Part 10, Landscape Irrigation
By Ed Butts, PE, CPI
In this month’s edition of Engineering your Business, we continue our series on irrigation with a discussion on landscape (turf and ornamental) irrigation techniques, fundamentals, and basic design concepts.
Although landscape (turf and ornamental) irrigation adheres to many of the previously stated fundamentals of irrigation design, there are specific differences associated with applying water to raise a cash crop as opposed to providing water for simply aesthetic purposes.
For one, the need for uniform and equal water distribution for landscape irrigation is critical. And since ornamental plants and shrubs do not possess the same deep and widespread root zones that many food crops do, and various state and municipal restrictions associated with this type of water use often exists, an intelligent and innovative design along with effective and efficient system operation become imperatives.
Unlike most forms of agricultural irrigation where growing a crop for revenue is the sole intent, landscape and turf irrigation is intended to maintain lush and green grassy areas and hardy ornamental plants to maintain appearances for residential lawn areas, commercial common areas, golf courses, cemeteries, fairgrounds, park grounds, grass medians, or sporting fields.
Four of the principal differences with designing an agricultural irrigation system versus a landscape irrigation system are:
- Constant and numerous obstructions and impediments to uniform coverage such as irregularly spaced sidewalks, trees, shrubs, buildings, and parking areas
- As city water is often used as the water source, it is more likely to be limited and fixed in capacity and pressure, generally leading to the required use of zones for larger lawns or irrigated areas
- The need for uniformity of sprinkler coverage is much greater as grass is grown in much closer spacings
- Different grasses and ornamental plants possess diverse and unique spacings, root depths, and water requirements than a single agricultural crop.
Landscape Irrigation Design
Landscape sprinklers are usually selected according to output volume in gallons per minute and the wetted diameter in feet. The output volume of a sprinkler is primarily determined by its nozzle size (the bore diameter is most common) and delivery pressure. Though also a function of nozzle size and pressure, the wetted diameter is greatly impacted by the type of sprinkler head.
For instance, impact sprinklers will typically deliver a greater flow rate and wetted diameter than spray heads even at identical pressures. Where sprinkler spacings for agricultural irrigation systems are generally dictated by the degree of wind exposure, landscape and turf irrigation design is typically conducted using 50% of the sprinkler’s rated diameter or 100% of the radius, if alternatively used.
This theoretically provides what is called head-to-head coverage (Figure 1) and assumes that water delivered from an adjacent sprinkler will just contact the closest head. This ensures adequate coverage under virtually all normal wind conditions and avoids the possibility of developing the appearance of scalloped or dried out areas common to underwatered turf grass. This is particularly important with systems that use zoned coverage where one operational sprinkler is inline and adjacent to another but may not be operating concurrently with that head.
As opposed to most agricultural irrigation systems that use and draw from the water storage available in the soil, many automated turf and landscape systems are designed to provide the replacement sum of plant consumptive use and evaporation each day, or apply water on an as- needed basis. Although this operation requires greater attention to observing daily rainfall events and monitoring crop use coefficients, modern irrigation controllers commonly provide this feature to enable efficient irrigation practices in the form of a rain switch.
This also allows taking advantage of the greater intake rate most soils possess on a semi-arid basis, which in turn allows higher instantaneous application rates needed with many landscape sprinklers. Hence, instead of applying 2-3 inches of water to span a rotation period of seven to 10 days before a repeat of watering, many landscape irrigation systems simply operate for less than 1 to 2 hours each day to replace the ½-inch or less of water lost during the previous 24 hours.
There are four common methods of applying water onto landscape and turf areas:
- Permanent heads, such as pop-up, shrub, stream, or strip spray head sprinklers
- Portable or solid set impact or rotor sprinklers, including plug-in (bayonet) or pop-up canned heads
- Portable hard hose travelers
- Drip, trickle, and micro-irrigation primarily used for ornamental and irregularly spaced plants.
Numerous types of sprinklers are available, and if properly selected, can be adapted to cover almost any area of grass or landscape. Sprinklers are usually the best choice for large turf areas and are capable of excellent coverage if applied correctly.
Plug-in styles of impact sprinklers or hard hose travelers are often the best choice when solid set systems are not feasible due to cost, installation limitations, or other factors. When using plug-in styles, access to the irrigated area must be maintained without tripping hazards, or the sprinklers must be constantly relocated to optimize the available water capacity with the irrigated acreage, a common factor with multi-use facilities.
Drip and micro-irrigation have been increasing in popularity during the past few years and are especially suitable for watering individual plants with non-uniform spacing such as potted or in-ground bedded shrubs, bushes, plants, gardens, and trees, and can also work well for ornamental suspended plants.
Disregarding economic factors, the use of a particular sprinkler head and system will vary based on the coverage area (width and length), available flow and pressure, and labor availability for portable systems.
Many large areas, such as permanent lawn areas and golf course fairways, are watered using a solid set, permanently buried sprinkler system consisting of pop-up impact or rotor sprinklers as this type of sprinkler will generally cover the largest area per gallon of applied water.
A step-by-step design procedure is not always a simple matter since numerous factors are generally involved. When designing a sprinkler system, the types and number of sprinklers must match the available pressure and capacity of the water supply.
If zones are employed, each zone should be flow balanced to the extent that sprinkler uniformity is maintained and hydraulic considerations (i.e., velocity and pressure drop) are not exceeded.
Also, the instantaneous rate and gross application depth from sprinklers or drip emitters should be low enough to prevent excessive runoff from the soil surface but high enough to satisfy the water requirements of the plants between watering.
Selecting the best sprinklers or emitters and spacing for a system is often a process of trial and error. Sometimes, such as with residential homes on an existing public water system, the water supply already exists, and the system must be designed with these inherent pressure and flow rate limitations in mind.
In other instances, the water source adequacy will be so great that the flow and pressure requirements can be determined from the irrigation design, with the water supply designed and installed to fit the irrigation demand.
At any rate, you must consider all relevant factors while planning an effective landscape irrigation system, including:
- Characteristics of plants and grasses: daily irrigation consumptive use requirements, and root depths
- Environmental factors like prevailing wind speed and direction, maximum temperature, and evaporation
- Shape and size of the irrigated area, boundaries, slopes, and physical obstructions
- Soil type, depth, and hydraulic characteristics such as intake rate and water-holding capacity
- Water source flow, pressure, and quality (i.e., is the use of zones, booster pumping, or filtration needed)
- Operational requirements such as minutes/hours per day, number of zones, automatic or manual controls).
Sprinkler Styles in Landscape Irrigation
Pop-up sprinkler heads are popular on both landscape and turf and are usually constructed from stainless steel, thermoplastic, or rubber components. The most popular types of landscape sprinklers are rotors and sprays (Figure 2).
The sprinkler body, or can, is installed below ground and the nozzle is attached to a fixed or adjustable height riser. When pressure is applied to the head through the inlet connection on the lower part of the base, the riser and nozzle extends above ground to a height between 2 and 12 inches and the sprinkler begins operation.
A wiper seal on the upper part of the body prevents water and sand from invading the sprinkler body and possibly disrupting rotation.
After irrigation is complete, the riser and nozzle are pulled back into the sprinkler body, retracted by the tension from an internal spring. Since the body is typically installed below ground, the sprinkler then becomes essentially inconspicuous when it is in the at-rest or not-in-use position.
An example of a pop-up rotary sprinkler head and cutaway drawing is illustrated in Figure 3.
This type of head offers three distinct advantages: the sprinkler doesn’t detract as much from the appearance of the landscape; the head is not as likely to cause a tripping hazard; and the sprinkler is not likely to be damaged by lawn care equipment. All are important considerations for mowed areas such as athletic fields and many common areas.
Rotary sprinklers, illustrated in Figure 4, generally operate at high pressures of 30 to 80 pounds per square inch and cover large areas (30- to 60-foot radius, 60- to 120-foot diameter). They usually produce an application rate between 0.2 and 0.5 inches per hour.
Although rotary and impact sprinklers are typically identical in performance and combined into a single group as rotary sprinklers, they will be outlined separately for this discussion.
Rotary sprinklers, often referred to simply as rotors, have largely replaced impact sprinklers in most landscape applications and are frequently the most economical choice for large gardens and large open-turf areas.
Typically, water moving through the sprinkler body spins an inline turbine, which turns a set of bevel gears that rotate the nozzle. These gear-driven rotors deliver one or more continuous streams of water which rotate silently across the landscape.
They provide uniform rotational speeds and coverage and do not produce the sudden jerks and stalls often associated with impact heads. Typically, rotors are used for full-circle sprinkler spacings from 20 to 60 feet apart, with smaller rotors typically limited to spacings of 40 feet.
Part-circle operation of rotors is performed by reversing the internal gear and causing the sprinkler head to drive backwards at the same forward speed until reaching the reverse stop and resuming normal rotation.
Rotor performance is greatly influenced by the degree of applied pressure. Therefore, a good rule of thumb to remember is: The water pressure (in psi) at the head should meet or exceed the distance or spacing (in feet) between the heads. This means a sprinkler with 40 psi of operating pressure should not be generally spaced any farther than 40 feet from adjacent heads.
Rotor bodies are available in two basic styles: pop-up and shrub. Larger rotors are also available in valve-in-head configurations. This option places a low voltage, AC or DC solenoid-controlled valve as a component of the valve body and permits on/off operation of the head from a remote controller or timer. This is a popular sprinkler for golf courses.
Fundamentally, rotors are not as durable as impact sprinklers as they use more internal moving parts. They are also much more prone to stalling or failure due to inferior water quality than impact sprinklers and should be used with caution and adequate filtration on sandy or surface water sources.
An impact sprinkler is mounted on a sealed bearing that allows the entire sprinkler body to spin in full or part circles. It is the oldest type of sprinkler still in use.
It is progressively rotated by the oscillating impact of a swinging arm with a counterweight on one end and a drive spoon on the other that repeatedly engages the flow stream from the nozzle. This momentarily strikes the body of the sprinkler, causing it to slightly rotate each time.
This type of sprinkler is available in sizes ½-inch up to 1 and 1/14-inch threaded inlet sizes with nozzle flow rates between 3 GPM to more than 125 GPM at operating pressures between 30 to 80 psi.
As with rotary sprinklers, they also usually cover a 30- to 60-foot radius with an application rate between 0.2 and 0.5 inches per hour. Impact sprinklers used for landscape irrigation are typically used for plug-in (bayonet) heads into quick coupling (QC) valves or as small pop-up heads (Figure 5).
Portable aluminum lateral lines are occasionally used with impact sprinklers to irrigate turf grass in unusual cases.
Larger impact sprinklers are frequently used in the 30 to 125 GPM flow range and are used on gun carts for small hard hose travelers or QC keyed-valve combinations that allow them to be rapidly disconnected and moved from one location to another. Common impact sprinklers in this range are the Nelson models P65, P85, and 75 Series Big Gun, and the Rainbird model P65PJ.
To avoid slick and hazardous surfaces or potential water damage, it is usually desirable to keep coverage away from adjacent areas such as sidewalks or sides of buildings. For this type of application, part-circle rotary sprinklers are available, which are usually adjustable with the arc of throw able to be varied between 20 to 360 degrees. Many nozzles can also be equipped with adjustable diffuser screws that enable lowering the distance of throw (radius) by disrupting the flow stream.
Spray heads, or sprays, are designed to operate at lower pressures than rotary sprinklers (usually within 15 to 35 psi). They cover a smaller area than rotary sprinklers (10- to 20-foot radius) and apply from 1 to 2 inches per hour. This characteristic may limit their use and length of application in heavier soils or in areas with excessive slopes.
Spray heads are most often used in small or narrow turf areas and irregularly shaped areas. Most use interchangeable nozzles installed on the sprinkler which determine the pattern (quarter, half, or full circle) and the radius of the water throw.
Some specialty patterns are available for long, narrow, or larger areas such as the conical, full-circle heads shown in Figure 6a for grass areas.
The basic physics of water spray limit the spacing distance between heads since they require between 20-30 psi of water pressure to operate properly. As with rotary sprinklers, spray heads can be mounted on permanent risers or are available as pop-up types to be used in turf or ornamental planting areas.
Spray heads are available in numerous configurations, so they may be adapted to cover almost any shaped area within reason. They are available in full circle, half circle, quarter circle as well as bubbler, bowtie, square, strip, conical, and rectangular patterns.
As a rule, all sprinklers used in the lawn and garden should be installed on a swing joint or flexible riser, especially for pop-up sprinklers. The swing joint allows the sprinkler to be adjusted flush with the ground level and prevents the sprinkler and underground pipe from being damaged in the event they are stepped on or run over by heavy equipment.
Many of the emitters and sprinklers that were discussed in the previous column on drip and micro-irrigation can be effectively applied to landscape watering. Small planters and narrow planting beds can usually be adequately irrigated with drip irrigation, flood bubblers, stream bubblers, or short-radius spray sprinklers.
If a planter is narrow with walled or bermed borders, flood bubblers (Figure 6b) can be used to fill the reservoir area under the plants. Slightly wider planting areas can use stream bubblers that can throw gentle streams out to a radius of 5 feet. Narrow lawn strips can be watered by short-radius spray sprinklers with strip pattern nozzles.
Landscaped strips can be irrigated in several ways. For strips that are 4 to 7 feet wide, low pop-ups for lawns and shrub sprinklers or high pop-ups for shrub areas can be used with center strip or end strip spray nozzles. These nozzles have a bowtie or half-bowtie pattern and are located down the center of the area. For strips with trees in the center of the shrub or lawn area, the side strip nozzle (Figure 6c) can
cover the area from each edge of the strip instead of the center where the trees would block the spray. Wider strips, more than 6 to 7 feet in dimension, can use half-circle spray sprinklers throwing in from both edges.
Narrow strips and confined areas often use low-angle trajectory or flat-angle spray sprinklers to reduce the chances of overspraying the area. Many strips and planter areas are bordered by walkways such as common areas between apartments, condominiums, and offices where overspray is unacceptable.
Hard Hose Travelers
Soft or hard hose (reel) irrigation systems, also called traveling gun systems or travelers, consist of a single, portable sprinkler head that sprays water over a semi-circular pattern, generally an arc between 210 to 320 degrees.
Although the larger sizes are commonly used for agricultural irrigation, the smaller sizes are popular for irrigating turf grasses in parks, cemeteries, athletic fields, and golf courses (Figure 7).
Since hard hose travelers are much more popular than soft hose machines for turf areas, they are exclusively discussed here. These movable sprinklers are installed on carts with a 1.1-inch to 2.5-inch polyethylene water hose with various lengths attached. The machines can be positioned and operated wherever necessary, so long as they remain connected to a water source.
Flow rates vary from 30 up to 150 GPM with common hose lengths between 200 to 600 feet depending on the manufacturer. This diversity in hose lengths permits irrigation on many types of grass areas including athletic fields, median strips, and large lawns.
These sprinklers are best suited for irregular-shaped areas such as golf courses, baseball and football fields, difficult-to-reach areas, and fields that cannot be otherwise irrigated because of physical obstructions like ditches, power lines, roads, trees, and buildings.
Most of the smaller hard hose systems use an inline turbine, bellows, or piston drive powered by a portion of bypassed water flow heading to the sprinkler. In certain cases, a small DC electric motor on very small reels or a diesel- or propane-powered engine is used to drive the machine, either directly through speed reduction with belts and pulleys or a closed loop hydraulic fluid driven system with hydraulic motors.
Regardless of the drive method, the machine steadily retracts and rolls up the hard hose while irrigating from the sprinkler affixed to the gun cart at the end of the hose. Depending on the desired application, sprinkler capacity, type of drive, hose length, and lane spacing, travelers typically pull in the hose at a rate between 0.5 to 1.5 feet per minute and can typically cover a travel run in 3 to 18 hours.
The example shown in Figure 8 reflects a 160-foot wide × 360-foot long football field, which, by using an 85 GPM nozzle at 70 psi with a 240° sprinkler coverage, will apply 2.55 inches of gross water (1.66 inches net) in 12 hours.
Turbine-drive systems operate with an inline water-powered turbine that uses part of the energy from the flow of pressurized water moving through the machine to the gun cart, slowly reeling in the hose whenever water moves through the system. To generate the needed power, turbine drives consume a portion of the incoming water pressure during operation, with 10 psi or less used through the turbine.
Although booster pumps can be used on small reels to raise the inlet pressure to the machine, the extra cost, operating complexity, fueling and transportation hassle, and added weight usually preclude their use on turf irrigation systems. Engine drives are primarily used in applications where the customer requires higher speed irrigation or has limited water pressure and cannot afford the pressure loss from the turbine.
The bellows drive is capable of retraction without consuming any pressure in the process. This allows 100% of the pressure provided at the machine to be available for irrigation rather than used or consumed by the drive mechanism. This is the preferred drive for machines that tend to operate at low pressure and flows (i.e., small-diameter models). The bellows drive discharges and dissipates a small quantity of water used in the retraction onto the ground or through a low-pressure sprinkler.
Operators often prefer the simplicity of the inline bellows or turbine drive, particularly when the machine is used for turf irrigation. In certain smaller reels, an electric drive using a solar recharged battery and 12-volt DC motor can be used.
This wraps up this month’s contribution to this series on irrigation fundamentals. Next month, we’ll continue with a review of the pumping systems used for irrigation purposes and the unique application of tailwater recovery and reuse.
Until then, work safe and smart.
Ed Butts, PE, CPI, is the chief engineer at 4B Engineering & Consulting, Salem, Oregon. He has more than 40 years of experience in the water well business, specializing in engineering and business management. He can be reached at email@example.com.