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The Hargreaves-Samani model is used in estimating reference evapotranspiration, and is very useful for irrigation management; it is considered to be of practical use as it uses meteorological elements, which are easily obtained, such as temperature. However, the equation is in need of regional calibration in order to accurately estimate evapotranspiration. The aim of this work was to calibrate the Hargreaves-Samani model for the State of Ceara, using meteorological data from 12 cities. The empirical parameters alpha (0.0023) and beta (0.5) were calibrated considering two processes: firstly, the simultaneous calibration of the two parameters, and then only the alpha coefficient. The simultaneous calibration returned a variation in the extreme values for three cities; alpha ranged from 0.0004 (Campos Sales and Crateus) to 0.0019 (Guaramiranga), and exponent beta varied between 0.51 (Guaramiranga) to 1.1977 (Campos Sales). The correlation between the Hargreaves-Samani model calibrated with the Penman-Monteith equation, had a maximum value for the city of Jaguaruana, 0.934 when calibrated with the two parameters, and 0.942 when calibrated with the alpha coefficient only, demonstrating that calibration of the Hargreaves-Samani equation is viable, and is necessary for improving estimates of evapotranspiration.