Abstract: Long-term observational records from the Huaihe River basin show that the evaporation rate from open pans of water has been steadily decreasing over the past 50 years.Traditionally,the Huaihe River valley is seen as the transition zone dividing the arid north from the tropical south in China.Due to its geographic location,studying of pan evaporation changes in the basin would be important to understanding the basin's water cycle and water resources conditions,especially in a changing climate.In this study,the pan evaporation change in the past 50 years is investigated using the observed pan evaporation,temperature,surface wind speed,sunshine duration,relative humidity,air pressure and rainfall from the China Meteorological Information Centre,as well as the monthly mean cloud amount data of UK Climatic Research Unit (CRU)and the NCEP/NCAR re-analysis data.The result confirms the pan evaporation in the basin has undergone a significant downward trend change over the past 50 years.The most pan evaporation occurs in springs and summers,and thus showing a strongest downward trend.Regionally,the greatest pan evapora-tion and the related downward trend change are found to be in the basin east and south.The study on the cause of decreased pan evaporation reveals that the downward trend change might be the result of the decreasing in surface wind speeds and sunshine duration,as well as the increasing in relative humidity and precipitation.The increase of both relative and specific humidity in the troposphere can result in an increase of surface relative humidity,cloud amount and precipitation,and thus weakening the intensity of solar radiation and shortening the sunshine duration.Such a process will eventually cause the reduction in pan evaporation.