Onion Water Requirement Calculator
Work out the seasonal and per-irrigation water requirement of onion (FAO-56 ETc = ET₀ × Kc, peak Kc 1.05) for your field, climate, soil and irrigation method.
Frequent light irrigation suits onion's shallow roots; stop watering 2–3 weeks before harvest once tops fall to cure bulbs and improve storage. Drip greatly improves bulb uniformity and saves water.
Indicative planning figures based on published research averages. Local soil tests, varieties and weather change actual requirements — confirm with your agronomist or extension officer.
Disclaimer: This tool is for general informational and estimation purposes only and is not professional financial, tax, accounting or legal advice. All figures are estimates — verify with a qualified professional before making decisions. Read the full disclaimer.
Farmers and irrigation planners use the free Onion Water Requirement Calculator to turn the FAO-56 crop coefficients for onion into a real seasonal water volume and an irrigation interval for their own field — no agronomy tables, works offline.
About Onion Water Requirement Calculator
Shallow-rooted onion needs 350–550 mm of carefully-timed water and is one of the worst crops to over- or under-irrigate near bulbing. This calculator uses the FAO-56 method — crop evapotranspiration ETc = ET₀ × Kc — with onion's own crop coefficients (Kc 0.7 initial, 1.05 mid-season, 0.75 late) across a 120-day season. Pick your climate band (which sets reference ET₀), irrigation method (drip, sprinkler, pivot or surface — each with its own efficiency) and soil texture, and it returns the seasonal water need in cubic metres, the peak daily demand, the net depth per irrigation and how many days to wait between irrigations.
How to use Onion Water Requirement Calculator
- 1Enter your field area and choose the climate band matching your season.
- 2Select your irrigation method and soil texture.
- 3Read the seasonal water volume, peak daily demand and irrigation interval, then follow the scheduling note.
Why use Onion Water Requirement Calculator?
- ✓Uses onion's real FAO-56 Kc curve, not a flat factor
- ✓Accounts for drip / sprinkler / pivot / surface efficiency
- ✓Gives seasonal volume, peak daily need and an irrigation interval
- ✓Free, instant and fully in-browser — works offline in the field
Frequently asked questions
How much water does onion need?+
Shallow-rooted onion needs 350–550 mm of carefully-timed water and is one of the worst crops to over- or under-irrigate near bulbing. The exact figure depends on climate and season length; this tool sums ET₀ × Kc across the crop's 120-day growth stages for your conditions.
When should I stop irrigating onion?+
When the tops begin to fall and necks soften — about 2–3 weeks before harvest. Late watering keeps onions growing soft, thick-necked bulbs that rot in storage rather than curing firm.
Why is drip good for onion?+
Onion's shallow, sparse roots can't exploit a large soil volume, so the frequent, localised wetting from drip keeps moisture ideal, improves bulb size uniformity and cuts water and disease versus flood irrigation.
What is ET₀ and Kc?+
ET₀ (reference evapotranspiration) is how fast a standard grass surface loses water in your climate. Kc (crop coefficient) scales it to a specific crop and growth stage. Crop water use ETc = ET₀ × Kc — the basis of FAO-56 irrigation scheduling.
Is this calculator free and private?+
Yes — free, no sign-up, and all calculation runs in your browser, so it works offline in the field and your data never leaves the device.
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