Abstract
Reference crop evapotranspiration (ETo) is essential for irrigation, water resources management and environmental assessment. The indirect estimation of ETo is based a) on energy budget approach using meteorological data and b) pan evaporation measurements (Epan) multiplied by pan coefficients (kp) adapted to the surrounding environmental conditions. Significant interest is shown for the kp equations, which have to be tested before their use. The purpose of this study is to evaluate six different kp equations, such as those of Cuenca, Allen and Puitt, Snyder, Pereira et al., Orang, Raghuwanshi and Wallender for the summer growing season (April to October) of Thessaloniki plain in Greece, which is characterized by a semi-arid Mediterranean environment. The evaluation of the kp equations is performed by two years Epan measurements, using as reference the daily ETo values estimated by the ASCE-standardized Penman-Monteith equation (ASCE-PM) in hourly time step. The results of this study showed that Cuenca’s equation provided more accurate daily estimations. Additional analysis is performed in other methods such as those of FAO-56 and Hargreaves based on the calculation time step (hourly or daily) and their correspondence to the ASCE-PM.
Highlights
The reference crop evapotranspiration (ETo) constitutes the major factor for the accurate estimation of the crop water requirements, which are essential in irrigation planning, scheduling, hydrologic balance studies and watershed hydrology
Many methods have been proposed for the estimation of evapotranspiration for short (ETos), based on the energy budget approach, such as the corrected FAO-24 Blaney-Criddle method [1], the Priestley-Taylor method [2], the corrected FAO-24 Penman method [3], the Shuttleworth and Wallace method [4], the Hargreaves method [5], the FAO-56 Penman-Monteith method [6] and the ASCEstandardized Penman-Monteith method [7]
The results showed a relative underestimation of the calculated ΕΤο for all the methods compared to the daily ΕΤο values of ASCE-standardized Penman-Monteith method (ASCE-PM) method in hourly time-step, for both cases of the reference crop
Summary
The reference crop evapotranspiration (ETo) constitutes the major factor for the accurate estimation of the crop water requirements, which are essential in irrigation planning, scheduling, hydrologic balance studies and watershed hydrology. Many methods have been proposed for the estimation of ETo, based on the energy budget approach, such as the corrected FAO-24 Blaney-Criddle method [1], the Priestley-Taylor method [2], the corrected FAO-24 Penman method [3], the Shuttleworth and Wallace method [4], the Hargreaves method [5], the FAO-56 Penman-Monteith method [6] and the ASCEstandardized Penman-Monteith method [7]. Additional studies have been performed for the development of new methodologies for the estimation of ETo using empirical equations such as the “Copais model” [12] or equations based on Penman formula with reduced parameters [13]
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