Abstract: Under the background of global warming and shortage water resources,the research on evaporation is not only conducive to understanding the variation of hydrologic cycle,but also could provide a theoretical basis for water resources planning and management.Based on the meteorological data in the west Horqin Sandy Land during the period of 1981-2016,the trends of pan evaporation and some meteorological factors,including precipitation,wind speed,temperature,cloud cover,relative humidity and vapor pressure deficit,were analyzed using the linear trend,Mann-Kendall trend and Sen’s slope tests.The main factors affecting pan evaporation were also determined by means of Partial correlation method,principal component analysis and stepwise regression analysis.The results showed that,in recent 36 years,pan evaporation and wind speed decreased significantly (P<0.001),while total cloud and low cloud were in a significant increase trend (P<0.001).Furthermore,the seasonal pan evaporation and wind speed also decreased significantly (P<0.001),while the seasonal low cloud was in a significant increase trend (P<0.001),and the decrease rates of pan evaporation in spring and summer were higher than those in autumn and winter.The result of partial correlation analysis showed that the annual and seasonal pan evaporation was more closely related to the wind speed and cloud cover.A further examination in which the principal component and stepwise regression analysis were used showed that the decrease of wind speed and the increase of cloud cover were the main meteorological factors affecting annual pan evaporation.Based on the results above,it was concluded that the decrease of wind speed and the increase of cloud cover were the main causes resulting in the decrease of pan evaporation in the west Horqin Sandy Land in recent 36 years.Additionally,the change of precipitation and relative humidity was not significant,the annual mean temperature increased (P<0.05) but the pan evaporation decreased,significantly (P<0.001),which revealed that the climate warming in the study area was significant,but the increase of aridity degree was not so obvious in recent 36 years.

Key words: pan evaporation, meteorological factor, partial correlation, Horqin Sandy Land