Penman-Monteith Evapotranspiration Calculator

Penman–Monteith Reference Evapotranspiration (FAO 56)

When agronomists need to estimate reference evapotranspiration ETo (mm/day), the FAO 56 Penman–Monteith equation is what they reach for. It is the international standard, defined over a hypothetical grass surface 0.12 m tall with fixed albedo and surface resistance: ETo = (0.408 Δ (Rn − G) + γ · 900/(T+273) · u2 · (es − ea)) / (Δ + γ(1 + 0.34 · u2)).

Here is what each input means. T is the mean daily air temperature (°C). u2 is wind speed at 2 m (m/s), Rn is net radiation (MJ m⁻² day⁻¹), and G is soil heat flux (close to 0 for daily steps). The term es−ea is the vapour pressure deficit (kPa), Δ is the slope of the saturation vapour pressure curve, and γ is the psychrometric constant.

Applications

ETo feeds into a lot of work. It drives irrigation scheduling (ETc = Kc · ETo), drought monitoring, hydrological modelling, water-balance studies and climate-change impact assessments. The FAO crop-water-requirement guidelines used worldwide rest on it.

FAQ

Why FAO 56 instead of older Penman or Thornthwaite formulas? Penman–Monteith is physically based, includes aerodynamic and surface resistance terms, and was validated against lysimeter data across many climates.

What if I only have wind at 10 m? Convert it to 2 m with the log-wind profile: u2 = uz · 4.87/ln(67.8z − 5.42) where z is the measurement height in metres.

Can I assume G = 0? Yes, on daily time steps soil heat flux is small relative to Rn and is usually set to zero. For hourly steps use FAO 56 daytime/nighttime coefficients.