What is the crop coefficient (Kc)?

Kc relates crop-specific ET to reference ET₀: ETc = Kc × ET₀. Three-stage Kc: Kc_ini (initial), Kc_mid (mid-season), Kc_end (late). For corn: Kc_ini=0.3, Kc_mid=1.2, Kc_end=0.35.

What is the seasonal water requirement for major crops?

Corn: 500–800 mm/season. Wheat: 400–600 mm. Soybean: 450–700 mm. Cotton: 700–900 mm. Rice (paddy): 900–1200 mm (includes percolation losses). Values are total ETc for the season.

Crop Water Requirement & Kc Calculator — FAO-56

Calculate crop evapotranspiration (ETc) from reference ET and crop coefficients (Kc) for major crops across growth stages.

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 Crop Water Requirement & Kc Calculator — FAO-56 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 Crop Water Req..
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.
Crop water requirement: A key concept referenced by the Crop Water Req.: Calculate crop evapotranspiration (ETc) from reference ET and crop coefficients (Kc) for major crops across growth stages.
ET crop calculator: A key concept referenced by the Crop Water Req.: Calculate crop evapotranspiration (ETc) from reference ET and crop coefficients (Kc) for major crops across growth stages.
Crop coefficient Kc: A key concept referenced by the Crop Water Req.: Calculate crop evapotranspiration (ETc) from reference ET and crop coefficients (Kc) for major crops across growth stages.
Crop evapotranspiration: A key concept referenced by the Crop Water Req.: Calculate crop evapotranspiration (ETc) from reference ET and crop coefficients (Kc) for major crops across growth stages.

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