Changes of pan evaporations and its impact factors over northern Tibet in 1971-2006

With the data of monthly 20cm pan evaporation and its impact climatic factors at 6 meteorological stations over northern Tibet from 1971 to 2006, the trend is analyzed through the linear trend analysis. The main results are as follows:(1)Significant negative trend of annual pan evaporation is detected at all stations of northern Tibet, and the mean rate of decrease is 61.7 mm per 10-year (99% confidence level),especially in summer (-20.8 mm per 10-year, 99% confidence level). The spatial distribution of the change rate shows that the decrease of annual pan evaporation mainly occurs in mid-western of northern Tibet, the decreasing rate declines with the increase of longitude and rises with the elevation of altitude.(2)Significant decrease trends of annual sunshine duration and wind speed are found over northern Tibet. Significant positive trends of annual mean relative humidity and precipitation are detected. So is significant increase trend of annual mean temperature, with its increase is 0.34℃ per 10-year. While the increase trend of annual mean minimum temperature(0.54℃ per 10-year)is bigger than that of annual mean maximum temperature(0.17℃ per 10-year), and the trend of daily temperature range decrease significantly(-0.37℃ per 10-year).And (3)it is thus obvious that the decrease of the pan evaporation is mainly caused by the significant decrease of the daily temperature range and sunshine duration as well as the obviously increase of the mean relative humidity. Precipitation and wind speed may also play an important role in the unknown mechanism.