A model for predicting the reference evapotranspiration ET_o in arid areas was developed and evaluated. The model was developed based on the Jensen-Heise model with added coefficients and the original coefficients of the model were calibrated for the conditions of the area.  For evaluation, the  ET_o values of the model were compared to the ET_o values obtained using the FAO Penman Monteith method. The model was also evaluated using a set of weather data (14 years of data) obtained from a location 350 km from the original site. The model improved the prediction at the original site reducing the overall Mean Absolute error (MAE)from1.62 using the Jensen-Heise model to 0.84 using the Modified Jensen-Heise (MJH) model. Calibrating the values of the coefficients to the new location improved the performance of the model and made it better than the Jensen-Heise model decreasing the overall MAE from 2.75 for the Jensen-Heise model to 1.24 and 1.22 for the 3 and the 14 years calibration, respectively.