RET usually represents the PET of the reference crop's most active growth.

The equation calculates evapotranspiration for a 'reference crop', which is taken as actively growing green grass of 8-15 cm height [2].

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

The most basic crop coefficient, K, is simply the ratio of ET observed for the crop studied over that observed for the well calibrated reference crop under the same conditions.

Named in the document issued by Pope Eugene III for the Abbey of Echternach in 1148 was, among other things, a lesser holding near Alflue or Afflue; another such reference crops up from 1161.