All posts by jstryker

How to Estimate Water Useage Required for an Irrigation System

The amount of water needed for irrigation depends on many different factors.  A reasonably accurate estimate of the amount of irrigation water needed can be made using Eto data for your actual zip code.  “Eto” is the amount of water needed for irrigation, based on scientific research.  You can find the historic Eto for any zip code in the USA at the website http://www.rainmaster.com/historicET.aspx courtesy of the Rainmaster irrigation controller company, who makes very good “Smart” irrigation controllers.   I use one of their Eagle model controllers on my own home.  (Rainmaster get a plug from me as well as a big “thank you” for providing the ETo look up service online.)  Unfortunately the Eto value only tells you how many inches per day are needed, which for most folks is a meaningless value. It makes more sense if you think about rainfall which is often also measured in inches.  If you find you need 0.20 inches of irrigation, then 0.20 inches of rainfall would provide the required water.  But most people in the USA want a value in gallons, which requires you to provide a little more information about your yard.  Then you plug the values into a simple formula, and do a little multiplication and division on any calculator.

Formula to calculate the gallons of irrigation water needed per day:
(Eto x PF x SF x 0.62 ) / IE  =  Gallons of Water per day

Values for the formula:
Eto: Get this from http://www.rainmaster.com/historicET.aspx .  Enter your zip code, or a nearby zipcode, and the website will give you the average daily ET value for each month of the year.  Use the highest value or the “suggested reference value”.  Usually they are the same thing.

PF: This is the plant factor.  Different plants need different amounts of water.  Use a value of 1.0 for lawn.  For water loving shrubs use .80, for average water use shrubs use 0.5, for low water use shrubs use 0.3.

SF: This is the area to be irrigated in square feet.  So for a 30 foot x 50 foot lawn you would use 1500.

0.62: A constant value used for conversion.

IE: Irrigation efficiency.  Some irrigation water never gets used by the plant, this value compensates for that.  I suggest using 0.75 as the value for this.  Very well designed sprinkler systems with little run-off that using efficent sprinklers can have efficiencies of 80% (use 0.80).  Drip irrigation systems typically have efficiencies of 90% (use 0.90).

Example:
A 1500 square foot grass lawn in zip code 85232 (Central Arizona)
Start by looking up the Eto for zip code 85232 at the Rainmaster website, which displays a suggested reference value of 0.3 inches per day using June, the driest month of the year in that area.

Now rewrite the formula inserting your values into it:

0.3 (Eto value)  x  1.0 (grass value)  x 1500 (sq ft)  x 0.62 ÷ 0.75 (efficency factor) = gallons of water per day

Now do the math, just punch the values into a calculator and get your answer:
0.3   x   1.0   x   1500   x   0.62   ÷   0.75   =   372 gallons per day



We could figure out the average daily water use for other months of the year also.  Just use the same formula but insert the Eto value from the Rainmaster website for the month you want to get a valve for.

Remember this calculation just gives you an estimated value.  There are many other factors that could make this value higher or lower.  When planning for how much water a system that has not yet been designed or installed will use, it would be very wise to allow for  error by adding 10% or more to the daily water use needed.  It is generally better to have too much water, than to have too little!  Play it safe!

A common related question is “how much water pressure will my irrigation system need?”  The answer depends on a lot of factors, but as a rule of thumb, I would suggest 50 PSI of water pressure as a good starting point for sprinklers, 45 PSI for drip systems.  If you have a large yard and want to put the sprinklers farther than 30 feet apart you will need more pressure.  For example, if you want your sprinklers 45 feet apart you will probably need 65 PSI of water pressure.  To get a real value you will need to create an actual sprinkler system design. See the Landscape Sprinkler Irrigation System Design Tutorial .

Never buy a pump, sprinklers, or any other materials before your sprinkler design is completed!

Drip tube blows off fittings.

Q:  The pressure is blowing off the pipes/tubes from the barbed fittings on my drip irrigation system.  This is only happening on hot days (30°C=86°F in the shade).  Pipe temperature could be as high as 45°C=113°F.  Our water pressure varies between 2.5 and 3.1 bar (35 and 45 PSI.)

A:  Drip tube should not blow off the barbs, even on a hot day when the temperature softens the plastic tubing (however the heat does make it easier for them to blow off!)  There are three common reasons the tubes blow off.

1. The most common problem is that the water pressure is too high.  This is probably your problem.  The water pressure should be around 1.3 to 2.0 bars (20 – 30 PSI).   You should install a pressure reducer after your valve to lower the pressure.

2.  The pipe and fittings may not be the same size.  This is one of the pitfalls I warn about in my Drip Guidelines.   16mm and 18mm tube  are both commonly referred to as 1/2 inch in the USA! The fittings for these two are not interchangeable.

3. Pressure spikes can pass through the less expensive pressure reducers often sold for drip systems.  If you have high water pressure this may be the problem.  The solution is to install a high quality brass pressure reducer valves.  These generally are sold in the plumbing department rather than the irrigation department of stores and cost $50.00 or more.

Common sizes are 12 mm (0.455″ or 3/8″), 16mm (0.620″ or 1/2″), 18mm (0.720″ or 1/2″), and 24mm (0.940″ or 3/4″). Do you see the problem? Two sizes are commonly referred to as “1/2 inch” in the USA! The fittings for these two are not interchangeable.

Drip emitter installation tools.

Q:  Have you come across tools to insert drippers in tubes or pipes?  Pushing the dripper’s into the tube is leaving my fingers bruised.

A:   You’re right, some of the hole punches are better than others.  I have one that has a nice big grip handle on it that is easy to hold.  But another inexpensive hole punch I have simply has a rounded top that you press with your palm to force the punch into the tube.  It makes your palm sore after just a few uses.  Pressing the emitters into the holes can be a pain too.  Inserting a couple dozen emitters into the holes can leave your finger tips hurting!  Some emitters have irregular, rough, or sharp edges that make it even worse.

Fortunately there are many tools made to help you out.

There are any number of simple hole punches.  Some are small and hard to hold, some have larger handles which makes them a bit easier.  All of these are very simple tools, you hold them in your hand and press a sharp tip through the wall of the tubing, a bit like an ice pick would work.  Actually an ice pick would work to make a hole, however I have found that any type of pointed punch tends to be too aggressive; it punches a hole through one side of the tube and out the other, making two holes!  Then you have to put a goof plug in one of them to plug it up.

Another thing to consider is the shape of the hole punched.  Most punches actually create a round hole about an 1/8″ in diameter.  Some create holes as large as 1/4″ for specific brands of emitters with larger barbs.  It is much easier to get the emitter into a round hole than one made with a pointed tip, such as a nail. When holes are made with a pointed tip the plastic tube tends to stretch as the tip goes through it.  Then the hole closes back up when you pull the tool out, the resulting small hole is hard to get the emitter barb into.  Also a pointed tip is more likely to tear the tubing wall as the tubing stretches around the tool, creating something more similar to a slit than to a round hole.  There is some debate as to if these tears in the tubing will enlarge over time (similar to how a tear in a plastic bag sill get larger if you pull on it.)  I tend to think you are better off with a round tipped punch that punches an actual round hole in the tube, as opposed to a pointed tip.

Note that any punch that you hold with your finger tips, or press a small “knob” with your palm to operate, is going to be hard on your fingers or palm if you install more than a dozen emitters at a time.  These super cheap punches work fine for a few emitters, but beyond that… ouch!

There are more sophisticated hole punch tools, like the Miracle Punch.  It holds the tube firmly in place which aligns the hole properly.  It operates similar to a pair of pliers, which is much easier on your hands.  There is at least one other “pliers type” tool I have seen on the market, however it does not hold the tubing as firmly.

There are a number of devices made to install the emitters.  Most of these are product specific, that is they only work with a particular brand and model of emitter. They generally have a handle on one end and a molded cradle that you place the emitter in on the other.  You place the emitter in the cradle and then press it into a hole you have already punched in the tube using a hole punch.  Some of these device have both the hole punch and emitter insertion cradle as part of the same tool.

Some companies, like Rainbird, make emitters that are “self punching”.  The barb on the emitter is sharp enough to create it’s own hole when pressed hard against the tube.  Rainbird makes a tool that you place the emitter in, then using the tool you press the emitter into the tube.  My experience is that you need this tool to use the self-punching feature of the emitter.  Without it I have not had much success getting the emitters lined up correctly and pressing them in with your fingers is near impossible.  You don’t have to use the tool to insert the Rainbird emitters, it works fine to punch a hole first using a punch and then stick them into the hole.

TIP: Try wearing heavy leather gloves when installing the emitters to reduce hand pain.  Also try putting a couple of pieces of cardboard in your palm between the hole punch and your hand, to help distribute the pressure over a larger area of your palm.