Evaporation characteristics and its influencing factor from 1957 to 2009 in Nan'ao County,Guangdong province

Based on evaporation data from a small evaporation pan in Nan′ao County from 1957 to 2009,the variation characteristics of evaporation in Nan′ao were analyzed.The results show that the maximum and the minimum evaporation appear in October and February,respectively.Among the four seasons,the highest evaporation is in autumn,second in summer,and the lowest in winter.The evaporation trend decreases clearly from 1957 to 2009 with an average rate of 7.2 mm per year,especially during the period of 1960s to 1990s,while it increases in the first decade of 21st century.The maximum decreasing amplitude of evaporation is in November and the minimum is in July for monthly variation,while the maximum is in winter,then in autumn and the minimum in summer for seasonal variation.On the other hand,the analysis indicates that the decrease of sunshine duration and average wind speed are related to that of evaporation,and they are the mainly influencing factors causing the decrease of evaporation.Furthermore,the increase of low cloud cover also leads to the decrease of sunshine duration.Evaporation has negative correlations with relative humidity,precipitation and water vapor pressure,especially with low and total cloud cover.