Field Capacity (FC) is the amount of water that a well-drained soil should hold against gravitational forces. This is the maximal amount of water the field can hold. In the Soil Water Balance model Manna uses a volumetric ratio (the volume of water/the volume of the soil) as an input, and specifies it based on the soil type input of the field.
To read more about the model in the FAO-56, click here.
Image by the FAO