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The main objective of this study is to investigate the effects of climatic parameters variability on evapotranspiration in five climatologically different regions of Iran. The regions include Tehran, Esfahan, Shiraz, Tabriz and Mashhad. Fifty four-year monthly records of temperature, relative humidity, sunshine duration, wind speed, and precipitation depth from 1951 to 2005 comprise the database. Trend and persistence analyses of the data are performed using the Mann-Kendall test, the Cumulative Deviation test, Linear Regression, and the Autocorrelation Coefficient. A sensitivity analysis of meteorological variables in these five regions is carried out using Penman-Monteith formula. In all of the studied regions, sensitivity analysis reveals that, temperature and relative humidity are the most sensitive parameters in Penman-Monteith formula respectively. The results of this study indicate that the effective climatic variables on evapotranspiration are changing, though in each region the variables have significant long-term trends and persistence. (C) 2011 Elsevier B.V. All rights reserved.