AE459, a 12-page fact sheet by Lincoln Zotarelli, Michael D. Dukes, Consuelo C. Romero, Kati W. Migliaccio, and Kelly T. Morgan, provides a step-by-step calculation of the reference evapotranspiration (FAO-56 method) for a given location from the available weather data. Includes references. Published by the UF Department of Agricultural and Biological Engineering, February 2010.
http://edis.ifas.ufl.edu/ae459