Study On Variation Of Runoff And Sediment In Coarser Sediment-Producing Area Of Shaanxi Province In The Middle Reaches Of The Yellow River

With the development of human society, people gradually realized the importance of ecological civilization construction. Since the end of the 20th century, China has implemented large-scale governance measures, especially in the middle reaches of the Yellow River. The implementation of large-scale water conservation measures has made the restoration of watershed vegetation effect. In this paper, 5 watersheds in the coarse sand area of the middle reaches of the Yellow River are studied, and the changing characteristics of hydrological and meteorological factors and the characteristics of land use types are analyzed to reveal the characteristics of watershed water and sediment and further quantify the impact of precipitation and human activities on changes in watersheds. The SWAT distributed hydrological model is used to simulate the water and sediment in the basin, and some scientific suggestions are provided for the comprehensive management of the river basin. The main contents and achievements are as follows:(1)The characteristics of precipitation, runoff and sediment in five basins were analyzed by using a variety of mathematical methods. The results showed that the average annual precipitation of the five watersheds was 423.7 mm ,and the average annual runoff and sediment transport volume which showed a significant decrease trend, and the rate of decrease of sediment load was higher.The precipitation?runoff and sediment load were mainly concentrated in the flood season.In this study, Mann-Kendall method was used to analyze the abrupt change and relasized the sudden change of water and sediment in the five basins occurre in the late 1970s.(2)Based on the land use transfer matrix, the change characteristics of land use types in the five river basins in the coarse sand area were analyzed. The results showed that the land use of the five land and urban construction land, which is not mainly converted into grassland. The internal transfer trend of land use in different periods is similar.(3)Based on the double cumulative curve method, the results show that the slope of the cumulative curve of the Kuye River Basin is the largest and the sedimentation rate is the largest.The slope of the accumulation curve of the Wuding River Basin is the smallest. The relationship between water and sediment in the year showed that the average monthly runoff and monthly sediment transport curves of the five watersheds showed the same trend, and the clockwise curve was presented. The relationship between water and sediment in flood season showed that the correlation coefficient of water and sediment in the three periods is basically a decreasing trend,that is, the correlation coefficient of the reference period> the correlation coefficient of the verification period> the correlation coefficient of the water-sediment reduction period. During the verification period, the relationship between water and sediment in the basin is very harmonious and can reach more than 90% .The interannual relationship between water and sediment showed that there is a positive correlation between the annual runoff and the annual sediment transport in the coarse sand area of 5 watershes that the correlation coefficient of water and sediment in the Huangfuchuan, Gushanchuan and Kuye River basins is over 90% and the correlation coefficient of water and sediment in Wuding River Basin is more than 80%.(4)The change of water and sediment is mainly affected by precipitation and human activities.With the development of social economy, the influence of water and sediment reduction is getting smaller and smaller, and the influence of human activities is getting bigger and bigger. In the 21st century, human activities become the main factors of water and sediment reduction in the basin.The impact rate of human activities on the five basins in the 21st century was more than 60%,especially for the Tuwei River Basin and the Wuding River Basin, the impact of human activities were more than 90%. The main form of human activities are returning farmland to forest (grass),silt dam, irrigation area, coal mining, reservoir and other forms.(5)The construction of SWAT distributed hydrological model in Wuding River Basin, which used the Baijiagawa hydrological station as the total export of the basin and divided the Wuding River Basin into 25 sub-watersheds, adopt the man-machine combination method to calibrated and verified the water and sediment parameters of each month in baijiachuan hydrological station of Wuding River Basin . The Nash-Sutcliffe efficiency coefficient of the runoff and the correlation coefficient R~2 are above 0.8, the relative error Re is 9.86, the Ens and R~2 of the sediment are all above 0.8, the relative error Re is -13.72, less than 20% .The simulated value is smaller than the measured value. The Ens for the runoff is 0.79, R~2 is 0.81, Re is 11.34, Ens is 0.75, R~2 is 0.8, the difference is -16.67, less than 20%, and the simulated value of sediment is smaller than the measured value. The simulation results of river water and sediment are better than the verification period, and the simulation effect of runoff is better than that of sediment.