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.