Irrigation Fundamentals

Part 4a, Sprinkler Irrigation, Small Systems

By Ed Butts, PE, CPI

Figure 1a. Rectangular sprinkler layout and spacing.

In the last three parts of this series on irrigation fundamentals and concepts, we’ve discussed irrigation basics beginning with terms and definitions; soil-plant-water relationships, water quality considerations, and application efficiency; determining required flow rate, volume, and distribution uniformity.

Now, beginning this month and continuing next month, we will expand upon our earlier discussions with a two-part subseries on what many of you are likely most familiar with: sprinkler irrigation techniques and design.

We’ll start with hand-move, side roll, and solid set irrigation, and follow with a discussion of “big gun” systems in Part 5 and finish the  sprinkler subseries in Part 6 with pivots and lateral moves (linears). Flood and furrow irrigation will be outlined in Part 7. A look at drip and trickle irrigation practices will follow in Part 8. An examination of turf and ornamental (landscape) irrigation practices will be in Part 9. Finally, we’ll wrap up the series in Part 10 with a review of pumping systems used for irrigation, including tailwater recovery pumping, and offer a final summary.

Sprinkler Irrigation Overview

Fresh and non-potable water used for sprinkler irrigation includes water applied and consumed for all aspects of crop production. These include but are not limited to: field preparation, seeding, crop propagation, frost protection, application of fertilizers and chemicals, weed control, crop cooling, dust suppression, leaching excess salts from the root zone, and water lost in conveyance and application.

Figure 1b: Triangular sprinkler layout

Irrigation of golf courses, parks, nurseries, turf farms, cemeteries, and other self-supplied landscape-watering uses are also included in this definition.

Likely the most visible form of irrigation techniques, sprinkler irrigation in all its various forms and methods is practiced throughout the world. Sprinkler irrigation using application methods as small as rotor, micro, and mini sprinklers to conventional impact and spray head sprinklers used on handmove, solid set, wheellines, center pivot, and lateral move systems—and all the way up to “big gun” solid set, portable, and mechanized systems—are in widespread use on all types of irrigable land across all continents.

Sprinklers are generally arranged in a rectangular configuration (Figure 1a) or triangular pattern (Figure 1b), each with inherent advantages and disadvantages.

Smaller Sprinkler Systems

Smaller sprinkler irrigation systems are defined as those that use a series of sprinklers on a multiple sprinkler lateral line with individual sprinkler capacities of 50 GPM or less. This basically encompasses most impact, rotator, spinner, and spray sprinkler heads with a single-nozzle or double-nozzle configuration with a single-nozzle diameter up to one-half inch and typical nozzle pressures between a range of 40 to
75 psi.

Lower pressure (LEPA) systems that operate between 2 to 30 psi are also being used with more frequency, but this overview will generally be limited to the most common type of system used for agricultural irrigation purposes—those on higher pressure.

Figure 2a. Typical aluminum mainline.

Irrigation systems may be designed in several ways. They may be permanent systems using both buried PVC or permanently set aluminum main and lateral lines (solid set); semi-permanent systems with buried PVC mainlines and portable or transportable aluminum laterals (hand-move or wheelline); and fully portable systems using portable aluminum mainlines and laterals (hand-move) that are in short
enough lengths to make them road legal for transport and use on other fields.

With all of these systems, water is initially pumped and delivered from the source, usually a well or surface water source—such as a river, lake, or similar water body—and sent through a mainline usually made from aluminum materials or PVC (Figures 2a and 2b). The water is then distributed through hydrants to individual or multiple “lateral” lines (Figure 3) where it is discharged above the crop or soil surface through individual risers and sprinkler heads attached to the laterals.

Figure 2b. Typical buried PVC irrigation mainline.

Aluminum mainlines on permanent or solid set systems are generally designed to remain stationary in the field throughout the entire growing season and utilize aluminum laterals that are fed from the mainline and through a hydrant and valve opening ell to each lateral (Figure 4a).

In certain cases, PVC pipe can be used for solid set sprinkler laterals and risers, although an exterior treatment or painting of the pipe is usually required to prevent long- or short-term ultraviolet (UV) degradation and weakening. Solid set systems are ideal for many high-valued nursery stock, ornamentals, trees, and similar plants and perennials where the plants remain stationary in one basic position over the course of their development or lifetime. This means that fertilizing, pruning, weed control, and harvest are not impeded or affected by the presence of the laterals. This is critical as labor costs and potential plant disruption and damage to continually move and reset the laterals would likely become cost prohibitive.

Each sprinkler head generally applies water in a full or part circular pattern with the diameter governed by the sprinkler type, nozzle size, and pressure. However, square pattern sprinklers are occasionally used, particularly for border watering.

Figure 3. Typical aluminum lateral.

For uniform coverage, the patterns are designed to overlap adjacent sprinklers from 35% to 70% of their respective diameter, depending on the sprinkler type, nozzle pressure, slope, soil and wind conditions.

Typical spacing is 60%–65% between laterals and 40%– 50% between sprinklers. Sprinklers are most commonly used in a single-nozzle configuration, but double-nozzle sprinklers are also available and used in many applications. Doublenozzle sprinklers are commonly used to provide better uniformity and coverage close to the head by employing a smaller spreader nozzle.

The spreader nozzle is typically 3/32 of an inch to 9/64 of an inch in diameter with a shorter coverage radius than the primary drive or range nozzle. In many cases, the distribution uniformity will improve up to 10%–15% with the use of a double-nozzle sprinkler with a spreader nozzle.

Figure 4a. Aluminum sprinkler irrigation components.

This type of improvement in distribution uniformity is critical to irrigated areas such as golf courses, nurseries, and turf grass. Examples of sprinkler types commonly used for agricultural irrigation can be seen in Figures 5a-5h.

Depending on the specific system design, the spacing between mainline hydrants can vary widely between 40 feet to more than 200 feet. Closer hydrant spacings are often used for aluminum mainline, with wider hydrant spacings often employed for buried PVC mainlines to save installation and material costs. Swing pipes are used with wider hydrant spacings to accommodate the offset distances between lateral line sets.

Lengths of aluminum mainline also vary but—from a practical standpoint of portability, weight, and legal road transfer—are generally limited to between 20 feet to 40 feet with a maximum of 50 feet in length. Aluminum mainlines may also use wider hydrant spacings of up to 150 feet to 240 feet with 20-foot to 40-foot blank pipe sections comprising the unused distance between hydrants. The length of individual aluminum lateral lines is generally made to coincide with sprinkler spacings, with a range of 20 feet to 40 feet most commonly used.

Figure 4b. Aluminum main and lateral line components.

Components for typical aluminum irrigation systems are shown in Figures 4a and 4b. Generally, aluminum mainline and lateral lines use cast aluminum couplings pressed into aluminum tubes with the joint typically good for pressures up to 150 psi.

As seen in Figure 4b2, many couplings are cast with an apron to evenly facilitate and spread the pipe’s drainage from the joint following a set.

Gaskets, shown in Figure 4b5, use a chevron lip to provide a compression seal to the pipe as the pressure behind the gasket pushes the lip of the gasket out to and against the pipe. Lateral gaskets are designed to be either self or slow draining and relax once the line pressure falls to approximately 5-10 psi, to provide lighter pipe sections in order to enable faster disassembly and transfer to the next set for portable laterals or permanently set for solid set applications. In some cases, sand will settle in laterals, making the self-draining operation between sets even more desirable.

Figure 5. Various styles of agricultural sprinklers.

Aluminum mainline gaskets are almost always designed to not permit low pressure drainage and keep the water within the pipe between sets. This not only conserves water but allows the system to reach operating pressure faster.

A wheelline or side roll system is an irrigation system that falls between and with attributes of a portable and semipermanent system. It is a semi-permanent system since the wheelline, analogous to a lateral, stays assembled between sets and is moved in one section. It is also a portable system, since it can be completely disassembled and relocated to another field, as with hand lines.

A wheelline, shown in Figures 6a–6c, is an assembled 4-inch or 5-inch aluminum lateral that is designed to physically roll between one irrigation set to the next set. Due to the transmitted twisting force or torque that must be applied to the aluminum tubing to cause the wheelline to roll, aluminum tubing used for wheellines must be able to withstand this force. This is why aluminum tubing used for wheellines is constructed using thicker wall tubing called torque tube.

Figure 6a. Wheelline components

Wheellines are supported by steel wheels provided with cleats to bite into wet ground; sprinklers are placed between the wheels. The wheels do not readily sink as the ground contact pressure is very low. Common wheel diameters include 57 inches, 64 inches, and 76 inches. Selection of the wheel diameter is primarily predicated on the degree of crop clearance needed.

Although most wheellines are equipped with levelers to maintain vertical sprinkler alignment, the wheels are designed for universal rotations to permit sprinklers to turn into an upright position over a fixed travel distance—57-inch and 76-inch wheels are designed for 60-foot moves while 64-inch wheels are designed for 50-foot moves.

Figure 6b. Typical wheelline assembly and components.

Wheellines are powered by a 5-7 HP gas or diesel engine positioned on a mover, generally located in the center of the span (center drive), although end drive can be used in certain circumstances.

Although most wheellines are securely bolted together at each joint using a lug-style coupling with four bolts, they can be equipped with ring-lock bands to permit rapid disassembly for relocation to other fields or sites or to shorten the line length on tapered fields. This style of wheelline is called a quick take-down line.

The initial cost and labor required to operate the system is directly related to the degree of desired portability of a sprinkler system. The more portable a system is designed to be, the lower the initial costs are, but with the impact of higher labor requirements needed to operate the system. Conversely, the permanent or solid set system has the highest initial cost with the lowest labor requirements.

Figure 6c. Wheelline sprinkler and line spacing.

Each person selecting or purchasing a sprinkler system has their own set of circumstances, available labor, and economic conditions to consider. Beyond the obvious considerations of source capacity and water quality, the ultimate selection of an irrigation system is often an economic compromise driven by the individual conditions of initial cost vs. operating costs.

Therefore, no two systems are likely identical or offer the same payback. However, after the type of system has been determined, there are basic common guidelines to follow in developing the details of the system.

Sprinkler System Design

Design and selection of an irrigation system and method is largely comprised of one of three factors: (1) what the client currently uses; (2) needs to fit or that must fit a unique set of circumstances; (3) use based on the designer’s recommendations or cost.

Through the process of factoring all elements associated with the crop’s water needs, soil limitations, available source capacity and quality, labor availability and costs, and other intangibles, a designer can provide an effective and efficient irrigation system that will operate for many years.

The first step generally taken is to collect all available information on the water source and proposed irrigable land, and only thereafter consider viable crop options for the field. Many designers try to conduct this process in a reverse manner by attempting to push a crop onto the wrong soil or with insufficient capacity or inferior water quality (the water supply) for irrigation.

Figure 7. Example wheelline system for 40 acres.

Since the land and water supply are generally considered to be “fixed” components of the design, an initial cursory evaluation of these elements can usually provide enough information to determine which crops can be grown. After all, in most cases the grower’s ultimate goal is to generate enough revenue to recover the costs of developing the irrigation system, plant the crop, and then operate the system to harvest. Only then can the grower hope to return a profit on the harvested crop.

Specific design factors, such as the soil’s water-holding capacity and maximum application rate, are crucial to effective operation of an irrigation system and must be included in the overall plan.

The initial examination of the water supply should include a sustained yield or production test on the source to ensure the source can provide adequate water volume over the entire growing season. For example, in some cases the reliable flow rate or yield from a pumped well may decline after 30 days of continuous pumping, far short of the 4-6 months typically needed to raise an irrigated crop. This may require a total
reappraisal of the source and crop options.

With water quality, many crops may not exhibit a minor or negligible impact from salts, sodium, boron, and other chemical constituents. This can require considering the use of more salt-tolerant plants or applying additional irrigation water needed to leach the offending elements from the soil and crop’s root zone. This leaching requirement places an additional burden on the irrigation system, especially the
water supply, as it must now operate at one set for a longer period than originally intended for the basic crop water needs—disrupting the planned number of operational hours per set as well as the irrigation rotation schedule.

A water quality examination to determine the possible presence of all potentially adverse elements or compounds is definitely warranted on a new irrigation system design or for a new or unknown source planned to be used on an existing system.

Next, the soil intended for use should be carefully scrutinized with an eye toward the structural elements of the soil’s first 1–2 feet of depth. Of particular importance are the type, gradation, texture, and consistency to ascertain the potential for the surface and first horizon to dry and seal from accumulated salts—forcing water to run downhill and away from the intended area of application and potentially starve the remaining horizon depths (layers) from receiving water.

From a hydraulic standpoint, since most crops will develop a root zone depth between 1–3 feet in order to seek out the water they require for sustained growth, it is necessary to have a full understanding of this specific zone, including the soil’s water acceptance rate, land slope, water-holding capacity, and drainage characteristics. Many of these same evaluation criteria are outlined in this series and should be considered before committing to planting a crop.

Once the client and designer have determined the water supply and intended irrigable land issues are satisfied, the client can proceed to selecting or confirming the planted crop. While a permanent or semi-permanent irrigation system is under design, adequate flexibility in the system’s design should be implemented and potential crop options should be discussed.

This means designing the system to permit variances in crop variety and rotation, multiple or mixed crops or plants on a single parcel such as those found in nurseries, and possibly revising or changing the crop for differing water demands or limited source.

The degree of flexibility will depend on factors usually not considered such as source production reliability and alternatives, water quality dynamics, labor availability, and costs. A good irrigation system designer will want to assist the client in adjusting the design, if necessary, to fit these possible future conditions.

Selection of the crop should consider aspects of water and salinity stress and tolerance, need for high uniformity of water application such as needed in nurseries, as well as fitting the previously determined limitations of source capacity, water quality, and soil characteristics.

From this juncture, the designer will apply the selected crop’s water requirements and root depth to the soil’s maximum water-holding capacity and application rate. The water-holding capacity within the crop’s root zones is particularly important since this retained water is what the crop uses to sustain itself and continue growth between irrigation applications.

This simply means that a soil with 2 inches of water-holding capacity per foot used to grow a crop with a 3-foot effective root zone depth can theoretically provide up to 2 inches/foot × 3 feet = 6 inches of available water to the crop. If a crop uses 0.30 of an inch of consumptive use water per day, the plant should fundamentally be able to last up to 10 days before starting to stress.

This is obviously an oversimplification, as other factors such as soil drainage, actual percentage of remaining water in the soil, competition from nearby plants, growth stage, and carryover moisture will all play a role in this element. The key is to use the water-holding capacity with the crop’s daily uptake rate and be able to return to irrigate the plant before the plant begins to endure any appreciable stress. Typically, this
period is around 75%–80% of the maximum time span.

Generally, the first step that should be performed in the actual system design is to work with the fixed design elements by selecting the sprinkler types and nozzles to fit the soil’s maximum application rate by factoring the application area or length and available source capacity.

Thus, if the maximum application rate for a given soil is 0.25 inches per hour, source capacity is 300 GPM, and the field acreage is 23 square acres, the width and length are considered to both be 1000 feet. This, along with the intended sprinkler and lateral spacing, will control the basic sprinkler discharge rate.

Referring to the formula in May’s column of Engineering Your Business and initially assuming an even 40-foot sprinkler and lateral spacing of 50 feet to evenly divide into 1000 feet:

Average application rate in inches/hour = 96.3 × GPM of sprinklerSprinkler spacing on laterals (ft) × Lateral spacings on mainline (ft) Assuming a standard sprinkler spacing of 40 feet and lateral spacing of 50 feet yields: Average application rate: 0.25 inches/hour = 96.3 × GPM of sprinkler40 feet × 50 fee = 50096.3

= 5.19 GPM/sprinkler

Next, referring to the upper portion of this month’s Table 1, the designer can select the nozzle bore and operating pressure or the one desired to match the application rate vs. the sprinkler and lateral spacing shown on the lower table. I would select a 5/32-inch nozzle at 50 psi, which equals 5 GPM per sprinkler at 0.22 inch per hour, 12% below the maximum rate.

Conversely, the next equation can be used to compute the nozzle discharge rate for a given pressure. Almost all versions of sprinkler irrigation utilize a specific diameter or area of round, rectangular, or slotted opening (nozzle that delivers a flow rate against a value of applied head, i.e., pressure). For sprinkler nozzles with the circular, smooth bore nozzles, the following equation is used to estimate the theoretical flow rate:

Flow rate from nozzle in GPM = 28.9 × D2 × √P
where:
D = Nozzle bore diameter (inches)
P = Pressure at nozzle bore (psi)

Example: What is the theoretical discharge rate from a 5/32-inch nozzle at 60 psi of pressure?

Solution: GPM = 28.9 × (5/32 inch)2 × √60 psi = 28.9 × (0.15625)2 × √60 = 28.9 × 0.0244 × 7.7459 = 5.46 GPM

The designer can proceed to the next step, which is to verify the initial sprinkler and lateral line spacing based on rectangular or triangular patterns, wind speed, and nozzle pressures.

Historically, spacings were governed primarily from prevailing wind speeds and direction, but low-pressure systems are a recent introduction. Primarily intended for center pivot applications, the low energy precision application (LEPA) irrigation concept was developed primarily to allow irrigators in arid and semi-arid areas to maximize the use of their total water resource and significantly increase irrigation efficiencies.

It was particularly targeted to those areas experiencing declines in water availability due to dropping water tables, dwindling surface supplies, or supply decline from other socio-economic reasons. Table 2a reflects recommended sprinkler and lateral spacings for higher pressure systems between 40 to 75 psi while Tables 2b and 2c reflect spacings for lower pressure systems up to 40 psi.

We will wrap up this discussion on smaller sprinkler systems with a design example next month 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 epbpe@juno.com.