Water Source

• Water Meters
• Service Line
• Point of Connection Size or Custom Maximum Available
• Pressure Available

To begin an irrigation system design, you must find out the flow and pressure your water source can provide. Your water source can be a water meter – typical in an urban environment – or a custom source, such as tapping into an existing pipe or a water well and booster pump.

Our Source Data tool provides an analysis of your project's water source. For more information, see our Source Data documentation.

Water Meters

If your water source is a water meter, you are limited by the size of the meter. Your allowable flow through the meter is limited by the fact that you should not exceed a maximum flow of 75% of the safe allowable flow the meter can handle. This 75% figure may vary from 50% to 80% depending on the water agency, but 75% is a generally accepted standard.

 

If your project has an existing water meter that you plan to use as its source, you should note its size. This information is usually stamped on the meter, or you can call the water company to find out the size. You should also note the size of the service line – the pipe serving as the connection between the water service in the street and the meter.

 

If your project is a new one, and/or you have the freedom to select the meter size that best suits your needs, you have a lot more flexibility in determining your available source flow. You will need to determine the total flow demand of your project and divide that number by the desired number of valves, or stations, that you feel is appropriate for the project. For a more in-depth discussion of this subject, see our Simple Initial Water Source Determination documentation.

 

Irrigation F/X allows you to complete this calculation in one of two ways:

• Using our Schematic Irrigation tool to determine a close estimate of your final total flow demand, dividing by the desired number of valves or controller size, to calculate an anticipated flow average per valve.
• Laying out all your irrigation heads and/or drip equipment, and then using the Circuit tool to determine your flow total. You can then divide by the desired number of valves as described above.

How much flow can you allow for certain meter sizes? Our Irrigation Design Rules of Thumb show the limits of various meter sizes at 75% safe flow.]

 

Water Meter Size/Flow

 

Service Line

As noted above, the service line is the pipe connecting the main in the street with your water meter. "Service line" does not refer to any of the piping within your project. The service line type and size can often be the limiting factors in determining the available flow for your system. You would match the water meter with the limits of the service line. The lowest number for flow will be the number that governs.

 

Our Irrigation Design Rules of Thumb describe two service line types and their related flow options for two velocity options: 7.5 fps (2.29 m/s) and 9 fps (2.74 m/s). These are higher velocities than you would allow within your site, but the service line is typically a straight run in the street, with little to no water hammer, and you want to maximize your flow as much as possible for the maximum efficiency of your system.

 

Service Line Size/Flow

 

Maximum GPM @ 7.5 FPS and 9 FPS

 

 

 

 

Maximum L/M @ 2.29 M/S and 2.74 M/S

 

 

 

You may have an existing meter and existing service line on your project. Let's say you have a 1-inch meter and a 1-inch copper service line. The meter can handle 36 gpm (136 l/m), and the service line can handle 18.2 gpm (69 l/m) at a velocity of 7.5 fps (2.29 m/s) or 21.8 gpm (83 l/m) at a velocity of 9 fps. Your limitation is the service pipe, and perhaps you would accept the higher velocity for the 21.8 gpm (83 l/m) flow maximum – the highest flow you can achieve in this situation.

 

If you have a new project that includes the installation of a new meter, you will normally be putting in a new service line in the street to a new water meter. This installation allows you to select the service line size of your choosing. Let's say the 1-inch meter at 36 gpm (136 l/m) would work for you, but you want a 1½-inch (40mm) service line of PVC to service this meter. As the limitation for that pipe at 7.5 fps (2.29 m/s) is 47.6 gpm (180 l/m) – which is higher than the 36 gpm (136 l/m) for the meter – your meter maximum flow will now govern, which is 165% more than the same meter with a smaller service line. Therefore, you'll need to look at both the service line and meter size for your available flow. If this is a new project, you can just look at the water meter flow allowable, as you will naturally add a service line size to maximize that flow.

Point of Connection Size or Custom Maximum Available

Another method of defining your available water source would involve determining either a point of connection (POC) size or the custom max available.

 

The POC Size is simply the size of the connection to your source. For instance, if you plan to tap into an 8-inch (200mm) service line, and you use a 1 1/2-inch (38mm) tap into this service line, your POC size is 1 1/2 inches. Using the Irrigation F/X Source Data for POC size, a flow of 47.66 gpm (180 l/m) would be indicated, which would be the equivalent flow for a velocity of 7.5 fps (2.3 m/s). This is essentially a rather high flow with a rather high initial velocity. You could choose a slower velocity, such as 5 fps (1.5 m/s), for the piping within your project, possibly resulting in a pipe sizes larger than 1 1/2 inches (38mm), but you would then be maximizing the flow available through the POC you selected.

 

Perhaps a better method is to simply select a custom max available in the Irrigation F/X Source Data, where you define the amount of flow that you know this source can generate. You could obtain this number by testing the source in the field.

Pressure Available

After determining your source, you could review the pressure available from the source, or you could estimate the pressure you may need for your project. If your source is a water meter, you could call the agency responsible for the meter and ask for the value of the static pressure at this location. Static pressure is the pressure available at the source before any demand has been placed upon it – thus the name “static,” or still. Once you put a demand on this service, such as turning on a valve, the water will start flowing and pressure loss will occur as the water moves through the pipe. The pressure will decrease depending on this loss.

 

Here are some typical design pressures for different irrigation system types:

Spray Heads: 30 psi (2,07 bars), although most spray heads can operate to an adequate level at 25 psi (1,73 bars), and perhaps even less.

Rotor Heads: 40 psi minimum (2,76 bars), but be sure to have at least 1 psi per foot of radius minimum (0,232 bars per meter of radius).

Drip Emitters: 20 psi (1,38 bars).

 

This design pressure represents the ideal pressure as a minimum for that head type – in other words, the pressure you should expect at the very last head in any system. You would add this design pressure to the anticipated pressure loss you would incur to get the water from the POC to this last head.

 

What would be a typical pressure loss to get water to that last head? This number would be the loss through the pipes and valves serving your system to supply this water. the following tables show the low and high averages for this loss in a typical situation.

 

 

 

 

 

 

 

 

Therefore, expect a typical system pressure loss to range from about 25 to 30 psi (2,93 to 3,45 bars). Add this number to your most demanding design pressure to obtain an approximate pressure necessary at your source to provide for your design. The following table shows average pressure requirements for a few types of equipment.

 

Typical pressure requirements

 

 

If this initial analysis of your source indicates that it will simply not provide the pressure necessary to operate your system, you will at least be aware of this information early on and can anticipate using a booster pump to boost the pressure. You may also change your mind on certain head types to use those that require a lower design pressure.