Field water use efficiency (FWUE) is calculated by

Answer: Option B

The field water use efficiency (WUE) is the ratio between the dry matter production of a plant (the output) divided by evapotranspiration. Thus the equation for WUE is:
$$\text{WUE}=\frac{\text{Y}}{\text{ET}}$$
Y = Dry matter production (kg/ha)
ET = Evapotranspiration (in mm)