Reference crop evapotranspiration (ETo) is an important component of the hydrological cycle and its estimation is widely used in engineering applications irrigation and consequently in water management in agriculture. In this study, in order to estimate the ETo evapotranspiration of reference crops and due to the few synoptic stations (four stations) and also the amount of recorded meteorological data (three stations with less than six years of data) in North Khorasan province , Iran, seven stations of the neighboring provinces were used. The reference evapotranspiration was calculated by six different methods, including: the method proposed by Pan Class A, Hargreaves-Samani, Priestly-Taylor, Turc, Makkink and Allen et al (1998) to estimate ETo with missing data (hereafter EPM). Due to the lack of lysimetric data, the standard FAO-Penman-Monteith method was used as the best method for evaluating other models. Since there was no agreement on the appropriate method to calculate ETo at the selected stations, using the significance test of regression lines, a linear regression equation for each month was suggested and the best calculation method converted into the FAO model -Penman-Monteith. The best calculation methods were Hargreaves-Samani in 9 months of the year and EPM in the other months. The evaluation of the proposed equations showed their adequate precision especially for the cold months (average absolute error values ​​ranged from 0.27 to 2.47 mm day-1). Furthermore, high R2 (0.87 to 0.98) and acceptable mean square error values ​​(0.507-1.44 mm day-1) indicate sufficient precision of the presented equations compared to the literature. The proposed equations can provide ETo estimates with acceptable accuracy using limited data, which will result in improved agricultural water management. Keywords: semi-arid climate, FAO-Penman-Monteith, Hargreaves-Samani, evapotranspiration of reference crops, regression.1. Introduction Reference crop evapotranspiration (ETo) is an important component of the hydrological cycle and its estimation is widely used in hydrological engineering and irrigation applications (Exner-Kittridge and Rains, 2010). A quantitative assessment of evapotranspiration is necessary for purposes such as agricultural water management, irrigation scheduling, and environmental assessments in irrigated areas around the world. Increasing competition among consumers for water use has highlighted the need for more accurate estimates of baseline evapotranspiration. The lysimeter is the most accurate and reliable method for measuring actual evapotranspiration. Several researchers have compared the accuracy of other methods including FAO-Penman-Monteith (PM) with lysimeter (Bakhtiari et al., 2011 for the semi-arid climate of Kerman province, southeastern parts of Iran; Lopez -Urrea et al., 2006 for the semi-arid climate of the province of Albacete, Spain; Yoder et al., 2005 for the Cumberland plateau located in the humid southeastern United States;.