IRRIGATION
DUTY CYCLE CONTROLLER
Precision pump scheduling for closed-loop hydroponics & field crops
← Return to Workshop Q5187947 | FAO-56THE THEORY
In precision irrigation, the pump is not a switch. It is a metronome. Every cycle must deliver exactly the volume lost to evapotranspiration plus the maintenance reserve for root zone saturation.
V_required = (ET₀ × Kc × A_root) − SM_deficit
T_pump = V_required ÷ Q_pump × 60
T_pump = V_required ÷ Q_pump × 60
Where:
- ET₀ = Reference evapotranspiration (mm/day, from Penman-Monteith)
- Kc = Crop coefficient (dimensionless, stage-dependent)
- A_root = Root zone area (m²)
- SM_deficit = Current soil moisture deficit (L)
- Q_pump = Pump flow rate (L/min)
Grounded in Q5187947 (crop coefficient) • FAO Irrigation Paper No. 56 • Allen et al. 1998
Field Note: In my Wasatch trials, I discovered that lettuce at mid-canopy (Kc ≈ 1.15) demands 4.2 mm/day in July. A 12 L/min pump servicing 50 m² must run exactly 11 minutes per 24 hours. Anything less stresses the roots. Anything more leaches nutrients.
THE CONTROLLER
mm/day • From ET Matrix or local station
square meters
liters remaining from last irrigation
liters per minute
CROP COEFFICIENT REFERENCE TABLE
Values sourced from FAO-56, adapted for controlled-environment agriculture.
| Crop | Initial Stage | Mid-Season | Late Season | Peak Demand |
|---|---|---|---|---|
| Lettuce | 0.95 | 1.15 | 1.00 | 4.2 mm/d |
| Tomato | 0.65 | 1.10 | 1.15 | 6.8 mm/d |
| Pepper | 0.60 | 1.05 | 1.00 | 5.4 mm/d |
| Corn | 0.50 | 1.15 | 0.85 | 7.2 mm/d |
| Wheat | 0.40 | 1.05 | 0.45 | 4.8 mm/d |
Machine-readable export: irrigation-controller.json
