Climatologically based controllers are also known as evapotranspiration, or ET, controllers (Fig. 1). Generally, ET is the process of transpiration by plants combined with evaporation that occurs from plant and soil surfaces. More information on the ET concept and definitions can be found in the publication, Evapotranspiration: Potential or Reference? http://edis.ifas.ufl.edu/AE256. There are generally three types of ET controllers: