Industrial & Fiber
Cotton Irrigation Guide
Cotton runs a 180-day season with water demand peaking at a crop coefficient of 1.2. On a hot mid-summer day (ETo of 6 mm), that is about 7.2 mm per day, or 50 mm (2.0 inches) per week. Every number on this page comes from the FAO-56 standard and is exactly what RainRule schedules with.
Season length
180 days
30 + 50 + 55 + 45 by stage
Root depth
1.3 m
≈ 4.3 ft at maturity
Peak daily use
7.2 mm
at mid-season, ETo 6 mm/day
Depletion threshold
65%
of available water before stress
Crop coefficient (Kc) through the season
Multiply any day's reference evapotranspiration (ETo) by the Kc below to get Cotton's actual water use that day.
| Stage | Length | Kc | What's happening |
|---|---|---|---|
| Initial | 30 days | 0.35 | Establishment: bare soil dominates, low water use |
| Development | 50 days | 0.35 → 1.2 | Canopy closes; demand ramps day by day |
| Mid-season | 55 days | 1.2 | Full canopy: peak water use, most stress-sensitive |
| Late season | 45 days | 1.2 → 0.7 | Maturation: demand tapers toward harvest |
Refill points by soil type
Total available water (TAW) is what your soil holds across Cotton's 1.3 m root zone. Irrigate when the running deficit crosses the refill point (RAW), 65% of TAW for Cotton, so the crop never enters stress.
| Soil | Holds (TAW) | Irrigate at (RAW) | Days of peak use per refill |
|---|---|---|---|
| Sand | 91 mm | 59.2 mm | ~8 days |
| Sandy Loam | 156 mm | 101.4 mm | ~14 days |
| Loam | 195 mm | 126.8 mm | ~17 days |
| Silt Loam | 221 mm | 143.6 mm | ~19 days |
| Clay Loam | 208 mm | 135.2 mm | ~18 days |
| Clay | 182 mm | 118.3 mm | ~16 days |
Sandy soils hold the least water, so Cotton on sand needs small, frequent irrigations; clay holds the most and supports longer intervals with bigger applications.
Worked example: a peak-summer week
Take Cotton at mid-season on a week averaging an ETo of 6 mm/day:
- Daily crop use: 6 mm × Kc 1.2 = 7.2 mm/day
- Weekly demand: 7.2 × 7 = 50.4 mm (2.0 inches)
- On loam (126.8 mm refill point), a full profile lasts about 17 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 industrial & fiber guides
Get a live schedule for your cotton 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.