The concept of reference evapotranspiration (ET) was developed in the 1970’s as a practical and definable replacement for the term potential ET. Reference ET is a function of local weather, represents the ET from a defined vegetated surface, and serves as an evaporative index by which engineers, hydrologists, water managers and other technical professionals can predict ET for a range of vegetation and surface conditions by applying “crop” coefficients for agricultural or landscaped areas.