What is basis for the Watering Index?

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.

Show All Answers

1. What is a Watering Index?
2. How Do I use the Watering Index?
3. What if my plants look stressed even though I'm using the recommended Watering Index?
4. What is basis for the Watering Index?
5. Is there anything else I can do to improve my watering practices and reduce my use of water?