The Watering Index is based on ET data (evapotranspiration) that measures the loss of water evaporated from the soil and transpired by plants. ET is calculated from specialized weather stations that measure such information as solar radiation, air temperature, relative humidity and wind velocity. ET values are primarily dependent on the amount of solar radiation. A watering schedule should be designed to periodically replace the amount of water lost via ET.
The Watering Index compares current ET data with the highest average ET value recorded over the last 10 years. The historical high occurs in July and equals 100 percent on the Watering Index. The concept behind the Watering Index was developed by Mr. John Wynn, of the State of California Department of Water Resources.
The graph below shows the amount of water that could be saved if the controller is adjusted to match the Watering Index instead of just twice a year, which is typical for many people.