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.

Explore the next module, or search for a specific topic or issue.

Before using the content of the Knowledge Hub, please familiarize yourself with the relevant disclaimers and additional safety and usage information contained within the modules/guides being viewed. Keep in mind that due to numerous variances for every application, before making any product or application decision, as always, you should consult with a local irrigation consultant for your specific needs. Case study outcomes are for informational purposes only and actual results may vary.

© 2026, Rivulis Irrigation Ltd, all rights reserved. 
Reproduction of this content in any form is prohibited without the written consent of Rivulis Irrigation Ltd.