Lawn Watering

Photo courtesy Hunter Industries

Why we water...

In the Santa Clara Valley, lawn watering makes up nearly half the water used for most homes during the warmer months. We believe that responsible landscapers should reduce the amount of space in our landscapes devoted to lawns, but lawns still serve an important role in moderating temperatures and cutting down dust.

Most of us run our sprinklers too often and too long. 

Less frequent, deeper watering is better for most varieties of turfgrass. Daily watering encourages weeds, and promotes shallower roots. Constantly saturated soil doesn't give roots a chance to "breathe" properly.

Turf studies have shown that most lawns need to be watered only once every three days. The guide below will help you determine how long to set your sprinklers to water on an "every third day" schedule.

Every time you water you are attempting to replace water lost to evapotranspiration.

The amount of water your lawn uses is influenced by local environmental factors such as
humidity, temperature, sun or shade and wind exposure. These are all considered in calculating what is known as the "evapotranspiration rate". or "ET rate", expressed in inches (as in the number of inches of rainfall or irrigation that need to be replaced.) Remember that whenever you water you are attempting to replace water which has been lost to ET.

ET data is gathered at a number of weather stations around the state and is often available according to Zip code, either as historical data or in "real time". ET is currently used by a new generation of sprinkler controllers that take out the scheduling guesswork. 

How much water are you currently using?

Before setting your timer you need to find out how much water your lawn is actually using.

The first step is to take care of obvious repair and maintenance issues.
Leaky sprinklers, clogged nozzles, and heads set too deep in the lawn all contribute to poor, uneven coverage. Repair, clean and adjust sprinkler heads as needed.

The next step is to determine how much water your sprinklers are applying. The amount of water falling on your lawn is called the "
precipitation rate" or PR.

Sprinkler manufacturers test and rate sprinkler performance (PR shown as inches per hour) in their product catalogs, based on their recommended spacing. Most feature "
matched precipitation rate" or MPR nozzles, which means that a quarter-circle head applies 1/4 of the water of a full circle head, a half-circle head applies 1/2 the water of the full circle (or twice that of the quarter circle), etc.

Deviations in spacing, running sprinklers at too high or too low pressure, or using differently rated nozzles on the same line can cause wide variations in the application of water.

Watering zones with
impact heads (Rain Bird MiniPAW), gear-driven rotors (Hunter PGP) or stream rotors (Toro 300) will require longer run times than spray heads (Toro 570 or Rain Bird 1800) because rotors and impact apply water more slowly over a greater area. This will be reflected in your catch can test. It's also why it's important to keep spray heads and rotors on separate watering zones.

The goal is "uniform coverage", sometimes called "head-to-head coverage", just two different ways of saying "applying water evenly".

How to do the "catch can" test:

  • Set three flat bottomed cans or coffee mugs at various places on your lawn.  
  • Try to place them halfway between sprinklers or in areas that appear to receive the least water.
  • For very large lawns, more than 3 cans may be required. (Tuna cans seem to work well.)
  • Turn on your sprinklers for 15 minutes.
  • Measure the depth of water in each can or mug, then determine the average water depth between the containers.
  • Consult the guide below to determine how much to water your lawn every third day per watering zone.
  • If you have separate zones with different types of sprinkler heads (rotors in one zone and spray heads in another) then the amount in the cans in a given period of time should vary. Adjust each zone's time accordingly.

A guide to running your lawn sprinklers, watering every third day, 
in the Santa Clara Valley:

Use these watering times as a guide only, since so much relies on local soil conditions, slopes, sun exposure, wind and coverage uniformity.

As you use this guide, think of the ground as a sponge. It can only absorb so much water at once....

Average watering depth: March 1 - May 15
May 16 - Sept. 15
Sept. 16 - Nov. 30
(In Inches, in 15 minutes) Minutes to run Minutes to run
Minutes to run
1 6 9 6
3/4 8 12 8
5/8 10 15 10
1/2 12 18 12
3/8 16 24 16
5/16 20 30 20
1/4 24 36 24
3/16 32 48 32
1/8 48 72 48

To avoid run-off, and make sure the ground absorbs all the water your lawn needs, separate your total watering time into two or three periods at least one hour apart. (This is particularly important on slopes.) For instance, if your lawn needs 18 minutes of watering, try watering 3 times for 6 minutes each time.

During winter, water only during extended dry periods.

Winter temperatures contribute to a lower ET rate. Your lawn may need extra water when it's unusually hot, and less when it's cool.

Avoid watering during daylight hours, especially in the afternoon. If you must water during the day, finish before 10:00 AM. 

Don't water during windy conditions. Wind distorts sprinkler spray patterns, resulting in erratic coverage. 
Keep soil porous by aerating lawns at least once per year.

Don't water when it's raining.

Allow your lawn to grow taller in the summer. We recommend  a 2" mowing height (which is probably the highest setting on your mower). When grass is allowed to grow taller, roots will penetrate the soil deeper, making better use of the water you apply. Appropriate pop-up (4" or taller) sprinkler heads will clear the grass blades as long as you mow weekly.

Some newer varieties of turfgrass do much better with less water and fertilizer than old standards. If you currently have a "thirsty" lawn, consider replacing it with a newer hybrid dwarf fescue. Then, as long as you are renovating, think about "downsizing" your lawn at the same time.

Much of the information on this page was adapted from a publication from the Santa Clara Valley Water District. For more water saving tips visit the following page on their website:

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