Landscape Irrigation Basics
Distribution Uniformity
The objective in designing a superior landscape irrigation system is to apply the same amount of water over the area being irrigated within the same window of time; this concept is called distribution uniformity. It is a key element for a high-quality irrigation design.
Poor uniformity results in over watering in some areas to get the proper amount of water in other areas. One well-known manufacturer refers to this as the “bog factor”. We measure the amount of water applied to the surface in terms of inches per acre per hour. The amount of water applied must not exceed the soil's ability to absorb it. This concept is called the soil's "infiltration rate" and will vary greatly based on soil type, degree of compaction, and whether it a “modified” soil. Different areas of the landscape may well have different infiltration rates and water holding capabilities, therefore will have differing water requirements. Beds are different than sod, for example. For this reason a basic rule of landscape irrigation design is to never water beds and sod at the same time (in the same zone). It is impossible to do both things well simultaneously.
Water Pressure
How much water at what pressure is available for your landscape irrigation system? More importantly, what is the dynamic flow: gallons per minute and PSI? In the same way that a pump has a curve that defines its performance, so also does every landscape water supply. This curve most often has to be determined in the field using a flow meter, gate valve, and a pressure gauge.
Poly Drip has developed a simple test kit that will allow you to determine a curve for your particular water supply for flow rates up to 50 gallons/minute. This inexpensive test kit (about $65) will allow you to record actual information for your water supply that will then be used to determine a “design basis” for your system. If your system is larger than 50 gallons/minute, we can suggest ways for you to accomplish the same thing using other components.
Flow Test Chart 1 | Flow Test Chart 2
Getting Started
It is best to begin with an accurate drawing. Choose the type of sprinkler head based on the dimensions of the area it will irrigate. The options are fixed arc sprays or variable arc rotating sprinklers. Once the proper irrigation device is determined, you can select the appropriate nozzle to fit the location where the sprinkler is to be placed. A key goal here is that we must match the flow rate of the sprinklers to the amount of water per square-foot irrigated within the pattern. This is called "matching precipitation rates". No single sprinkler is by itself very uniform in its distribution pattern. For this reason sprinklers are spaced so that their patterns of distribution will overlap to improve overall application uniformity. The most common rule of thumb for spacing is called "radius spacing". This simply means that if a sprinklers radius is 15 feet you should space them 15 feet apart. This spacing must be modified depending on a location’s potential wind conditions.
Choosing the appropriate nozzle for fixed arc sprays (also called pop-up’s) is really straightforward. Choose a nozzle that will match the distance you want the sprinkler to spray and the arc you need to cover with the sprinkler. All manufacturers in the industry currently offer match to precipitation rate nozzles for these types of sprinklers. In other words, of 15Q nozzle will automatically put out the correct amount of water per square-foot, as will a 15H or a 15F.
Nozzle selection for variable arc rotating sprinklers (also called gear drives) is a little bit more involved. Refer to your manufacturer’s performance chart to determine the flow rate at various pressures for each of the nozzles available. Based on the arc you need the sprinkler to cover and the distance you need it to throw, you must choose the appropriate nozzle. A sprinkler that is watering a 90 arc should have 1/2 the flow rate of one watering a 180 arc, and one 1⁄4 the flow rate of one watering a 360° arc. Careful attention must be given to selecting the appropriate nozzle based on the arc desired.
Flow Rates
Sprinklers do not determine flow rates, nozzles do. Fixed arc sprays or pop-ups should not be operated at pressures exceeding 30 PSI. The droplet size becomes so small that much of it aerates if there's any wind at all. This aeration is called misting and is most undesirable for landscape irrigation.
However, variable arc sprinklers that rotate, called gear drives, may be operated at pressures up to 60 to 65 PSI. In fact, most gear drives do not perform very well at pressures below 40 PSI, even though the manufacturer’s literature may indicate they will work down to 20 PSI! There must be enough pressure to allow the stream of water to break up into smaller droplets and fall along the length of the sprinkler’s pattern. As with pop-ups, the nozzle flow rate must be selected based on the sprinklers desired arc. Particular care must be taken to match the flow rates to the arcs desired in the same manner as with pop-ups. However, it is more difficult because a choice is involved, rather than having it “built in” to the equipment.
Example: #3 nozzle at 40 PSI might flow 1.2 gallons per minute. If the arc assigned to this nozzle is 90°, then the nozzle chosen for a 180° pattern should flow 2.4 gallons per minute at 40 pounds).
Another less desirable method is to have all gear drives with the same arc watering at the same time. If this method is chosen, you could use the same nozzle for all the sprinklers and vary the amount of water applied by changing the time to match the arc. This way it becomes very difficult to pipe all gear drives with the same arc so they will run at the same time. This can become a piping nightmare! The one place where this method seems to be a convenient way to nozzle up the sprinklers is on athletic playing fields where the piping challenges are not so great.
Review
- The flow rates and pressures for your water supply
- Which type sprinklers based on dimensions of areas to be watered
- What nozzles you will use in each sprinkler
- The pressure at which you want to operate those sprinklers
- Accurate dimensions of your site, and any changes in elevation. You're making great progress!
Zoning
Which sprinklers we will operate at the same time? Two rules that must not be broken:
- YOU CAN’T USE MORE THAN YOU’VE GOT. This can be applied to many areas in life, but for now let’s just stick with irrigation. The application of this rule in irrigation is that you can only operate the number of sprinklers simultaneously that will be equal to or less than the flow rate and pressure available from your specific water supply. (An example: you want to operate 15-15 H. nozzles on 4 inch pop-ups in an area between your sidewalk and the street, assume each nozzle will operate at 30 PSI and have a flow rate of 3.5 gallons per minute, 3.5 times 15 equals 52.5 GPM, your water supply is a 1 inch dedicated irrigation meter. You have flow tested the meter and know that at 52.5 gallons per minute, the pressure of available is 30 PSI. You can't run all of these sprinklers at once! You have violated rule #one! The pressure available at the supply is equal to the operating pressure of the sprinklers without factoring in any friction loss. You must break these 15 sprinklers and the two zones one of seven and one of eight.)
- Never, never, never put sprinklers having different precipitation rates in the same zone. And never put turf and bed areas in the same zone.
Zoning can be a time-consuming and tedious process but it must be done, and done correctly.
Connecting the system
Now it's time to “pipe up” the sprinklers you have selected. Remember your goal of good distribution uniformity. Sprinkler flow rate and distance of throw are both affected by pressure; so, you will strive, within reason, to have all of the sprinklers operating at the same pressure. In fact, a variation of 10 percent in pressure is quite acceptable and still provides a high degree of uniformity. As water flows through the piping from the valve to the first sprinkler and down the line to subsequent sprinklers, pressure is lost due to the friction between the water and the inside wall of the pipe.
Some of this "friction loss" is unavoidable. However, by properly sizing the pipe, you can minimize its effects. To reduce friction loss at any given flow rate, pipe size can be increased. The larger pipe reduces the speed, or velocity, of the water flowing to the pipe and as velocity is reduced so is friction loss. Friction loss may be calculated using standard friction loss charts for the type of piping chosen for your system. It should be calculated from sprinkler to sprinkler until the end of the zone line. This loss is cumulative meaning all of the losses in pipe segments from sprinkler to sprinkler must be added together to get the overall friction loss for a zone.
Ttip: Position the valve in the hydraulic center of the zone. Being in the center will help you even out the performance of all the sprinklers in the zone and often allow the use of smaller size zone pipe. Control valves should be selected based on the flow rate for the zone. The valves do not have to be the same size as either the pipe upstream or the pipe downstream of them.
With your sprinklers placed, nozzles chosen, pipe connected to your sprinklers, and valves selected and placed in the proper hydraulic locations, it's time for you to run your mainline and your control wire to the electric valves. Check the location of your water supply and locate a most critical component; the back flow prevention device near the water supply.
This is where your irrigation system actually begins. After determining which type of back flow preventer you are required to use (either by local code or application profile), you will usually want to install it physically as close to your meter as possible. The device should always be installed with an isolation valve upstream of it so that in the event of freezing weather or the need for repairs, the system may be de-pressurized on demand. The path for your mainline should avoid obstacles, be the shortest route possible, and must continue to the last valve.
Looping the mainline, if economically or physically possible, will usually result in a more uniform pressure throughout the system. Trench depth should be determined by local frost line conditions and should be of sufficient depth to eliminate damage from freezing. Electrical wire to operate the control valves is normally placed in the trench with the mainline pipe. One wire, called the common, must connect to one of the leads from each valve: doesn't matter which lead. A separate wire, called the signal or hotwire, much must run to each valve and connect to the other lead.
Example: Nine control valves-one common (usually white) and nine hotwires (one for each valve).
All of the wiring must terminate at the proposed location of the irrigation controller. This device is the “brains” of our system and will carry out all of your instructions flawlessly. With it you can schedule your run times for the valves based on the amount of water you want to apply to each zone. Multiple “cycles” can be programmed to eliminate runoff due to low infiltration rate areas. You can tell the controller what days to irrigate, what days to skip and what time of day to water.
You can add sensors that will not allow irrigation to take place if a prescribed amount of rain has fallen, if the temperature falls to near freezing, if the wind is blowing to hard, or if the soil moisture is already at an acceptable level (system wide or by zone). A new generation of controllers will soon be available that will actually calculate the ET for the region of installation and automatically adjust run times for the zones to supply water loss due to ET, automatically selecting cycles as needed to prevent runoff.
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