中国综合农业分区下的Ångström-Prescott公式系数逐月校正与优选

Monthly calibration and optimization of Ångström-Prescott equation coefficients for agricultural comprehensive area in China

Ångström-Prescott公式是联合国粮食及农业组织(FAO)推荐的计算地表有效总太阳辐射(Rs)数据以支持参考作物需水量估算等研究的简便方法。本文以优选中国综合农业分区的Ångström-Prescott公式系数as、bs为目标,采用最小二乘回归方法,以全国范围内121个地面气象站点1957—2010年的逐月Rs和日照百分比数据计算获得了各农业区的逐月as、bs系数,并以2011—2016年的Rs观测值为真值,比较验证了分别以as、bs系数回归值和FAO的建议值计算的Rs相对精度。结果表明,38个农业子区逐月的站点平均as、bs系数取值无论是在时间上还是在空间上均存在不稳定性,且与FAO的推荐值存在明显差异。整体上,以as、bs系数回归值计算的Rs相对精度要优于FAO的建议值计算的Rs相对精度,但是在各农业子区内,前者并不是在所有月份均优于后者。综合考虑二者的精度比较结果,建议在全国大规模的实践中,东北区、内蒙古及长城沿线区、黄淮海区、黄土高原区、甘新区仍然使用FAO推荐的Ångström-Prescott公式系数,而在长江中下游区、西南区、华南区、青藏区则建议以矫正后的站点均值作为Ångström-Prescott公式的系数;在局部区域高精度的计算中,建议使用优选值作为Ångström-Prescott公式的系数以获得最优的Rs估算值。本研究方法简单、可操作性强,在现有的数据条件下,对提高地表太阳辐射和参考作物需水量的计算精度有一定的参考价值。

ngström-Prescott equation is the recommended algorithm for calculating the radiation coefficients for the Penman-Monteith formula, which is the standard method for reference crop evapotranspiration recommended by the Food and Agriculture Organization(FAO) of the United Nations. The calibration and optimization of asand bscoefficients in the equation is the key to accurately calculate the surface solar radiation. This study aims at obtaining the Ångström-Prescott equation coefficients asand bs, which are optimized for China’s comprehensive agricultural areas. The monthly average solar radiation(Rs)(from Dataset of Monthly Values of Radiation Data from Chinese Surface Stations) and srelative sunshine duration data(from Dataset of Monthly Values of Climate Data from Chinese Surface Stations) at 121 stations during 1957-2016 were collected. Using the data from 1957 to 2010,we calculated the monthly asand bscoefficients for each area through the least squares regression. Then, taking the observation values of Rsfrom 2011 to 2016 as the true values, we estimated and compared the relative accuracy of Rscalculated by regression values of coefficients asand bsand that calculated by FAO suggested coefficients asand bs. The results showed that the monthly average coefficients asand bsof each area are significantly different from the FAO recommended coefficients both temporally and spatially. There are some differences between regions and within regions, and the relative value of asand bsshows the opposite state. The relative error range(0-54%) of solar radiation calculated by the regression asand bscoefficients is small, while the relative error range(0-77%) of solar radiation calculated by the FAO recommended value is large. So, overall, the relative accuracy of Rs calculated by regression values of asand bscoefficients is better than that calculated by the FAO suggested coefficients. The relative error was reduced by 1% to 6%, and the relative error decreases more in winter and spring than in summer and autumn. However, regression values of asand bscoefficients perform worse in some months and some agricultural areas for verification in application. It is said that the regression values of asand bsare not entirely reliable. For each month and each agricultural area, the best scheme is to combine the regression values of asand bscoefficients with the FAO recommended values. Therefore, we chose the asand bscoefficients with the minimum Rsestimation error as the final coefficients and made a coefficient recommendation table for 38 agricultural production and management areas in the Chinese mainland. This study further illustrates the necessity of localization modification of Ångström-Prescott equation coefficients in application, and enriches the case study of coefficient calibration of Ångström-Prescott equation in China, which is helpful for improving the accuracy of calculation of surface solar radiation and reference crop evapotranspiration based on existing data.