Changes Of Runoff And Sediment Yield Pattern And Its Influencing Factors In The Upper Reaches Of The Yangtze River

The Yangtze River basin occupies an important strategic position in the sustainable development of China's economy and society.The upper reaches of the Yangtze River is the main source of runoff and the important hydroelectric development base in China,which is of great significance for the rational allocation and comprehensive development and application of water resources in the whole basin.In recent years,under the background of climate change,the temperature and precipitation in the upper reaches of the Yangtze River have changed significantly.At the same time,the implementation of ecological projects such as Key water and soil conservation prevention project and Natural forest protection project have changed the underlying surface environment of the river basin.And the increase of hydropower development intensity in the upper reaches of the river basin has also affected the natural hydrological cycle process,which caused the obvious changes the regional runoff and sediment yield process.Therefore,studying the changes of runoff and sediment yield patterns and its influencing in the upper reaches of the Yangtze River can provide support for the development and utilization of water resources and hydropower resources in the basin and the sustainable development of the region.Taking the upper reaches of the Yangtze River as the main research object,we have analyzed the change trend of runoff and sediment yield,mutation characteristics and periodic characteristics by using Mann-Kendall mutation test,Mann-Kendall trend test and wavelet analysis,which was based on hydrological data from 1954 to 2018 of Gaochang station,Beibei station,Pingshan station,Wulong station and Yichang station in the basin.And the main influencing factors of change in runoff and sediment yield were discussed in combination with meteorological data and socio-economic data.Based on the daily runoff process data,we adopted the IHA/RVA method to quantitatively analyze the changes in the hydrological situation of the monthly flow rate,extreme flow rate and other indicators before and after the mutation of runoff in each basin.Based on parameter calibration and model validation of SWAT model in Jialing River Basin,the runoff simulation under different climate and land use change scenarios was carried out,and the contribution of climate change and land use change to runoff was quantitatively evaluated.Due to the lack of daily process data of sediment yield,the change degree and model simulation of sediment yield process have not been carried out.The main research results and conclusions of this study were as follows:(1)The results of Mann-Kendall trend test showed that runoff and sediment yield in all basins have shown a downward trend.The most significant declines in runoff and sediment yield were at Gaochang station and Pingshan station.From 2010 to 2018,the runoff increased except for Beibei station,and the sediment yield of each station showed a decreasing trend.The runoff at Beibei station and Gaochang station had mutations in 1986 and 1968 respectively.The years of runoff at Wulong station,Pingshan station,and Yichang station were 1961 and 1982,1968 and 1997,1961 and1972 respectively.There was no mutation in sediment yield at Yichang station,while the mutations in Beibei station,Wulong station,Pingshan station and Gaochang station occurred in 1968,1964,1958 and 1966 respectively.The main cycles of runoff changes at Beibei station,Wulong station,Pingshan station,Gaochang station,and Yichang station in the past 60 years were 30 a,15a,20 a,21a and 21 a,and the main cycles of sediment yield change were 29 a,21a,22 a,30a and 20 a respectively.(2)The results of attribution analysis of runoff and sediment yield showed that precipitation had a significant positive correlation with runoff and sediment yield in each basin(p<0.05),which had a strong correlation with runoff and it was the main driving factor for the change of runoff.The construction of water conservancy projects and the implementation of ecological projects have caused the water and sediment double accumulation curve change in different years,resulting in an overall reduction of 2.09% and 28.37% of runoff and sediment yield in the basin.Setting the mutation year of runoff and sediment yield as the critical year,we estimated the runoff and sediment yield at each control station based on the linear regression equation fitted by cumulative curve before the critical year.The results showed that the contribution of precipitation to runoff and sediment yield weakened and was lower than that of human activities eventually after the critical year.Except for the Gaochang stations,the sediment reduction benefits of human activities at other stations exceeded 75% after2000.(3)After the mutation,the runoff of all the control stations in the upper reaches of the Yangtze River had indicators with high alteration.Among them,the Gaochang station has the most changes in height,with 11,and the Beibei station with the least.The results of the overall hydrological change degree showed that all control stations had medium degree of change,and the Gaochang station was close to high degree of change(62.53%),and the hydrological situation changed the most in all of the basins.The analysis of the change degree and precipitation difference in each basin showed that except for Jinsha River Basin,the average precipitation in other basins decreased significantly.The Minjiang River basin had the most significant decline,whose hydrological situation also had the largest change,indicating that precipitation was the main reason for the changes in the hydrological situation of the basin.(4)We used the SWAT model to simulate the runoff process in the Jialing River Basin.The results showed that the Ens of Xiaoheba station,Wusheng station,Luoduxi station and Beibei station were 0.54,0.62,0.93 and 0.87 respectively in the regular period,which reached the acceptable level.And the Ens of verification period were 0.82,0.79,0.90 and 0.93 respectively,which showed that the SWAT model has good accuracy.The simulation effect of the outlet station was better than that of the upper river basin,and the simulated runoff of each station in the dry season was slightly lower than the measured value.The runoff simulation results under different climate and land use change scenarios showed that climate change and land use change were the reasons for the significant decrease in runoff in the Jialing River basin,and their contribution rates to the decrease in runoff at Beibei Station were 42.81% and 57.19% respectively.