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

Abstract

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 as and bs 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 as and bs obtained from the different time scales corrected data affected the calculating solar radiation (Rs_c) precision, determined the optimal time scale for the corrected data, and compared and selected the as and bs with the minimum estimation error as the recommended values. The results show that our corrected as and bs coefficient values significantly reduce the range of the relative error of Rs_c, with 10 years being the best time scale for the corrected data. Further, the Rs_c precisions estimated by as and bs 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).