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Drip Irrigation Design GuidelinesBackground
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? !!" 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! However, I will provide English measurements also. 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! I think you will still find that this tutorial is pretty complete.
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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.
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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 (3/8"), 16mm (1/2"), 18mm (1/2"), and 24mm (3/4"). Do you see the problem? Two sizes are commonly referred to as "1/2 inch" in the US! 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. Nine times out of ten, 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.
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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?
Two 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 two per plant in order to maintain the minimum spacing between emitters. Minimum spacing for emitters: Install emitters at least 18" apart. Rule of thumb- install emitters 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 3/4" valve for most systems. Any type of valve may be used. See the sections on Valves and Backflow Preventers above.
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 400 feet. So you could have 300 feet of mainline and 60 feet of sub-main, for a total of about 360 feet of both. But you could not have 250 feet of mainline and 200 feet of sub-main because the total of both would be more than 400 feet. 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 200 feet from the point the water enters the tube to the end of the tube. Thus you could have 400 feet of tube if the water entered the tube in the middle (that would be 200 feet 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 200 feet.
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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
Feeder tubes, spaghetti, and distribution tubing are all names used for small diameter tubes, that is, anything less than 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.
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Size of Fittings
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, 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.
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Stake Down the Tubes
Stake the drip tubes to the ground once about every 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 12-inch length of wire into a "U" shape to make a tubing "staple".
Technical Data
For those who want to know all the details (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/m flow for 20 mm valve, and 0,9 l/s for 25 mm valve.
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Copyright © Jess Stryker, 2001
