Variation And Abrupt Change Of Reference Evapotranspiration On Tibetan Plateau During1967-2008

Reference evapotranspiration is an important part of the hydrologic cycle whichis used to evaluate the drought degrees, the vegetation water consumption, thepotential productivity,and the supply and demand balance of water resources.Recently, the trends of the climate factors which affect ET0have been changed due toglobal climate changes.The Tibetan Plateau is the highest giant plateau in the world. Because of theunique landscape and the great elevation, Tibetan Plateau is regarded as the mostsensitive area to climate change and has a significant impact to the areas around. Toanalysis the trend of ET0could enhance our comprehensive of climatic change and itsfeedback on environment on the Tibetan Plateau and its surrounding regions.According to the daily meteorological data of77stations from Tibetan Plateauduring1967-2008， we estimated the annual reference evapotranspiration usingPenman-Monteith recommended by FAO. The temporal and spatial distributioncharacteristics and the trend of ET0were analyzed by using Mann-Kendall. MultipleRegression Analysis was employed to attribute the effects of the variations of airtemperature, wind speed, sunshine hours and relative humidity on ET0. We dividedTibetan Plateau into two parts with the East Kunlun Mountains-Bayan Har Mountain,and calculated the trends of ET0and the factors separately. We also tested the abruptchange of ET0and the factors with DistributionFreeCUSUM test、CumulativeDeviation test、Worsley Likelihood Ratio test. Results show that，(1)In recent42years,the trends of wind speed and sunshine hours are decline, and the trends of temperatureand relative humidity are increase; the ET0mainly towards an decrease in the northernand southern parts of Tibetan Plateau while increasing in the MID-eastern part.Thedecline trend in wind speed is the main cause of the ET0declining, and the increase oftemperature and sunshine hours result increase trend.(2)The mainly year of the ET0abrupt changeoccurred during1984-1990and1993-1997, of the temperature occurredduring1986-1989and1992-1997, of the wind speed occurred during 1984-1987,1988-1992and1993-1996, of the sunshine duration occurred during1984-1990and1993-1997.(3)The analysis about the upstream of the yellow riverbasin shows: the highest ET0is in summer, followed by spring, autumn, winter; theET0has shown significant increasing trend in the whole year except winter, and themost significant trend is in autumn.