| He-Long reach is the most serious area of soil erosion in Yellow River Basin,which is the main source region of coarse sediment and is the key area of water and soil conservation and ecological construction in Yellow River Basin.Since 2000,the amount of sediment discharged in He-Long reach has dropped sharply.In order to explore the characteristics of sediment variation in He-Long reach and to reduce the causes,this study selects erosive rainfall,erosive rainfall in the main raining season,runoff modulus,sediment yield and other indicators.Pettitt test,ordered clustering analysis and Morlet wavelet analysis were used to analyze the trend mutation and periodicity of meteorological and hydrological series in He-Long reach from 1956 to 2015.The relationship between climate change and watershed sand-water response was analyzed by slope change rate comparison method and redundancy analysis(RDA).Based on the Revised Universal Soil Loss Equation(RUSLE),the sand reduction contribution rate of the slope measures of He-Long reach was determined,and the calculation model of the sediment reduction rate of the silt dam was further constructed,and the contribution rate of the channel measures was determined.The main findings obtained are as follows:(1)Explain the trend of meteorological and hydrological sequences in He-Long reach from 1956 to 2015.The annual rainfall in He-Long reach showed a de-creased trend,that was not significant(p>0.01).Both the annual runoff and the annual sediment discharg significantly(p<0.001).The year of abrupt change of annual sediment discharge in He-Long reach is 1979 and 1999.Based on the abrupt change test of annual sediment discharge and previous studies,the period 1956-1979 is divided into T1 period,1980-1999 is T2 period,2000-2015 is T3 period.(2)The relationship between precipitation,runoff and sediment discharg in typical watershed of He-Long reach is analyzed.In the three time scales of year,raining season(June-October)and main raining season,(July-August),runoff-sediment discharg and rainfall-runoff in Dali River Basin showed significant linear positive correlation(p<0.05).The relationship between runoff and sediment discharg had changed from the year to the raining season and then to the main raining season.The coefficient of determination and correlation had been increasing.The slopes of rainfall-sediment linear regression at different time scales during the main raining season was better than other time scales.The annual erosive rainfall-runoff coefficient of determination(R2)was the highest and the fitting effect was good in 2003-2015.(3)It is clear that human activities are the main factor for the reduction of sediment discharg in He-Long reach.The contribution rate of human activities to the reduction of sediment discharg is greater than that of precipitation.After the sudden change of sediment discharg in each basin,human activities play a dominant role in the reduction of sediment discharg in the basin.From 1956 to 2015,the contribution rate of precipitation change in He-Long reach and its tributaries to the reduction of sediment discharg in the basin ranged from 0.58%to 41.31%,and the contribution rate of human activities to the reduction of sediment discharg was 58.69%-99.42%,with an average contribution rate of 82.45%.(4)The contribution rate of sediment reduction by water and soil conservation measures such as terraces,vegetation and silt dams in He-Long reach was quantified.From 2000 to 2015,the range of sediment reduction in terraced fields in He-Long reach increased year by year,from 14.02%to 25.99%.Under the influence of vegetation and terraced fields,the amount of soil erosion on slope decreased year by year,changing from 190 to 513 million t/a,with an average of 321 million tons for many years.From 1956 to 2011,the annual sediment interception capacity of the silt dam in He-Long reach changed from 0.04 to 391 million tons,the average sediment interception capacity for many years was 131 million tons,and the cumulative sediment interception capacity was 7.312 billion tons.Compared with the reference period T1(1956-1979),the average contribution rates of vegetation and terrace sediment reduction in T3 period(2000-2015)were 54%and 17%,respectively,the average contribution rates of silt dam sediment retention were 17%,and the total contribution rates of reservoir sediment retention and water diversion and sediment extraction were 12%. |
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