Variation in reference crop evapotranspiration caused by the Ångström-Prescott coefficient: Locally calibrated versus the FAO recommended

Abstract

Accurate estimation of the reference crop evapotranspiration (ET0) is investigated due to its critical role in affecting calculation of crop water use and efficiency in agricultural ecosystems. The main emphasis in this paper is to clarify the possible uncertainty in the estimation of ET0 associated with using un-calibrated Ångström-Prescott (A-P) coefficients. We first calibrated the coefficients using long-term data records from 34 sites in the Yellow River basin in China, and then applied these coefficients to estimate short wave irradiance (Rs) and ET0 at 16 sites to evaluate the difference in ET0 between the FAO recommended and the locally calibrated. We found that the direct use of the FAO recommended coefficients significantly affected the estimation of ET0 at most sites, which differed from -3% to 15% at daily scale and from -4% to 16% at monthly scale from the locally calibrated ones. These differences are comparable with or larger than those caused by some alternatives of the FAO recommended algorithms for net irradiance or vapor pressure, which further highlights the importance of using the locally calibrated coefficients. The degree of difference in ET0 showed a significant threshold relation with altitude and longitude in such a way that relatively small impact lies around 2233m and 98°E, and away from these, the effect begins to increase. Given the large overestimation in water use as a consequence of the significant overestimation in ET0 associated with the direct use of the FAO coefficients, especially in those high yield production areas with altitudeReference crop evapotranspiration Ångström-Prescott coefficients Penman-Monteith