Sensitivity Analysis of Reference Crop Evapotranspiration Estimation Methods to Meteorological Factors in Greenhouse Conditions

One of the most important factors in calculation of greenhouse crop water requirement is the accurate estimation of reference crop evapotranspiration. Numerous parameters affect the evapotranspiration rate of the reference crop; with the knowledge of the most important factor the simulation error will be reduced. Because of the differences among mathematical structure and underlying assumption of indirect reference crop evapotranspiration estimation methods, behavior of the response variable to changes in climatic parameters may become inconsistent. Therefore, it is mandatory to analyze the sensitivity as well as determining the influence of meteorological factors on reference crop evapotranspiration. In this study, the sensitivity of Penman-Monteith, Irmak, Copais and Valiantzas methods to changes in meteorological factors including maximum and minimum daily temperature, maximum and minimum relative humidity and short-wave radiation was analyzed in the research greenhouse of the College of Agriculture and Natural Resources of the University of Tehran, located in Karaj, Iran. Sensitivity coefficients were calculated by changing meteorological factors in the range of [20, -20] percent with 5% intervals. Results showed that in all methods, the internal radiation was the most influential variable in determining the reference crop evapotranspiration under greenhouse conditions. The obtained sensitivity coefficient for this variable in Penman-Monteith, Irmak, Copais and Valiantzas methods was equal to 0.78, 0.71, 0.68 and 0.63, respectively. In contrast, the least important climatic variable was the minimum humidity in Penman-Monteith, Copais and Valiantzas methods and the minimum temperature in Irmak method. The results also showed that Penman-Monteith is the most sensitive method to changes in meteorological factors. As a result, in case of using of Penman-Monteith method for reference crop evapotranspiration estimation in greenhouse conditions, ensuring the correct operation of measuring instruments and their accuracy is essential.