Calibration and Optimization of the Ångström–Prescott Coefficients for Calculating ET₀ within a Year in China: The Best Corrected Data Time Scale and Optimization Parameters

• Main Authors: Xingsheng Xia, Xiufang Zhu, Yaozhong Pan, Xizhen Zhao, Jinshui Zhang
• Format: Article
• Language: English
• Published: MDPI AG 2019-08-01
• Series: Water
• Subjects: solar radiation; Ångström–Prescott formula; parameter calibration; reference crop evapotranspiration
• Online Access: https://www.mdpi.com/2073-4441/11/8/1706

This study used meteorological data from official data sets to correct Ångström−Prescott formula parameters for China’s agricultural zones for which existing research encountered the problem of spatio-temporal scale disunity. The data, collected from 124 stations, were used to correct the a_s and b_s coefficients of the Ångström−Prescott formula, by area, at 5−50 year-scales, the former taking into account China’s comprehensive agricultural zones. We focused on how the a_s and b_s obtained from the different time scales corrected data affected the calculating solar radiation (R_{s_c}) precision, determined the optimal time scale for the corrected data, and compared and selected the a_s and b_s with the minimum estimation error as the recommended values. The results show that our corrected a_s and b_s coefficient values significantly reduce the range of the relative error of R_{s_c}, with 10 years being the best time scale for the corrected data. Further, the R_{s_c} precisions estimated by a_s and b_s coefficients based on the Food and Agriculture Organization of the United Nations (FAO) and the regression result of the best time scale corrected data are inconsistent in different months by area. The best choice in practice is combining the two coefficients and optimizing their use. This study provides a research-based process for standardizing the correction of Ångström−Prescott formula parameters and selecting the corrected data time scale in China. It would be helpful in improving the calculation accuracy for reference crop evapotranspiration (ET₀).