Crops

FAO 33 Yield Model

Attaching a yield model to a crop model allows the conversion of soil water stress into the corresponding yield loss, and hence, to quantify the costs of a soil moisture deficit. The yield model currently available is the FAO 33 Yield Model.

This model is based on a so-called potential yield (Y_p), which is the crop yield under optimal conditions (no soil moisture stress). The sensitivity of a crop to soil moisture stress depends on the growth stage. A crop will usually be more sensitive to soil moisture stress in early growth stages than in late stages. This is taken into account with a yield response factor (K_y). A yield response factor has to be specified for each of four growth stages. Each stage is assigned a length that may, but does not have to, be the same as the growth stages in the Crops model to which it is related.

The crop yield is calculated as:

(A.3.2)   

Where:

Y_a: Actual yield [kg/ha]

Y_p: Potential yield [kg/ha]

Et_a: Actual transpiration [mm/day]

Et_p: Potential transpiration [mm/day]

Index i is the i’th growth stage in a growing season with a total of G growth periods.