The standardized ASCE Penman–Monteith and FAO-56 equations were used to estimate reference evapotranspiration (ET
0) using estimated and measured net radiation (
Rn) and soil heat flux (
G), based on hourly and daily meteorological data. The estimates were evaluated against lysimeter measurements. The results indicate that using measured or estimated values of
Rn and
G can have significant effect on the accuracy of the ET
0 estimations, especially when calculations were made on an hourly basis. The FAO-56 version performed very well during the irrigation season on a daily basis. The use of measured
Rn and
G did not improve ET
0 estimation on a daily basis, therefore, the use of estimated
Rn and
G appears to be dependable when calculations are based on 24-h weather data. When daily ET
0 was calculated from hourly estimations, the results were different depending on the version used. The ASCE version was more accurate, especially when
Rn and
G were measured. Therefore, measurement of
Rn and
G may have potential to improve estimation only when daily ET
0 is calculated from hourly estimations. The PM FAO-56 version was always a little less accurate than the ASCE version. For hourly calculations, using a constant surface resistance (as in FAO-56 version), the PM method underpredicted for high evaporative demand and vice versa. The ASCE version performed better than PM FAO-56 version when
Rn and
G were measured and estimated. Therefore, ASCE version tended to provide quite accurate values of hourly ET
0, even using estimated values of
Rn and
G. As conclusion, the methods proposed by FAO-56 for estimating
Rn and
G tended to produce accurate estimates for daily and hourly ET
0 under semiarid conditions and can be used with some degree of confidence for estimating ET
0. In addition, results suggest that the ASCE standardized equation on an hourly basis improved the accuracy of ET
0 estimation with respect to the FAO-56 version.
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