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1. What is a Watering Index?

The Watering Index (%) is a simple way to represent and understand plant water needs. It is based on weather conditions, such as: solar radiation, temperature, relative humidity, wind, and other factors. Summer represents 100% of maximum water needed for plants. Over the course of the year the Watering Index changes to reflect the landscape’s changing need for water as climatic conditions change. As new Watering Index values are published, the controller’s water budget adjustment feature should be changed to match the current Watering Index value.  The watering times for all valves controlled by one landscape controller will change by the same percentage when the water budget adjustment feature settings are changed.

Use the chart below, along with site conditions, to find the right watering index value for your yard! 

Many new(er) sprinkler timers have a seasonal adjust option (or % adjust) that you can use. To do so, first program your sprinkler times for summer conditions (100% seasonal adjustment factor). You can then just use the seasonal adjust feature to adjust the watering index to the appropriate percentage. For example, if it’s October, you use a watering index of 50-60% and your sprinkler runtimes will automatically be cut by about half based on your summer (100%) schedule.

2. How Do I use the Watering Index?

Most modern sprinkler timers have a feature that lets you use a Watering Index – it is often called a "Seasonal Adjust" or "Water Budget" feature. The Watering Index (%) modifies your sprinkler controller's watering runtimes. It does this by controlling the watering runtimes for all electric valves managed by your sprinkler controller. By adjusting the Watering Index on your sprinkler controller, all your watering runtimes are either increased or decreased. The adjustment is effectively a percentage (%) of the maximum watering runtime allowed by the sprinkler controller for each valve. As an example, if you water for 6 minutes in summer (at a 100% Watering Index), a 50% Watering Index setting would reduce sprinkler runtimes to 3 minutes.

3. What if my plants look stressed even though I'm using the recommended Watering Index?

The Watering Index (%) is a tool to help you adjust watering times for the current weather. The Watering Index is not  hyper dependent on local conditions. Watering run times will be different to reflect your particular climate, but the Watering Index generally goes up or down by the same percentage throughout region. The Watering Index (%) suggests when adjustments should be made, but you have to be proactive by paying attention to the health of your landscape. If your plants look stressed after setting your Watering Index (%) on your sprinkler controller (i.e. “Seasonal Adjust” or “Water Budget” feature), then make practical adjustments from there based on what your landscape is telling you. You may increase the Watering Index (%) slightly, or if the issue is specific to a certain sprinkler valve, go back to the sprinkler controller and adjust the runtime for that zone.

For help on how to program your sprinkler controller, consult your controller's instructions, view videos on how to adjust your specific type of sprinkler controller, or contact your landscaping professional

4. 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.

5. Is there anything else I can do to improve my watering practices and reduce my use of water?

In addition to having a good watering schedule and adjusting it as the Watering Index changes, it is important to periodically (at least monthly) turn on each irrigation valve to see how it is working. Look for sprinklers that are not popping up properly, shrubbery or grass that is interfering with the watering pattern, broken or clogged nozzles, sprinklers that are spraying sidewalks and driveways, etc.

If your water pressure is high, use pressure regulation devices to bring the sprinkler operating pressure down to the optimal pressure range specified by the manufacturer. Pressure that is too high causes the water exiting the sprinkler to turn to mist, which, can be blown away by even a gentle breeze.

 Water in the early morning hours and when the air is still. Do not water in the afternoon, or much of your water will be lost to evaporation by the sun.

 Watch for run-off.  If you water for longer periods, water may run off and be wasted.  Many controllers have a feature that allows for multiple start times so you can “cycle and soak”.  Watering for shorter periods allows the water to soak in.  To prevent run-off, multiple start times of shorter duration may be set before and/or after any time restrictions in your area.  Avoiding runoff not only reduces your water use and improves the appearance of your landscape, but, equally important, it avoids runoff that carries pollutants into our storm drains and onto our beaches