Pan Evaporation Variation Characteristic and Climatic Influencing Factors in Shijiazhuang City

The variation characteristics of the pan evaporation and its climatic influencing factors in Shijiazhuang city in the last 37 years are analyzed in this study by using the methods of cumulative anomaly and linear trendency estimation and complete correlation coefficient.The results show that the annual pan evaporation presented obviously decreasing trend,and most significantly decrease occurred in spring,then in summer and autumn.The pan evaporation in a year and different seasons has a positive correlation with sunshine duration,average temperature daily range,average wind speed and average land surface temperature,but a negative correlation with average humidity.The major reason of pan evaporation decrease in Shijiazhuang city is the decrease of sunshine duration and average temperature daily range,and the second is the decrease of average wind speed and average land surface temperature.