Change in Pan Evaporation and the Influential Factors over China:1956-2000
中国水面蒸发量的变化

Trend of pan evaporation and the related climate factors for the entire country and ten discharge basins of China are analyzed using a data set of total 600 stations.A significant decreasing trend in pan evaporation for the country as a whole has been found for the period from 1956 to 2000,with eastern,southern and northwestern China experiencing the most marked decrease.Eight of the ten discharge basins are found to be weaker in evaporating presently as compared to 1950's and 1960's,while two of them,the Songhuajiang Basin and the rivers of southwestern China,witnessed no significant change.The most significant decline occurred in the Yangtze River,Haihe River,Huaihe River,Zhujiang River,and northwestern rivers basins.The decrease is most obvious in summertime in the Yangtze and Huaihe rivers,in springtime in the Zhujiang and Liaohe rivers,and in both summertime and springtime in the Haihe River and northwestern rivers.The Yellow River,southeastern rivers and southwestern rivers are characterized by a decrease with no season being dominant,and the Songhuajiang River has no season when pan evaporation decreased significantly.In most parts of the country,a significant positive correlation of pan evaporation with sunshine duration,average wind speed and daily temperature range can be found.With the change of pan evaporation,these variables also experienced an evident negative change,indicating that they might have played an important role in the change of pan evaporation.It is more uncertain for air temperature and relative humidity,which show a good relationship with pan evaporation only in areas where no significant decrease occurred,indicating that they are generally not the main climate factors affecting pan evaporation.Possible causes for the evident decrease of sunshine duration and mechanisms for the climate factors to affect pan evaporation are also discussed in the paper.