Hargreaves公式的全局校正及适用性评价

Global calibration of Hargreaves equation and its applicability in China

Hargreaves公式是计算参考作物腾发量(ET0)最为简便的经验公式之一,但目前对于该公式在不同气候类型区域的适用性及误差影响因素尚缺乏系统评价。在月时间尺度上,以Penman-Monteith公式计算的ET0为基准值,利用洗牌复合形进化算法(SCE-UA),在中国105个气象站对Hargreaves公式进行了全局校正。分析了校正后Har-greaves公式参数的地区分布规律和影响因素;同时,从长系列过程和年内季节性过程两方面,综合说明了校正后Hargreaves公式在中国7个区域的适用性;讨论了影响Hargreaves公式计算精度的气象因素。结果表明:校正后Hargreaves公式可以有效提高计算精度,但计算精度的地区差异仍然比较明显。校正后的Hargreaves公式,对于青藏高原及毗邻区和西南、西北地区大部分站点,在年内各月均具有较强的适用性;对于东北、华北和新疆地区,夏秋季节的计算精度较高,而在冬春季节计算精度较低;对于华南地区的绝大多数站点,仅在年内2～3个月份具有较高计算精度。同时,对于大部分站点,Hargreaves公式无法有效描述ET0中的空气动力学项是计算误差产生的主要原因。校正后Hargreaves公式在年内各月计算精度与空气动力学项占ET0的比例(K)、月平均降水量(P)具有显著相关性。一般而言,K值越大,Hargreaves公式计算精度越低;P越大,Hargreaves公式计算精度越高。各月计算精度与风速、相对湿度的关系则比较复杂,在不同区域表现各异。

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 factors on the parameters are investigated.The applicability of the calibrated Hargreaves equation in seven different climatic regions is synthetically evaluated for the long-term 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.