Comparison of ten potential evapotranspiration models and their attribution analyses for ten Chinese drainage basins

Potential evapotranspiration (PET) is usually calculated by empirical methods from surface meteorological variables, such as temperature, radiation and wind speed. The in situ measured pan evaporation (ETpan) can also be used as a proxy PET. In this study, PET values computed from ten models are compared with observed ETpan data in ten Chinese river basins for the 1961-2013 period. The daily observed meteorological variables at 2267 stations are used as the input to those models, and a ranking scheme is applied to rank the statistical quantities (ratio of standard deviations, correlation coefficient and ratio of trends) between ETpan and modeled PETs in different river basins. There are large deviations between the modeled PETs and the ETpan in both the magnitude and the annual trend at most stations. In 8 of the basins (except for Southeast and Southwest China), ETpan shows decreasing trends with magnitudes ranging between -0.01 mmday-1 yr-1 and -0.03 mmday-1 yr-1 while the decreasing trends in modeled PETs are less than -0.01 mmday-1 yr-1. Inter-comparisons among different models in different river basins suggest that, PETHam1 is the best model in the Pearl River basin, PETHam2 outperforms other models in the Huai, Yangtze and Yellow River basins, and PETFAO is the best model for the remaining basins. Sensitivity analyses reveal that wind speed and sunshine duration are two important factors for decreasing PET in most basins except in Southeast and Southwest China. The increasing PET trend in Southeast China is mainly attributed to the reduced relative humidity.