ET stands for Evapotranspiration, which is the amount of water that is evaporated from the soil and transpired by plants due to local weather conditions.

Four indicators calculate ET:
Temperature
Solar Radiation
Wind Speed
Humidity

The weather-based controller compiles the daily ET reading along with each watering zones specific features like: Landscape type (lawn, flower bed, slope etc.), Soil Type (clay, sand etc), and Sun Intake (shady or direct sunlight area )