The Blaney–Criddle equation (named after H. F. Blaney and W. D. Criddle) is a method for estimating reference crop evapotranspiration.
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.
[3] 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.