Irrigation Hydraulics and Zone Design

Reading a nozzle chart is easy. Making sure the last head on the last zone actually gets the pressure on that chart is the job. A mid-level installer stops guessing here. You add up every loss between the meter and the far head, and you know before you dig whether the zone will work. These same numbers also explain most complaints about weak heads, misty spray, and dry patches. Remember the basics as you read: measure static pressure and available flow first, stay within 75 percent of available flow on a zone, and keep pipe speed under 5 feet per second.

What You Will Be Able to Do

• Convert elevation to pressure and read a friction loss chart
• Work out pipe velocity and friction loss for a real run
• Write a pressure budget from the point of connection to the last head
• Calculate precipitation rate for a zone
• Lay out zones by plant, sun, slope, and head type
• Choose and place backflow protection as part of the design

Three Numbers Set the Limit

Your zone flow can never be higher than the lowest of these three limits.

1. Measured flow. Take 75 percent of what you measured at the property.
2. The water meter. Each meter has a safe flow. One published AWWA operating table lists a safe maximum of 20 GPM for a 5/8-inch positive displacement meter and 30 GPM for a 3/4-inch one. Your meter's own data sheet and the water supplier decide the real limit.
3. Pipe velocity. The pipe size must keep speed at or below 5 feet per second.

Worked example. A bucket test shows 16 GPM. 75 percent of that is 12 GPM. The meter is a 5/8-inch, so its limit is above that. A 1-inch Class 200 PVC line carries 12 GPM at about 3.5 feet per second, which is fine. Your design flow is 12 GPM.

Elevation and Pressure

Water weighs something, so height costs pressure. A column of water 1 foot tall pushes down with 0.433 psi. Said the other way, 1 psi lifts water 2.31 feet.

• Going uphill, you lose 0.433 psi for each foot of rise.
• Going downhill, you gain 0.433 psi for each foot of fall.

Worked example. The far heads sit 8 feet above the point of connection. 8 x 0.433 is about 3.5 psi lost. A site on a steep hill can gain or lose more pressure this way than through all its pipe.

Friction Loss and Velocity

Moving water rubs on the pipe. The loss depends on flow, pipe size, and the inside surface. Published charts do the math for you, using the Hazen-Williams formula.

These values are for PVC Class 200 pipe (also called SDR 21) at C = 150, taken from a manufacturer's published chart. Other pipe has other values. Schedule 40 PVC has a smaller inside diameter than Class 200 in these sizes, so it loses more. Use the chart for the pipe you are installing.

Two things stand out. Friction does not rise in step with flow. Going from 6 to 10 GPM in 3/4-inch pipe raises flow by about 67 percent, but the loss more than doubles. A small jump in pipe size also cuts loss sharply, so a short run of larger pipe is often the cheapest pressure you can buy.

Working Out Velocity

Charts give velocity, but you can also work it out:

Velocity in feet per second = 0.408 x GPM divided by (inside diameter x inside diameter), with the diameter in inches.

Worked example. 1-inch Class 200 pipe has an inside diameter of 1.189 inches. Squared, that is about 1.41. For 12 GPM, 0.408 x 12 is 4.9, and 4.9 divided by 1.41 is about 3.5 feet per second. That passes the limit. At 18 GPM, the same pipe runs about 5.2 feet per second, so that flow needs the next size up.

Sizing a Lateral

Flow in a lateral falls at each head, because each head takes its share. So the far end needs less pipe than the near end. Size it from the last head back toward the valve, adding flow as you go.

Worked example. A zone has 6 heads using 2 GPM each, for a total of 12 GPM. Heads sit 15 feet apart, and the first head is 40 feet from the valve.

The total is about 1.5 psi. For each section, the math is loss per 100 feet multiplied by the length divided by 100. For the first section, that is 1.82 x 40 divided by 100, or 0.73 psi.

The Pressure Budget

A pressure budget lists every loss from the point of connection to the worst head. The worst head is usually the highest or the farthest one. Subtract all the losses from static pressure, and you get what that head really sees.

Worked example. Static pressure is 65 psi, and the design flow is 12 GPM. The far head is 8 feet above the connection. The mainline is 80 feet of 1-inch pipe.

The losses add up to about 21 psi. 65 minus 21 leaves about 44 psi at the far head. The meter, backflow, and valve numbers here are samples, so read yours from each maker's chart at your actual flow.

Now compare 44 psi with what the head needs. Rotary nozzles are often rated around 40 psi, so they will be happy. Fixed sprays are built to run near 30 psi, so 44 psi will mist. Fix that with pressure-regulating bodies or a regulator at the valve.

If the budget had come out at 25 psi, you would have four choices: fewer heads per zone, larger pipe, a lower-loss device, or a pump. Find that out on paper, not after the trench is backfilled.

Pressure Across the Zone

Heads on one zone should see close to the same pressure. A common design rule keeps the difference between the highest and lowest pressure in a zone within about 20 percent. Flow changes with the square root of pressure, so 20 percent in pressure is about 10 percent in flow.

Worked example. At the far head, you have 44 psi. 20 percent of that is about 9 psi. With the zone climbing 8 feet (3.5 psi) and 0.7 psi of pipe loss past the first head, the spread is about 4 psi, so this zone passes. A zone climbing 30 feet would not, since that alone is 13 psi.

Precipitation Rate

Precipitation rate is how fast a zone puts water down, in inches per hour. You need it for two reasons. You cannot set a run time without it, and you must not mix rates on one zone.

The formula uses a constant of 96.3, which turns gallons per minute over square feet into inches per hour.

Precipitation rate = 96.3 x GPM divided by area in square feet.

Whole zone: Use the total zone GPM and the area it covers. A zone flowing 12 GPM over 800 square feet gives 96.3 x 12 divided by 800, or about 1.44 inches per hour.

Square spacing: Use the full-circle GPM of one head and the spacing. A nozzle using 3.7 GPM at full circle, on a 15 by 15 foot grid, gives 96.3 x 3.7 divided by 225, or about 1.58 inches per hour.

Triangular spacing: Rows sit closer together, so multiply the area by 0.866. The same nozzle gives 96.3 x 3.7 divided by (225 x 0.866), or about 1.83 inches per hour.

With part-circle heads, use the full-circle GPM for the nozzle series. A half-circle nozzle puts out half the water on half the area, so its rate matches. That is what matched precipitation means. A nozzle that is not matched will flood or starve its part of the zone.

Zone Layout

A zone is a group of heads that open together, so everything on it gets the same schedule. A good zone meets all of these.

A zone that is over budget gets split. Do not split a zone across two exposures just to save a valve. A lawn with one sunny half and one shaded half on one zone will always be wrong somewhere.

Drip Zones

Drip is sized in gallons per hour (GPH). Divide by 60 to get GPM.

Worked example. A shrub bed has 90 emitters at 1 GPH. That is 90 GPH, and 90 divided by 60 is 1.5 GPM. That flow is so low that a standard valve may not close reliably. Check the valve's low flow limit, and use a low-flow control zone kit with a filter and pressure regulator if you need one.

Backflow Is Part of the Design

Backflow protection is one of the largest losses in the budget, and its location can decide what device is even possible. Choose it early. The model codes accept vacuum breakers or a reduced pressure zone assembly (RPZ) on a lawn system, and they call for an RPZ where chemicals are put into the water. The water supplier and the local code decide what is allowed where you work, and some cities require an RPZ on every lawn system.

The height for a PVB varies. The International Residential Code and common maker's instructions call for at least 12 inches above the highest downstream piping. The International Plumbing Code's text gives a lower figure. Read your adopted code and the device sheet, and use the higher number.

Worked example. A yard climbs 9 feet behind a house, and the water connection is at the house wall. A PVB at the wall would sit below the top heads, so it fails. You have two options. Run the supply uphill and set the PVB at least 12 inches above the highest head, or use an RPZ at the house. Both must be drained or protected from freezing where you work. The RPZ costs more pressure, so go back and rerun your budget.

Testable assemblies are tested by a certified tester, usually when installed and yearly after that. Pull any required permit, and send the test report to the water supplier.

Common Mistakes

Check Your Understanding

1. The highest head sits 12 feet above the meter. How much pressure do you lose to that height?

12 x 0.433 is about 5.2 psi.

2. A run of 3/4-inch Class 200 pipe carries 10 GPM for 60 feet. How much pressure does it lose? What happens if you go up to 1-inch pipe?

At 4.29 psi per 100 feet, 60 feet loses about 2.6 psi. In 1-inch pipe at 1.30 psi per 100 feet, it loses about 0.8 psi. That one size saves about 1.8 psi.

3. A nozzle uses 3.0 GPM at full circle on a 14 by 14 foot square grid. What is the precipitation rate?

96.3 x 3.0 is 288.9. Divide by 196 and you get about 1.47 inches per hour.

4. Your budget shows 52 psi at the far head of a fixed spray zone. Is that good news?

Not really. Sprays are built for about 30 psi, so they will mist and drift. Add pressure-regulating bodies or a regulator at the valve.

Key Takeaways

• Zone flow is capped by measured flow, the meter, and pipe velocity.
• Every foot of rise costs 0.433 psi.
• Friction climbs faster than flow, so sizing up pays off.
• Write a pressure budget for the worst head before you dig.
• Precipitation rate is 96.3 x GPM divided by area.
• Choose backflow early, because slope and hazard can decide it.