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

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 (ET_a) on a rainfed crop field. A generalized complementary relationship (GCR) was employed to quantify the water stress coefficient (K_s) for the FAO-56 method with no use of soil moisture data. Then, the K_s values were multiplied to the crop ET (ET_c) 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 (ET_o) estimated by the corrected temperatures was within a range of 89–95% of that from non-corrected ones, implying that ET_o could be inflated when using the mean air temperatures measured under sub-humid conditions. The K_s values estimated by GCR showed consistent patterns with temporal rainfall variation on the field. The multiplication of the K_s from GCR and ET_c showed higher performance in ET_a 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 ET_a prediction. This study suggests that combination of the complementary principle and the FAO-standard method, which are seemingly in separate paradigms, may improve ET_a prediction on crop fields.