Penman-Monteith Equation

ET_0 = \frac{0.408\Delta(R_n - G) + \gamma\frac{900}{T+273}u_2(e_s - e_a)}{\Delta + \gamma(1 + 0.34 u_2)}

Reference evapotranspiration combines radiation, temperature, humidity, and wind for crop water needs.

By Howard Penman, John Monteith

Agricultural Sciences

1948/1965 · Howard Penman

Why it matters: Enables efficient water use for feeding billions.

Discoverers: Howard Penman, John Monteith (1948/1965)

What does it mean?

Reference evapotranspiration combines radiation, temperature, humidity, and wind for crop water needs.

Enables efficient water use for feeding billions.

Symbol	Name	Unit	Meaning
$ET₀$	Evapotranspiration	—	mm/day
$R_n$	Net radiation	—	MJ/m²/day
$u₂$	Wind speed	—	m/s at 2m
$e_s - e_a$	VPD	—	Vapor pressure deficit

Hot dry windy day → high ET₀ → more irrigation needed.

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Equation Universe

ET_0 = \frac{0.408\Delta(R_n - G) + \gamma\frac{900}{T+273}u_2(e_s - e_a)}{\Delta + \gamma(1 + 0.34 u_2)}

Real-world impact

Precision agriculture

Equations optimize food production for billions.

Photo: Unsplash — precision farming

Reference evapotranspiration combines radiation, temperature, humidity, and wind for crop water needs.

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