Precipitation rate

Why do we provide irrigation recommendations as inches of water?

The Inland Valley Garden Planner provides irrigation recommendations as “inches of water”. You can think about this measurement as an inch deep of water covering the entire irrigated area in your yard or landscape. We provide irrigation recommendations in inches, instead of recommending how long to run your irrigation system, because the flow rate at which different irrigation systems provide water varies greatly. Even within the same irrigation system type, like spray systems, different spray nozzles and sprinkler layouts can cause significant differences in how fast water flows in any specific irrigation zone on a property. Inches of water is a more uniform measurement that can be applied to any irrigation system, on any property.

Here are a few useful terms to help apply the Inland Valley Garden Planner irrigation recommendations to your own irrigation system. The rate at which an irrigation system puts out water in inches is the “precipitation rate” and is measured here as inches per minute. A “zone” refers to an area of your yard that is irrigated with one single valve. A “hydrozone” refers to an area in which all plants have similar water needs. Setting up hydrozones is the most water efficient and simple way to irrigate your yard, and it also results in the best plant health.

For the reasons listed above, we provide recommendations for the volume of water (in inches) required to properly irrigate a zone of plants, and provide additional resources to help you determine the schedule and precipitation rate for each zone of your irrigation system. Once you know how much and how oSen a zone of your yard needs water (supplied by the schedule charts in the Inland Valley Garden Planner), and the precipitation rate of that zone, you can calculate how long you will need to water (also known as run time) for each zone of your irrigation system for different seasons of the year.

How can I convert the “inches of water” recommendations into a usable irrigation schedule for my irrigation system?

Depending on the type of irrigation you have in your yard (spray, rotor, or drip irrigation), different methods are available for you to use to calculate your irrigation schedule. Some require a bit more math and some a bit less – use whatever method is best for you. Remember, if you live within the area served by the Chino Basin Water Conservation District (find information here), and you have a functional irrigation system, you can sign up for a free “Irrigation Efficiency House Call,” and as part of the service, we will calculate precipitation rates for you and provide you with irrigation timer recommendations.

Part A: Determine your precipitation rate for each zone

Before starting, briefly run the irrigation for the areas you will be measuring to make sure there are no clogged or broken sprinklers or leaks. If anything is broken or clogged, fix it before testing. You can check for leaks in drip lines buried in mulch by running the system and walking the area both looking for visual leaks, or for smaller leaks, listening for a hiss or other unusual sounds. Don’t be intimidated if the process below seems confusing to you. Visit our “Additional Resources” page (link) to see videos that will walk you through each of these approaches as well as lots of other information.

If you have a spray or rotor system, you can determine the precipitation rate using a simple “catch can” method. The simplest method is to use 15 to 20 jars with wide mouths and straight sides, such as cat food cans, tuna cans, or pint-size wide-mouth mason jars. Many containers will work, as long as the opening is the same size as the rest of the container and you have a large number of matching containers (see image). One convenient option is to buy a 16-pack of wide-mouth pint size (NOT quart size) mason jars. To test your precipitation rate, place the identical jars or cans in roughly a grid in one zone of your yard, ensuring that some cans are placed close to the sprinklers and that some are placed in the center of the irrigation areas. Run the corresponding zone of your irrigation system for 10 minutes. After 10 minutes, measure and record the depth of water in each container (in inches or part-inches) by using a ruler either along the side of the jar, or dipped into the can. Round to the nearest one-eighth of an inch. Add up all the recorded numbers (keeping units in inches) and divide the total inches by the number of jars or cans you put out to get the average depth. Divide that number by 10 (because the system ran for 10 minutes) to calculate the precipitation rate in inches per minute. See below for how to use this information to calculate your runtime.

9) Create a chart with the measurements, converting the records for each jar (originally noted in eights of an inch) into decimals by dividing the top number of the fraction by the bottom number (8). Add up all the decimals to get the total number of inches of water for all of the jars added up.

Note: An efficient irrigation system will relatively evenly water an area, so there should not be a huge difference between the depths of the water caught in each jar. If the recorded numbers are extremely varied, you may want to consider making improvements to your irrigation system, especially if you are planning to replant the area.

10) Find the average depth of the water for the jars by dividing the total depth from all jars (9.625”) by the number of jars (12):

9.625” / 12 = 0.80” Average jar water depth = 0.80”

11) Find the precipitation rate (per minute) by dividing the average jar water depth (0.80”) by the number of minutes the irrigation system ran for the test (20).

0.80” / 20 = 0.04” Precipitation rate = 0.04” per minute

See below for how to use this information to calculate your runtime.

If you have a drip irrigation system, or if you want to use an alternate method to test your spray or rotor system, you can use your water meter to get your precipitation rate. This method requires additional math, because it relies on the meter reading and the area (in square feet) of the irrigation zone you are testing. First, use a measuring tape or measuring wheel to get a measurement of the total area irrigated by the valve you are testing, in square feet.

Next, find your water meter box, and carefully open it with gloved hands (black widow spiders often live in them). You will need a screw driver or other tool to open the lid. Check the underside of the lid and the inside of the box for spiders to make sure the area is safe. Then, pull up the faceplate of the meter to reveal the numbers. You may need to clean off the faceplate to clearly read the numbers. Record, in cubic feet, the current numbers shown on the meter before starting your irrigation valve (this is referred to as “beginning meter reading”). Run your irrigation system and record the how long it ran. We recommend at least five minutes for sprays, ten minutes for rotors, and fifteen minutes for drip. Record the new number shown on the meter after running the valve for the area you are measuring. This is referred to as “End meter reading”. Below is how to do the math to figure out that zone’s precipitation rate in number of inches per minute. If the math below is intimidating, don’t worry, we will walk you through the process in words below it.

\begin{align*} \frac{\textsf{End meter reading} - \textsf{Beginning\ meter\ reading}}{\textsf{Number of minutes tested}} &= \textsf{CFPM (cubic feet per minute)}\\ \\ \frac{\textsf{CFPM}}{\textsf{square feet of landscape area tested}}*12 &= \textsf{precipitation rate, (inches per minute)} \end{align*}

1) Start by subtracting the end meter reading by the beginning meter reading to get the total amount of water (in cubic feet) that was put out by the irrigation system in the time you ran it.

2) Next, divide the number you got in step #1 by the number of minutes you ran the system. This will give you the number of cubic feet per minute.

3) Divide the number of cubic feet per minute (from step #2) by the total square feet of the landscape area irrigated by that zone.

4) Multiple the number you got from step #3 by 12 to get your precipitation rate, in inches per minute. The number will be a decimal that is less than 1.

14) Find the amount of water used over the time of the test period by subtracting the “meter start reading” from the “meter end reading.”

\begin{align*} \textsf{Meter end reading} - \textsf{Meter start reading} &= \textsf{Water used}\\ \\ \textsf{40636 cubic feet} - \textsf{40597 cubic feet} &= \textsf{39 cubic feet of water used over the 20 minute test} \end{align*}

(Note: Most meters read cubic feet. like the one in the example. If you water meter reads hundreds of cubic feet, multiply the end number, after subtracting, by 100 to get the number of cubic feet. If your reader reads gallons, multiple the end number, after subtracting, by 0.134 to get the number of cubic feet. If you are having trouble telling what kind of units your meter reads, call your water provider and ask them.)

15) Divide the number of cubic feet used over the test by the number of minutes the test ran to get the number of cubic feet per minute.

\begin{equation*} \frac{\textsf{cubic feet}}{\textsf{number of minutes}} = \textsf{cubic feet per minute} \end{equation*}

\begin{equation*} \frac{\textsf{39 (cubic feet) }}{\textsf{20 (minutes)}} = \textsf{1.95 cubic feet per minute} \end{equation*}

16) Divide the cubic feet per minute by the total area of the zone found in step 9. This will give the depth of water put out over the en5re area, per minute.

\begin{equation*} \frac{\textsf{cubic feet per minute}}{\textsf{total area}} = \textsf{feet per minute over the area} \end{equation*}

\begin{equation*} \frac{\textsf{1.95 (cubic feet per minute)}}{\textsf{601 (square feet)}} = \textsf{0.0032 feet per minute} \end{equation*}

This answer is the depth of water per minute (in feet) applied over the whole area as opposed to the cubic feet per minute (volume) of water that goest through the pipes. You don’t really need to understand this, so ignore the explanation if you find it confusing.

17) Multiple the feet per minute answer from step (16) by 12 to get the precipitation rate in inches per minute.

\begin{equation*} \textsf{feet per minute} * 12 = \textsf{inches per minute (precipitation rate)} \end{equation*}

0.0032 (feet per minute) × 12 = 0.38 inches per minute precipitation rate

We will round this up to a precipitation rate of 0.04 inches per minute

See below for how to use this information to calculate your runtime.

Part B: Determine the runtimes for each zone

To calculate your runtime for the zone, divide the desired number of recommend irrigated inches of water per run (from the Inland Valley Garden Planner chart) by the precipitation rate (inches per minutes) that you just calculated.

\begin{equation*} \frac{\textsf{inches per run}}{\textsf{precipitation rate}} = \textsf{runtime} \end{equation*}

For this example, we will use a calculation to apply 1” per run.

     \begin{equation*} \frac{\textsf{1 (inch per run)}}{\textsf{0.04 (inches per minute)}} = \textsf{25 minute run time} \end{equation*}

So, for this example, with a 1 inch per run recommendation for the selected time of year, you would program you 0mer to run for 25 minutes each time it irrigates the zone. How oYen to irrigate, for established plants, is also provided in the recommendation charts.

Important notes:

  • The recommendations on the charts are for established plantings. Younger plants will need water more frequently during the first year or two because they have a smaller and shallower root systems. In general, most low water plants will want a deep watering approximately once a week in warm weather. Medium water plants may require water up to twice a week during establishment in warm weather. All of these recommendations are general and may require more or less frequent irrigation depending on weather, sun exposure, and soil conditions. It is important to observe your plants and dig a bit in the soil periodically to check that plants are getting proper watering during establishment. When plants get to a point where they put on a spurt of significant growth and shrubs and perennials are well on their way to their mature size, establishment can be considered over and you can gradually transition to the mature watering recommendations supplied in the charts.
  • Your runtimes will change throughout the year, following the provided charts. You will need to learn how to change your irrigation controller, or timer, to make these runtime adjustments. Once you learn how, it is easy. If you don’t have the manual that came with your timer, write down the name and model number and search for it on the internet. Most manufacturers have free downloads of manuals for all their controllers. If you cannot find it online, call the manufacturer directly. If you need or want a new controller, check with your local water provider. There are often rebates available to help pay for newer and easier to use irrigation controllers. 
  • If you are irrigating a sloped area or compacted soil, the suggested runtime may cause water to runoff because it is more water than the soil can absorb in a small amount of time. You do not want this to happen. To prevent this, use a “cycle and soak” approach. Try breaking the single run5me up over three or more shorter runtimes which occur after a short break (usually minutes or hours) to let smaller amounts of water soak in before the area receives more water. Some timers have this function built in as an option. Most others will have an option for multiple start times. So, if you have an area that needs to run for 30 minutes, and you need to use the multiple start time function, try setting the controller to run for 10 minutes with three start times, one hour apart. If you are running multiple areas on the same program, it is okay to break them all up into three start times running on the same program. Even if other zones are not on slopes or areas that runoff, there is no negative effect to doing this on areas that don’t really need it.

See our Additional resources for more useful information about irrigation, plants, and garden care.

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