Canopy Resistance and Actual Evapotranspiration over an Olive Orchard

Τhis study evaluates the hourly actual evapotranspiration (AΕT), predicted either by the two modified Penman-Monteith models (PM) which take into account the canopy resistance (r_c) from the Katerji-Perrier (KP) or Todorovic (TD) models, or the simplified PM model with zero r_c, as proposed by Priestley and Taylor (PT). The evaluation is based on comparisons with experimental measurements of AΕT applying the ‘Bowen ratio’ method. Hourly experimental data, of air temperature, humidity, wind speed and radiation balance measurements, taken at a 0.5 ha olive orchard in the rural area of Sparta (37° 04΄ N, 22°05΄ E), during the period from June 2010 up to July 2014, are used. The r_c estimated by KP model is parameterized by a semi-empirical approach which requires a simple calibration procedure, while r_c from TD model is parameterized using a theoretical approach. For estimating AET from minimum data (air temperature, humidity and radiation balance components) the PT model is also employed, since r_c is not required and the aerodynamic term of PM is taken into account in the empirical parameter of the model. The results show that PT and KP models are the most appropriate [Refined Index of Agreement (RIA) equal to 0.89 or 0.88, respectively] followed by the TD model (RIA = 0.78). PT or KP models underestimate AET by 9.3% or 9.8%, respectively, while TD model overestimates AET by 15.0%, increased up to 25.8%, during warm period.