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Abstract
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Shortage of water is a challenge to agriculture in arid and semi-arid regions. For these regions, improving irrigation management to use water more precisely is needed. Improving evapotranspiration (ET) models and identifying factors governing reference ET can lead to formulation of strategy to precision irrigation and prevent water losses. A study was conducted in a pilot farm in Karaj, Iran, during 2014 to evaluate the factors affecting reference ET of the FAO Penman-Monteith (P-M) model in a semi-arid environment. The corn ET obtained from the P-M model applying single and dual crop coefficients and pan evaporation were compared with lysimeter measurements. In addition to providing the correction between these methods, a simple equation was presented to estimate crop ET using FAO P-M model and applying the effective meteorological parameters in the model. According to the results corn ET obtained from P-M model applying dual crop coefficients with RMSE = 0.69 mm/day had the best performance compared to lysimeter measurements. Considering the statistical indicators, corn ET using dual crop coefficient had lower RMSE and a greater correlation compared to other methods. Corn ET obtained from pan evaporation with applying single and dual crop coefficients was in the next place. Sensitivity analysis of reference ET to changes in meteorological variables was conducted by increasing and decreasing ± 10, ± 20 and ± 30% an individual climate variable holding the other variables constant. Results indicated that under Karaj climate condition, maximum temperature, wind speed, and relative humidity had stronger effects on reference ET, than sunshine duration and minimum temperature. After sensitivity analysis and determining the sensitive factors in the FAO P-M equation, multivariate regression was performed using SPSS software between sensitive parameters and the calculated ET0 by FAO P-M. Although the FAO P-M equation is widely used around the world, but due to its large me
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