Evapotranspiration (ET) represents the loss of water from the Earth’s surface from the combination of direct evaporation and plant transpiration. ET is usually expressed as a rate such as inches per day. Knowledge of ET is important for irrigation scheduling but it is also an important factor for other land use applications such as septic tank drain fields, water shed water budgeting, and climate and weather models. ET can be used as a historical tool but usually it is predicted or used in a forecast to help irrigators optimize irrigation.