Spatiotemporal Variation Of Climatic Trend Rate Of Pan Evaporation And Its Relationship With Climatic Factors In China

Global warming and its effects on weather and climate have aroused great concern from all walks of life.Theory speculates that the increase of energy received by the surface and the intensification of water cycle will promote the pan evaporation in the context of global warming.However,a large number of observation facts have obtained the opposite result.It is reported that the pan evaporation shows a significant decline trend in many parts of the world,which has aroused the scientific community's debate on the phenomenon of "evaporation paradox".At the same time,the causes of the phenomenon of "evaporation paradox" have become a scientific issue of great concern.2016 Zhang et al.found in the "LOPEX " that the pan evaporation trend is different from most other parts of the world,appeared the phenomenon of "reverse evaporation paradox ",in order to explore whether the phenomena of "reverse evaporation paradox" are ubiquity in the summer monsoon transition region,and analyses the reasons for this phenomenon,we use 277 sites across the country observation data in China,the distribution of the pan evaporation climate tendency rates and theirs climate factors are studied,main conclusions are as follows:1.On the whole,the trend of pan evaporation is mainly downward in China.After the sliding T test,it is found that 1996 is the turning point.The pan evaporation in summer monsoon affected areas and non-summer monsoon affected areas basically presents the phenomenon of "evaporation paradox",but the pan evaporation in summer monsoon transition areas basically presents the phenomenon of "reverse evaporation paradox".Taking 1996 as the turning point to analyze the spatial distribution characteristics of the pan evaporation climatic trend rate in China,it was found that the spatial distribution of the pan evaporation climatic trend rate showed,before 1996,the distribution characteristics of "descending-ascending-descending" from the non-monsoon area to the monsoon area,but after 1996,it showed the distribution characteristics of "ascending-descending-ascending".Both before and after the turning point,the trend of pan evaporation in the summer monsoon transition areas was always opposite to that in other regions.2.The study found that temperature as the dominant factor,with the effect of other factors leds to the rising trend of pan evaporation in the summer monsoon transition area.In the summer monsoon affected area and the non-summer monsoon affected area,the temperature change alone is conducive to the pan evaporation as well,but other factors are not conducive to the pan evaporation with a greater impact,which result in downward trends of the pan evaporation in the two regions.Besides,in the summer monsoon influence area,the change of wind speed contributed the most to the pan evaporation change rate,while the change of precipitation in non-monsoon area contributed the most.In addition,the changes of low cloud cover among the summer monsoonarea,non-summer monsoon area and summer monsoon transition area are very important.3.According to the distribution of the pan evaporation in the physical space composed of every 2 meteorological factors,the critical value of the influence of each meteorological factor on the pan evaporation was preliminarily determined.When the sunshine duration is higher than 6h or the wind speed is higher than 1.8 m/s,the pan evaporation is larger;when the relative humidity is higher than 81 % or the cloud cover is higher than 5.5,the pan evaporation is smaller.