Wash-off

A portion of the pesticide on plant foliage may be washed off during rain events. The fraction washed off is a function of plant morphology, pesticide solubility, and the timing and intensity of the rainfall event. Wash-off will occur when the amount of precipitation on a given day exceeds 2.54 mm.

The amount of pesticide washing off plant foliage during a precipitation event on a given day is calculated:

$pst_{f,wsh}=fr_{wsh}*pst_f$ 3:3.1.1

where $pst_{f,wsh}$ is the amount of pesticide on foliage that is washed off the plant and onto the soil surface on a given day (kg pst/ha), $fr_{wsh}$ is the wash-off fraction for the pesticide, and $pst_f$ is the amount of pesticide on the foliage (kg pst/ha). The wash-off fraction represents the portion of the pesticide on the foliage that is dislodgable.

Table 3:3-1: SWAT+ input variables that pertain to pesticide wash-off.