A hybrid approach combining the FAO-56 method and the complementary principle for predicting daily evapotranspiration on a rainfed crop field

Abstract

In this study, we proposed a hybrid approach combining the standard FAO-56 method with the complementary principle of evapotranspiration (ET) to predict actual ET (ETa) on a rainfed crop field. A generalized complementary relationship (GCR) was employed to quantify the water stress coefficient (Ks) for the FAO-56 method with no use of soil moisture data. Then, the Ks values were multiplied to the crop ET (ETc) estimated with the corrected air temperatures for the assumption of adequate land-surface moisture. The proposed hybrid method applied to a rotational maize-soybean field where an eddy-covariance flux tower was installed. Results showed that the reference ET (ETo) estimated by the corrected temperatures was within a range of 89–95% of that from non-corrected ones, implying that ETo could be inflated when using the mean air temperatures measured under sub-humid conditions. The Ks values estimated by GCR showed consistent patterns with temporal rainfall variation on the field. The multiplication of the Ks from GCR and ETc showed higher performance in ETa prediction than when the GCR was solely applied. By inverting the hybrid approach, a local crop coefficient curve for the rainfed maize field was developed, and led to further improvement in ETa prediction. This study suggests that combination of the complementary principle and the FAO-standard method, which are seemingly in separate paradigms, may improve ETa prediction on crop fields.