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Estimating actual transpiration of apple trees based on infrared thermometry Published In Journal of Irrigation and Drainage Engineering, 141(8), 2015, by Y. Osroosh, R. Peters, and C. Campbell

Abstract. A method was developed based on the radiative properties and energy budget of a single apple leaf to calculate the actual transpiration (T) of apple trees. The model uses canopy temperature (Tc), air temperature (Ta) measured in the orchard, and other meteorological data from a local weather station as inputs. The model was applied to two scenarios, as follows: (1) well-watered, young Fuji apple trees in the 2007 and 2008 growing seasons; and (2) older apple trees, bearing little fruit in the 2013 growing season. Simulated transpiration rates at both scenarios were compared with Penman-Monteith (PM) model predictions corrected by regionally adjusted crop coefficients, i.e., values of ETc. In 2007 and 2008, a linear regression analysis of the relationship between daily mean transpiration (Tavg) and ETc revealed that they better agreed on warm and dry days (correlation coefficient R2=0.57, slope=1.16, and intercept=0.4) than during cold and humid periods (R2=0.48, slope=0.69, and intercept=2.3). Combining the results of the 2007 and 2008 seasons, Tavg and ETc presented a fairly good agreement, with the relationships R2, slope, and intercept of 0.77, 1.0, and 1.08, respectively. In 2013, the actual water use calculated by a soil water budget approach (ETWB) was considerably less than ETc while there was no significant difference between the total simulated transpiration (ΣTavg) and ETWB. In 2013, a linear regression analysis of the relationship between midday T (Tmid), Tavg, and midday stem water potential (Ψstem) showed they were highly correlated (Tmid, R2=0.85; Tavg, R2=0.87). The experiments presented varied results on the linear relationship between air vapor pressure deficit (Da) and Tavg from year to year. On the other hand, canopy and air temperature difference (ΔTm) was linearly related to Tavg in all of the seasons. According to the model for the actual transpiration (i.e., the T-model), the apple trees had an intense transpiration in the morning and then there was a decline around solar noon. The transpiration increased again late in the afternoon. Real-time estimations of water use using the T-model can provide a basis for a fully automated system of irrigating apple orchards.

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