**4. Discussion**

At the beginning of this research, we were able to design the Karla watershed within the APEX model. The first step was based on the automatic delineation of the watershed using the ArcAPEX interface. After this initial step, the input files generated by the interface required some modification to better represent features of the watershed that were not correctly captured by the ArcAPEX interface, such as the presence of a reservoir in the lower part of the watershed. After setting up all the input data required by the model, we started the calibration procedure. Beginning with the calibration of the PET, which we consider as the starting point to have a good simulation of the water balance, we were able to obtain good results with R<sup>2</sup> of 0.9 and Ef of 0.85. The good results obtained for the PET are followed by satisfactory results in the simulation of the yields of the three main crops cultivated in the studied area. The APEX model was able to produce reasonable results for maize, cotton, and wheat yields with some overestimation that required analysis for the later harvest. The calibration process will be completed by including the analysis of runoff and nitrate leaching, and the model will be used to assess how climate change will affect crop production and water and nutrient losses.
