Major Meteorological Factors Affecting Pan Evaporation in the Tibetan Region

As one of the most important components of surface energy and water balance,evaporation is a most sensitive factor to be affected by underlying surfaces and climate change.The investigation of evaporation and related factors is significant to the study of climate change and energy balance in Tibetan Plateau.Based on the analysis of spatial distribution and long-term trend of evaporation in the region,factors including air temperature,wind speed,sunshine duration and relative humidity are investigated.The results indicate that although the average temperature increased 0.03 ℃/a during the period from 1974 to 2001,pan evaporation decreased by-1.98 mm/a,indicating that the so-called evaporation paradox exists in the region.The annual mean evaporation ranges from 1 400 mm to 2 600 mm,and the long-term trends from-16 mm/a to 20 mm/a.Correlation relationship between air temperature and evaporation is significant,showing that air temperature is the main factor affecting the evaporation.The ranking of various meteorological factors affecting pan evaporation is that air temperature is the greatest one,wind and sunshine duration are the 2nd and 3rd.Relative humidity has no significant effect on pan evaporation.The air temperature determines the pan evaporation in monthly scale,resulting in the changes of permafrost by affecting ground temperature,which further results in the change of liquid water concentration on land surface.However,the decrease of wind speed and sunshine duration are the most important reasons for the decrease of the pan evaporation in annual scale.Besides,owing to the heterogeneous land surface and the different meteorological conditions,the importance of various meteorological factors differs from place to place.The decrease of pan evaporation means that the velocity of locally hydrological cycle becomes slow and the impact on global hydrological cycle need further investigation.