Phenology and canopy conductance limit the accuracy of 20 evapotranspiration models in predicting transpiration

Transpiration is a fundamental biophysical process, directly measured in plants by dividing sap flow by total leaf area. Under non-limiting conditions, transpiration and reference evapotranspiration (ETo) are hypothesized to be equal when ETo is normalized by the leaf area index of the reference crop, i.e., LAI = 2.88. Known as the E-2.88 model, it has only been tested with ET(o )derived from Penman-Monteith FAO56. Phenological influences on canopy conductance potentially decouple transpiration from atmospheric evaporative demand and lower the accuracy of E2.88. This study tested the accuracy of 20 E-2.88 models in predicting apple (Malus domestica (Suckow) Borkh. var. Granny Smith) and pear (Pyrus communis L. var. Beurre Bosc Pear) transpiration over the 2020-2021 austral growing season. For apple, the Penman-Monteith ASCE-EWRI model had the highest predictive power with 7% error and r(2) = 0.89; whereas for pear the Valiantzas (2018, Eq. (7)) showed 2% error and r(2) = 0.96 evaluated via linear regression. Generally, models that included a humidity parameter had stronger predictive power than models excluding humidity. Yet, the predictive power of the E-2.88 models decreased considering the phenological phases for each crop. For apple, early and late season E(2.88 )models underestimated transpiration by at least 27%. For pear, late season error increased to 7% as the E-2.88 models overestimated transpiration. Canopy conductance and the atmospheric decoupling factor were significantly greater in early and late season for apple and significantly lower in late season for pear. Therefore, phenology decreased the predictive power of the E-2.88 model in early and late season by decoupling physiological processes from atmospheric evaporative demand.