Changes of Pan Evaporation and its Influence Factors in China

Evaporation is an important component both in surface energy balance and terrestrial water cycle, and it is also a key factor determining weather and climate conditions. It plays a significant role in the studies of water and energy balance within a certain area and estimations of water resources. In this study, based on pan evaporation data and their variation trends from 499 stations in China, the correlations between evaporation and various influence factors, including the amount of precipitation, temperature, wind speed, sunshine duration and relative humidity, were studied in depth by means of correlation analysis and set pair analysis methods. The analysis results indicated that there was a decline of pan evaporation in both of the whole China and its seven major river basins, and it has decreased significantly since mid-1970s, mainly due to the decreasing sunshine duration and wind speed. In addition, increased precipitation and relative humidity may inhibit the reduction to a certain degree. In most regions of China, temperature change presented a contrary trend compared with the variation of evaporation, as described in 'evaporation paradox'; however, the correlation between them was not very striking, which implied that the variation of water surface evaporation was slightly affected by temperature.