Nearly Four Decades, Pan Evaporation And Climate Change And Potential Evapotranspiration Estimates

The ground's moisture-content which governs agricultural productivity depends on the difference between the rainfall and surface evaporation. Both of which may be affected by global warming. You might expect evaporation to increase as a result of the temperature rise. But, on the contrary, during the past half century, Pan evaporation, measured by pan evaporimeters, has decreased over the United States and former Soviet Union(Peterson et al., 1995) and India (Ghattopashyay and Hulme, 1997) and China( liu et al.,2004). The decline in pan evaporation is at odds with the expectation that global warming caused by increasing emissions of greenhouse gases would increase potential evaporation. How the hydrological cycle change? Is the hydrological cycle accelerating or decelerating? How the actual evapotranspiration change? It is very important for the meteorologists and hydrologist to resolve them.one purposes of this paper is to present a new analysis of the trends of pan evaporation over the last forty years in China. On a regional basis and for the country as a whole. We compare pan evaporation measurements with other climate parameters such as mean, maximum and minimum temperature, solar irradiance, water vapor press deficit, cloud cover, relative humidity, precipitation, sunshine time, and wind speed. In our analysis we explore the possible causes of changing pan evaporation rates, considering these alternative explanations..The other purpose of this paper is to compare the Thornthwaite method and pan data with the standard FAO Penman-Monteith estimates of reference evapotranspiration in China. Various methods are available to estimate reference evapotranspiration(ETO) from standard meteorological observations. The Penman-Monteith method is considered to be the most physical and reliable method and is often used as a standard to verify other empirical methods. So here the Penman-Monteith method is used as a reference, and its spatial and temporal differences with the Thornthwaite method and pan measurement are evaluated.The decrease is statistically significant in whole China and all of its eight climatic regions, pan evaporation declines the most in the northwest, with successively smaller rates of decline moving south and east. The lowest rate of decline was in the northeast.The most possible contributing factors for whole China are wind speed, DTR, sunshine time, solar irradiance.The most possible contributing factors for all the eight climate regions in the last forty years are different.ETO estimated by the Penman-Monteith method as recommended by the FAO shows large regional differences temporal variation over China.There are obvious differences between the estimations of Thornthwaite method and the estimations of FAO Penman-Monteith method. In southern China, the values of Thornthwaite method are less than the values of FAO Penman-Monteith method, by contraries, in northern China, the values of Thornthwaite method are more than the values of FAO Penman-Monteith method.The temporal variation of ETO is much better represented by pan measurement than by the Thornthwaite method. The positive and high correlation in time plus the consistent regional and temporal deviation indicate that pan measurement can be a good substitute for the Penman-Monteith method if appropriate corrections are made to account for the systematic errors. The pan coefficients for 8 regions in China can be used to estimate regional ETO from pan measurement. The annual mean pan coefficient ranges from 0.35-0.85, 70% values between 0.55 and 0.85, the average mean of China are about 0.6.