Crop Water Requirement Calculator

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, which is 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.

Do not use Kc_mid for the whole season. This is the most common mistake in back-of-envelope irrigation planning, and it is worth understanding because it inflates a water budget by a fifth or more. A wheat crop spends roughly 15% of its days as bare soil at Kc 0.30, another 25% ramping up, only 40% at full canopy, and the last 20% senescing back down. Weighted across those stages the season-average Kc is about 0.83, not 1.15.

At ET₀ = 5 mm/day over 130 days that is the difference between 537 mm and 748 mm. The first figure sits inside the 450 to 600 mm that wheat is known to use; the second does not. This calculator reports both numbers, and they answer different questions: peak daily ETc at Kc_mid sizes your pump and pipe, the stage-weighted season total sizes your water budget.

Perennials are the exception. Alfalfa and citrus hold near Kc_mid year-round, so no stage weighting is applied to them.

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): the portion of rainfall that infiltrates and is available to crops. Typically Pe = 0.7 to 0.8 of total rainfall for most cropping situations. Gross irrigation = NIR / irrigation efficiency. Drip: 90 to 95% efficient. Sprinkler: 70 to 85%. Furrow or flood: 40 to 60%.

This calculator uses Pe = 0.80, the optimistic end of that range. That is the right assumption for gentle rain on a soil with room to take it, and the wrong one for a summer downpour on a slope, where half of it can run off before it soaks in. If your rain arrives in heavy bursts, take the irrigation figure here as a floor rather than an answer.

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 to 600 mm of water over its growing season.

Water Stress If soil moisture falls below the stress threshold, usually 50 to 70% of available water depleted, plant water stress begins and yield falls roughly in proportion to it. Ks (stress coefficient) = 1 when there is no stress, and below 1 under deficit irrigation.