Step 4. Offset rainfall

Rainfall/effective precipitation (Pe) is used to offset the irrigation requirement.

During the season, this is a straightforward calculation based on rainfall events. However, it is also a factor when planning your irrigation system, although nature is impossible to predict with certainty, so factor in a buffer.

  • Effective rainfall

The question is how much rainfall is sufficient to deduct from your scheduled irrigation plan.

As a general rule of thumb, effective rainfall is when there is a minimum of 5 mm (0.2 in) precipitation in a day. Below this amount of rain is generally not considered ‘effective’, and not sufficient enough to deduct from your irrigation planning.

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