Abstract:

The daily reference crop evapotranspiration (ET0) was estimated using the FAO-56 Penman-Monteith and five other methods (Irmak-Allen, Makkink, Turc, Jensen-Haise and Hargreaves) and meteorological data from seven sites located in inside and surrounding areas of the Qinhuai River Basin for the period of 2000-2013.Taken the FAO-56 Penman-Montieth method as a reference, the original empirical coefficients of five other methods were calibrated. The results were analyzed with mean absolute error (MAE), mean relative error (MRE), correlation coefficient (r) and non-parametric Wilcoxon test at an annual and a monthly scale respectively. We aimed to obtain one method which requires less data with high accuracy for the Qinhuai River Basin. With daily results of five methods as independent variable and daily results of FAO-56 Penman-Montieth method as dependent variable, monthly linear regression equations were established. Monthly correction parameters could be found based on these equations. This research indicated that, Irmak-Allen, Makkink, Turc and Hargreaves methods were not applicable at an annual scale when the original empirical coefficients were used. At a monthly scale, when original empirical coefficients were used, large biases and significant differences were found for most of the methods in most months except May to August for Irmak-Allen method, September to November for Turc method, April and September to November for Hargreaves method. After model calibration, the Makkink method performed best followed by Turc method at annual scale. The MAE,MRE and r was 14.9mm·y-1,1.42% and 0.89 respectively. No significant difference existed between the results of Makkink and the FAO-56 Penman-Montieth method. In contrast, there were significant differences between the results of Irmak-Allen and FAO-56 Penman-Montieth method. The Hargreaves method was still not applicable due to a poor correlation. At a monthly scale, considering the accuracy of estimation, Makkink and Turc methods were used by collocation. Turc method was recommended to use from April to October, MAE and MRE was 2.1-6.1mm·mon-1 and 2.9%-4.3%, Makkink method was recommended to use in the period from January to May and November to December, MAE and MRE was 1.2-2.4mm·mon-1 and 3.2%-5.7%. No significant difference existed and coefficient of variation of monthly MRE was small in the two periods for each method. Considering continuity of time, Hargreaves method was recommended to use from January to December, MAE and MRE was 1.9-10.4mm·mon-1 and 4.0%-9.2%.No significant difference existed and coefficient of variation of monthly MRE was small from January to December for Hargreaves method.

Key words: Qinhuai River Basin, Penman-Monteith, Radiation-based method, Temperature-based method