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Irrigation Requirement Calculator

Net irrigation (mm) from ET0, crop coefficient & rainfall.

Introduction

Irrigation Requirement Calculator — Net irrigation (mm) from ET0, crop coefficient & rainfall. Enter Reference ET₀ (mm/day), Crop coefficient (Kc), Effective rainfall (mm) to get an instant, accurate result.

Formula

Crop evapotranspiration: ETc = ET0 × Kc. Net irrigation requirement (mm) = ETc − effective rainfall, floored at zero. This is the standard FAO-56 crop water requirement approach.

Step-by-step

  1. Enter the Reference ET₀ (mm/day).
  2. Enter the Crop coefficient (Kc).
  3. Enter the Effective rainfall (mm).
  4. Click Calculate to see your result instantly.

Real-world example

Example: With Reference ET₀ (mm/day) = 5 mm, Crop coefficient (Kc) = 1, Effective rainfall (mm) = 2 mm, the Irrigation Requirement Calculator gives Etc Mm: 5, Net Irrigation Mm: 3.

Frequently Asked Questions

What is the crop coefficient (Kc)?
It adjusts reference evapotranspiration for a specific crop and growth stage — young or low-water crops use a lower Kc, while full-canopy or high-water-use crops use a higher one, typically referenced from FAO-56 crop tables.
Why is effective rainfall subtracted rather than total rainfall?
Not all rainfall is usable by the crop — some runs off or drains below the root zone, so only the effective (plant-available) portion offsets the irrigation need.
What inputs does the Irrigation Requirement Calculator need?
You'll need to provide: Reference ET₀ (mm/day), Crop coefficient (Kc), Effective rainfall (mm). All fields use sensible defaults, so you can see a working example immediately and then adjust the values to match your own numbers.
How accurate is the Irrigation Requirement Calculator?
The Irrigation Requirement Calculator applies the formula exactly as calculated — Crop evapotranspiration: ETc = ET0 × Kc. Net irrigation requirement (mm) = ETc − effective rainfall, floored at zero. This is the standard FAO-56 crop water requirement approach. — so results are precise for the inputs you provide. Accuracy depends on entering correct, realistic input values.
Is the Irrigation Requirement Calculator free to use?
Yes, the Irrigation Requirement Calculator is completely free, requires no signup, and runs instantly in your browser.

About the Irrigation Requirement Calculator

The Irrigation Requirement Calculator uses a real, verifiable formula — Crop evapotranspiration: ETc = ET0 × Kc. Net irrigation requirement (mm) = ETc − effective rainfall, floored at zero. This is the standard FAO-56 crop water requirement approach. — so results are accurate every time, not an approximation.