Analysis Of Runoff And Sediment Variations In The Upper Reaches Of The Yangtze River And The Driving Factors

The runoff and sediment of land rivers seriously affect the delta landform,biogeochemical cycle and land-sea interaction,which has attracted worldwide attention.As the largest river in China,the Yangtze River supports the economic and social development of the Yangtze River basin.The upper reaches of the Yangtze River is the main source of water and sediment flux in the whole Yangtze River,which controls the change of water and sediment flux in the middle and lower reaches.With the development of economy and the advancement of modernization,climate change and human activities in the Yangtze River basin have greatly disturbed it,and the river runoff and sediment discharge have changed seriously.Based on this,this paper selects the representative hydrological stations in the upper reaches of the Yangtze River to analyze the variation law of water and sediment in the upper reaches of the Yangtze River and study its driving factors.The main research contents and conclusions are as follows.(I)Analysis of the evolution law of runoff and sediment regime in the upper reaches of the Yangtze River.TFPW-MK trend test,Lee-Heghinian mutation test and wavelet period analysis were used to analyze the evolution law of water and sediment.The results show that there is no significant change in the interannual distribution of runoff in the upper reaches of the Yangtze River,while sediment load shows a significant decreasing trend;abrupt changes in runoff of the upper Yangtze River are concentrated in the 1970 s,1980s and early 21 st century,while abrupt changes in sediment load are concentrated in the 1980 s and early 21 st century;annual runoff mainly has 10-18 years and 23 years main cycle changes,and annual sediment load mainly has 10-20 years and 30 years main cycle changes,30-year main cycle variation;around the late 1980 s and early 21 st century,the water-sand relationship changed twice,and its correlation kept weakening.(Ⅱ)Analysis of variation degree of runoff and sediment situation in the upper reaches of the Yangtze River.Based on IHA,32 hydrological indexes,RVA method and biodiversity index SI are used to reflect the variation degree of runoff and sediment situation and its ecological response in the upper reaches of the Yangtze River.The results show that the annual extreme value,change rate and frequency,high and low frequency and diachronic variation of runoff and sediment in the upper reaches of the Yangtze River are great.The overall variation degree of runoff and sediment in each hydrological station is greater than 33%,that is,the variation degree of river runoff and sediment is in the middle height;SI values of biodiversity indexes in the upper reaches of the Yangtze River all showed a downward trend,that is,biodiversity in the upper reaches of the Yangtze River showed a downward trend.(Ⅲ)Quantitative identification of influencing factors of runoff and sediment in the upper reaches of the Yangtze River.The elastic coefficient method,sediment attribution decomposition method and BP neural network are used to quantitatively identify the attribution of water and sediment variation.The results show that the contribution rate of runoff change is 52.53～84.21% for human activities,20.10～63.66% for precipitation and-16.77 ～-4.31% for evaporation.For the contribution rate of sediment change,the sediment yield is 64.1～138.65%,the runoff yield is-41.03～14.22,and the precipitation is-32.77 ～ 76.92%.The results of neural network calculation at Yichang station show that the contribution rate of human activities to the change of water and sediment is above 65%,which means that the change of water and sediment flux in the upper reaches of the Yangtze River is mainly affected by human activities.(Ⅳ)Study on the driving factors of runoff and sand situation in the upper Yangtze River.River impact factor RI,landscape pattern and least squares were used to evaluate the degree of reservoir construction and land use impact on rivers and the driving mechanism.The results show that: river connectivity is seriously affected by reservoir construction;the overall landscape pattern fragmentation increases in the upper Yangtze River due to land use change;reservoir construction positively affects the variability of water-sediment situation,i.e.reservoir operation increases the variability of water-sediment situation;land use change negatively affects the variability of water-sediment situation,i.e.land use change slows down the variability of water-sediment situation.