Estimating Potential Transpiration of Apple Trees Using Theoretical Non-Water-Stressed Baselines

To maximize irrigation efficiency, applied water has to be precisely adjusted to the crop water use. This study develops a method based on the energy balance of a single apple leaf to calculate potential transpiration ($E_p$) for the whole apple tree. The $E_p$ model was based on two main submodels predicting canopy temperature ($T_c$) and total canopy conductance ($g_T$). The $g_T$ model was derived by simplifying the energy budget to rely on only climatic data and an empirical coefficient. These submodels were evaluated using the canopy temperature data collected in a Fuji apple orchard during the 2007, 2008, and 2013 growing seasons. The applicability of the $E_p$ model was examined on (1) well-watered, young Fuji apple trees, and (2) well-irrigated, older apple trees bearing little fruit. Predicted potential transpiration rates at both scenarios were compared with those predicted by the ASCE standardized Penman–Monteith values for alfalfa (${\mathrm{ET}}_r$). Daily average weather data collected during the three growing seasons provided the inputs to the $E_p$ model and its components. With the exception of air temperature measured in the orchard, the rest of the meteorological data were obtained from a local weather station. The canopy temperatures of the fully watered trees were predicted during midseason with mean absolute errors (MAEs) of about 0.41, 0.33, and 0.23°C in 2007, 2008, and 2013, respectively. These MAEs were better than the individual IRT accuracy of $\pm 0.6°\mathrm{C}$. The coefficient of variation (CV) of the predictions averaged 2% over the experiment $\mathrm{plots}/\mathrm{years}$, being better than that of the measurements ($\mathrm{CV}=4.8\%$) with the exception of one plot in 2007 with little difference (3% versus 2%). $E_p$ was fairly correlated with ${\mathrm{ET}}_r$ on warm and dry days ($R^2=0.58$, $p<0.001$) with slope and intercept values of close to 1.0 and 0.0, respectively. The model was able to reflect the high degree of coupling between the apple trees and the humidity of the surrounding air during cold and humid periods as $E_p$ resulted in significantly lower values. The overall results of the experiments with Fuji apple trees showed that the non-water-stressed baselines and potential transpiration of Fuji apple trees can be estimated using the proposed approach.