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What is the Watering Index?

The Watering Index is a percentage of the maximum amount of water you would apply on your landscape in July or August, when temperatures are high, rainfall is scarce and days are long. The Watering Index is adjusted weekly using data collected from a local weather station.

Think of it this way: during a hot, dry week in July, you are watering at 100% your landscape’s maximum need. But if we get a week of foggy weather, the Watering Index reflects that and tells you how to adjust your irrigation controller to prevent over watering. Over the course of the year, the Watering Index changes weekly to reflect the landscape’s changing need for water as climatic conditions change.  As new Watering Index values are published each week, the irrigation controller’s water budget feature should be changed to match the current Watering Index value.

If you water with a hose or your irrigation controller does not include the “water budget adjustment” feature, then you can use the Watering Index as a guideline for manually adjusting your watering duration or frequency.

The Watering Index is a scientifically based guide to help people adjust watering schedules for landscape irrigation controllers (also commonly known as timers) that include a water budget adjustment feature. 

This feature (either a button or a dial) permits the watering run times for all electric valves managed by a controller to be increased or decreased with just one adjustment.  The adjustment is by percentage.

The Watering Index represents the recommended percentage setting for the watering budget adjustment feature.  The index is normally 100% for much of July and August.  Over the course of the year the index changes to reflect the landscape’s changing need for water climatic conditions change.  As new Watering Index values are published weekly, the controller’s percentage adjust feature should be changed to match the current index value.      

Note: the Watering Index does not specify that watering times should be increased or decreased by a specified number of minutes per electric valve.  Instead, it simply specifies a value that the percentage adjust feature should be set at.  The watering times will automatically change when the percentage adjust setting is changed.

What is the basis for the Watering Index?

The Watering Index is based on ET (evapotranspiration) data 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 and wind speed. A watering schedule should be designed to periodically replace the amount of water lost via ET.

The Watering Index compares the average ET data for the most recent week with the highest average weekly ET value recorded over the last 10 years.  The historical high occurs in July or August and equals 100% on the Watering Index.  

The graph below shows a typical ET curve and the step-curve created from Watering Index values calculated over a twelve-month period.   The third curve shows the water used when the irrigation schedule is adjusted just twice a year, a common approach for many people.   

Where does the ET data come from that is used in the Watering Index?

Both the historical and actual ET data is collected from a weather station located at the City of Santa Barbara’s Municipal Golf Course.  This station was selected because of its long history of data and its average weather location in the County. 

Wouldn’t a weather station that is closer to me than the City of Santa Barbara reflect my weather conditions more accurately?

Yes, but the Watering Index is not dependent on local conditions.  Even though Santa Barbara’s climate is different than, say, the climate of an inland community, as the ET in Santa Barbara increases or decreases, the ET for the entire Santa Barbara County area generally tends to increase or decrease.  The watering run times (expressed in minutes per electric valve or per station and the number of days of watering per week), which are set by you or your gardener, reflect your particular climate.  The Watering Index simply specifies the value at which the percentage adjust feature should be set as climatic conditions change.  The change in percentage value will automatically change the run times as the climatic conditions change.   

What if my irrigation controller is set at the suggested Watering Index value and my plants looked stressed?

Realize the Watering Index value is only an educated guide - a helpful tool.   It is not an absolute command that is always right for all situations.  The index coaches you when adjustments should be made and how large those adjustments might be, but you have to be pro-active by paying attention to the health of your landscape.  If your plants look stressed after setting your percentage adjust feature at the Watering Index value, increase the current percentage adjust amount on your controller by a notch.  (Common signs of plant stress are droopy leaves, or a grayish blue tinge to your grass, or grass that stays flat after being stepped on.)  Wait several days to see if the stress disappears.  If signs of stress remain, increase the adjust feature up another notch.  Make these periodic adjustments until the landscape appears healthy.  (Note: for some controllers a “notch” represents one percent, for others it
can represent 5 or 10 percent.  For controllers with one percent increments, make notch adjustments of 5%.)There is a flip side to this technique.  If your percentage adjust feature is set at the value of the Watering Index, and if you’ve made no “notch” adjustments, and if the landscape looks healthy, the watering system may be actually be applying more water than is necessary, meaning you are paying for more water than you need to.  To check if this is the case, reverse the process described above.  Reduce the percentage adjust feature, notch by notch, watching your landscape for several days between each change.  Once you’ve reduced the percentage to a point where the plants are beginning to show signs of stress, increase the percentage adjust feature by a notch.

Could the Watering Index ever be greater than 100 percent?

Yes.  If the year is hotter than the 10-year average, or if there are exceptionally hot and dry periods (like when Santa Ana winds are in town), the Watering Index may exceed 100%.  The percentage adjust feature can be set for values greater than 100%.    

Where will I find the latest weekly Watering Index?

The index will be published on www.sbwater.org and other local water purveyors websites.

How can I reduce my irrigation schedule if I don’t have an irrigation controller or have an irrigation controller that lacks the percentage adjust feature?

There is an on-line program called the Landscape Watering Calculator that was developed by the City of San Diego.  After plugging in your zip code and answering a few simple questions about your landscape and watering system, the program provides a weekly irrigation schedule in minutes of watering time per station.  The schedule is custom tailored to your specific situation. 

Where do I find this Landscape Watering Calculator?

Go to http://www.santabarbaraca.gov/WaterCalc/

Is it a good idea to use the Landscape Watering Calculator first before adjusting my controller to match the Watering Index?

Yes, assuming you have an irrigation controller with a percentage adjust feature.  It is a good idea to use the calculator to confirm that your current watering program is reasonable.  Once you are certain that you have an efficient watering schedule for the peak watering months of July and August, you can input the weekly Watering Index information to keep your watering schedule current as climatic conditions change.  However, if the program in your controller is not efficient, following the weekly Watering Index values will merely shift the inefficient program up and down, with an end result of inefficient watering. 

If you have a controller that lacks the percentage adjust feature, the Landscape Watering Calculator is definitely a must.  It will not only provide a starting schedule, but it will also provide a long-term schedule to follow over time.  Like the Watering Index, the Landscape Watering Calculator should be used together with ongoing observation of the health of your landscape to make schedule adjustments to overcome signs of stressed landscape. 

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

Definitely, yes.  In addition to having a good watering schedule and adjusting it as the Watering Index would suggest, 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 of a sprinkler, broken nozzles, clogged nozzles, sprinklers that do not pop up vertically, 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 just a gentle breeze. 

If your water pressure is low check for the following: broken or leaking sprinkler heads, broken pipe, or too many sprinkler heads on one valve.

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

Your local water agency is a good resource for assistance in water efficiency including free irrigation system evaluations. 

Also, it is very important to look for runoff, especially if you have sloping landscape and clay soil.  If runoff occurs before the appropriate watering time is completed, break the watering time into increments that do not exceed the time it takes before runoff appears.  Then, reprogram the controller to run this station (the electric valve) however many times is necessary at the shorter run times to apply sufficient water, allowing enough time between run times to permit the water to soak into the soil.  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.