American Society of Civil Engineers


Evaluation of Reference Evapotranspiration Estimation Methods under Southeast Australian Conditions


by Aftab H. Azhar, (corresponding author), (Assistant Professor, Centre of Excellence in Water Resources Engineering (CEWRE), Univ. of Engineering and Technology (UET), Lahore, Pakistan E-mail: aftab_azharpk@yahoo.co.uk) and B. J. C. Perera, (Professor/Associate Dean, Faculty of Health, Engineering and Science, Victoria Univ., Melbourne, Victoria 8001, Australia.)

Journal of Irrigation and Drainage Engineering, Vol. 137, No. 5, May 2011, pp. 268-279, (doi:  http://dx.doi.org/10.1061/(ASCE)IR.1943-4774.0000297)

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Document type: Journal Paper
Abstract: Reference evapotranspiration ( ETo) estimates are often required for use in water resources planning and irrigation scheduling. Ten ETo estimation methods ranging from simple temperature-based to data-extensive combination methods, including Hargreaves (HAR), improved Hargreaves (IHA), FAO-24 Radiation (RAD), Ritchi-type (RIT), FAO-24 Class-A Pan with pan coefficients of Doorenbos and Pruitt (PEV) and empirical regression coefficient (SEV), combination methods McIlroy (McI), FAO-Penman with wind functions of Watts and Hancock (W_H) and Meyer (M_PY), and the Penman-Monteith (P_M) were evaluated at three sites, namely, Aspendale, Griffith, and Tatura in the Goulburn-Murray Irrigation Area (GMIA) of southeastern Australia. At Aspendale, 4 out of 10 ETo methods (McI, M_PY, SEV, and RAD) overestimated the ETo estimates; at Griffith no method overestimated them, whereas at Tatura only the RAD method overestimated ETo. The overestimations were at Griffith, McI (1%), M_PY (10%), and SEV (4%); at Tatura, RAD (2%). At the Griffith and Tatura sites, almost all methods showed a strong tendency to underestimate daily ETo estimates throughout the entire range of evaporative demand. Overall, the underestimation ranges observed were McI (12–27%), W_H (7–22%), RIT (6–25%), PEV (19–31%), HAR (18–31%), and IHA (8–11%). The underestimation of daily ETo estimates by the P_M method ranged from 21 to 29%, raising caution about its use as a base method (without calibration against measured data under local conditions) to evaluate other ETo methods, as has been advocated in recent literature. The use of the McI method as the top-ranked method at Aspendale and Tatura, and the W_H method at Griffith, indicated that no single daily ETo estimation method using meteorological data was satisfactory for all three sites. Generally, the combination methods proved to be the most accurate ETo estimates. At Tatura, the fact that the RAD method was ahead of the W_H and M_P combination methods indicates how a less data-intensive ETo method, if calibrated, can perform even better than a physically based combination method. All ETo estimation methods required local calibration against measured lysimeter ETo data for better performance.


ASCE Subject Headings:
Irrigation
Scheduling
Temperature effects
Evapotranspiration
Water management
Evaluation
Radiation
Australia

Author Keywords:
Evapotranspiration
Evaluation
Temperature
Radiation
Combination
Irrigation scheduling
Australia