Reference crop evapotranspiration (ETo) is a major component of the hydrological cycle, and its estimation is widely used in crop applications. irrigation engineering and, therefore, in agricultural water management. In this study, in order to estimate the evapotranspiration of ETo reference crops and due to the few synoptic stations (four stations) and also the amount of meteorological data recorded (three stations with less than six years of data) in the North Khorasan province, Iran, seven stations in neighboring provinces were used. Reference evapotranspiration was calculated by six different methods, including: the method proposed by Hargreaves-Samani, Priestly-Taylor, Turc, Makkink and Allen et al (1998) to estimate ETo with missing data (hereinafter EPM). Due to the lack of available lysimetric data, the standard FAO-Penman-Monteith method was used as the best method for evaluating alternative models. Since there was no agreement on the appropriate method to calculate ETo in the selected stations, using the significance test of regression lines, a linear regression equation was suggested for each month and the best method calculation was converted to the FAO-Penman-Monteith model. The best calculation methods were Hargreaves-Samani for 9 months of the year and EPM for the other months. Evaluation of the proposed equations showed their appropriate accuracy, especially for cold months (mean absolute error values ​​ranged from 0.27 to 2.47 mm day-1). Furthermore, high R2 (0.87–0.98) and acceptable root mean square error values ​​(0.507–1.44 mm day-1) indicate sufficient accuracy of the presented equations compared to literatures. 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 a major component of the hydrological cycle and its estimation is widely used in hydrological engineering and irrigation applications (Exner-Kittridge and Rains, 2010). Quantitative assessment of evapotranspiration is necessary for purposes such as agricultural water resources management, irrigation planning, and environmental assessments in irrigated areas across the world. Increasing competition for water use among consumers 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 the FAO-Penman-Monteith (PM) method with a lysimeter (Bakhtiari et al., 2011 for the semi-arid climate of Kerman province in southeastern Iran; Lopez-Urrea et al., 2006 for the semi-arid climate of the province of Kerman, in southeastern Iran; semi-arid climate of the province of Albacete, Spain; ., 2005 for the Cumberland Plateau located in the humid southeastern United States ;.