Crop Water Requirement (ET) Calculator
Calculate crop evapotranspiration (ETc) using the FAO-56 Penman-Monteith reference evapotranspiration (ETo) and crop coefficient (Kc) method.
Formulas Used
FAO-56 Penman-Monteith Reference Evapotranspiration (ETo):
ET₀ = [0.408·Δ·(Rn − G) + γ·(900/(T+273))·u₂·(es−ea)] / [Δ + γ·(1 + 0.34·u₂)]
- Δ = slope of saturation vapour pressure curve (kPa/°C)
- Rn = net radiation at crop surface (MJ/m²/day)
- G = soil heat flux (MJ/m²/day) — 0 for daily
- γ = psychrometric constant = 0.000665 × P
- T = mean daily air temperature (°C)
- u₂ = wind speed at 2 m height (m/s)
- es = saturation vapour pressure (kPa)
- ea = actual vapour pressure = (RH/100) × es
Crop Evapotranspiration: ETc = Kc × ETo
Gross Water Requirement: GWR = (ETc × days × area × 10) / (efficiency/100) [m³]
Assumptions & References
- Based on FAO Irrigation and Drainage Paper No. 56 (Allen et al., 1998).
- Reference surface is a hypothetical grass crop with height 0.12 m, fixed surface resistance 70 s/m, and albedo 0.23.
- Soil heat flux G = 0 is standard for daily ET₀ calculations (FAO-56 §3.2).
- Wind speed input must be measured or corrected to 2 m height. Use u₂ = u_z × (4.87 / ln(67.8·z − 5.42)) to convert from height z.
- Typical Kc values (FAO-56 Table 12): Wheat mid-season 1.15, Maize mid-season 1.20, Cotton mid-season 1.20, Rice mid-season 1.20, Tomato mid-season 1.15, Potato mid-season 1.15.
- 1 mm of ET over 1 hectare = 10 m³ of water.
- Irrigation efficiency accounts for conveyance, distribution, and application losses.
- Extraterrestrial radiation Ra is computed from latitude and day of year (FAO-56 Eq. 21).
- Net longwave radiation uses the FAO-56 simplified formula (Eq. 39) with Stefan-Boltzmann constant σ = 4.903×10⁻⁹ MJ/K⁴/m²/day.