Variations Of River Sediment And Its Response To Climate Change And Vegetation Coverage In The Three-River Source Region

Runoff and sediment play an important role in the supply of water resources,shaping the river morphology,maintaining the regional environmental and ecological system.The three-river source is the most important water source conservation areas of the Yangtze River,the Yellow River and the Lancang River,which is also called the "Chinese water tower".Strengthening the nature conservation and ecological environment construction in the Three-River Source and its downstream regions is closely related to the survival and development of Chinese,which requires better understanding the relationship between river and its environmental factors.The objectives of this study were(1)to detect change in sediment discharge of the Three-River Source Region and(2)to explore the influence of climate change and vegetation cover to the variability of river sediment.The result can be useful reference to plans and ecological conservations in this region and research concerning sediment concentration in reservoirs of its lower reaches.Taking the three-river source as the research area and using observation data of runoff and sediment at 9 hydrologic stations,the meteorological data of 45 meteorological stations in the Three-River Source Region and its adjoining area and vegetation coverage data,the change trends,change-point and periodic changes of sediment discharge and sediment concentration were analyzed by using the Mann-Kendall trend test.Double-mass curve and rating curve method were used to explore the relationship between water and sediment in Three-River Source region.The Mann-Kendal trend test and correlation analysis were used to quantify the effect of climate change and vegetation cover on the change in sediment discharge.The main results are as following:(1)For long-term annual sediment discharge,there were no significant change trend and change-point in sediment discharge except Xinzhai which shown a decreasing trend and changed greatly in 1998.In terms of monthly sediment discharge,the monthly average sediment discharge of 9 hydrological stations in the Three-River Source Region is very uneven during the year,concentrating mainly in May to September.There are 14a.7a and 3a period changes of the annual sediment discharge in Zhimenda hydrologic station in the source region of the Yangtze River and the main cycles are,successively.The annual sediment discharge of Tangnaihai hydrologie station,in the source region of the Yellow River exsits 12a,28a and 7a period changes.Besides,there are 15a and 8a period changes of the annual sediment discharge in Xiangda hydrologic station in the source region of the Lancang River.(2)As for the annual sediment concentration,in Xinzhai hydrologic station,in the source region of the Yangtze River,reduced significantly,while it increased conspicuously in Huangheyan and Tangnaihai hydrologic stations in the source region of the Yellow River,and the sediment concentration did not change apparently in other 5 hydrologic stations.A change-point of sediment concentration existed in Xinzhai hydrologic station of Batang river,in 1999,and in Tongren hydrologic station of Longwu river,in 1989.Sediment concentration also has significant periodicity with 14a,7a and 3a period changes of the annual sediment concentration in Zhimenda hydrologic station in the source region of the Yangtze River,successively.The annual sediment concentration of Tangnaihai hydrologic station,in the source region of the Yellow River,exists 12a,28a and 7a period changes.Bseides,there are 15a,8a and 30a period changes of the annual sediment concentration in Xiangda hydrologic station in the source region of the Lancang River.Relationship between runoff and sediment did not change obviously in basins controlled by Zhimenda hydrologic station in the source region of Yangtze River and Xiangda hydrologic station in source region of Lancang River.Adversely,the relationship varied obviously in Xinzhai control area in the source region of the Yangtze River and control areas in the source region of the Yellow River.The change of water-carrying capacity showed an obvious spatial heterogeneity,which could be mainly divided into three types,including weakening,enhancing and stabilizing.(3)The climate factors that are closely related to the change of sediment discharge in the Three-River Source Region are precipitation and evapotranspiration.A significant positive correlation was observed between sediment and precipitation in the Three-River Source Region except the area controlled by Tuotuohe hydrologic station.Besides,there was a significant negative correlation between annual sediment transport and evapotranspiration in Xinzhai control area in the source region of the Yangtze River and in the source region of the Yellow River.The relationship between sediment discharge and temperature in the Three-River Source Region was not significant.In addition,the correlation between sediment discharge and vegetation coverage in the Three-River Source Region indicates that vegetation restoration has no significant effect on the variation of river sediment discharge.