Recent change of pan evaporation and possible climate factors over the Huang-Huai-Hai watershed,China

The tendency and the possible causes of the pan evaporation change over the Huang-Huai-Hai watershed from 1956 to 2000 are analyzed by using the observed monthly data.The results show a significant decrease in the pan evaporation in the area.The rate of decrease is about-50 mm/10a,and the maximum rate of decrease reaches-80mm/10a over the plain area.It is also found that the maximum decrease of the pan evaporation occurred in the southeast of the Haihe basin and the lower Yellow River during spring,and in the Huaihe basin during summer.The main climatic factors for the change of the pan evaporation are analyzed by studying the relationship between the pan evaporation and such elements as the sunshine duration,the average wind speed,the diurnal temperature range(DTR),the mean maximum and minimum temperature,the mean temperature and the precipitation.It is thus obvious that the decrease of the pan evaporation was mainly caused by the weakening solar radiation and the sunshine duration.The arerage wind speed and the DTR may also play an important role in an unknown mechanism.