Irrigation Water Requirement Calculator
Calculate daily and weekly irrigation requirements using the FAO Penman-Monteith ET₀ method with crop coefficient (Kc).
Results
What is it?
The FAO-56 method calculates crop water use as ETc = ET₀ × Kc, where ET₀ is reference evapotranspiration and Kc is a crop-specific coefficient. Net irrigation = max(0, ETc − effective rainfall). Volume (m³) = depth (mm) × area (m²) ÷ 1,000.
How to use
Enter the reference ET₀ from your local weather station or FAO database, the Kc for your crop and growth stage, effective daily rainfall, and the irrigated area. The result shows net irrigation depth in mm/day and total water volume per day and week.
Example scenario
Maize at mid-season in a semi-arid region: ET₀ = 7 mm/day, Kc = 1.20, effective rainfall = 2 mm/day. ETc = 8.4 mm/day. Net irrigation = 6.4 mm/day. Over 1 hectare: 64 m³/day = 448 m³/week. A drip system at 4 L/plant/hour for 8 hours delivers 32 L per plant — check against this result.
Pro tip
Kc varies throughout the crop growth cycle — it starts low (establishment), peaks at mid-season, then drops (late season). Always use the stage-appropriate Kc. Soil moisture sensors (tensiometers, TDR probes) at key depths provide real-time feedback and typically reduce water use by 15–30% compared to calendar-based scheduling.