Water And Sediment Evolution Of The Yangtze River And Its Response To Climate Change And Human Activities

River system is an important link between water,land and air energy conversion.However,under the influence of global climate change and human activities,the water cycle of the basin has undergone a great transformation,and the evapotranspiration,precipitation,runoff,sediment and their spatial and temporal distribution have undergone dramatic changes.The evolution law of water and sediment and its driving factors were discussed to provide reference for integrated river basin management.Based on the hydrometeorological data of 199 meteorological stations and 12 main control stations of the main and tributaries in the study area from 1956 to 2015,this paper takes the Yangtze River Basin as the research object,and combines the feature information of the main large reservoirs and the data of the five phase of land use.The characteristics of water and sediment changes and the impacts of climate change and human activities in the study area in recent 60 years were analyzed.The main results and conclusions are as follows:(1)Change in precipitationThe precipitation in the Yangtze River basin in the past 60 years fluctuated above and below the annual average.The trends of each tributary vary.The rising trend of Jinsha River,Dongting Lake and Poyang Lake is not obvious,while the Min River,Jialing River,Wu River and Han River showed no significant downward trend.The trend spatial distribution shows that the annual precipitation in most areas has not changed significantly with time.Only the source area of the Yangtze River has changed significantly,and the upper reaches of the Jinsha River and the middle and lower reaches of the Yangtze River have a slight upward trend,while the upstream of the mainstream and the Wu River showed insignificant downward trend.The results of the mutations showed that the abrupt change of annual precipitation in the mainstream of the Yangtze River was not significant,and the years of possible mutations in the tributaries were roughly in the mid-to-late 1980 s and 1990 s.(2)Changes in water discharge and sediment loadThe distribution of annual runoff and sediment load in the Yangtze River is extremely uneven during the year,mainly distributed from May to September,and the concentration of runoff and sediment load in the upper reaches of the basin is higher than that in the downstream.The interannual variation of runoff on the mainstream of the Yangtze River is relatively stable,and the interannual extremum ratio and Cv value of the runoff on the tributary are relatively large.In general,the average annual runoff variation amplitude of the 12 control stations in different ages was small(most stations was within 10%).The amount of sediment load varies greatly from year to year,and the variation coefficient of each station was between 0.38 and 1.54.Except for Hukou Station,other stations showed a decline trend of more than 99% confidence level,and decreased from upstream to downstream.Significant mutations in runoff were less,and most of the sediment load in 12 control stations experienced significant mutations in the late 1960 s,mid 1980 s,1990s,and early 21 s.(3)Impacts of climate change and human activities on the water and sediment of the Yangtze RiverThe impacts of climate change on runoff and sediment transport in the Yangtze River Basin are different in time and space.The runoff and sediment transport in the upper reaches of the Yangtze River Basin are more affected by climate change than in other regions.During the period of variation I,the decrease about 50% of runoff in the Yangtze River is attributed to the decrease of precipitation,while the other 50% is due to human factors.And the significant decrease of sediment transport is attributed to the decrease of precipitation.The decrease of runoff in variation period II is about 30% due to the decrease of precipitation,and the decrease of sediment transport over 80% is due to the influence of human activities.Precipitation is the main source of runoff,and the correlation between the two is significant.The correlation coefficient of most control stations in the Yangtze River basin is higher than 0.80.However,the correlation between precipitation and sediment load is poor,and the double cumulative curves showed that precipitation and sediment load in the variability phase II have a large downward shift,indicating that the impact of human activities is increasing during this period.Among the different human factors,the construction of water conservancy projects is the main reason for the annual decline in sediment load in the Yangtze River,especially after entering the 21 st century.After the construction of the water conservancy project,the runoff and sediment load in the Yangtze River have changed significantly in the year and ages.Not only does the distribution of runoff during the year has played a role in peaking and replenishing the dry-up,but also significantly reduces sediment load in each month,especially the reduction in each month of the flood season(June-September)is higher than 50%;After the impoundment of many large reservoirs in the Yangtze River basin,the annual average sand interception of reservoirs in the main and tributaries of the Yangtze River basin has increased year by year,which greatly reduces the amount of sediment load of the control stations of the main and the tributaries rivers.In addition,the water and sediment reduction benefits of soil and water conservation measures in the Yangtze River basin are relatively small and significant,which is an important factor leading to a significant reduction in sediment transport in the Yangtze River basin.Moreover,the land use types of the Yangtze River Basin changed from 1980 to 2015.The land use types mainly changed between forest land,grassland,paddy field and dry land,as well as the transfer of forest land,paddy field and dry land to construction land,which also had a certain impact on the water and sediment changes in the Yangtze River basin.