Estimates of potential evapotranspiration are essential for planning, designing, and managing irrigation and drainage plans and water resource management. Evapotranspiration involves evaporation from the Earth’s surface and water, and plant transpiration, which is of particular importance in irrigated arid and semiarid areas. In this research, nine methods of potential evapotranspiration estimation, including FAO Penman-Monteith (FAO PM), Penman-Kimberley (1996), Penman (1948), Penman-FAO 24, FAO radiation 24, Blanney Criddle-FAO, Hargreaves (1985), Priestly Taylor, and Makkink (1957) were calculated for the Zabol Synoptic Station. The FAO PM is accepted as a standard method by many researchers and international institutes, hence, it was adopted as a reference approach in this study. Estimates of reference evapotranspiration were determined by different empirical methods using the REF-ET model and were then compared with each other. It was found that the Penman (1948), Penman-FAO 24, Hargreaves (1985), Penman-Kimberly (1996), Blanney Criddle-FAO, FAO radiation, Priestly Taylor, and Makkink (1957) were more suitable methods, respectively, in Zabol synoptic station. Also, regression coefficients and linear equations were presented between each method and the FAO PM approach to convert potential evapotranspiration measured by the other methods into the FAO PM method.