# Blaney–Criddle equation

The **Blaney–Criddle equation** (named after H. F. Blaney and W. D. Criddle) is a method for estimating reference crop evapotranspiration.

## Usage

The Blaney–Criddle equation is a relatively simplistic method for calculating evapotranspiration. When sufficient meteorological data is available the Penman–Monteith equation is usually preferred. However, the Blaney–Criddle equation is ideal when only air-temperature datasets are available for a site.

Given the coarse accuracy of the Blaney–Criddle equation, it is recommended that it be used to calculate evapotranspiration for periods of one month or greater.^{[1]}

The equation calculates evapotranspiration for a 'reference crop', which is taken as actively growing green grass of 8–15 cm height.^{[2]}

## Equation

*ET _{o}* =

*p*·(0.457·

*T*+ 8.128)

_{mean}Where:

*ET _{o}* is the reference evapotranspiration [mm day

^{−1}] (monthly)

*T _{mean}* is the mean daily temperature [°C] given as

*T*

_{mean}= (T_{max}+ T_{min})/ 2*p* is the mean daily percentage of annual daytime hours.^{[3]}

## Accuracy and bias

Given the limited data input to the equation, the calculated evapotranspiration should be regarded as only broadly accurate. Rather than a precise measure of evapotranspiration, the output of the equation is better thought of as providing an order of magnitude.^{[2]}

The inaccuracy of the equation is exacerbated by extreme variants of weather. In particular evapotranspiration is known to be exaggerated by up to 40% in calm, humid, clouded areas and depreciated by 60% in windy, dry, sunny areas.^{[2]}

## See also

- Jensen–Haise equation (M. E. Jensen and H. R. Haise, 1963)
- Penman–Monteith equation

## External links

## Notes and references

- ↑ "Chapter 2 - FAO Penman-Monteith equation". http://www.fao.org/docrep/X0490E/x0490e06.htm.
- ↑
^{2.0}^{2.1}^{2.2}"CHAPTER 3: CROP WATER NEEDS". http://www.fao.org/docrep/S2022E/s2022e07.htm. - ↑ "Irrigation Water Management: Irrigation Water Needs - FAO 1986". http://www.fao.org/docrep/s2022e/s2022e07.htm.

Original source: https://en.wikipedia.org/wiki/Blaney–Criddle equation.
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