Part 9, Micro-Irrigation, Drip and Trickle Irrigation
By Ed Butts, PE, CPI
I trust everyone enjoyed a safe and sane holiday season and is ready to return to work. So with that, here we go!
In this month’s edition of Engineering Your Business, we will continue our ongoing series on the fundamentals of irrigation with a focus on micro-irrigation techniques, specifically drip and trickle irrigation techniques, fundamentals, and design.
In general, micro-irrigation systems distribute water uniformly to a crop or plant using low-volume, low-pressure calibrated devices that control the rate of water output. Water can be slowly applied over the plant using micro-sprinklers (as is done with sprinkler irrigation), or at or below ground level with drip irrigation. Examples of such devices include drip emitters, drip tape, and micro-sprinklers as shown in Figure 1.
Some advantages of micro-irrigation:
- Water conservation as it uses one-third to one-half less water compared to other methods, allowing use on more limited or restricted water sources or greater acreage with the same flow
- Since lower pressure and flows are used, it is far more energy efficient than other pressurized methods
- High application efficiency with drip irrigation, usually between 80% to 90%
- High coefficient of uniformity on well-designed drip systems, up to 95%
- Fewer weed issues since systems do not generally wet the entire ground or application surface
- Can effectively apply water to diverse crop types including row crops, orchards, berries, and vineyards
- Can operate on virtually all soil types and slopes
- Area between rows remains drier, facilitating spraying, harvesting, and other cultural operations
- Can also be used for fertigation and chemigation purposes
- Reduced labor requirements compared to other methods, especially portable sprinkler irrigation
- Can be easily and fully automated, which lowers labor costs.
There are many variations of micro-irrigation, but they all typically fall into one of two general categories we will outline: micro-sprinklers and drip (or trickle) irrigation.
Introduction to Micro-Sprinklers
Micro-sprinklers are a broad descriptive term encompassing many types of small sprinkler heads including micro-jets, bubblers, stream sprays (Figure 2a), spinners, and mini- and micro-sprayers.
Like drip emitters, micro-sprinklers operate at low pressure, but designed for areas where drip irrigation is not advisable or practical for the need to keep plant foliage constantly moist or when overhead watering is required. Micro-sprinklers and sprayers are rated by flowrate; wetted diameter; strip patterns such as full or part circle, flat, circular, or square; and the spray method including mist, spinner, or streams.
Mist heads are low-flowrate sprinklers that create a fine mist with a low application rate and cover a small, more concentrated area than other micro-sprinklers. Spinner heads resemble impact sprinklers as they typically spray water throughout a circular area and are driven by rotary action. Stream heads disperse water through small individual fingers of water in the selected pattern and coverage.
Like conventional sprinklers, micro-sprinklers and sprayers are available in 360°, 180°, and 90° coverage and strip patterns that aim left and right—similar to a bow tie.
Micro-sprinklers and micro-sprayers provide low precipitation rates, allowing longer watering time with less runoff on tight soils.
A micro-sprinkler is installed at the base of a tree to water only that tree in most cases. However, in some cases a microsprinkler is installed between two trees.
Water can be provided to micro-sprinklers from above ground polyethylene (poly) pipe, or by PVC pipe risers attached to buried pipe below ground. Deciding which type of irrigation system is best requires a farmer and designer to consider many different aspects—water source, budget, crops grown, watering requirements, setup and convenience, toname a few.
When considering buying a new micro-irrigation system, it is helpful to work with an experienced company that regularly provides design assistance, installation, and maintenance on this particular type of system.
Micro-sprinkler systems are similar to drip irrigation systems except—rather than discharging water at discreet points—the water is sprayed out through a small sprinkler device at much lower rates than conventional sprinkler heads.
Micro-sprinklers are typically made of plastic and available in a multitude of flowrates. But, as opposed to other irrigation sprinklers in which the flowrate is measured in gallons per minute, micro-sprinkler flowrates are generally measured in gallons per hour. Rates typically range between 2 to 50 GPH or 0.03 to 0.83 GPM.
Connection styles vary but generally range from ⅛-inch up to ¾-inch pipe size. They are attached to the irrigation line using stake supports (Figure 2b) in which the sprinkler head is fed by a small section of poly tubing. They are attached to a ground stake permitting relocation to any field position and height press or bayonet-fit styles where the sprinkler is plugged into a small plastic line or attached directly to a rigid riser (Figure 2c).
Micro-sprinklers are designed for operation at much lower pressure than conventional sprinklers—20 to 30 psi the generally recognized maximum pressure range. When used on domestic or commercial water systems, a pressure regulator (pressure-reducing) valve is needed to lower pressure from the water system to the irrigation system.
One advantage of micro-sprinklers compared to drip irrigation is that they disperse water over a larger surface area, with 3 to 10 feet of diameter the most typical (Figure 2d). This is especially advantageous on sandier soils where water applied from a drip emitter tends to move vertically downward rather than laterally, which can cause insufficient volume within the root zone needing irrigation.
For this reason, micro-sprinklers are used extensively on citrus crops where sandy soils are prevalent.
Micro-sprinklers may also be advantageous when water quality is an ongoing concern. Because they have larger openings than drip emitters, micro-sprinklers tend to be less prone to clogging, although their relatively small port size still requires inline screened or cartridge filtration to remove particle sizes of roughly 80 mesh and larger. Since the water is sprayed above ground, a farmer can more easily detect when there is a problem using visual inspection.
Micro-sprinklers also include devices called adjustable micro-spray heads (Figure 2e), which spray water over a smaller area but are adjustable to control pattern and coverage. These are generally used on tree and vine crops with wider root zones. In addition to trees and vineyards, micro-sprinklers have been used on pecans and a few other orchard crops such as peaches.
Since they must be installed above ground, they may be more prone to physical damage than some types of drip irrigation systems, especially for permanent set systems.
Introduction to Drip Irrigation
Drip irrigation is one of the most efficient types of agricultural and ornamental irrigation systems in use today. Also commonly known as trickle or low-volume irrigation, water is rationed and applied to the plants at the root zone as they need it (Figure 3a).
This reduces or eliminates wind drift and evaporation losses, particularly on hot or windy days, and enables the grower to only water the desired plants and not the row alleys, non-planted areas, or roadways. Therefore, weed control is easier and workers can conduct fieldwork while the irrigation system is operating.
Drip irrigation offers an excellent alternative to sprinkler or micro-sprinkler irrigation for many trees, vegetable and small fruit crops, and it is popular on many vineyards (Figure 3b).
Drip irrigation systems typically use up to 30%-50% less water than sprinkler systems on comparable acreage. The ability for continuous operation at low flowrates and operating pressures makes the system ideal for irrigating small acreages using a conventional domestic water system. This allows the grower to irrigate at a lower cost and use less source capacity and smaller pumps than most irrigation systems.
Pipes measuring 1 to 2 inches can be used depending on the system design and deliver as little as 5 to 30 GPM. The irrigation pumping requirement then drops from the normal range of 4 to 7 GPM per acre at 50-70 psi discharge pressure—typical for conventional impact sprinklers—down to 2 to 5 GPM per acre at 5-30 psi discharge pressure for drip irrigation systems.
Thus, a 15 GPM capacity water well that was solely dedicated to supplying three to four impact sprinklers may now be used to drip irrigate 2 to 4 acres of vegetables or small fruits with enough reserve capacity to meet the normal household demands.
As a rule of thumb, 1 inch of water applied over one acre = 6,000 to 13,500 gallons depending on the row and emitter spacing.
With proper installation and use, there are many advantages with using this method of irrigation. However, there are also certain issues that may arise if and when used in improper applications.
Drip irrigation involves small-diameter polyethylene (poly) tubing with specialty-designed water release devices placed into or onto the lines. These are called online or inline emitters (Figure 4) and used to apply a specific low-volume rate of water, generally 0.5 to 2 GPH, directly to the root zone of a crop.
Emitters can be individually installed or punched into the tubing by hand in the field to water a specific tree or plant using point-source or online emitters (Figure 5a) or inline emitters (Figure 5b) installed onto drip tubing, which has the emitters generally preinstalled at the factory (or occasionally in the field) on a specific spacing to reduce installation costs. Inline drip emitters are the most popular type and usually simplify the installation for drip irrigation systems.
The emitter’s rate of flow depends on the inlet pressure. With higher pressure the flow will increase, and with lower inlet pressure it will decrease. Nominal flowrates of most drip emitters are designed for 0.5, 1, and 2 GPH at a 15 psi inlet pressure.
Many emitters are pressure-compensating; they adjust the flowrate to maintain emitter flow uniformity by compensating for pressure differences due to friction or changes in elevation. Pressure-compensating emitters typically use a labyrinth where the water must travel around a maze that progressively destroys the excessive pressure energy.
Mini-inline emitters have a barbed inlet and outlet for field installation, as seen in Figure 5b, and are used with ¼-inch micro-tubing.
Multiple drip emitters can be placed on the same micro-tubing line but at different spacing intervals in series and wound through vegetation or circled around larger plants to fit the application. These emitters are ideal for anywhere a consecutive run of drip emitters is needed such as in pots, baskets, planter boxes, vegetable gardens, and for low-pressure gravity feed systems.
The selection between the two styles often comes down to plant spacing. When plant spacings are irregular or greater than 3 feet on center, using online drip emitters may be the preferred product.
Online drip emitter outlets are usually punched into the tubing near the plant and installed at or near the plant’s root zone. This helps to eliminate wasteful irrigation between plants. This method also provides greater flexibility for emitter placement since it is equally as important to consider the hydraulic limitations when installing emitters online.
Drip systems can be installed above ground or buried to reduce damage to the tubing. Drip tape is a type of drip irrigation which uses drip emitters that are installed in a thin tube that is shipped flat in coils or rolls with emitters spaced from 6 to 12 inches apart. Standard drip tape is available in several thicknesses to match the application, including 6, 8, 10, 15, 20, and 25 mils for lengths up to 500 feet.
Standard flowrate for drip tape is 0.45 GPM at 8-10 psi per 100 feet of row or 0.0045 GPM per foot with 12-inch emitter spacing.
To obtain the flowrate, multiply the linear feet of drip tape by the unit flowrate. Therefore, 5000 feet of drip tape × 0.0045 GPM per foot = 22.5 GPM flow requirement.
Certain types of drip tape may be capable of higher capacity. For instance, the flowrate of high flow T-tape drip irrigation ribbon with 8-inch emitter spacing at 10 psi is 0.74 GPM per 100 feet, 64% greater than standard drip tape.
Drip tape also requires filtration as water flowing through the drip lines runs through a small turbulent channel to make each emitter drip the same amount of water. Thus, unfiltered water may clog these channels.
Drip tape is usually used to irrigate vegetable crops and gardens but can be buried to irrigate other crops such as cotton or corn. Subsurface drip irrigation uses permanently or temporarily buried dripper-line or drip tape located at or below the plant roots. It is becoming popular for row crop irrigation, especially in areas where water supplies are limited or where recycled (tailwater) water is used for irrigation.
As opposed to many conventional sprinkler irrigation systems, careful study and consideration of all relevant factors such as land topography, soil, water, crop, and climate conditions are needed to determine the most suitable drip irrigation system and components to be used in a specific installation.
When needing to irrigate rows of plants, drip irrigation can be a good solution for certain watering requirements. This type of irrigation applies water close to the ground, fed from waterlines laid in rows near the plants, allowing water to be absorbed into the soil surrounding the plants. The rows of water lines are connected to a pipe manifold at the end of each row, fed by a water pump or gravity in the appropriate
circumstances.
The lines deliver seeping drips through emitters directly onto the soil to water the planted rows. Irrigation rates can be controlled by the port size of emitters that can be selected by crop need, yielding a thorough watering at the roots without having to water the entire planted acreage at the same rate.
Drip Irrigation Benefits and Drawbacks
Drip irrigation provides many benefits when properly designed and adjusted to account for soil type, plant needs, and other concerns and is a preferred choice in certain growing situations. It is highly efficient, stretching irrigation water as far as possible, allowing individual plants to be watered close to the roots. It minimizes the amount of water lost due to evaporation or runoff.
Drip irrigation also eliminates plant damage caused by water being applied directly to the plant and lessens pest damage. Water consumption is kept low and the risk of evaporation inherent with other types of irrigation systems is all but eliminated.
But even with these advantages, there are distinct disadvantages related to water quality resulting in plugging of emitters, limited use with many crops, and the biggest disadvantage—cost.
Advantages of Drip Irrigation
- With proper scheduling, water is applied and used at the optimum growth level of the plant.
- Leaching is reduced or eliminated; fertilizer and nutrient loss is minimized.
- Weeds cannot compete for water; they grow in less numbers.
- Water application and use is optimized so a lower source flow requirement and crop yield is maximum.
- Water and fertilizers can be applied with high efficiency, up to 90%, and result in less waste, up to 50% less.
- Operating cost is minimum on automatic systems.
- Less or no topsoil erosion occurs.
- Soil infiltration capacity is maintained or even increased.
- Fertilizers and groundwater are not mixed, lessening potential for groundwater contamination.
- Seed germination is improved.
- Recycled water can be safely used with appropriate treatment.
- Fields do not need to be level when using pressure-compensating devices.
- Water can be applied onto irregularly shaped or variable length fields.
- Energy cost is reduced as the system typically uses up to 75% less pressure than other irrigation methods.
Disadvantages of Drip Irrigation
- Drip irrigation is one of the most expensive methods on a per acre basis, $500 to $1500 per acre.
- The method requires special technical knowledge and maintenance for successful management.
- Inferior water quality such as rust, silt, or sand can cause plugging of emitters and increased maintenance.
- When used in heavier soils, problems of flow and water blockages can occur.
- Plants and roots are often unable to receive proper nutrition, except in a very limited area.
- The method is not suitable for every crop or direct high-pressure service.
- Utmost care and line flushing must be taken to maintain emitter openings, as soil or inferior water quality may block the ports at any time, which may prevent water dripping from the emitters.
- Animals may cause damage to exposed branch pipelines and drip lines.
- Removal and disposal of drip lines and related components may be difficult and expensive.
Hydraulic Design and System Components
The hydraulic design of a drip irrigation system begins with the source. Whether a well, spring, or surface water, the safe and available capacity of the source determines the design flow for the drip system and whether or not zoned automatic or manual stations are used to distribute the available flow.
This is extremely important for residential water systems with limited yield that must compete with domestic fixtures or other demands for water. The hydraulic and piping layout will depend on the specific system design (see Figure 6).
Mainlines are typically sized for the maximum capacity of the system and will usually range in size for smaller systems from 1 to 2 inches, with buried PVC being the predominant pipe type. Larger acreage drip irrigation systems can use mainline sizes between 4 inches up to 12 inches.
Flow estimation is conducted using a typical value of 3 to 4 GPM per acre. Therefore, depending on the number and spacing of emitters, a 100-acre drip irrigation system would be estimated to require about 300-400 GPM of source and mainline capacity.
Water quality can play a huge role in the success of a drip irrigation system. Inferior water quality including rust (iron) or manganese, or physical issues such as grit, silt, or sand, can cause plugging of emitters and drip lines, calling for frequent flushing and chemical treatment. If required, primary filtration and chemical treatment are incorporated into the system after the source pump and on the mainline to filter all source water heading to the system.
Larger drip systems often use centrifugal sand separators to remove larger sand and debris particles of 74 microns (200 mesh) and greater and granular media (sand) filtration to remove smaller particulate sizes down to 10-20 microns. Occasionally, both methods are used together with a sand separator preceding a media or screened filter. Inline passive screens are generally used for submain and smaller system filtration.
The method of treatment and filtration size will vary with the quality of the source. However, chlorination is often employed to lower iron plugging with 200 mesh (74 microns) screens a fairly typical size for submains and 80-100 mesh often used for smaller mainline filters.
Submains distribute water from the mainline to the separate laterals and typically use ½-inch or ¾-inch PVC or poly tubing. They are often equipped with secondary filtration, pressure-regulating valves, and solenoid control valves for automatically controlled zones. Submains measuring ½-inch are typically limited to flows of 3 GPM or less and lengths of 1200 feet or less, while ¾-inch submains are limited to 5 GPM or less with lengths not exceeding 1500 feet.
Manifolds are used to attach the lateral lines to the submains. Manifolds can consist of a simple piping and fitting arrangement where all laterals are routed from a single location or a pipe network with each separate lateral feeding from the pipe. Lateral lines are fed from the manifold and generally made from short lengths of ¼-inch poly (spaghetti) tubing with the installed online or inline emitters used for watering of plants.
Air release and vacuum prevention control is critical for drip systems as trapped air can drastically reduce or disrupt the carrying capacity of a pipeline or tubing, and vacuum can collapse soft tubing, rendering it useless or possibly damaging it beyond salvage.
When a gravity source is used for pressure generation, the pressure is determined by the drop of elevation with every 2.31 feet of fall equal to 1 psi.
The capacity of a drip system is determined by the number of emitters that will be in simultaneous operation at any given time. For example, a 500-foot aggregate length of lateral line with 1 GPH emitters spaced 4 inches apart will total 500 feet/0.33 feet (4 inches/12 inches per foot) × 1 GPH = 1501 GPH/60 or 25 GPM.
The head design adheres to most other hydraulic designs with the primary elements of well lift, main, submain, and lateral friction loss, elevation gains or falls, emitter pressure (usually 8-15 psi), and filter head loss included. Most drip irrigation systems end up operating at a pressure of 50 psi or less, and pressure regulators are usually needed to lower pressure to submains or lateral lines to 10 to 15 psi.
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This concludes this month’s column. Next month, we will continue this series with an overview on ornamental and turf irrigation practices.
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.