YielixHow to Calculate Crop Water Requirements
Start with weather demand, crop stage and a measured area
Crop-water planning starts with weather-driven reference evapotranspiration (ET₀), then applies a crop coefficient (Kc) for the crop and growth stage. The Irrigation Water Requirement Calculator makes those inputs, efficiency and irrigated area explicit.
Use this as a planning estimate. Rainfall, soil-water storage, salinity, irrigation uniformity and crop stress can change what a field actually needs.
Calculate reference ET₀ from location and weather conditions
ET₀ Calculator
Calculate reference evapotranspiration using the FAO-56 Penman-Monteith equation for irrigation planning
Keep reference ET, crop ET and delivered water distinct
ET₀ is a weather-based reference rate in mm/day; ETc is ET₀ adjusted by Kc. Kc is crop- and stage-specific, so do not use a mid-season coefficient across the season. FAO's single-Kc approach is intended for planning and basic scheduling, while field observations remain necessary when conditions depart from standard assumptions.
ETc is not automatically the depth delivered at the field. Divide by stated irrigation efficiency to estimate gross application. Keep field effective rainfall separate from roof supply: the Effective Rainfall Calculator estimates rain reaching the root zone, whereas the Rainwater Harvesting Calculator estimates a collected supply that still needs storage and delivery.
Formula and units
ETc [mm/day] = ET₀ [mm/day] × Kc [dimensionless]
gross irrigation requirement [mm/day] = ETc ÷ efficiency [fraction]
daily volume [m³/day] = gross requirement [mm/day] × area [ha] × 10
A millimetre over 1 hectare is 10 m³. Use the same day, crop stage and area basis for every input.
Worked example: the retained 10-hectare fixture
Use the calculator's verified fixture: ET₀ 5.0 mm/day, Kc 1.15, 85% efficiency and 10 ha.
ETc = 5.0 × 1.15 = 5.75 mm/day.gross requirement = 5.75 ÷ 0.85 = 6.765… mm/day; the live calculator reports 6.76 mm/day.daily volume = 6.7647… × 10 × 10 = 676.5 m³/day.weekly volume = 676.5 × 7 = 4,735.3 m³, or 4,735,294 L/week.
This is a gross demand before subtracting verified effective rainfall or changing the schedule for observed root-zone water.
Calculate a gross daily and weekly irrigation requirement from ET₀, Kc, efficiency and area
Irrigation Water Requirement Calculator
Calculate crop water needs from ET₀, crop coefficient, and irrigation efficiency
Turn the estimate into a field decision
The 6.76 mm/day result is a planning rate, not a command to irrigate every day. Compare it with measured soil-water depletion, effective rainfall and system capacity. Use the Soil Moisture Depletion Calculator to relate ETc to an effective root zone and allowable depletion, then use Irrigation Runtime Calculator for a separate runtime estimate.
Assumptions and limitations
- ET₀ represents the selected weather period and Kc represents the selected crop and growth stage under the single-coefficient method.
- Efficiency is an entered planning assumption; it does not prove system uniformity, pressure, infiltration or application performance.
- The calculation does not model rainfall timing, salinity, soil-water measurements, restrictions, crop stress, runoff or deep percolation.
Common mistakes
- Using ET₀ as crop ET: apply a stage-appropriate Kc first.
- Mixing net and gross depths: apply efficiency once, and identify whether a reported value is at the root zone or at the system inlet.
- Counting roof capture as field rainfall: account for storage, delivery and application separately.
- Using one Kc all season: update the coefficient and weather input as crop stage and conditions change.
Sources
- FAO Irrigation and Drainage Paper 56, Chapter 6: ETc — single crop coefficient — primary method for
ETc = Kc × ET₀and its planning context. - FAO Irrigation and Drainage Paper 56, Chapter 5: introduction to crop evapotranspiration — definition of reference ET and standard-condition limitations.