# 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.

## 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

ETo = p ·(0.457·Tmean + 8.128)

Where:

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

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]