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Knowledge of evapotranspiration, ET, is crucial for hydrological and micrometeorological studies including model calibration, because ET links two fundamental equations; the surface energy balance and the water balance. However, regardless the use of direct and indirect methods, measuring ET is difficult due to a number of limitations and/or shortcomings involved in methodologies and instrumentation. For irrigation planning, ET has traditionally been estimated using crop coefficient, Kc, values, where the actual evapotranspiration, ETa, is determined as, ETa = Kc x ETo [ETo is the reference evapotranspiration]. ETo is estimated from weather or climate data using semi-empirical equations (Doorenbos and Pruitt, 1977; Allen et al., 1998; Allen et al., 2005). Weighing lysimeters have been traditionally used to measure ETa and estimate the Kc = ETc/ETo, where ETc = ETa assuming little or no loss of evaporation due to stress. However, acquisition and maintenance of a large weighing lysimeter is expensive (Scott et al., 2005), and they are not transportable. Therefore, it is difficult to assess Kc over different agricultural areas because Kc evaluation is constrained to a unique crop per season, to a given climate and to a site-specific crop management. It is of interest to mention that it is often difficult to achieve uniformity within and outside of the lysimeter for tree orchards because of the size of the plants, and that under windy conditions lysimeters may not be reliable. For many agricultural institutions, the eddy covariance, EC, method offers an alternative to lysimeters, however, the method is somewhat complex, instrumentation is stringent and gas analyzers are expensive. Less costly micrometeorological methods, based on similarity theory (e.g., the Bowen ratio -energy balance method, aerodynamic method, etc.) have been investigated for use in agriculture (Meyers and Baldocchi, 2005; Drexler et al., 2004; Brustaert, 1988), but neither method has been widely adopted likely due to shortcomings under some climate features. It is not straightforward to measure accurate local humidity gradients and similarity based methods operate within the inertial sub-layer. Therefore, its application is often constrained to non-shallow inversions and to extensive fields to meet fetch requirements. Thus, full season monitoring over a growing crop with a relatively tall canopy requires sensor mounting at high levels, and the lack of fetch often limits application of similarity-based methods. To better interpret the measurements obtained using direct micrometeorological methods, including the EC method, it is highly recommended to take the measurements required to evaluate the closure of the surface

Is It Worthy to Apply Different Methods to Determine Latent Heat Fluxes? - A Study Case Over a Peach Orchard