Journal of Irrigation and Drainage Engineering, Volume 141, Issue 9 (September 2015)

Y. Osroosh,1; T. R. Peters2; and C. S. Campbell3

1Postdoctoral Research Associate, Dept. of Biological Systems Engineering, Washington State Univ.-Prosser, 24106 N Bunn Rd., Prosser, WA 99350 (corresponding author). E-mail:
2Associate Professor/Extension Irrigation Specialist, Dept. of Biological Systems Engineering, Washington State Univ.-Prosser, 24106 N Bunn Rd., Prosser, WA 99350. E-mail:
3Vice President, Research and Development, Decagon Devices Inc., 2365 NE Hopkins Ct, Pullman, WA 99163. E-mail:


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 (Ep) for the whole apple tree. The Ep model was based on two main submodels predicting canopy temperature (Tc) and total canopy conductance (gT). The gT 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 Ep 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 (ETr). Daily average weather data collected during the three growing seasons provided the inputs to the Ep 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 ±0.6°C. The coefficient of variation (CV) of the predictions averaged 2% over the experiment plots/years, being better than that of the measurements (CV=4.8%) with the exception of one plot in 2007 with little difference (3% versus 2%). Ep was fairly correlated with ETr on warm and dry days (R2=0.58p<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 Ep 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.

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