The Variation And Causes Of Small Pan Evaporation Over The Past 45 Years In China

Based on the observational small evaporation pan and other meteorological elements data of 465 meteorological stations in china from 1957 to 2001,the spatial and temporal distribution characters of evaporation, long-term changing tendency and the reasons leading to the changes are studied. At the same time the dynamic and energy items of Penman formula are analyzed. And then the conversion coefficient of small evaporation pan is studied and the regression model is given using different meteorological factors.The trend of evaporation is decreased at all seasons, especially in summer. Evaporation's tendency and range of variation are different according to season and area.The analysis of dynamic and energy items of Penman formula indicated that the evaporation decreasing tendency causing by energy item mostly in the south and by dynamic item mostly in the west.The decline of radiation, wind speed, saturation deficit and temperature daily range are major reason for evaporation's decreased tendency though analyzing the change tendency of other meteorological elements and simple correlation coefficient.The conversion coefficient is stabilized about 0.6 and the change is small. The value is larger in southern than the northwest. The regression model can use to caculate water evaporation.