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.
 R. Lopez-Urrea, F. Martin de Santa Olalla, C. Fabeiro and A. Moratalla, “Testing Evapotranspiration Equations Using Lysimeter Observations in a Semiarid Climate,” Agricultural Water Management, Vol. 85, No. 1-2, 2006, pp. 15-26. http://dx.doi.org/10.1016/j.agwat.2006.03.014
 P. C. Sentelhas, T. J. Gillespie and E. A. Santos, “Evaluation of FAO Penman-Monteith and Alternative Methods for Estimating Reference Evapotranspiration with Missing Data in Southern Ontario, Canada,” Agricultural Water Management, Vol. 97, No. 5, 2010, pp. 635-644.
 P. Droogers and R. G. Allen, “Estimating Reference Evapotranspiration under Inaccurate Data Conditions,” Irrigation and Drainage Systems, Vol. 16, No. 1, 2002, pp. 33-45. http://dx.doi.org/10.1023/A:1015508322413
 G. Landeras, A. Ortiz-Barredo and J. J. López, “Comparison of Artificial Neural Network Models and Empirical and Semi-Empirical Equations for Daily Reference Evapotranspiration Estimation in the Basque Country (Northern Spain),” Agricultural Water Management, Vol. 95, No. 5, 2008, pp. 553-565.
 G. H. Hargreaves and Z. Samani, “Reference Crop Evapotranspiration from Temperature,” Journal of Applied Engineering in Agriculture, Vol. 1, No. 2, 1985, pp. 96-99. http://dx.doi.org/10.13031/2013.26773
 T. Raziei and L. S. Pereira, “Estimation of ETo with Hargreaves-Samani and FAO-PM Temperature Methods for a Wide Range of Climates in Iran,” Agricultural Water Management, Vol. 121, 2013, pp. 1-18.
 G. H. Hargreaves and R. G. Allen, “History and Evaluation of Hargreaves Evapotranspiration Equation,” Journal of Irrigation and Drainage Engineering, Vol. 129, No. 1, 2003, pp. 53-63.
 APAC, “Water and Climate Agency of Pernambuco,” 2013. http://www.apac.pe.gov.br
 J. C. B. Santos, V. S. Souza Júnior, M. M. Corrêa, M. R. Ribeiro, M. C. Almeida and L. E. P. Borges, “Carachterization of Entisols of Semiarid Region of Pernambuco,” Brazilian Journal of Soil Science, Vol. 36, No. 3, 2012, pp. 683-695.
 Z. Samani, “Discussion of ‘History and Evaluation of Hargreaves Evapotranspiration Equation’ by George H. Hargreaves and Richard G. Allen,” Journal of Irrigation and Drainage Engineering ASCE, Vol. 130, No. 5, 2004, pp. 447-448.
 C. J. Willmott, S. G. Ackleson, J. J. Davis, K. M. Feddema and D. R. Klink. “Statistics for the evaluation and comparison of models,” Journal of Geophysical Research, Vol. 90, No. C5, 1985, pp. 8995-9005.
 M. E. Moeletsi, S. Walker and H. Hamandawana, “Comparison of the Hargreaves and Samani Equation and the Thornthwaite Equation for Estimating Dekadal Evapotranspiration in the Free State Province, South Africa,” Physics and Chemistry of Earth, Vol. 66, 2013, pp. 4-15.
 P. Gavilán, I. J. Lorite, S. Tornero and J. Berengena, “Regional Calibration of Hargreaves Equation for Estimating Reference ET in a Semiarid Environment,” Agricultural Water Management, Vol. 81, No. 3, 2006, pp. 257-281. http://dx.doi.org/10.1016/j.agwat.2005.05.001