Reference evapotranspiration (ETo) is an important parameter used in numerous applications, such as climatological and hydrological studies, as well as for water resources planning and management. There are several methods to estimate ETo, being that the FAO Penman-Monteith (EToPM) method is considered standard. This method needs many parameters (solar radiation, air temperature, humidity and wind speed), however there are still many uncovered areas, suggesting the need for methods of calculating evapotranspiration based on few meteorological elements, such as air temperature. Therefore, this study aimed to determine the ETo by Hargreaves-Samani method in the experimental watershed of the “Riacho do Papagaio” farm, in county of Sao Joao, in north-eastern Brazil, using data of 2011 and 2012. Reference evapotranspiration estimated by non-calibrated Hargreaves-Samani method (EToHS) was overestimated in all months (RMSE = 1.43 mm·d-1), mainly in months of lower evaporative demand (from May to July). Because of these tendencies, this method cannot be used in its original form to estimate ET for this region; therefore, a calibration of radiation adjustment coefficient (kRs) was performed. The calibrated Hargreaves-Samani method (EToHSc) had better performance (RMSE = 0.52 mm·d-1), being suitable for predicting ETo in this region.