Evapotranspiration, or ET, is the process by which water evaporates from the soil and plants. Plants use transpiration of soil to cool themselves, much like humans use perspiration. Plants also use water in photosynthesis. ET measures all these water losses from the landscape.
ET controllers are irrigation controllers that use some method of weather-based adjustment of irrigation. These adjusting methods include:
The ET rate is dependent on such weather factors as sunlight, temperature, wind, and humidity, and varies throughout the day. ET is usually measured in inches per day and is highest in the summer and lowest in the winter. Some ET controllers have the watering schedule programmed into their computer chip based upon historical ET data for a given region. Other ET controllers can change the water schedule weekly based on the past week’s weather for a region via radio signal.
By scheduling watering based on ET rates, plants can receive the required amount of water to remain healthy without over or under watering since the ET controller adjusts the watering schedule automatically. Because many homeowners do not adjust a controller’s watering schedule often enough, ET controllers have the potential to increase watering efficiency.
Rain shut-off devices can be wired to a controller to shut off the system when it is raining. Contacts inside the sensor open when rainfall is detected and close again when they dry out.
A soil moisture sensor is designed to detect the available moisture in the soil. It should be installed in a representative area of the landscape and will prevent irrigation until the soil moisture drops below a set threshold level.