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Saeed Kazemi Mohsenabadi, Mohammad Reza Biglari

Estimation of evapotranspiration has always been regarded as an important phase of hydrological cycle in different areas. Besids, dominant of this phenomenon in some agriculture and civil domains such as determination of plants water need, designi water canals reveals and estimation of the main part of water loss in dams backside duplicated the importance of this phenomenon and its estimation. The precise Method in estimating evapotranspiration is using lysimetric data, but regarding the lack of this kind of data for regions, experimental equations have been accounted as common Method. in this research, 14 experimental Methods have been used in estimating the potential evapotranspiration of Qazvin. The main goal of this study after comparing the result of empirical equations with pan measured values is presentation of corrective equations using linear regression Method for all equation and evaluation of accuracy of these equations before and after revision. Based on the result of this study, Blaeny-Criddle Method in order to estimate the potential evapotranspiration of Qazvin in Oct, Jul and Jun, and Romanenko Method for Sep and Aug are suggested. Results of the corrective equations showed that using linear regression Method and presentation of corrective equations predominantly increase the accuracy of empirical equations. So that revised Thorenthwait Method with correlation coefficient of 0.988 (R2= 0.988) obtained as the best corrective equation and Jensen-Hais Method was specified as the study area. In addition, after revising this equation and obtained correlation coefficient of 0.868 (R2=0.868), salient decrease of error percentage in all evaluated months has been indicated.