Vegetables
Watermelon Irrigation Guide
Watermelon runs a 110-day season with water demand peaking at a crop coefficient of 1. On a hot mid-summer day (ETo of 6 mm), that is about 6.0 mm per day, or 42 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
110 days
20 + 30 + 30 + 30 by stage
Root depth
0.8 m
≈ 2.6 ft at maturity
Peak daily use
6.0 mm
at mid-season, ETo 6 mm/day
Depletion threshold
40%
of available water before stress
Crop coefficient (Kc) through the season
Multiply any day's reference evapotranspiration (ETo) by the Kc below to get Watermelon's actual water use that day.
| Stage | Length | Kc | What's happening |
|---|---|---|---|
| Initial | 20 days | 0.4 | Establishment: bare soil dominates, low water use |
| Development | 30 days | 0.4 → 1 | Canopy closes; demand ramps day by day |
| Mid-season | 30 days | 1 | Full canopy: peak water use, most stress-sensitive |
| Late season | 30 days | 1 → 0.75 | Maturation: demand tapers toward harvest |
Refill points by soil type
Total available water (TAW) is what your soil holds across Watermelon's 0.8 m root zone. Irrigate when the running deficit crosses the refill point (RAW), 40% of TAW for Watermelon, so the crop never enters stress.
| Soil | Holds (TAW) | Irrigate at (RAW) | Days of peak use per refill |
|---|---|---|---|
| Sand | 56 mm | 22.4 mm | ~3 days |
| Sandy Loam | 96 mm | 38.4 mm | ~6 days |
| Loam | 120 mm | 48 mm | ~8 days |
| Silt Loam | 136 mm | 54.4 mm | ~9 days |
| Clay Loam | 128 mm | 51.2 mm | ~8 days |
| Clay | 112 mm | 44.8 mm | ~7 days |
Sandy soils hold the least water, so Watermelon on sand needs small, frequent irrigations; clay holds the most and supports longer intervals with bigger applications.
Worked example: a peak-summer week
Take Watermelon at mid-season on a week averaging an ETo of 6 mm/day:
- Daily crop use: 6 mm × Kc 1 = 6.0 mm/day
- Weekly demand: 6.0 × 7 = 42.0 mm (1.7 inches)
- On loam (48 mm refill point), a full profile lasts about 8 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 watermelon field
The free calculator combines these coefficients with your field's actual 7-day forecast. Or start a trial and let RainRule track your real water balance every hour.