Change of Pan Evaporation and Its Impact Factors in the Mt. Qomolangma Region in China during 1971-2006

Using the data of monthly 20-cm pan evaporation and other relevant climatic factors(for instance,sunshine duration,wind speed,relative humidity,precipitation and daily temperature,etc.)at five meteorological stations near the Mt.Qomolangma from 1971 to 2006,the distribution and linear changing trend of evaporation was analyzed,and the impact climatic factors for the changing trend of evaporation were also discussed.The following main results were obtained: 1)The changing trend of annual pan evaporation was detected insignificantly in most stations near the Mt.Qomolangma,and the mean rate of decrease was 15.6 mm·(10a)-1,but the rate was evaluated to be lower than that in East China,north part of Northwest China and South China.However,the decreasing rate had a maximum in spring and a minimum in summer.2)In terms of inter-decadal variation,the annual pan evaporation was higher in the 1990s and lower in the 1980s.3)The sunshine duration and wind speed has shown a decreasing tendency during recent 36 years in the Mt.Qomolangma region,and the mean relative humidity increased significantly.Also,the annual mean temperature increased evidently,and the increasing trend of mean minimum temperature was larger than that of mean maximum temperature.4)It is thus obvious that the decrease of the annual pan evaporation was mainly caused by the significant decrease of the sunshine duration and wind speed as well as the increase of mean relative humidity.