How to use this calculator
- Enter or choose reference evapotranspiration (ET₀).
- Enter or choose crop coefficient (Kc).
- Enter or choose field capacity.
- Enter or choose wilting point.
- Enter or choose effective root depth.
- The days between irrigations updates as you type. Use Copy result or Share to keep it.
Method
ETc = Kc × ET₀; TAW = 1000 (θFC − θWP) Zr; RAW = p × TAW; Interval = RAW ÷ ETc; Gross depth = RAW ÷ Efficiency
- θFC, θWP
- water content at field capacity and wilting point (m³/m³)
- Zr
- root depth (m)
- p
- depletion fraction before stress
- ETc
- crop evapotranspiration
Worked example
- ETc = 1.15 × 5 = 5.75 mm/day.
- TAW = 1000 × (0.30 − 0.15) × 0.8 = 120 mm; RAW = 0.5 × 120 = 60 mm.
- Interval = 60 ÷ 5.75 = 10.43 days; gross depth = 60 ÷ 0.85 = 70.59 mm (705.88 m³/ha).
Irrigate about every 10 days, applying roughly 71 mm gross (706 m³ per hectare).
What the result means
Round the interval down to a whole number of days, and recalculate as ET₀ and Kc change through the season. Rain that soaks in extends the interval.
Assumptions and limits
- The root zone starts at field capacity after each irrigation.
- No rain, capillary rise or deep percolation between irrigations.
- Uses one soil layer and one ET₀ value; real schedules should be updated with weather data.
Questions
Where do I get ET₀?
Many national weather services and irrigation advisory services publish daily reference evapotranspiration. It can also be calculated with the FAO Penman–Monteith equation.
What is the difference between net and gross depth?
Net depth is what the root zone needs; gross depth is what you must pump, allowing for losses in application.