The Research Of Variation And Impact Factors Of The Runoff In Source Region Of The Yellow River For Recent Nearly60Years

Based on1956～2012monthly runoff data of Tangnag and climate data of8stations in Source Region of the Yellow River, also with NDVI and circulation field data from NOAA and ERA. Through trend analysis, calculated contribution of climate factor and boundary vapor transportation, analyzed the characteristics of the runoff in Source Region of the Yellow River for recent nearly60years. On the basis of the contribution of climate factors’ change to the runoff, than analyzed the influence of plateau summer monsoon and the surface sensible heat flux to boundary vapor, temperature and runoff. At last the conceptual models of the cause for bimodal pattern is given, also obtained the main conclusions are as following:1)There are2linear rise(1957-9167,2002-2012) periods,1linear decline period and1oscillation period of the variation of the runoff.During the rise periods, the periodicity of the runoff turned to2-4years from8years.2)The maximum coefficient of variation appeared in80’s,accordingly, the annual contribution the runoff take on bimodal. The minimum coefficient of variation appeared in50’s and90’s, the peak value is the least in the between several decades.3)The annual contribution of the runoff mainly have two types,one is the unimodal of July,anther is the bimodal of July and September. In normal years, the annual contribution mainly is unimodal type, in the high flow year, the annual contribution mainly is bimodal,in the low flow year, the annual contribution also is bimodal, but the peak value appears random.4)The climat became warm and wet. The precipitation takes on increase in the21st century. The precipitation in the south and middle region in Source Region of the Yellow River is stronger than other parts. The annual contribution of the precipitation mainly have two types,one is unimodal of July,another is bimodal of July and September The temperature and evaporation in Source Region of the Yellow River take on uptrend.The annual contribution of the temperature and evaporation are unimodal, the peak value appears on July. The vegetation in Source Region of the Yellow River takes on recover tendency from1980’s to21st century both in time and space. The annual contribution of vegetation is unimodal of August in1980’s and1990’s, and it turned into unimodal of July in21st century.5)The positive contribution of precipitation to runoff takes on rising trend, meanwhile, the contribution of the temperature turned from weak positive to notable negative. The precipitation contribution to the runoff annual distribution is bimodal, the peak value appears on July and September, also the temperature take on unimodal, the peak value appears on September, the precipitation and temperature work together, makes the annual runoff distribution takes on bimodal of July and September.6)When the surface sensible heat flux in Source Region of the Yellow River increase, the heat of atmospheric to the ground is enhanced, so makes the geopotential height field over the Source Region of the Yellow River decrease, the evaporation convergence and updraft. At the same time, the strengthen of the summer plateau monsoon makes the climate warm and wet, the Autumn runoff increase. And the position of the summer plateau monsoon just north, makes the input vapor of north boundary increase, the output vapor of east boundary decrease, caused precipitation increase, so the runoff increase, form the bimodal pattern.