How Much to Irrigate: Calculating Crop Water Needs From ETo and Kc

Irrigating by feel tends to overwater early and underwater at peak demand. The standard method, from the FAO-56 guidelines, starts from the weather and the crop's growth stage.

The steps

1. ETo — reference evapotranspiration, in mm/day. It comes from a weather station or your local agricultural service and reflects temperature, sun, wind and humidity.
2. Kc — the crop coefficient for the current stage, from FAO-56 tables. Low at emergence, about 1.0–1.2 at full canopy, lower again near maturity.
3. ETc = ETo × Kc — what the crop uses per day.
4. Net depth = ETc × days between irrigations − effective rain.
5. Gross depth = net ÷ system efficiency (drip about 90%, sprinkler or pivot about 80–85%).

Worked example

ETo 5 mm/day, Kc 1.1 (full canopy), irrigating every 3 days, no rain, sprinkler efficiency 85%, application rate 8 mm/h, area 10 ha. In the irrigation calculator:

• ETc: 5.5 mm/day
• Net depth: 16.5 mm per irrigation
• Gross depth: 19.4 mm
• Run time: about 2 h 26 min
• Volume: about 1,940 m³ (1 mm on 1 ha = 10 m³)

Practical checks

• Don't apply more than the soil can hold in the root zone; sandy soils need smaller, more frequent irrigations.
• If the application rate exceeds the soil's infiltration rate, water runs off — split the irrigation.
• Count only effective rain; a light shower on hot soil adds little.
• Verify with soil moisture sensors or by digging when you can.

Pumping costs belong in the crop budget — add them in the crop cost and profit calculator.