Changes of Evaporation and Its Influencing Factors in Shiyang River Basin

Mann-Kendall trend test,Wavelet analysis and Gray relation analysis were applied to determine the change characteristics and influencing factors of 6 stations over the Shiyang River Basin from 1959 to 2005.The results show that the average annual evaporation of 47 years has a decline trend over the basin.The time of abrupt change of the evaporation happened in the 1960s and late 1980s and around the year 2000.The evaporation of Summer and Spring declines clearly,which is the main contribution of annual evaporation decreasing.The spatial distribution of evaporation and change trend of evaporation both have increasing change from the westsouth to northeast,which is closely related to the terrain conditions and meteorological factors in the basin.The first order main periods of evaporation are 28 years and 20 years,which contain small periods from 10 years to 14 years.There is a close relation between air temperature,sunshine duration,wind speed of the main influencing factors and evaporation.The average temperature and sunshine hours have significant increasing trend,and average wind speed has remarkable decrease.The change of precipitation,relative humidity and vapour pressure are relatively weak.Remarkable decrease of average wind speed compensates the increased evaporation resulted from rise of air temperature and sunshine duration,which eventually reduce the evaporation.