Vegetables
Onion Irrigation Guide
Onion runs a 150-day season with water demand peaking at a crop coefficient of 1.05. On a hot mid-summer day (ETo of 6 mm), that is about 6.3 mm per day, or 44 mm (1.7 inches) per week. Every number on this page comes from the FAO-56 standard and is exactly what RainRule schedules with.
Season length
150 days
15 + 25 + 70 + 40 by stage
Root depth
0.4 m
≈ 1.3 ft at maturity
Peak daily use
6.3 mm
at mid-season, ETo 6 mm/day
Depletion threshold
30%
of available water before stress
Crop coefficient (Kc) through the season
Multiply any day's reference evapotranspiration (ETo) by the Kc below to get Onion's actual water use that day.
| Stage | Length | Kc | What's happening |
|---|---|---|---|
| Initial | 15 days | 0.7 | Establishment: bare soil dominates, low water use |
| Development | 25 days | 0.7 → 1.05 | Canopy closes; demand ramps day by day |
| Mid-season | 70 days | 1.05 | Full canopy: peak water use, most stress-sensitive |
| Late season | 40 days | 1.05 → 0.75 | Maturation: demand tapers toward harvest |
Refill points by soil type
Total available water (TAW) is what your soil holds across Onion's 0.4 m root zone. Irrigate when the running deficit crosses the refill point (RAW), 30% of TAW for Onion, so the crop never enters stress.
| Soil | Holds (TAW) | Irrigate at (RAW) | Days of peak use per refill |
|---|---|---|---|
| Sand | 28 mm | 8.4 mm | ~1 days |
| Sandy Loam | 48 mm | 14.4 mm | ~2 days |
| Loam | 60 mm | 18 mm | ~2 days |
| Silt Loam | 68 mm | 20.4 mm | ~3 days |
| Clay Loam | 64 mm | 19.2 mm | ~3 days |
| Clay | 56 mm | 16.8 mm | ~2 days |
Sandy soils hold the least water, so Onion on sand needs small, frequent irrigations; clay holds the most and supports longer intervals with bigger applications.
Worked example: a peak-summer week
Take Onion at mid-season on a week averaging an ETo of 6 mm/day:
- Daily crop use: 6 mm × Kc 1.05 = 6.3 mm/day
- Weekly demand: 6.3 × 7 = 44.1 mm (1.7 inches)
- On loam (18 mm refill point), a full profile lasts about 2 days with no rain before irrigation is due.
Real weeks aren't averages: ETo swings with heat waves and rain changes everything. That's the gap between a rule of thumb and a live schedule.
Related vegetables guides
Get a live schedule for your onion field
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