Legumes
Alfalfa (Hay) Irrigation Guide
Alfalfa (Hay) runs a 75-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
75 days
10 + 30 + 25 + 10 by stage
Root depth
1.5 m
≈ 4.9 ft at maturity
Peak daily use
7.2 mm
at mid-season, ETo 6 mm/day
Depletion threshold
55%
of available water before stress
Crop coefficient (Kc) through the season
Multiply any day's reference evapotranspiration (ETo) by the Kc below to get Alfalfa (Hay)'s actual water use that day.
| Stage | Length | Kc | What's happening |
|---|---|---|---|
| Initial | 10 days | 0.4 | Establishment: bare soil dominates, low water use |
| Development | 30 days | 0.4 → 1.2 | Canopy closes; demand ramps day by day |
| Mid-season | 25 days | 1.2 | Full canopy: peak water use, most stress-sensitive |
| Late season | 10 days | 1.2 → 1.15 | Maturation: demand tapers toward harvest |
Refill points by soil type
Total available water (TAW) is what your soil holds across Alfalfa (Hay)'s 1.5 m root zone. Irrigate when the running deficit crosses the refill point (RAW), 55% of TAW for Alfalfa (Hay), so the crop never enters stress.
| Soil | Holds (TAW) | Irrigate at (RAW) | Days of peak use per refill |
|---|---|---|---|
| Sand | 105 mm | 57.8 mm | ~8 days |
| Sandy Loam | 180 mm | 99 mm | ~13 days |
| Loam | 225 mm | 123.8 mm | ~17 days |
| Silt Loam | 255 mm | 140.3 mm | ~19 days |
| Clay Loam | 240 mm | 132 mm | ~18 days |
| Clay | 210 mm | 115.5 mm | ~16 days |
Sandy soils hold the least water, so Alfalfa (Hay) on sand needs small, frequent irrigations; clay holds the most and supports longer intervals with bigger applications.
Worked example: a peak-summer week
Take Alfalfa (Hay) 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 (123.8 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 legumes guides
Get a live schedule for your alfalfa (hay) field
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