Variation Charicteristic and Influence Factors of Pan Evaporation in Xingtai of Hebei Province

The variation characteristics of pan evaporation and its influencing factors from 1972 to 2011 in Xingtai were analyzed by using methods of a linear tendency,accumulative anomaly,slide T test and a complete correlation coefficient.The results show that the annual and seasonal pan evaporation presented decreasing trend,especially in spring,then in summer.Decrease of annual pan evaporation was mainly due to spring and summer pan evaporation decrease.The abrupt change of annual pan evaporation occurred in 1983,and after that the pan evaporation decreased obviously.Annual pan evaporation in Xingtai was positively correlated with sunshine hours,average wind speed and temperature daily range,and negatively correlated with precipitation,and relative humidity.The decreasing average wind speed,temperature daily range and sunshine hours are the major reasons for the downward pan evaporation in Xingtai.