The discrepancy between the demand for water and available resources constitute a major challenge for all nations, but it is particularly harsh in dry areas where water supplies are limited. Agriculture is the most affected sector, and this is due to the high amount of water intended for the irrigation process. To deal with this situation, irrigation strategy should be improved. Model based irrigation management system can be the best solution, as it allows farmers to determine the appropriate quantity of water to be applied, which will reduce water consumption and increase food productivity. In this work, two models are developed to estimate the precise values of reference evapotranspiration, since it is the key element to determine plant water needs. The four basic environmental factors, namely: temperature, wind speed, relative humidity and insolation duration during July, in arid zone are used; besides a comparative analysis is conducted to decide which model is the most effective.