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 [2].
The equation calculates evapotranspiration for a 'reference crop', which is taken as actively growing green grass of 8–15 cm height [3].
Equation
ETo = p ·(0.46·Tmean + 8)
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.[1]
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 [4]
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 [5].
See also
- Jensen–Haise equation (M. E. Jensen and H. R. Haise, 1963)
- Penman–Monteith equation