Drip Irrigation Design Guidelines

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!

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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 drier regions of Australia. 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 irrigation will work but 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.

Parts of a Drip system

Valves

Valves turn on or off the water flow through a pipe.

Isolation valves are manually operated valves used for infrequent shut-off of the water. Typically an isolation valve is located at the water source so the water can be shut off for repairs or shut off during the non-irrigation season. Isolation valves may also be installed anywhere on the irrigation system to allow the shut down of sections for repair, this is common on large systems where shutting down the whole system for a repair would be inconvenient.

Control valves are the valves that turn on and off the water to individual “circuits” or areas of the yard that are irrigated separate from one another. The control valves can be automatic (usually electric-powered using a solenoid) or manually operated (hand-powered, ie; turn, turn, turn!) There may be just one control valve or there may be several control valves on a drip irrigation system. For example one control valve may turn on and off the water to emitters/drippers in a vegetable garden. Another control valve might turn on and off the water to emitters for some hanging pots on a patio. Another control valve might turn on and off the water for the emitters at shrubs around the house. Another could even turn on and off water for sprinklers in the lawn, or water for filling the swimming pool or pond.

Illustration of a very simple drip system

A more complex home drip system

Prescriptive Drip Design Guidelines:

These guidelines will provide you with all the information necessary to design a residential drip system for a typical yard. These guidelines are what is termed a “prescriptive standard” in the building industry. A prescriptive standard is a set of rules and/or methods that, when followed, allow you to skip the engineering calculations for a design. Obviously this saves a lot of time and effort in preparing a design. The downside to a prescriptive standard design is that it tends to “over-design” in order to make the design “one size fits all”. Unlike sprinkler irrigation, drip irrigation systems are much more forgiving of design error, the cost of over sizing the materials is minimal, and so a prescriptive design method works very well for almost everyone. To prepare a fully engineered drip irrigation design requires a massive number of difficult mathematical calculations. If there was ever a great place to use prescriptive standards for the design, it is drip irrigation!

Emitter Type and Flow:

Use pressure compensating emitters if you have an elevation difference of over 1,5 meters (5 feet) in the area you are irrigating. For more level areas turbulent flow emitters will work great and are often less expensive. For gravity flow systems use short-path emitters, they typically work better than the others at very low water pressures.

For most soil types 2,0 l/hr (0.6 gph) emitters work well and are more economical. For sandy soil use 4,0 l/hr (1 gph) emitters.

For more information see http://www.irrigationtutorials.com/drip-irrigation-emitters/.

How Many Emitters are Needed?

1 or 2 emitters per plant, depending on the size of the plant. 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: In most situations install emitters at least 450mm (18″) apart. A good default spacing for quick and dirty design is to space the emitters 600mm (24″) apart. For supplemental watering of low-water-use plants, use one emitter per plant. Supplemental watering is used for establishment of drought tolerant plants that are not likely to need irrigation once they have developed a good root system, or might be used to apply a little extra water now and then to make them a bit more lush. Use of low-water plants with supplemental drip irrigation is considered very “green” and is the current trend in landscape design.

Rule of thumb- install emitters 600mm (24″) apart under 80% of the leaf canopy of the plant. That’s where the roots are, and the roots need water. If the soil is very permeable install emitters 300mm to 450mm (12-18 inches) apart. For more information and a better method of determining spacing see http://www.irrigationtutorials.com/drip-emitter-spacing/.

Backflow Preventer:

Drip emitters rest directly on the soil so it is especially important to have a backflow preventer to prevent water contamination by soil-borne disease. There are several types that will work depending on your situation and local codes. For more information see http://www.irrigationtutorials.com/irrigation-backflow-preventers/.

What valve type and size to use:

Use a 20mm (3/4″) valve for most systems. Any type of valve may be used. For more information see http://www.irrigationtutorials.com/drip-irrigation-valves/.

 

How many emitters per valve?

Use the charts below to determine how many emitters to install on each valve circuit. If you don’t know what size your water supply pipe is, see

http://www.irrigationtutorials.com/how-to-find-the-size-of-a-pipe/.

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″) water supply. Use a 20mm (3/4″) valve.

25mm (1″) water supply. OK to 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

*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 http://www.irrigationtutorials.com/pump.htmtutorial for more information about using pumps.

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!

Mainlines & Laterals.

Use 25mm (1 inch) PVC, PEX or polyethylene irrigation pipe for mainlines (“mains”) and laterals. The total length of the mainline and the lateral together should not be more than 120 meters (400 feet). So you could have 100 meters of mainline and 20 meters of lateral, for a total of 120 meters of both. But you should not have 80 meters of mainline and 60 meters of lateral because the total of both would be more than 120 meters. Remember mainline is the pipe before the control valve, lateral is pipe after the control valve. Many drip systems won’t need mainlines or laterals.

Maximum drip tube length.

The length of drip tube (or 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. If you bury the emitter roots will grow into it and clog it. If you do want to bury the emitters do a search for “subsurface drip irrigation” to find specialty drip products designed to be buried. Follow the manufacturer’s recommendations for those products as they must be designed and installed to very exacting standards to avoid problems.

Buried Tube

 

Don’t bury the drip tube. If you do bury drip tube don’t complain to me if gophers, moles or other rodents 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 over it 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, Spaghetti, and Distribution Tubing

Avoid using feeder, spaghetti, or distribution tubing if possible.

Fittings- Use the 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. Never heat the drip tube or use oil on it to make it easier to insert into or onto the fittings.


Stake down the Drip 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 its 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”. Or you can buy metal staples that are made for holding down erosion control blankets, they work great.

Check Valves, Slopes, Hillsides

Install check valves if the drip system is on a hillside of slope to prevent the water in the tubes from draining out through the lowest emitter each time the system stops running.

Air Vents

Install an air vent at the highest point on each drip valve circuit. If there are multiple high points you an air vent installed at each one. Air vents should always be used for drip systems on sloped areas. Air vents are often not installed on small homeowner drip systems without any slopes. If air vents are not used be sure the emitters at the highest points are not installed where dirt could be sucked into them.

Flush Valves and End Caps

Install a flush valve or end cap at the end of each drip tube. Automatic flush valves are available, however my personal preference is for manual flush valves.

Patios with Potted Plants and Trellises

You will probably want 6mm (1/4″) feeder/spaghetti/distribution tube running to the plants if they are in pots just to make it less obtrusive visually. Try to use as little 6mm (1/4″) distribution tube as possible, keep the tube lengths short as much as possible, and only put 2 emitters on a single 6mm (1/4″) tube. If a 6mm (1/4″) tube is longer than 5 feet, use only one emitter on it. I like to staple the tubes to something to keep them in place if possible (like stapling the tube to a trellis for hanging plants.) Use a wire stake to hold the emitter in place in a pot. Don’t pull any of your tubes tight, snake them a little, leaving some slack in them to allow movement. The tubes will expand and contract with temperature changes, you don’t want them to tear or pop the fittings off.

So for example, I run standard 15-16-17mm (1/2″) tube along the patio perimeters, trying to put it in places it will be out of the way or I can hide it. I also run it up onto the trellis if there are lots of hanging plants, putting it on the back side out of view and clamping it to the trellis using standard conduit or pipe clamps. (I’ve found conduit clamps are cheapest, look in the electrical dept at any hardware store.) From the 15-16-17mm (1/2″) tube I run short lengths of 6mm (1/4″) tube to the potted plants. Remember: more 6mm (1/4″) tube = more problems.

Backflow preventers are always an issue if you have hanging plants and trellises. Vacuum breaker or anti-siphon type of backflow preventers must be installed above the trellis or they won’t work. Both those types of backflow devices must be installed at least 150mm (6″) higher in elevation than any of your emitters. This is generally not very practical to do. I have seen people run copper pipe up a trellis and put an anti-siphon valve 150mm (6″) above the trellis. But in most cases you need to use a double check, or preferably a reduced pressure type of backflow preventer. Those can be installed at any elevation (a reduced pressure type should be above ground.) I recommend a reduced pressure type.

Beyond these issues, the other basic drip guidelines in this tutorial all apply to patio and trellis drip systems.

Gravity Flow Systems

If you are using a gravity flow water source like a rain barrel see the suggestions on the http://www.irrigationtutorials.com/gravity-flow-rain-barrel-drip-systems/.