Blaney–Criddle equation

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


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]


ETo = p ·(0.457·Tmean + 8.128)


ETo is the reference evapotranspiration [mm day−1] (monthly)

Tmean is the mean daily temperature [°C] given as Tmean = (Tmax + Tmin )/ 2

p is the mean daily percentage of annual daytime hours.[3]

Accuracy and bias[edit]

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[edit]

External links[edit]

Notes and references[edit]

  1. ^ "Chapter 2 - FAO Penman-Monteith equation". Retrieved 3 August 2019.
  2. ^ a b c "CHAPTER 3: CROP WATER NEEDS". Retrieved 3 August 2019.
  3. ^ "Irrigation Water Management: Irrigation Water Needs - FAO 1986".