One of the most important componenets of water management in farms is estimating crops’ exact amount of  evapotranspiration (water need). The FAO-Penman-Montheis (FPM) method is a standard method to evaluate other techniques which are used for easy calculation of potential evapotranspiration, when lysimeter datasheets are not available. This study was carried out based on 18 years’ climatic data of Birjand (1998-2016), to determine the best experimental method based on coefficient of evaporation pan amongst methods proposed by Cuenica, Allen and Pruitt, Snyder and Orang. Then, the results were compared with the FPM method. The best method was used to assess the capabilities of gene expression method for prediction of daily coefficient of evaporation pan, which was the major goal of this study. The best equation for calculation of daily coefficient of evaporation pan was determined using these statistical factors: coefficient of determination, root mean squared error, mean absolute error, and percent error. The results showed that in Birjand’s climatic conditions, it is best to use the equation of generated by the gene expression method with a coefficient of determination of 0.77 and mean squared error of 0.021 millimeter per day.