Crop Coefficient Method

\[\begin{aligned} ET_0 &= 0.0023(T_{mean}+17.8)\sqrt{T_{max}-T_{min}}R_a \\ ET_c &= K_cET_0 \end{aligned}\]

Variables

ET0HarHargreaves reference evapotranspiration (mm/day)

ETcKccrop evapotranspiration from crop coefficient method (mm/day)

Kccrop coefficient (-)

Tmeanmean daily air temperature (°C)

Tmaxmaximum daily air temperature (°C)

Tminminimum daily air temperature (°C)

Raextraterrestrial radiation (MJ/m²/day)

Description

What is this formula?

The Crop Coefficient Method estimates crop water use by first calculating reference evapotranspiration and then applying a crop-specific coefficient.

This approach is recommended by FAO and is the most widely used methodology for agricultural irrigation planning.

When to use it

Use this method when determining irrigation requirements for specific crops and growth stages.

Example

Given:

Tmean = 24 °C

Tmax = 32 °C

Tmin = 16 °C

Ra = 36 MJ/m²/day

Kc = 1.10

Step 1:

ET0 = 0.0023 × (24 + 17.8) × √(32 − 16) × 36

ET0 = 13.84 mm/day

Step 2:

ETc = 1.10 × 13.84

ETc = 15.22 mm/day

Result:

The estimated crop evapotranspiration is 15.22 mm/day.

Applications

- Irrigation scheduling

- Crop water requirement estimation

- Agricultural water management

- Irrigation project design

- Water resource planning

Note

This formula represents a calculation procedure rather than a fundamental physical law.

The FAO methodology normally computes ET0 using the Penman-Monteith equation. In this implementation, the Hargreaves equation is used because it requires fewer meteorological inputs and is more suitable for general-purpose calculators.

The coefficient Kc varies throughout the crop life cycle and should be selected according to the crop growth stage.

Using locally calibrated Kc values generally improves irrigation accuracy.