What is the FAO Penman-Monteith equation?

ET₀ = [0.408Δ(Rn−G) + γ(900/(T+273))u₂(es−ea)] / [Δ + γ(1+0.34u₂)] where Δ=vapor pressure slope, Rn=net radiation, G=soil heat flux, γ=psychrometric constant, u₂=wind at 2m, es=saturation VP, ea=actual VP.

What is the allowable depletion level?

Allowable depletion (AD) = RAW = p × TAW where p = depletion fraction before stress (typically 0.5–0.7 for most crops), TAW = total available water = (θFC − θWP) × RZD. Irrigate before AD is exhausted.

Irrigation Scheduling Calculator — ET-Based Water Management

Schedule irrigation using the soil water balance method. Calculate evapotranspiration, allowable depletion, and irrigation intervals from FAO-56 Penman-Monteith ET₀.

Tap constants to insert

Physical Constants Reference

Constant	Symbol	Value	Units
Speed of light	c	2.99792458×10⁸	m/s
Planck's constant	h	6.62607015×10⁻³‴	J·s
Boltzmann constant	kʙ	1.380649×10⁻²³	J/K
Avogadro's number	Nₐ	6.02214076×10²³	mol⁻¹
Gravitational constant	G	6.6743×10⁻¹¹	N·m²/kg²
Gas constant	R	8.31446	J/(mol·K)
Elementary charge	e	1.602176634×10⁻¹⁹	C
Electron mass	mₑ	9.1093837015×10⁻³¹	kg
Proton mass	mₚ	1.67262192369×10⁻²⁷	kg
Fine-structure constant	α	7.2973525693×10⁻³	dimensionless

How to use this calculator

1
Enter the required values
Enter the required values into the Irrigation Scheduling Calculator — ET-Based Water Management input fields.
2
Adjust settings
Adjust any settings, select applicable options, or choose units as needed.
3
Calculate the result
Press Calculate to compute the result using the underlying formula.
4
Interpret the output
Review and interpret your result using the provided context, reference ranges, or explanatory notes.

Formula

Result = f(physical quantities, constants, conditions)

physical quantities: Measured values with units entered into the Irrigation Scheduling.
constants: Physical or chemical constants (e.g. CODATA values, gas constant, Avogadro's number).
conditions: Temperature, pressure, concentration, or environmental state of the system.
Result: Computed physical or chemical quantity with associated units.

Compare results with standard reference values or published experimental data.

Glossary and Definitions

Physical quantity: A measurable property of a physical system, expressed as a numeric value with units.
Constant: A fixed, well-defined value (e.g. a CODATA constant) used across scientific calculations.
Significant figures: The digits in a measurement that carry meaning, reflecting the precision of the instrument.
Reference value: A published standard or experimental value used to validate a calculated result.
Irrigation calculator: A key concept referenced by the Irrigation Scheduling: Schedule irrigation using the soil water balance method. Calculate evapotranspiration, allowable depletion, and irrigation intervals from FAO-56 Penman-Monteith ET₀.
ET calculator: A key concept referenced by the Irrigation Scheduling: Schedule irrigation using the soil water balance method. Calculate evapotranspiration, allowable depletion, and irrigation intervals from FAO-56 Penman-Monteith ET₀.
Penman-Monteith: A key concept referenced by the Irrigation Scheduling: Schedule irrigation using the soil water balance method. Calculate evapotranspiration, allowable depletion, and irrigation intervals from FAO-56 Penman-Monteith ET₀.
Soil water balance: A key concept referenced by the Irrigation Scheduling: Schedule irrigation using the soil water balance method. Calculate evapotranspiration, allowable depletion, and irrigation intervals from FAO-56 Penman-Monteith ET₀.

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