Analysis Of Different Pan Evaporation And Longterm Trend Of20Cm Pan Evaporation In Northern China In Winter

China’s analysis on the longterm trend of pan evaporation comes from the integrated data of two different evaporation pans (20cm and E601) for a long time. The evaporation pans have significant differences in the design, installation and environmental impact. Basing on the law of conservation of energy, this study creates a physical model of different pan evaporation to analyze the differences in the physical processes of the interaction between evaporation pan and environment. Then it verifies the above-mentioned results based on the law of conservation of energy with hour evaporations observed by different evaporation pans in the field observation experiment, the average evaporations observed by20cm evaporation pan and Closer E601evaporation pan in the same climate zone in Zhejiang province, and the average evaporation observed by20cm evaporation pan and Closer E601evaporation pan in long-term use throughout the country.The main conclusions are as follows:①Due to different installation height can lead to different sensible heat flux, and difirent heat exchange from the sidewall and bottom with the environment, so observation results of different evaporation will necessarily have a large gap;②The field observation experiment indicates that hour evaporation during the day E20> E601and during the night E601>E20, E20maximum appears in PM14:00, and E601at18:00;③From1985to2009pan evaporation appeared a continue upward trend for E20in all different stations, and for E601as the obvious downward trend from1985to1999, as the obvious rise from2000to2009, and for all seasons E20/E601presentes that summer>spring>autumn>winter.Comparative analysis found that evaporation from different evaporation pans has difference in some way, and then it is bound to some uncertainty when we analysis the change trend of pan evaporation according to integrate observations with different evaporation pans. So this study made detailed analysis on the longterm trend of pan evaporation with the data from the long-term use20cm evaporation pan of195stations in winter in northern China. The main conclusions are as follows:①The average pan evaporation in winter throughout the northern China is118.3mm. Compared pan evaporation:extreme arid zone>semi arid zone>semi-humid area>humid area; ②Pan evaporation showed a weak downward trend between1960-2008in winter throughout the northern China; For the trend of sub-period, in1960-1984a downward trend, the decline in the diurnal and sunshine hours play a dominant role; In1985-2008, pan Evaporation appeared an upward trend, that the rise in temperature and the drop in relative humidity played a dominant role;③For humid area, pan evaporation showed a rising trend in winter in1960-2008, the remaining districts were as a weak downward trend; For different districts in1960-1984pan evaporation showed a downward trend but the dominant meteorological factors were different greatly; In Extreme arid region pan evaporation showed a downward trend in1985-2008, that the rising in relative humidity and the decline in diurnal played a dominant role, the remaining districts were on the rise, and the rise in temperature and the drop in relative humidity played a dominant role;④In the winter in northern China there is one rising trend area of pan evaporation, located in the Northeast; There are two decline trend area of pan evaporation, located in the northwest and the middle and lower reaches of the Yellow River. Temperature and relative humidity are the dominant factors in the Northeast rising area; Diurnal, relative humidity, sunshine hours and precipitation are the dominant factors in northwest decline area; Relative humidity, diurnal and wind speed are the dominant factors in the decline area of the middle and lower reaches of the Yellow River.