The harvest index predicted with equation 5:2.4.1 is affected by water deficit using the relationship:

$HI_{act}=(HI-HI_{min})*\frac{\gamma _{wu}}{\gamma _{wu}+exp[6.13-0.883*\gamma _{wu}]}+HI_{min}$ 5:3.3.1

where $HI_{act}$ is the actual harvest index, $HI$ is the potential harvest index on the day of harvest calculated with equation 5:2.4.1, $HI_{min}$ is the harvest index for the plant in drought conditions and represents the minimum harvest index allowed for the plant, and $\gamma _{wu}$ is the water deficiency factor. The water deficiency factor is calculated:

$\gamma_{wu}=100*\frac{\sum^m_{i=1} E_a}{\sum ^m_{i=1} E_o}$ 5:3.3.2

where $E_a$ is the actual evapotranspiration on a given day, $E_o$ is the potential evapotranspiration on a given day, $i$ is a day in the plant growing season, and $m$ is the day of harvest if the plant is harvested before it reaches maturity or the last day of the growing season if the plant is harvested after it reaches maturity.

In the harvest only operation (.mgt), the model allows the user to specify a harvest efficiency. The harvest efficiency defines the fraction of yield biomass removed by the harvesting equipment. The remainder of the yield biomass is converted to residue and added to the residue pool in the top 10 mm of soil. If the harvest efficiency is not set or a 0.00 is entered, the model assumes the user wants to ignore harvest efficiency and sets the fraction to 1.00 so that the entire yield is removed from the HRU.

$yld_{act}=yld*harv_{eff}$ 5:3.3.4

where $yld_{act}$ is the actual yield (kg ha$^{-1}$), $yld$ is the crop yield calculated with equation 5:2.4.2 or 5:2.4.3 (kg ha$^{-1}$), and $harv_{eff}$ is the efficiency of the harvest operation (0.01-1.00). The remainder of the yield biomass is converted to residue:

$\Delta rsd=yld*(1-harv_{eff})$ 5:3.3.5

$rsd_{surf,i}=rsd_{surf,i-1}+\Delta rsd$ 5:3.3.6

where $\Delta rsd$ is the biomass added to the residue pool on a given day (kg ha$^{-1}$), $yld$ is the crop yield calculated with equation 5:2.4.2 or 5:2.4.3 (kg ha$^{-1}$) and $harv_{eff}$ is the efficiency of the harvest operation (0.01-1.00) $rsd_{surf,i}$ is the material in the residue pool for the top 10 mm of soil on day $i$ (kg ha$^{-1}$), and $rsd_{surf,i-1}$ is the material in the residue pool for the top 10 mm of soil on day $i-1$ (kg ha$^{-1}$).

Table 5:3-3: SWAT+ input variables that pertain to actual plant yield.

In the plant and harvest only operations (.mgt), the model allows the user to specify a target harvest index. The target harvest index set in a plant operation is used when the yield is removed using a harvest/kill operation. The target harvest index set in a harvest only operation is used only when that particular harvest only operation is executed.

When a harvest index override is defined, the override value is used in place of the harvest index calculated by the model in the yield calculations. Adjustments for growth stage and water deficiency are not made.

5:3.3.3

where is the actual harvest index and is the target harvest index.