A comparison between experimental and simulated data, considering the Priestley and Taylor (PT) and Penman-Monteith (PM) Reference-Evapotranspiration (ET) approaches was carried out. Experimental data, obtained from an irrigation assessment, conducted during the 1995 and 1996 maize growth-seasons at Zaragoza, Spain, was compared to the mechanistic-model SWAP simulation-results, considering each of the ET 0 calculation approaches in the model input. Soil hydraulic properties, meteorological data, seeding and harvest dates, crop water management and other experimental data were used as SWAP input. As corresponding to the windy and dry conditions found in many Mediterranean landplanes, PT ET 0 values were significantly lower than PM ET 0 calculations. Furthermore, simulated actual evapotranspirations considering the PT approach (PT-ET c) were lower than those found in the simulations that consider the PM approach (PM-ET c). Correspondingly, simulated drainage flux and soil water contents were higher when the PT ET 0 approach was used. The correlation coefficients between simulated and measured actual maize evapotranspirations and soil water contents were statistically significant, but the same for both ET 0 calculation approaches. Mean and median differences between actual and simulated maize water-use were not statistically different from zero for both considered ET 0 calculation approaches. Experimental data variability was significantly higher than simulated variability. The comparisons among the evaluated irrigation options, made with the experimental water-use data, lead almost to the same conclusions than those achieved from the simulated maize water-use. Considering PM-ET c rather than PT-ET c yields no statistical difference in the modeling-based conclusions. According to the obtained results, the PT approach could be used under Mediterranean conditions for comparative assessments aimed to support irrigation decision-making.

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