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The Potential Evapotranspiration (ETo) is a powerful and useful tool in order to estimate and manage the amount of water that it is necessary for irrigating crops and trees. Several equations can be used for calculating ETo, although Penman-Monteith equation is the most accurate equation, function of the temperature, relative humidity, wind velocity and net radiation. Moreover, ETo is currently obtained from sparsely located weather stations around different zones or states. These values are often extrapolated for calculating the requirements within a zone or it is directly taken into account the nearest station from the crop. However, the limitations in the number of stations make sometimes that this procedure does not fit perfectly with the real water requirements of the plants.

On the other hand, meteorology mathematical models for predicting weather behaviour have been developed over the last years based on basic fluid dynamics equations (Conservation of the mass, momentum and energy equations). Its usage has been validated in many studies for environmental and climatology studies.

In the present paper, the authors have been used a mathematical model, RAMS, for evaluating the ETo around a wide zone, using the standard and homogeneous data in the Region of Valencia, Spain. The results show that this tool is a very powerful way to calculate ETo from sparsely stations. This ETo map has been obtained from a mathematical model, instead of extrapolating empirical values from the stations around a zone.