Comparing Potential Evapotranspiration and Pan Evaporation at Several Research Stations in Western China.

Potential evapotranspiration (ET_o) is a very important parameter in meteorologic and hydrologic studies, as it is the upper limit of evaporation. Using 50 years of meteorological data from 7 stations in western China, ET_o was calculated with the Penman-Monteith equation. The objective of this study was to compare ET_o and E_pan, pan evaporation. The results showed: (1) There were linear relations between E_pan and ET_o at all 7 stations. The ratio of ET_o to E_pan (k_p) varied from 0.46 to 0.79 at the 7 stations, and its variation was not correlated with any single meteorological or geological factor; (2) The value of k_p was found to be time scale dependent. For longer time scales there was a closer relationship between ET_o and E_pan, and k_p increased slightly; (3) Over the 50 years the data showed a trend of increasing air temperature, decreasing relatively humidity, and decreasing sunlight hours for all 7 stations. The trends in precipitation differed for the 7 stations. The trend in ET_o increased at one station but decreased at the other 6 stations. The results of this study indicate that ET_o can be accurately estimated from E_pan for this part of China.