Modifying Hargreaves model considering radiation to calculate reference crop evapotranspiration in hilly area of central Sichuan Basin

Abstract: Reference crop evapotranspiration (ET0) is an important parameter in water cycle and water balance. Accurate estimation of ET0 becomes vital in planning and optimizing irrigation schedules and irrigation system management. Numerous methods have been proposed for estimating ET0, among which the Penman-Monteith (P-M) method recommended by Food and Agriculture Organization of the United Nations (FAO) in 1998 provides the most accurate results across the world wherever in arid or humid environment. But the main problems when computing ET0 by the P-M method are its complicated nonlinear process and requirements of many climatic variables. Thus, there is an urgent need to develop a much simpler and more appropriate method in areas with limited data, such as the Hargreaves-Samani (H-S) method. In order to improve the ET0 calculation accuracy of the H-S method, in this research the daily data of 13 representative meteorological stations from 1954 to 2013 in hilly area of central Sichuan was used to calculate the ET0. The H-S method was modified based on the Bayesian principle and the impact of the radiation, and at the same time the accuracy of the modified method was evaluated. The result showed that in ZoneⅠ the parameter a was 0.915, in Zone Ⅱ was 0.922, in Zone Ⅲ was 0.912, and the parameter b was the function of radiation. The trend of the modified method's computing result was similar to the P-M method and both showed a parabolic shape. Compared with the H-S method, the average absolute error of ten-day ET0 with the modified H-S method decreased from 0.93 to 0.15 mm/d in Zone Ⅰ, from 0.95 to 0.19 mm/d in Zone Ⅱ and from 0.93 to 0.28 mm/d in Zone Ⅲ. The fitting equation slope of ten-day ET0 was 0.89, 0.94 and 0.90 respectively in these areas using the modified method, and the calculation accuracy was enhanced remarkably. During the period between March and October, the average absolute error of monthly ET0 decreased from 0.89 to 0.46 mm/d in ZoneⅠ, from 1.14 to 0.29 mm/d in ZoneⅡ and from 1.28 to 0.21 mm/d in Zone Ⅲ, respectively. In January, February and December, the average absolute error of monthly ET0 changed from 0.14 to 0.35 mm/d in ZoneⅠ, decreased from 0.31 to 0.30 mm/d in Zone Ⅱ and from 0.32 to 0.28 mm/d in Zone Ⅲ. The fitting equation slope of monthly ET0 was 0.83, 0.89 and 0.90 respectively in these areas using the modified method. The accuracy of modified method decreased when the altitude increased. In the whole year, the accuracy increased by 47%, the accuracy in Zone Ⅰ increased by 35% and the accuracy in Zone Ⅱ increased by 48% especially in the main crop growth period. Therefore, the modified H-S method based on the Bayesian theory is more adaptive and accurate, and can be used as a simple method for the calculation of ET0 in hilly area of central Sichuan Basin.