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. The Blaney–Criddle equation is however ideal when only air temperature data is available for a site.
ETo = p ·(0.46·Tmean + 8)
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.
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 
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 .