Irrigation water requirement
Crop water need from evapotranspiration and crop factor, and how long to run your system.
From your nearest weather station. As a guide across much of Africa: 2 to 4 mm/day in the cool season, 5 to 8 mm/day in hot summer.
FAO-56 value for maize (grain), mid-season: 1.2. Edit it if local advice differs.
Days between irrigations: the water need is worked out for one cycle.
Only rain that soaks into the root zone. Light showers under about 5 mm mostly evaporate; count them as 0.
Share of pumped water the crop can use. Drip 85–95, centre pivot 75–85, sprinkler 70–80, furrow or flood 50–65.
From your system design or a catch-can test.
Enter ET0, Kc, interval, rainfall, efficiency and area to see the water requirement.
How this is calculated
This uses the FAO-56 crop coefficient method (Allen et al., 1998, FAO Irrigation and Drainage Paper 56). The crop coefficients (Kc) offered are from its Table 12, which assumes a sub-humid climate with moderate wind; in hot, dry, windy conditions mid-season values run slightly higher.
- Crop water use: ETc (mm/day) = ET0 × Kc. With ET0 of 6 mm/day and mid-season maize (Kc 1.2), ETc = 7.2 mm/day.
- Net irrigation per cycle = ETc × interval (days) − effective rainfall, never below zero. Over 7 days: 7.2 × 7 = 50.4 mm, less 10 mm of rain = 40.4 mm.
- Gross irrigation = net ÷ efficiency. A centre pivot at 85% must apply 40.4 ÷ 0.85 = 47.5 mm to put 40.4 mm where the roots can use it.
- Volume (m³) = gross mm × area (ha) × 10, because 1 mm of water on 1 ha is 10 m³. 47.5 mm on 30 ha = 14 259 m³.
- Run time (hours) = gross mm ÷ application rate (mm/h), or volume (m³) ÷ system flow (m³/h). At 5 mm/h, 47.5 mm takes about 9 h 30 min.
Kc values by stage (initial / mid / late): maize 0.3 / 1.2 / 0.6, wheat 0.3 / 1.15 / 0.4, potatoes 0.5 / 1.15 / 0.75, citrus 0.7 / 0.65 / 0.7. Efficiencies used as defaults: drip 90%, centre pivot 85%, sprinkler 75%, furrow or flood 55%.
This is a water budget, not a schedule: it doesn't account for how much water your soil holds. Don't apply more in one cycle than the root zone can store, or the extra drains away below the roots.
Read the guide: Scheduling irrigation with evapotranspiration and crop factors
Results are estimates for planning. Check product labels and local recommendations before you apply anything.