The amount of water used in irrigation systems is usually calculated using the local reference evapotranspiration (ETo) values. Several methods can be used for ETo calculation, and Hargreaves method is one of the most popular. This method uses estimated values of global solar radiation (Rs), based on the difference between the maximum (Tmax) and minimum (Tmin) air temperature. However, the Rs may be calculated by other methods, such as the Bristow-Campbell, which also is based on the difference between Tmax and Tmin. In this study were estimated daily values of ETo by the method of Hargreaves in Jales - SP, Brazil, with and without the use of the Bristow-Campbell equation to calculate the Rs. These values were compared with those from the Penman-Monteith method, which is considered standard for the estimation of ETo. The evaluations had considered the wet and dry periods in the region. The use of the Bristow-Campbell equation improved the performance of the Hargreaves method, in relation to the Penman-Monteith, and the performance during the dry period of the year was higher than that of the wet season.
Hargreaves; Penman-Monteith; irrigation
