Accepted Manuscript Not Copyedited 1 Submitted 20-Mar-2010 to: Journal of Hydrologic Engineering Local calibration of the Hargreaves and Priestley–Taylor equations for estimating reference evapotranspiration in arid and cold climates of Iran based on the Penman-Monteith model Hossein Tabari 1 and Parisa Hosseinzadeh Talaee 2 1) Department of Irrigation, Faculty of Agriculture, Bu-Ali Sina University, Hamedan 65174, I. R. Iran. E-mail: hosseintabari@gmail.com, Tel: +98 911 2528074, Fax: +98 811 4227012. 2) Department of Irrigation, Faculty of Agriculture, Bu-Ali Sina University, Hamedan 65174, I. R. Iran. Abstract: The FAO-56 version of Penman–Monteith (PMF-56) model has been established as a standard for calculating reference evapotranspiration (ET o ). An important constraint to application of the PMF-56 model is the requirement of solar radiation, wind speed, air temperature, and humidity data which may not be available for a given location especially in developing countries. The Hargreaves (HG) and Priestley– Taylor (P-T) equations are simple equations that require few weather data inputs, although regional calibration of the equations is needed for acceptable performance before applying them for ET o estimation. In this study, the HG and P-T equations were calibrated based on the PMF-56 method in arid and cold climates of Iran using data from 12 stations during 1994-2005. After calibration of the HG equation, the average value of the adjusted HG coefficient for arid climate was 0.0031, which is about 34% higher than the original value (0.0023). Similarly, the average value of the new HG coefficient for cold climate was 0.0028, which is about 22% higher than the original value. The results showed that the original P-T coefficient of 1.26 was very low for the climatic regions and the new P-T coefficients of 1.82 and 2.14 have the best fit as compared with the PMF-56 method in cold and arid climates, respectively. Overall, calibration of the HG and P-T equations resulted in improvements of the equations by reducing the errors of the ET o estimates. Keywords: Reference evapotranspiration; Calibration; Penman–Monteith FAO 56; Hargreaves; Priestley and Taylor; Arid and cold climates Introduction Evapotranspiration (ET) is one of the basic elements of the hydrological cycle, a key factor for water balance and for estimating irrigation water requirement (Chattopadhyay et al. 2009; Sabziparvar et al. 2010a, b). It is a very important and necessary parameter in many scientific fields in general and is essential in the context of many issues, for example irrigation scheduling, management of irrigated areas, crop production and environmental assessment (Irmak et al. 2003; Temesgen et al. 2005). 1 Hossein Tabari (Corresponding author) Journal of Hydrologic Engineering. Submitted March 20, 2010; accepted December 29, 2010; posted ahead of print January 3, 2011. doi:10.1061/(ASCE)HE.1943-5584.0000366 Copyright 2010 by the American Society of Civil Engineers