2.2.3. Description and Set-Up of the Crop Simulation Model

AquaCrop, a water-driven crop simulation model, was developed in 2009 by the Food and Agriculture Organization (FAO) of the United Nations [38–40]. The development of the AquaCrop model is based on the algorithm of yield response to water in FAO Irrigation and Drainage Paper No. 33 [66]. AquaCrop evolves from the previous Doorenbos and Kassam [66] Ky approach (Equation (2)), where relative evapotranspiration (ET) is pivotal in calculating yield.

$$\frac{\left(\mathbf{Y\_{x}} - \mathbf{Y\_{a}}\right)}{\mathbf{Y\_{x}}} = \mathbf{K\_{Y}} \left[\frac{\left(\mathbf{ET\_{x}} - \mathbf{ET\_{a}}\right)}{\mathbf{ET\_{x}}}\right] \tag{2}$$

where Yx and Ya are the maximum and actual yield, respectively; ETx and ETa are the maximum and actual evapotranspirations, respectively; and Ky is the proportionality factor between relative yield loss and relative reduction in evapotranspiration.

AquaCrop simulates crop yield in four steps: crop development, crop transpiration, biomass formation, and yield formation [40]. Four water stress response coefficients are considered in the model. These are related to canopy expansion, stomatal conductance, canopy senescence, and harvest index [67].
