Global calibration of Hargreaves equation and its applicability in China

As one of the simplest empirical methods for positional evapotranspiration(ET₀) calculation,it is still lack of systemic evaluation for Hargreaves equation's computational accuracy and applicability evaluation in different climatic regions.In this paper,the Hargreaves equation is calibrated using the global optimization algorithm SCE-UA(Shuffled Complex Evolution method developed at University of Arizona).The value of ET₀ calculated by the Penman-Monteith equation is used as the benchmark.The global calibration is done using a monthly time step at 105 climate stations distributed over the continental China as well as on Hainan Island.The regional distribution of the calibrated parameters is analyzed and influencing-actors on the parameters are investigated.The applicability of the calibrated Hargreaves equation in seven different climatic regions is synthetically evaluated for the longterm consistency and the seasonal consistency.The influencing factors on the accuracy of the calibrated Hargreaves equation are investigated using the method of correlation analysis.Results show that globally calibrated parameter values of the Hargreaves equation using SCE-UA are essentially different from that recommended by Food and Agriculture Organization(FAO) of the United Nations.However,the accuracy of the equation is highly promoted and the calibrated parameters at the 105 stations exhibit a certain regional regularity and may vary with annual precipitation and temperature.At almost all the stations on the Qinghai-Tibet Plateau,Northwest and Southwest China,the calibrated Hargreaves equation is applicable all year round.In contrast,poor calculation of ET₀ is found in all seasons in South China.The calibrated Hargreaves equation is only valid during summer and autumn seasons over Northeast and North China and the Xinjiang Autonomous Region.Generally,the disability in calculating the aerodynamic component of ET₀ is the main source of errors causing computational defect in the Hargreaves equation over different regions.The accuracy of the calibrated Hargreaves at most stations is notably related to the ratio(K) of the aerodynamic component to ET₀ calculated by the Penman-Monteith equation.The computational accuracy deteriorates with decreasing of K.The opposite is true for annual precipitation and temperature.While,effects of relative humidity and wind speed on the computational accuracy are rather complicated,varying with regions.