Variation Analysis on Pan Evaporation and It's Influence Factors in Jangxi Province Last 46 Years

Based on the observational data of 79 meteorological stations in Jiangxi province from 1960 to 2005,the variation of annual pan evaporation and the relationships among other meteorological elements,including percentage of sunshine duration,cloud cover,temperature(diurnal temperature range,mean temperature,minimum temperature and maximum temperature included),wind speed at 10m above ground,saturation vapor pressure difference,annual precipitation and relative humidity,were analyzed.The results showed that the annual evaporation in west and center of Jiangxi province was less than in south and the Poyang Lake region.But in last 46 years,the annual pan evaporation decreased continuously at a rate of 4.57mm/y.68 stations showed decreasing of 79 stations.The correlation coefficient between annual pan evaporation and energy supply factor(the percentage of sunshine duration) was largest,followed by the dynamic factor(wind speed at 10m above ground) and the small one was the relative humidity.Their complete correlation coefficients were 0.37,0.33 and 0.23 under 0.05 significant levels.Jiangxi province was affected by the percentage of sunshine in general,but for the southwest and Poyang Lake nearby,it was affected more by the relative humidity and the wind speed.The decrease of pan evaporation resulted from the decline of sunshine percentage and wind speed as well as the increase of relative humidity.The multiple regression equation,established based on the percentage of sunshine,wind speed and relative humidity,could express the regional averaged annual pan evaporation with high precision.