The Relationship between Pan Evaporation and Actual Evapotranspiration in Xinjiang since 1960

Pan evaporation, an indictor of potential evaporation, has decreased during the last several decades in many parts of the world; the trend is contrary to the expectation that global warming will be accompanied by an increase in evapotranspiration, known as the pan evaporation paradox. What is the essential relation between pan evaporation and evapotranspiration? This is still an uncertain problem. In this paper, the trends of pan evaporation and evapotranspiration in Xinjiang from 1960 to 2005 are investigated using observational data and the observation-constrained simulations using of the NCAR Community Land Model (CLM). Our analysis suggests that decreasing trend of annual pan evaporation accompanies increasing trend of annual evapotranspiration, the tendency has statistical significance. We also find that there is the same turning point of 1986 for precipitation, pan evaporation and evapotranspiration, and either before the point or after the point, pan evaporation has the inverse trend with evapotranspiration and precipitation has the same trend with evapotranspiration. The above analyses indicate pan evaporation and evapotranspiration have complimentary relationship. These results support the solution of the evaporation paradox described by Brutsaert and Parlange (1998) and suggest that decreases in pan evaporation indicate an increase in terrestrial evaporation in Xinjiang. The correlation analyses show that diurnal temperature range (DTR), wind speed, low cloud cover and precipitation are most likely the driving force for the reduction of pan evaporation and the ascending evapotranspiration.