Sediment Source Analysis And Comprehensive Evaluation Of Models In Typiacl Dam Control Basin Of Loess Palteau

Loess plateau is the most serious soil erosion areas in China,the erosion sediment mainly comes from the slope gully watershed system.Theref’ore,it is of great significance to study the rules of erosion and sediment production in small watershed and the source of sediment for establishing erosion prediction model of watershed scale and optimizing slope and gully governance structure.This study is based on the Zhaizi check dam in Hengshan County,Shaanxi Province.Sewage sediments,grassland,ditch walls,branch trenches and dam sediments were collected from the research area.Based on the measured deposition layer thickness,137Cs content,soil particle size indicators and historical rainfall data,we established the time coordinates of each sediment.Based on the results of three sediment source models,we studied the evolution characteristics of the erosion environment in the small watershed of the Loess Plateau and reveal the variation of soil erosion and sediment processes and their sources.The calculation accuracy and reliability of different sediment source models are clarified by the method of comprehensive evaluation.The main conclusions take follow:（1）Based on soil particle size,¹³⁷Cs content,measured check layer thickness and historical rainfall data,the time coordinates of sediments of check dams were established,and the siltation process of check dams was clarified.During the operation period（2006 to July 2017）,the check dam will contain about 664,000 tons of sediment,and the average annual erosion modulus in the dam-controlled basin is 14,000 t/（km2 a）.There was a significant difference in annual erosion,and its peak appeared in 2016,which blocked about 93,730 tons of sediment.During the observation period,the erosive rainfall mainly concentrated on rainfall events with rainfall of 12-15mm,15-20mm and 20-30mm,accounting for 78.76%of the total rainfall.（2）Based on the statistical analysis of nutrient elements and particle size indicators of-sediments in dams,the dynamic changes of nutrient elements and particle size indicators with the operation of check dams are clarified.During the observation period,the total nitrogen content of sediment in the dam was positively correlated with the total phosphorus content,the correlation coefficient was 1.00,and the correlation coefficient between organic carbon content and total nitrogen content and total phosphorus content was 0.199（P<0.01）.With the operation of the check dam,the sediment particles in the dam have a trend of refinement under similar rainfall conditions;the organic carbon content and total nitrogen content show a decreasing trend;the total phosphorus content has almost no change.（3）Based on the statistics of soil physical and chemical properties of sediment source,three mixed sediment models of Collins,MIXSIAR and Modified Hughe were established,and the sediment source of the dam-controlled small watershed was analyzed.In the study area,the best factor combination of sediment source is Zn+Mg+Mn+d（0,1）,and the total discrimination rate is 83.10%.The average contribution rate of sediment in the check dam is 54.22%of the ditch wall,23.56%of the sloping land,15.54%of the grassland and 6.68%of the ditch channel.Under the torrential rain event,the ability of sand sloping land and grassland to change sand has changed.Under the conditions of sub-rainfall,the average sediment yield of the four sediment sources is 4321.91t for the ditch wall,1795.43t for the sloping land,1167.13t for the grassland and 531.38t for the ditch channel.（4）Established a comprehensive evaluation index system of sediment source model based on relative error,deterministic coefficient,consistency index and other indicators,and comprehensively evaluated the fitting effect of the three models.In the evaluation system,the most sensitive indicator is the relative square mean square error MSESQ,and the least sensitive indicator is the relative log mean square error MSELN.On the whole,the Modified Hughe model has the best fitting effect,followed by the Collins model,and the MIXSIAR model has a poor fitting effect.The MIXSIAR model has the best fitting effect under the condition of 12-15mm rainfall,and the Modified Hughe model has the best fitting effect under the condition of 15-40mm rainfall.The Collins model has the best fitting effect under the rainfall of more than 40mm.