ET is an abbreviation for the word evapotranspiration. Evapotranspiration is the combination of the words evaporation and transpiration. Evaporation is the process of water in the soil changing to a vapor. Transpiration is the process of plants taking up water through their roots and expelling excess water through their stomata. Evapotranspiration is the water loss caused by the two processes. ET represents the amount of water that must be replaced by rainfall or irrigation. Plants have different water requirements, i.e. different ET rates. This is known as the plant coefficient.

The United Nations’ agency ”Food and Agriculture Organization” did much of the early research on ET. They were wrestling with how to estimate plant water loss across all climates. This produced many formulas for calculating ET. Each is usually a minor improvement over a previous formula. This culminated in ET controllers for irrigation systems; computers using an ET formula and data from weather stations to estimate the amount of water being lost. This technology is often referred to as “SMART Irrigation”.

ET controllers look at the estimate of the water loss and the precipitation received and modify the watering schedule to replace the difference. There have been many studies on the effectiveness of ET controllers. No study has said ET controllers do not save water. Many people, working in the landscape irrigation industry, believe the ”clock” style controllers will be completely replaced by ET controllers simply because of the amount of water they save.

See How to Select an ET Controller