Crop Water Requirement Calculator
Calculate crop water requirements and irrigation needs using the FAO Penman-Monteith method.
Find reference ET and crop ETc from Kc coefficients.
What Is Evapotranspiration (ET)? Evapotranspiration is the combined water loss from soil evaporation and plant transpiration. ET₀ (reference ET): the theoretical water use of a well-watered grass reference crop (0.12 m tall). ETc (crop ET): the actual water use of a specific crop = ET₀ × Kc. Kc (crop coefficient): accounts for crop type, growth stage, and canopy cover.
FAO Penman-Monteith Method The FAO 56 Penman-Monteith equation is the international standard for ET₀ calculation. ET₀ = [0.408Δ(Rn−G) + γ(900/(T+273))u₂(eₛ−eₐ)] / [Δ + γ(1+0.34u₂)] Where: Rn = net radiation, T = air temperature, u₂ = wind speed at 2m, eₛ−eₐ = vapor pressure deficit. This simplified calculator uses monthly average inputs for a quick estimate.
Simplified ET₀ (Hargreaves Method) ET₀ ≈ 0.0023 × (T_mean + 17.8) × (T_max − T_min)^0.5 × Ra Where Ra = extraterrestrial radiation (depends on latitude and month). The Hargreaves method requires only temperature data — useful when weather station data is limited.
Crop Coefficients (Kc): FAO 56 Kc is divided into three growth stages: Initial (Kc_ini): seedling stage, low ground cover. Mid-season (Kc_mid): full canopy, maximum water use. Late season (Kc_end): ripening, senescence.
Common Kc_mid values: Wheat: 1.15. Maize (corn): 1.20. Cotton: 1.20. Soybeans: 1.15. Tomatoes: 1.15. Potatoes: 1.15. Sunflower: 1.00. Grapes: 0.85. Citrus: 0.70. Alfalfa: 1.20.
Irrigation Requirement Net irrigation requirement (NIR) = ETc − effective rainfall. Effective rainfall (Pe): portion of rainfall that infiltrates and is available to crops. Typically Pe = 0.7–0.8 of total rainfall for most cropping situations. Gross irrigation = NIR / irrigation efficiency. Drip: 90–95% efficient. Sprinkler: 70–85%. Furrow/flood: 40–60%.
Units ET is measured in mm/day (depth of water). 1 mm/day = 10 m³/ha/day. For 1 hectare, 1 mm of ET = 10,000 liters = 10 m³ of water. A typical wheat crop needs 450–600 mm of water over its growing season.
Water Stress If soil moisture falls below the stress threshold (usually 50–70% of available water depleted): Plant water stress begins → yield reduction proportional to stress. Ks (stress coefficient) = 1 when no stress, < 1 under deficit irrigation.
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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