 |
Jess Stryker's
Drip Irrigation Design Guidelines
|
Background:
Drip irrigation is the most efficient method of irrigating. While sprinkler systems are around 75-85% efficient,
drip systems typically are 90% or higher. What that means is much less wasted water! For this reason drip is the
preferred method of irrigation in the desert regions of the United States. But drip irrigation has other benefits
which make it useful almost anywhere. It is easy to install, easy to design, can be very inexpensive, and can reduce
disease problems associated with high levels of moisture on some plants. If you want to grow a rain forest, however,
drip might not be the best choice!
Drip irrigation (sometimes called trickle irrigation) works by applying water slowly, directly to the soil. The
high efficiency of drip irrigation results from two primary factors. The first is that the water soaks into the
soil before it can evaporate or run off. The second is that the water is only applied where it is needed, (at the
plant's roots) rather than sprayed everywhere. While drip systems are simple and pretty forgiving of errors in
design and installation, there are some guidelines that if followed, will make for a much better drip system. The
purpose of this tutorial is to guide you toward materials and methods that will increase the benefits of your new
drip system, while steering you away from some common misconceptions and practices that can cause you trouble.
"What's with the Metric measurements? !!" Come on, quit whining, the rest of the world uses
metric without problems!!! OK, don't flame me, I give up, I'll compromise... While a lot of drip irrigation research
has occurred in the USA, most of the credit for making drip irrigation what it is today really should go to Israel
and South Africa. So I'm going to honor that contribution by using the metric system as the primary measurement
units for these guidelines. After all, metric is really the "native" measurements of drip irrigation.
When I started using drip irrigation (back in the dark ages of irrigation) all drip data and products were in metric!
But because I'm such a nice guy (inflated ego alert!! Dump some ice water on this guy!), I will provide English
measurements also. So don't panic. Now, on with the tutorial...
This tutorial is setup in a multilevel format. Each of the guidelines below describes a basic rule for drip
irrigation design, with the guidelines in the logical order for creating a design. You can think of the guidelines
as design steps if it helps. This page is the top level, here you will find a brief description of each design
guideline. For many of the guideline topics there is a link to another page with expanded information on the guideline
topic. There may be additional links from there to allow you to dig even deeper into the drip irrigation knowledge
base. So you choose how much you want (or need) to learn. My recommendation is that if you want to print out something,
print this page. Then refer to the other levels (and print them if necessary) as needed. That will save you a lot
of unnecessary wear and tear on your printer. It might also save a tree from going to the paper mill!
Note: This tutorial isn't finished yet! You will find that detailed information is not available for
all of the guidelines. Sorry! I wanted to make available to you the information I have now. Unfortunately that
means it is not complete! Don't wait for additional information to show up, it may take months (opps- it's now
been well over a year since I wrote that!) depending on my schedule. This web site is not my source of income,
so it is low priority in comparison to, say, feeding my family! That said, I think you will still find that this
tutorial is pretty complete. Let's get started with a graphic:
Illustration of a very simple drip system
The Basic Parts of a Drip System:
Valve-
Use any valve you want! They can be automatic or manual. If you use an anti-siphon valve it has a built-in backflow
preventer which saves money! (But be sure to read up on backflow preventers first, as anti-siphon valves won't
work in some places. For more information on valves, click here.
Backflow Preventer-
You need to use a backflow preventer on ALL drip systems. No exceptions! For more information on backflow preventers,
click here.
Pressure Regulator-
Most drip systems will need a pressure regulator. You need one if your water pressure is over 2,8 bars (40 PSI).
If in doubt, install one. The regulator can go before or after the valve. Traditionally it is installed after the
valve.
Filter-
You must use a filter. Drip emitters have very small openings that are easily clogged. City water is not
free from stuff that will clog your emitters! Use a 150 mesh screen or one with a higher mesh number like 200 mesh.
The filter may be installed before the valve or pressure regulator, but the inexpensive plastic filters often sold
for drip systems should be installed after the pressure regulator. I like to use a top quality filter and install
it right at the water source so it protects the valves and the pressure regulator too. Most valve failures result
from sand or rust particles clogging of the tiny passages inside the valve! Use a filter that is the same size
as, or larger than, the valve. For more information there is a separate, free, tutorial on filters. Click
here for the Filtration Tutorial .
Emitters-
Most emitters emit 4 liters/hour (4,0 l/hr) of water. That's about 1 gallon per hour (1 gph). I prefer a lower
flow rate than that, and use mostly 2,0 l/hr (0.6 gph, often referred to as "1/2 gallon per hour") emitters
on my drip system designs. Use pressure compensating emitters if you are irrigating a hilly area. There are many
different types and brands available. For more information click here.
Multi-Outlet emitters are very popular. They also suck! (That's a personal opinion, based on years of observation.)
Use single outlet emitters for a less troublesome drip system.
Mainline-
The mainline is the pipe that goes from the water source to the valves. In the illustration of a very simple
drip system above the gray colored vertical pipe under the valve is a mainline. The mainline may be made of galvanized
steel, copper, SCH 40 PVC, SCH 80 PVC, Cl 315 PVC, Heavy Wall Polyethylene (SDR 7 or SDR 9). Do not use polyethylene
for the mainline unless approved by your local building officials. Polyethylene has a low burst pressure and should
only be used for mainlines where local conditions are appropriate.
Sub-Main-
The sub-main is the pipe that goes from the valves to the connection point of the drip tube. Many small drip
systems do not have a sub-main, in those systems the drip tube connects directly to the valve The illustration
of a very simple drip system above shows a system without a sub-main. Sub-mains are usually Cl 200 PVC pipe or
standard weight polyethylene (poly) pipe. You use a sub-main when multiple drip tubes are needed.
Drip Tubing-
Drip tubing is a thin wall polyethylene tube, and is generally produced in metric sizes. 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. So make sure you know what you're getting when you buy it! Do not bury drip tubing underground- gophers and moles love to chew buried tubing!
Adapters and fittings-
Used to attach the drip tube to the other parts. Important- make sure the fittings are the exact right
size! Using fittings made for a different tubing size will result in the tube blowing out of the fitting. 9 times
out of 10, when a tube blows out of a fitting it is because the fitting is the wrong size.
End Cap-
The end cap is important. Without it the water all runs out the end of the drip tube. (Well, duhhh...)
Prescriptive Drip Design Guidelines:
Be sure to read through the brief descriptions above of the basic drip system parts before continuing. These
guidelines will provide you with all the information necessary to design a residential drip system for a typical
yard. This is what is termed a "Prescriptive Standard" in the building industry. In other words, following
these guidelines will allow you to skip the engineering calculations for design. The downside to a prescriptive
design method is that it is more restrictive, it tends to "over-design" in order to make the design "one
size fits all". Unlike sprinklers, drip irrigation systems are much more forgiving of design error, so a prescriptive
design method works well for almost everyone.
How Many Emitters are Needed?
2 emitters per plant, minimum. Trees and large shrubs may need more. Obviously, using two allows for a backup
if one clogs up (which happens now and then, even on the best designed and maintained drip systems.) But just as
important, more emitters also wet more soil area. This results in more roots, and a healthier, happier plant. Exception:
if the plants are very close together you may need to use less than 2 per plant in order to maintain the minimum
spacing between emitters. Minimum spacing for emitters: Install emitters at least 45cm (18") apart.
Rule of thumb- install emitters 45 cm (18") apart under 80% of the leaf canopy of the plant. That's
where the roots are, and the roots need water. For more information click
here.
What valve size to use?
Use a 20mm (3/4") valve for most systems.
Any type of valve may be used. See the sections on Valves and Backflow Preventers above.
How many emitters per valve?
Use the charts below to determine how many emitters to install on each valve circuit.
Click here to find out the correct way to determine the size of the supply pipe.
* Pumps can be tricky. This is a conservative figure in order to make it work with the majority of pump fed systems.
You may be able to use a larger number of emitters by calculating the actual output of your pump. See the Irrigation
Pumping Systems tutorial. Water supplies coming out of a building are also a problem. The piping in
buildings is almost never designed to carry large amounts of water such as is used by irrigation systems. To be
safe I assume you have significant restrictions. 95% of buildings have these restrictions so don't increase the
flow unless you really know what you're doing. Increasing the flow could cause extreme damage to the plumbing in
the building!
|
Emitter volume used
|
Any water supply that comes out of a building, such as a hose bib. Any system with a pump*.
|
20mm (3/4") Valve w/ 20mm (3/4") water supply.
|
25mm (1") water supply. Use a 20mm (3/4") valve.
|
|
2,0 l/hr (0.6 gph)
|
300
|
300
|
700
|
|
4,0 l/hr (1 gph)
|
180
|
180
|
420
|
PVC Mainlines & Sub-Mains.
Use 25mm (1") pipe for mains and sub-mains. The total length of the mainline
and the sub-main together should not be more than 120 meters (400 feet). So you could have 100 meters of mainline
and 20 meters of sub-main, for a total of 120 meters of both. But you could not have 80 meters of mainline and
60 meters of sub-main because the total of both would be more than 120 meters. Many drip systems won't need mainlines
or sub-mains. Or they may need just a mainline, or just a sub-main.
Maximum tube length.
The length of drip hose may not exceed 60 meters (200') from the point the water enters the tube to the end
of the tube. Thus you could have 120 meters (400') of tube if the water entered the tube in the middle (that would
be 60 meters from the point the water enters the tube to the end of the tube in each direction, which would be
OK). You can extend one tube off of another as long as the total length of the tubes that are connected is not
more than 60 meters (200').
Buried Emitters
Never bury emitters underground unless they are made to be buried. Even if they say they can be buried I do
not recommend it. It causes too many maintenance problems.
Buried Tube.
If you bury drip hose don't complain to me if gophers chew it up. I've seen them gnaw to pieces a buried drip
system over night. One day it works, the next, it's garbage. It only takes one gopher (or mole, squirrel, etc.),
and one evening! You've been warned! Other wildlife (and most dogs), will also chew the tubes. It helps if you
provide a water source for them to drink from if possible. A water bowl with an emitter to keep it full sometimes
will distract wildlife from the tubes. You may need to train your dog not to chew the tubes, dogs seem to chew
on the tubes for no real reason other than to annoy you. If you want to hide the tube, dig a shallow trench for
it, so that it is just below the level of the surrounding soil. Don't put dirt over the tube. Throw some mulch
or bark over the top to hide the tube, or plant a low spreading plant that will grow over it and hide it.
Feeder Tubes, Spaghetti, and Distribution Tubing
I hate this stuff! Feeder Tubes, Spaghetti, and Distribution Tubing are all names used for small diameter tubes,
that is, anything less than 10mm (3/8") in diameter. The problem is with maintenance. These small tubes tend
to be easily cut, broken, pulled loose, etc. and are generally a nuisance. This is the type of tubing that is connected
to the outlets of multiple-outlet emitters. This tubing is the reason I do not recommend the use of multiple outlet
emitters. If you are a meticulous type person who can be very careful, and you don't have pets or kids in the yard,
you may not have any problems. But for most of us, regret soon sets in as repairing these small tubes becomes a
weekly maintenance chore. There are a couple of exceptions where the tubes work well. One is when they are stapled
above ground to a trellis or arbor for watering hanging plants. They need to be firmly attached, in a location
where they will not be damaged. The other exception is a special thick-walled distribution tube which is used for
emitter "risers". This tube is sold in short, straight lengths, it does not coil easily as the thick
wall makes it very rigid. If it's sold in a coil, chances are it's not the right product. A buried PVC pipe (called
a "lateral") is installed between each plant in the landscape. The thick-walled tube is then installed
from a PVC fitting almost directly under the plant. The tube goes straight up to an emitter, which is installed
at ground level. This type of installation is commonly used in Arizona (USA), and the Coachella Valley of California
(USA), on desert landscapes. Few plants are used with this type of installation, and they are spaced very far apart.
Because most of the piping is PVC, it can be pretty expensive.
Fittings- Correct size.
This is really important! There are many different sizes of drip tubing sold, and the fittings have to be made
for the exact size tube you are using! If they aren't, they will either be very hard to install, or the tube will
blow off the fitting. Sometimes it takes a week or so for the tube to come loose, but if the fitting is even 1mm
too large, the tubing will come off eventually.
There are two types of fittings used. The first and most commonly used is the "compression"
type. On this type the tubing shoves inside the fitting. Compression type fittings tend to be harder to install,
but they also tend to hold up longer. The second type is "barbed" fittings. With barbed fittings
you shove the tubing over a barb on the fitting. The problem with this is that it stretches the thin wall drip
tubing as it goes over the barb. This weakens the tube, and over time it splits and comes off. It usually takes
a few years for this to happen. OK you ask, if barbs are a problem then why do they use barbed fittings with standard
polyethylene pipe? Standard poly pipe has a much thicker wall than drip tube and doesn't stress as much when stretched.
You also clamp standard poly pipe to the fittings, which helps keep the pipe from splitting (that's why you need
to clamp poly pipe even if it seems to stay in place without the clamps). Drip tubing is not clamped to the fittings
(clamping doesn't help prevent splitting because of the thin drip tubing wall).
I recommend using compression type fittings unless the tubing manufacturer recommends otherwise. To the best
of my knowledge, Netafim is the only tubing brand that recommends using barb type fittings. Their tubing is easily
recognized as it is brown colored, as are the special fittings they recommend be used.
Lubrication: Some people just don't have the strength to shove the tubing into a compression fitting. First
make sure the fitting is the right size, as that is very often the problem. If it is, then you can use a water
soluble lubricant on the tube. Do not use oil, silicon sprays (WD-40) or soap! Absolutely do not heat the tube
with a flame, heat blower, or hot water as that will stretch the tubing and create weak spots! What's a water soluble
lubricant? Try K-Y Jelly. Attention guys! Avoid terminal embarrassment! Do not head for the hardware store for
K-Y Jelly. Try the drug store, err, lady's personal hygiene department. Might want to take along the wife. Need
I say more?
Stake down the Tubes!
Stake the drip tubes to the ground once every meter (about 3 feet). This keeps the tubes from wandering. No
kidding, they tend to move around by themselves! Staking them also helps protect them from damage. I prefer to
use metal stakes as the plastic ones I've tried pull loose too easily. Wire that rusts holds even better, as the
rust binds the wire to the soil. After a few days they can be almost impossible to remove. They will rust away
in a few years, but by then the tubing has adapted to it's position and stays in place pretty well. Standard 12
gauge wire works well, as does pieces of wire coat-hangers. Buy some coat-hangers at a yard sale or thrift store
and help recycle! Bend a 300mm (12 inch) length of wire into a "U" shape to make a tubing "staple".
Don't forget to read the Irrigation Installation Tutorial. It covers a lot
of important information on how to install your drip system!
Technical Data:
For those who want to know all the details. Everyone else can ignore this information. Assumed pressure losses
for prescriptive drip system design per these guidelines:
Valve 0,4 bars
Backflow Preventer 0,8 bars
Pressure Regulator 0,0 bars
Filter 0,2 bars
Mainline & Sub-Main 0,4 bars
Drip Tube 0,2 bars
Emitters 1,0 bars
Total Pressure required 3,0 bars (44 PSI)
Based on 0,2 l/s flow for 20 mm valve with smaller supply, 0,4 l/s flow for 20 mm valve, and 0,9 l/s for 25 mm valve.
Glossary Conversion Formulas
About the Author
Comments? Questions? Product Data & Reviews More Irrigation Tutorials!
www.IrrigationTutorials.com
IrrigationTutorials.com is solely supported by users like you. This website is not affiliated
with any irrigation product manufacturer, distributor, or retailer. In order to provide relatively unbiased opinions,
this website does not accept support, payments, fees, royalties, etc. from manufacturers, distributors, or retailers
of irrigation products. Free product samples may have been provided to IrrigationTutorials.com by manufacturers
for the purpose of evaluation and review.
By using this tutorial you agree to be bound by the conditions and limitations listed on the
disclaimer page.
Jess Stryker's Irrigation Tutorials
Copyright © Jess Stryker, 2005
All rights reserved.