and composite sediment fingerprinting technique played an important role in soil erosion research.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)According to the bulk density of couplets,capacity-curve the thickness of the sampling profile,based on the principle of “the stronger the precipitation,the thicker the flood couplets” to calculate the annual sedimentation of the dam and each flood couplet.The results showed that Huangjiagou,constructed in 1972 with the control area of is 1.04 km2,the accumulated sedimentation volume is 485,600 m3 and the cumulative sedimentation is655,800 t.The thickness of the sampling profile is 2.97 m which corresponded to the sediment volume of 152,700 m3,and the cumulative sedimentation is 20.47 million t,wasformed by precipitation event from 2001 to 2014.The average annual sedimentation since the beginning of the operation till now is 15,400 t km-2a-1,and the average annual sediment yield is about 14,100 t km-2a-1from 2001 to 2014.The erosion and sediment yield in each year is closely related to the cumulative rainfall and showed a trend of fluctuation.The total rainfall in 2012 reached 646.9mm,caused total sediment yield about 4.47 million t,accounting for21.8% of the total sediment yield in 2001-2014,of which on a single day(2012-7-21)rainfall reached 124.7mm,corresponding to sediment yield 27,900 t,accounting for about 62.4% of the year.However,there was no significant sediment in 2011.(2)The sediment fingerprinting tracers in different sediment sources(Weathered sandstone,grassland and farmland)and each flood couplets were analyzed,then processed by nonparametric test(K-W H test)to select the statistically significant fingerprinting tracers,and then by means of stepwise discriminant screening,finally determined that the optimal fingerprinting tracers group for Huangjiagou dam was χhf,TN and137 Cs combination.According to the concentration of the optimal fingerprinting tracers in the couplets,the contribution ratio and the overall contribution of the sediment source area to each sampling layer in the dam were calculated by means of the multiple mixed model.The results showed that the contribution of the three sources were very different,the weathered sandstone was the main contributor with contribution ratio of 73.4%.The contribution ratio of farmland and grassland was 15.3% and 11.3%,respectively.The GOF of the model was 0.88> 0.8,which indicated that the actual accuracy of the analysis results suited the requirement.(3)The SEDD model was used to simulate soil erosion and sediment yield under different land use/cover types in Huangjiagou dam,and the sedimentation of the profile was used to calibrate the simulated values.The correlation coefficient between annual rainfall and sediment yield is 0.74,and the Nash coefficient of the model is 0.54,which indicates that the model is suitable for the study of erosion and sediment yield in this basin.The results showed that the spatial distribution of soil erosion and sediment yield was closely related to the land use types in the watershed.The weathered sandstone accounted for 94.2% of the total sediment yield,followed by farmland(3.3%),grassland(1.1%),woodland(0.8%)and alluvial plain(0.6%). |
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