evaluation of potential evapotranspiration and pan evaporative methods by lysimeter data in a semiarid climate (case study: qazvin plain)

iran was located in the arid belt and desert region of the world. this climatic situation was presented in several parts of iran such as qazvin. evapotranspiration is one the most important parameter which was effect on agricultural development. there are different methods for estimating this item which selected by climatic conditions. this research concentrates to evaluate 28 empirical and combined methods in qazvin plain. lysimeter data was selected as a base data and daily results of 28 methods was evaluated with lysimeter. the results showed that hargreaves-samani as empirical method and temperature group with r=0.87, rmse=0.34 mm/day and mbe=-0.71 mm/day is best method in qazvin. orang from pan evaporative method was more appropriated daily results with lysimeter data. to estimate evaporate from free water surface, penman 1948 is the best method which has highest correlation coefficient (0.7) and lowest standard error (1.65 mm/day) and also mean absolute error (8.63e-6 mm/day). amount of average evaporating from water surface with penman method was calculated 10.18 mm/day.