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Microirrigation
Dean E. Eisenhauer, Derrel L. Martin, Derek M. Heeren, Glenn J. Hoffman
Pages 297-319 (doi: 10.13031/ISM.2021.14) in Irrigation Systems Management. ,
Abstract. See https://www.asabe.org/ISM for a PDF file of this entire textbook at no cost.
14.1 Introduction
Microirrigation is a rapidly increasing method of irrigation, particularly for high value crops like vegetables and fruit and nut trees. A paramount question for a producer considering whether to invest in an expensive microirrigation system is whether or not the increase in crop production will be sufficient to pay for the system. A second concern is, Can the system be designed to filter the irrigation water to prevent the small emitters from clogging? Another important decision is which emitters, from the large array available, are appropriate for the intended purpose. Solving these issues will provide an excellent irrigation system for decades.
Microirrigation systems deliver water at low flow rates through various types of water applicators by a distribution system located on the soil surface, beneath the surface, or suspended above the ground. Water is applied as drops, tiny streams, or spray, through emitters, sprayers, or porous tubing and then flows through the soil by capillarity and gravity. Water pressure within the delivery lines is reduced by the design of the applicator to create a low discharge. Microirrigation is characterized by water being applied: (1) at low rates, (2) over prolonged periods of time, (3) at frequent intervals, (4) near or into the plant root zone, and (5) at relatively low pressure.
Most crops are adaptable to microirrigation. However, because the initial cost of these systems is high, microirrigation is best suited for high-valued crops, expensive land, or where environmental concerns are significant. Microirrigation systems are found on all soil types. These systems are particularly useful on very sandy and rocky soils that have a low water holding capacity or on salt-affected soils. Microirrigation is also excellent on steeply sloping land or where evaporation from the soil surface is a concern.
Potential advantages of microirrigation over other irrigation systems include: increased beneficial use of available water; enhanced plant growth, quality, and yield; reduced salinity hazards to plants; improved application of fertilizer and other chemicals; limited weed growth; decreased energy requirements; utilization of odd shaped areas; and improved cultural practices. Potential disadvantages include high initial costs, persistent maintenance requirements, restricted plant root development, and salt accumulation near plants (Bucks et al., 1982).
Microirrigation systems can be highly efficient and typically operate over prolonged periods of time; thus, low to moderate discharging water supplies can be utilized. This system offers maximum flexibility in chemigation. Frequent or nearly continuous application of plant nutrients, insecticides, fungicides, or other chemical amendments along with the irrigation water is feasible, and in most cases, beneficial for crop production. The low water discharge rates dictate a water applicator design with small openings; this can lead to clogging problems. Solutions to clogging include emitters that require less maintenance, adequate filtration of the water supply, and chemical treatment of the water.
14.2 History and Impact
Whereas surface irrigation by gravity began about 8,000 years ago, microirrigation is a relatively new concept. In 1913, E.B. House, at Colorado State University, experimented with subsurface trickle irrigation without raising the water table in the process, but concluded that it was too expensive. With the development of plastics during and following World War II, the idea of using plastic for irrigation pipe became plausible. The discovery of low-density polyethylene pipe in 1948 provided a suitable, economical material for drip irrigation. In the mid-1950s, an irrigation manufacturing firm in New York began supplying polyethylene tubing to water plants in greenhouses.
Publications on modern day trickle irrigation systems began to appear in Israel and the United States in the 1960s. Sterling Davis, working in California, installed the first field experiment with a subsurface trickle irrigation system in a lemon orchard in 1963. From 1968 into the 1970s, numerous inventors and companies developed drip irrigation emitters. By the mid-1970s more than 250 different water application devices had been produced.
In 1977, the plastic industry introduced linear, low-density polyethylene. This new plastic was less expensive, had improved strength, was resistant to stress cracking, and was flexible. With the addition of appropriate additives, such as carbon black, antioxidants, and stabilizers, clear plastic could be converted into a black, durable and economical pipe, well suited for microirrigation.
The first survey of microirrigation in the U.S. in 1974 (Irrigation Journal, 1974) reported about 70,000 acres of microirrigation. California had more than half of the national total (41,000 acres). Besides California, only Arizona, Florida, Hawaii, and Texas, had more than 1,000 acres of microirrigation at that time. By 1990 microirrigation had increased to 1.5 million acres. In the 2000 irrigation survey (Irrigation Journal, 2001), California still accounted for more than half of the 3.1 million acres of micro-irrigation in the U.S., but 40 states had at least 1,000 acres.
In 2018, nearly 56 million acres were irrigated in the U.S. (USDA, 2019). Of this total, microirrigation was used on nearly 6 million acres. Nearly 3 million acres were irrigated with surface drip; subsurface drip systems were installed on just over 1 million acres; and 1.5 million acres were irrigated by low-flow micro-sprinklers. Microirrigation was used on over 4 million acres in California. Fourteen other states had microirrigation on over 20,000 acres.
Although microirrigation accounts for less than 11% of the irrigated land in the United States, it is used frequently on high-value crops and for landscaping. The list of microirrigated crops include tree crops of avocado, citrus, stone fruit, nut, olive, coffee, and mango; and row crops like cotton, grape, melon, pineapple, sugar cane, tomato, and strawberry.
A global survey (FAO, 2021) indicates that in 2017, 20 countries had at least 1 million acres of microirrigation. The total area under microirrigation in these countries is about 25 million acres. Countries where the area of microirrigation exceeds that of sprinkler irrigation include Brazil, Chile, Cyprus, Egypt, Iran, Israel, Morocco, Palestine, Peru, Philippines, Poland, Portugal, Tunisia, and Uruguay. Microirrigation in Israel, for example, expanded from 25,000 acres in 1975 to about 400,000 acres in recent years. Estimates in 1991 indicated that microirrigation was used on 70% of the irrigated land in Israel (Stanhill, 1992).
14.3 System Types
Microirrigation includes trickle, drip, subsurface, bubbler, and spray irrigation systems (ASABE, 2019). Trickle and drip irrigation are often considered synonymous and we will refer to it as drip irrigation in this book. Here we differentiate these systems into four major types based on installation method, emitter design, or mode of operation. The four types are:
- Surface drip—water applied slowly through small emitter openings to the soil surface.
- Microspray—water sprayed over the soil surface at relatively low pressure (also called microsprinkler).
- Bubbler—a small stream of water applied to flood the soil surface in localized areas.
- Subsurface drip—water applied through emitter openings below the soil surface.
Examples of these four types of microirrigation are shown in Figure 14.1.
14.3.1 Surface Drip
A surface drip system consists of water applicators designed directly into or attached to lateral lines that are laid on the soil surface (Figure 14.1a). This method is also called trickle irrigation and is the most prevalent type. The maximum application rate for individual emitters is normally less than 3 gallons per hour (gal/hr). For porous tubing and other multiple outlet systems, the maximum application rate is generally less than 1 gallon per hour per foot of lateral. Advantages of surface drip irrigation over other microirrigation systems include ease of installation, inspection, repair, and cleaning of emitters. A major advantage is the ability to check soil surface wetting patterns and to measure individual emitter discharge rates. In some crops like grapes, which are trellised, the laterals are suspended above the soil surface by attachment to the trellis. This eliminates physical damage to the system from human traffic and cultural operations.
(a)  (b)  (c)  (d)  |
| Figure 14.1. Examples of microirrigation systems: (a) surface drip (photo courtesy of Toro); (b) microspray (photo courtesy of Texas International Irrigation); (c) bubbler (photo courtesy of Anchor Commodities); and (d) subsurface drip (photo courtesy of Freddie Lamm, Kansas State University). |
14.3.2 Microspray
Microspray systems apply water to the soil surface as a small spray, jet, fog, or mist (Figure 14.1b). Aerial distribution is a major component in the distribution of water by microspray systems compared to the other types where distribution of the water occurs primarily in the soil. Discharge of individual microspray applicators is typically less than 50 gal/hr. A comparison between the soil wetting pattern for surface drip and microspray systems is given in Figure 14.2. Microspray systems are used frequently to irrigate trees and other widely spaced agricultural crops. Microspray systems are vulnerable to drift and evaporation losses. Major advantages of microspray systems are minimal filtration needs and that maintenance requirements are small. Like drip, microspray systems are sometimes suspended above the soil surface.
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| Figure 14.2. Comparison of one half of the soil wetting pattern between surface drip and microspray irrigation systems. Image adapted from Keller and Bliesner (1990) with permission from Blackburn Press. |
14.3.3 Bubblers
With bubbler irrigation, water is applied to the soil surface in a small stream or fountain from a single discharge point (Figure 14.1c). Application rates are much greater than for drip systems but usually are less than 60 gal/hr. The discharge, applied at one point, normally exceeds the soil's infiltration rate and, therefore, a small basin is normally created by building an earthen dike to control the distribution of water on the soil surface. Advantages of the bubbler system include minimal filtration requirements, very little maintenance or repair, easy visual inspection, and extremely low energy requirements compared to other pressurized systems. However, large-size lateral lines are normally required to minimize pressure loss associated with high discharge rates. In well-designed systems only a few feet of pressure head are required to operate a bubbler system.
14.3.4 Subsurface Drip
A subsurface drip irrigation (SDI) system is basically a surface system that has been buried (Figure 14.1d). Comparison between the soil wetting pattern for surface and subsurface trickle systems is illustrated in Figure 14.3. This method of irrigation is not to be confused with subirrigation where an irrigation is accomplished by raising the water table to the bottom of the crop root zone. Subsurface systems are buried at a depth of a few inches to 18 inches or more. Shallow systems are installed in planting beds that are maintained over time. Deep installations do not require the crops to be placed in the same planting beds. Advantages of SDI include a permanently installed system that can last a decade or more, little interference with cultivation and other cultural practices, and irrigation and harvesting can occur at the same time on crops like melon and tomato. Figure 14.4 shows a schematic of a subsurface drip system with laterals placed in every other row between planting beds and photographs of equipment used to install subsurface drip laterals and a manifold that delivers water from the mainline to the laterals.
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| Figure 14.3. Typical wetting patterns for surface and subsurface drip systems. Image adapted from Coelho and Or (1997) with permission from John Wiley and Sons. |
 (a)  (b)  (c) |
| Figure 14.4. Installation of subsurface drip irrigation laterals: (a) schematic of a typical subsurface drip installation; (b) equipment for installing three subsurface drip laterals; and (c) laterals connected to the manifold. (Photos a and b courtesy of Suat Irmak, Nebraska Extension; photo c courtesy of Steve Melvin, Nebraska Extension.) |
14.4 System Components
The basic components of a microirrigation system are the pump station, control station, mainline, manifold, laterals, and water applicators. Schematics of microirrigation systems are given in Figure 14.5. Typically, filters, chemical injection equipment, and control and monitoring devices are all located in close proximity at a control station. Refer to Chapter 15 for a discussion on chemicals that may be stored and injected into the irrigation system. Automatic controllers that activate the pump or clean the filters, backflow prevention equipment, valves, and injection equipment, are also common features at the control station. Activating signals may be time or water volume based or dependent on sensors placed in the plant root zone.
(a) |
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| Figure 14.5. Basic components of (a) a control station and (b) an entire microirrigation system. (Image courtesy of Toro.) |
Water delivery from the control station proceeds through the main pipeline to the irrigated area. From the mainline, water is distributed through manifolds to laterals where water application occurs. Water discharge, in most systems, is controlled by adjusting the pressure or by regulating the flow at the manifold inlets. The regulators used for this purpose are usually preset for a given pressure or flow rate and are not adjustable.
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1 - Manual control valve 2 - Pump (not needed if water supply is pressurized) 3 - Pressure gauge 4 - Back flow prevention unit 5 - Chemical injection equipment |
6 - Media filter 7 - Strainer or screen filter 8 - Automatic regulating valve |
Figure 14.6. Example of an arrangement at the control station for a microirrigation system. | |
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Figure 14.7. Examples of a gravity water supply system for surface drip irrigation on smallholder farms. (Photos courtesy of iDE, International Development Enterprises.) | |
14.4.1 Control Station
A sketch of a typical control station for microirrigation is given in Figure 14.6. A control station may include a pumping unit and controls, a backflow prevention device, water meters, filtration units, chemigation equipment, flow and pressure control devices, and irrigation controls. Depending upon the water source, a pump (no. 2 in Figure 14.6) may or may not be required. For some low-pressure surface drip irrigation applications, water from a small tank, barrel, or bucket can be delivered by gravity (Figure 14.7) and a pump is not needed. When the water source is an open body of water or a well, a pump may be at the control station or at some distance. When the water source is adjacent to the control station, the pump and its controls will typically be part of the control station (no. 2 in Figure 14.6). If the pressure is excessive, a pressure regulator or flow control valve will be required (no. 10 in Figure 14.6).
When an irrigation system is connected to a potential source of drinking water or when chemicals are to be injected into the irrigation water, a back flow prevention unit (no. 4 in Figure 14.6) is required. This device prevents any water and chemicals from flowing back into the water supply (see Chapter 15 for more detail).
Common at the control station is equipment for injecting chemicals into the mainline. Chemicals typically injected are for fertilization, water treatment, or pest control. Chemicals are injected into the irrigation water before the primary filter.
Filters are installed at the control station to prevent the passage of unwanted particles into the system. Filtration of water from municipal sources frequently only require precautionary filtration that is frequently accomplished by a screen filter. Water taken directly from wells or open bodies of water generally require primary and secondary filtration. The primary filter should be located after the pump and chemical injection equipment. Secondary filters are installed downstream from the primary filter to remove particles which may pass through the primary filter during normal or cleaning operations. The most popular filtration system for microirrigation is a primary media filter (no. 6 Figure 14.6) followed by a secondary screen filter (no. 7 in Figure 14.6). See Figure 14.10 for typical filtration systems.
The last major item at the control station is a water measuring meter (no. 9 in Figure 14.6). Types of water meters used for irrigation systems are described in Chapter 3.
Items in Figure 14.6 labeled as no. 3 indicate possible locations for pressure gauges. For well-designed control stations pressure gauges are placed at nearly all of these locations. Measures of pressure are particularly important on each side of the filters. A significant decrease in pressure on the discharge side of a filter compared to before the filter indicates that the filter is becoming clogged and cleaning is required. Of course, the pressure gauge reading as water leaves the control station will provide assurance that the pressure is appropriate for proper operation of the microirrigation system.
Also important at the control station are strategically located valves. The location of valves depends upon the complexity of the control station and the desire of the irrigator to shut down the system to clean, repair, or replace components.
14.4.2 Mainline and Manifolds
The mainlines and manifolds for example microirrigation systems are illustrated in Figure 14.5. Actual systems may be different and far more complex than the illustration. The mainline carries water from the control station to manifolds which distribute the water to each lateral. Normally, there are no fixtures along the mainline other than elbows or tees. If the system is large and there are a number of manifolds, flow control valves and shutoff valves are located at the head end of each manifold. These fixtures assure the correct flow enters each manifold and accommodates manual or automatic control of irrigation water. The diameter of mainlines and manifolds are normally large, in the range of 1 to 6 inches. To determine the friction loss in plastic pipe, both polyvinyl chloride (PVC) and polyethylene (PE), refer to Chapter 8. The same procedures can be used to determine the appropriate pipe diameter for mainlines and manifolds for microirrigation as described in Chapter 8.
14.4.3 Laterals
Typically, emitters are spaced systematically along laterals in microirrigation systems. For row crops where plants are spaced uniformly in short intervals, emitters are spaced uniformly one to a few feet apart. Many laterals are currently manufactured with the emitters within the lateral itself as for single- or dual-chamber emitters (Section 14.4.4). For widely-spaced crops, like trees, emitters may be closely spaced near the tree with no emitters positioned between tree canopies. As the trees grow, additional emitters may be added. These emitter patterns apply for both surface and subsurface systems. Microspray and bubbler type emitters are also common for widely-spaced crops.
| Table 14.1. Friction loss for small diameter PE pipe based upon the Darcy-Weisbach equation for pipe with an e (absolute roughness) = 0.0005 in. | |||||
| Nominal Size (in) | 0.5 | 0.75 | 1.0 | 1.5 | |
| Inside Pipe Diameter (in) | 0.622 | 0.824 | 1.049 | 1.61 | |
| Flow Rate, Q (gpm) | Friction Loss (psi/100 ft) | ||||
| 0.5 | 0.13 | 0.03 | |||
| 1.0 | 0.56 | 0.15 | |||
| 1.5 | 1.13 | 0.30 | 0.09 | ||
| 2.0 | 1.86 | 0.49 | 0.15 | ||
| 2.5 | 2.76 | 0.72 | 0.23 | 0.03 | |
| 3.0 | 3.81 | 0.99 | 0.31 | 0.04 | |
| 4.0 | 6.38 | 1.64 | 0.51 | 0.07 | |
| 5.0 | 9.55 | 2.43 | 0.76 | 0.10 | |
| 6.0 | 13.3 | 3.37 | 1.05 | 0.14 | |
| 7.0 | 4.45 | 1.38 | 0.18 | ||
| 8.0 | 5.67 | 1.76 | 0.22 | ||
| 9.0 | 7.02 | 2.17 | 0.28 | ||
| 10 | 2.62 | 0.33 | |||
| 15 | 5.47 | 0.68 | |||
| 20 | 9.26 | 1.14 | |||
| 25 | 1.71 | ||||
Regardless of the emitter type, the flow within the lateral decreases from the beginning of the lateral to zero at the lateral terminus. If the laterals are fairly long, it may be advantageous to decrease the size of the lateral as the flow decreases along the lateral. In most microirrigation systems, however, the laterals are relatively short so only one small-diameter lateral is used. As in Chapter 8, where friction loss and pipe size were determined for various types of irrigation pipe, the same procedure can be used for microirrigation laterals. In Chapter 8, the Hazen-William equation was used to calculate friction loss in pipes. For small-diameter, smooth-walled pipe used in microirrigation (e.g., laterals), the Hazen-Williams equation with a C value of 150 underestimates the friction loss (Keller and Bliesner, 1990). They recommend the Darcy-Weisbach equation for microirrigation laterals as was used in Table 14.1.
The flow rate within the lateral decreases as the flow moves past water applicators; thus, the friction loss changes. When the lateral has uniformly spaced and uniformly discharging outlets, the friction loss can be estimated by:
PL= F L Pf (14.1)
where: PL= pressure loss due to friction for laterals with uniformly spaced and uniformly discharging outlets,
F = multiple outlet reduction factor (Table 8.3),
L = lateral length, and
Pf = pressure loss per unit length of a conveyance pipe without outlets.
For a pipe with no outlets, F = 1.0. There is a slight difference between values of F depending on the distance down the lateral from the manifold to the first outlet. If the spacing between the outlets is s, then the outlet factor is higher when the first outlet is a distance s from the manifold compared to a distance of one-half s for about the first 20 outlets on a lateral. Typical values of F are given in Table 8.3.
There are also minor pressure losses in laterals with emitters caused by flow constrictions for in-line emitters and by barbs for emitters inserted in the tubing. Keller and Bliesner (1990) present a method for estimating losses caused by in-line emitters and emitters with barbed insertions. Their method adds to the effective length of the pipe.
14.4.4 Water Applicators
For microirrigation, adequate pressure must be maintained in the pipelines to overcome friction losses and elevation differences to distribute water throughout the field. Once the point of application is reached, the difference in pressure inside the lateral and the atmosphere must be dissipated by a water applicator device.
There are three common types of applicators: emitters, line-source tubing (drip tape and porous tubing), and sprayers. Many different emitter designs have been devised and manufactured with the requirements that the emitters be inexpensive and reliable. Emitters are designed to dissipate pressure while discharging small uniform flows of water at a constant rate. They are often classified according to the mechanism used to dissipate pressure. Long-path emitters have a long capillary-size tube or channel to dissipate pressure. Orifice emitters rely on an individual opening or a series of openings. Vortex emitters dissipate pressure by creating a whirling or circular motion that tends to form a cavity or vacuum in the center of the swirling.
Many emitters are designed with additional features. Some are designed to provide a flushing flow of water to clean the discharge opening every time the system is turned on. Continuous-flushing emitters permit the passage of relatively large particles while operating. Another special feature is pressure-compensating emitters which discharge water at a constant rate over a wide range of pressure. Some emitters have multiple outlets and supply water through small diameter auxiliary tubing at various points. Examples of various types of emitters are illustrated in Figure 14.8.
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| Figure 14.8. Example of emitters with various designs and features for line-source microirrigation laterals (images courtesy of Toro). |
There are three types of line-source tubing, all of which are normally less than 1-inch in diameter. The wall thickness of tubing is available from 0.004 to 0.025 inches. The thin wall tubing is frequently discarded after each crop. The most common wall thickness is 0.008 to 0.010 inches. Recommended operating pressures depend on wall thickness. Several manufacturers recommend a maximum continuous operating pressure of 15 pounds per square inch (psi) for tube walls that are 0.015 inches thick, and 8 to 12 psi for thinner walls. Porous wall tubing is constructed of porous material with pores of capillary size that ooze water when under pressure. Single chamber tubing has orifices punched through the hose wall or emitters fabricated or inserted at fixed intervals along the hose. Double chamber tubing has both a main and an auxiliary passage. Widely spaced inner orifices are punched through the wall common to both passages. Typically, 3 to 6 exit orifices are punched at short intervals in the outer wall of the auxiliary passage for each inner orifice. Sketches of line-source tubing are provided in Figure 14.8. Early developments included double-chamber tubing and porous-wall tubing (illustrated in Figure 14.9).
Sprayers are designed to discharge a small spray of water to cover an area of 10 to 100 ft2. Aerosol emitters, foggers, spitters, misters, microsprayers, or miniature sprayers are examples of spray devices. Ideally, sprayers apply a relatively uniform depth of water throughout its wetted area and should have a low water trajectory angle.
The construction and materials used for water applicators are very important because they are exposed to sunlight, chemicals in and applied with the irrigation water, extremes in environmental conditions, and physical abuse. Emitter performance is a dominant factor in the uniformity of water applications and the life expectancy of the system.
14.5 Preventing Clogs
| Table 14.2. Plugging potential of irrigation water for microirrigation (Bucks et al., 1979). | ||||
| Potential Problem | Unit of Measure | Minor | Moderate | Severe |
| Physical | ||||
| Suspended solids | ppm | <50 | 50–100 | >100 |
| Chemical | ||||
| pH | - | <7 | 7–8 | >8 |
| Salts | ppm | <500 | 500–2000 | >2000 |
| Manganese | ppm | <0.1 | 0.1–1.5 | >1.5 |
| Iron | ppm | <0.1 | 0.1–1.5 | >1.5 |
| Hydrogen sulfide | ppm | <0.5 | 0.5–2.0 | >2.0 |
| Biological | ||||
| Bacteria populations | number/ml | <10,000 | 10,000–50,000 | >50,000 |
Clogging of emitters is one of the major concerns for microirrigation. Obviously, the smaller the orifice or the longer the capillary section, the more prone the emitter is to clogging. Emitters can be clogged by particles, bacterial slimes, algae, water-borne organisms, or precipitation of various chemicals. Filtering to prevent mineral and organic particles from entering the system or chemical injections to prevent mineral precipitation or the growth of slime are the normal management schemes to prevent clogging. If emitters become clogged, water distribution is not uniform and in severe cases, crop loss from water stress occurs. Clogging problems are frequently site specific and economical solutions are not always available. Table 14.2 is a water quality classification scheme to predict potential emitter plugging (Bucks et al., 1979).
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| Figure 14.9. Porous-wall tubing, an example of line-source tubing for microirrigation systems. |
14.5.1 Filtration
Most irrigation water requires filtration for microirrigation. Normally, filtration equipment is located just downstream of the pump at the control station. Domestic water, particularly municipal supplies, are already filtered so the homeowner or proprietor does not normally have to filter the water supply for microirrigation. In rural settings, filtration is almost always required. The filter system commonly used in microirrigation is a media filter followed by a screen filter. Examples of filtration systems for a large and a small field are shown in Figure 14.10. If the irrigation water has a heavy sand load, the water should pass through a sand separator or a settling basin before passing through media and screen filters.
 (a)  (b) |
| Figure 14.10. Typical control station and filtration system of media filters for (a) a subsurface drip system for a large field in Nebraska (photo courtesy of Laszlo Hayde, IHE Delft Institute for Water Education), and (b) a microirrigation system for a small field in India (image from Indiamart, https://www.indiamart.com/proddetail/drip-sprinkler-irrigation-system-20348028048.html). |
Suspended particles that might plug a system can be either inorganic or organic. Algae, bacteria, diatoms, larvae, fish, snails, seeds and other plant parts are the major organic solids while sand and soil particles are the primary inorganic solids. Because a consistently clean water supply is vital, filtration and chemical treatment must be furnished for the worst possible conditions. In a few cases, chemical coagulants are required to control silt, clay, or suspended colloids. Chlorine may be required sometimes to control algae and other organic materials.
Well water is usually low in organic materials, but it can contain sand. Therefore, a screen filter is frequently adequate. Irrigation water may be saline or be chemically unstable thereby producing chemical precipitates. In some cases, water supplies contain chemical constituents that provide nutrients for bacterial growth. For these waters, chemical treatment is required.
The size of particle that can be tolerated by a water applicator should be indicated by the manufacturer because it depends on applicator construction. Typically, the recommendation is to remove all particles larger than one-tenth the diameter of the orifice or flow passage of the emitter. This is necessary because particles may become grouped and bridge the passageway. Many manufacturers recommend removing particles larger than 0.003 to 0.006 inches in diameter.
In addition to the main filtration system, small screen filters should be installed at the inlet to each lateral or manifold as a precaution against plugging. These auxiliary screens prevent debris from entering the system when the main filters are cleaned or if breaks or openings occur in the distribution system.
Fine particles settle out when flow is slow or stops. The clogging that results may not be rapid, but it is inevitable. As a safeguard, either manual or automatic flushing devices should be installed at the end of each lateral. These protective devices are particularly important to clean the system after installation and repair.
Settling basins, ponds, or reservoirs can remove large quantities of sand and silt. They should be long and narrow with water discharged into the basin at one end and removed from the opposite end to provide settling time for the suspended materials. If water remains in the basin for at least 15 minutes, most inorganic particles larger than about 0.003 inches will settle out.
About 98% of the sand particles intercepted by a screen with 0.003-inch openings can be removed by a vortex separator. Centrifugal force is the principal employed by a vortex separator to remove high-density particles from the water. Organic materials, however, cannot be removed by this method because they have low density.
Media filters are used frequently in microirrigation systems. The filter consists of fine gravel and sand of selected sizes placed in graded layers inside a cylindrical tank (Figure 14.10). These filters are very effective in filtering inorganic and organic materials because they can be trapped throughout the depth of the media bed. Long, narrow particles, such as algae and diatoms, are more likely to be caught in the multilayered media bed than on the surface of a screen.
A drop in pressure of 2 to 3 psi occurs from the inlet to the outlet of a clean media filter. As the pores of the media become plugged with contaminants, the pressure drop increases. It is normally recommended that the media filter be flushed to remove the accumulated contaminants when the pressure drop reaches 10 psi. If the water is relatively clean and flushing is not needed frequently, manual flushing may be suitable. Where frequent cleaning is required, automatic flushing can be actuated by a timer or by sensing the pressure differential across the media filter.
Where suitable, screen mesh filters provide a simple and efficient means for filtering. Hole size and the total amount of open area in the screen determine a screen filter's efficiency and operational limits. Screen filters are used to remove fine sand or small amounts of algae. They are commonly used where the water is expected to be clean, i.e., pumped groundwater, municipal supplies, and following other filter systems.
Screen filters differ by their configuration for cleaning. The need for cleaning, as with media filters, is determined by the rate at which the filter clogs. This rate of plugging is normally monitored by the drop in water pressure across the filter. It is customary to clean screen filters whenever the pressure difference between the inlet and outlet to the filter is between 3 and 5 psi. Manual cleaning by opening the filter, removing the screen, and washing it is satisfactory when cleaning is not required frequently. If frequent cleaning is required, an automatic flushing system is normally installed. Back flushing, blow down, and gravity flow are examples of configurations for automatic cleaning. The flow of water is reversed in a backflushing filter to remove the collected materials. A high velocity jet of water is run over the screen to sweep away collected particles without opening the filter for blow down filters. A gravity flow filter functions by discharging the water supply onto and through a large screen before pumping it into the irrigation network. Some gravity flow filters have jets under the screen to lift particles and move them off the screen.
The screening material can be constructed of stainless steel, nylon, polyester, or other noncorrosive materials. A stainless steel screen offers strength. Nylon mesh in some blow down filters flutters during flushing which aids to dislodge collected particles.
The flow rate through a screen filter should not exceed 200 gallons per minute (gpm) per square foot of screen open area. The wire or plastic mesh itself obstructs much of the open area. For example, a screen constructed of stainless steel with 0.003-inch openings has 58% open area. An equivalent nylon mesh with the same size openings has only 24% open area. Thus, it is important to consider the actual open area of a screen when sizing a filter.
The total area of screen (As) needed for a screen filter can be calculated from:
(14.2)
where: Q = flow rate through the filter,
Qm = minimum flow rate permissible per unit area, and
Oa = fraction of open area within the screen.
14.5.2 Precipitation of Dissolved Solids
Dissolved solids are a problem when they precipitate as a solid mineral or serve as a source of nutrients for algae and bacterial slime. Clogging and eventual plugging of water applicators by precipitates and organic deposits are problems that cannot be solved by filtration. Precipitates form inside pipes or emitters as a result of changing pH or temperature, but a major cause of mineral deposits is evaporation of water at the outlet of the water applicators during nonirrigation periods.
Calcium and iron precipitates are a common problem with many well waters. An analysis of the water can indicate if the bicarbonate (the typical source of calcium) or iron concentration is high enough to cause precipitation. A bicarbonate concentration greater than 2 meq/L (120 ppm) coupled with a pH greater than 7.5 is likely to produce calcium deposits. Injecting inexpensive acid to lower the pH to between 5.5 and 7.0 effectively prevents calcium precipitation. Acid treatments at the end of each irrigation or on a periodic basis is frequently practiced to reduce costs. Typically, acid is injected at roughly 0.02 to 0.2% of the system capacity. If more acid than 0.2% is required to lower pH where bicarbonate concentrations are high, it is generally more practical and less expensive to aerate the water and hold it in a reservoir until it reaches chemical equilibrium and the precipitate settles out rather than adding acid.
As little as 0.3 ppm of iron present in the soluble ferrous form in the irrigation water can cause precipitation in a microirrigation system. In the presence of oxygen, the iron oxidizes to the insoluble ferric form which causes a reddish-brown precipitate. If iron is a potential hazard, it should be precipitated deliberately and filtered out before the water enters the microirrigation system. A chemical feeder can be set to provide a measured volume of a chlorine solution to oxidize iron and other organic compounds present. A residual chlorine concentration of 1.0 ppm is normally provided to avoid precipitation. Sodium hyperchlorite is preferred over calcium hyperchlorite as a source of chlorine because of the potential for calcium precipitation. Where iron concentrations are as high as 10 ppm, aeration by a mechanical aerator and sufficient settling time in a reservoir is another practical method of controlling iron.
14.5.3 Organic Materials
Algae and slime created by bacteria can cause severe clogging. Algae is common in almost all surface waters. Small pieces of algae can pass through filters and grow inside a microirrigation system. Since sunlight is required for algae to grow, light must be prevented from entering the system.
Slime is a general term for long filament microorganisms produced primarily by bacteria. The slime acts as a “glue” for suspended particles to combine into larger particles that plug emitters. The more common microorganisms that result in slime problems are airborne, thus, systems using open water supplies are susceptible.
Both algae and slime can be controlled by chlorination. Maintaining a residual chlorine concentration of 1.0 ppm, measured at the far end of the system, usually prevents problems from organic materials. An alternative practice is to inject sufficient chlorine to bring the concentration in the irrigation system to between 10 and 20 ppm during the last 20 minutes of the irrigation cycle.
14.5.4 Flushing and Maintenance
Flushing is an important part of starting up the system after installation and for maintaining performance. After installation or repairs, the system should be flushed to remove any foreign materials. To ensure adequate flushing, valves placed at the ends of all pipelines should be opened momentarily. Flushing provisions, which can be very inexpensive, should also be placed on every lateral.
In addition to flushing periodically, adequate maintenance requires that filters be cleaned routinely to ensure that water applications are uniform and appropriate to meet crop water requirements. In addition to main filters, all secondary filters at inlets to manifolds and laterals must also be cleaned routinely. A water velocity of at least 1 foot per second is recommended to flush fine particles.
In addition to all these precautions, systematic field checks are required to detect malfunctioning emitters. Emitter discharge may be altered by blockage or wear of the emitter parts. Discharge from emitters should be checked periodically to maintain uniform applications.
14.6 Uniformity
The primary objective of a microirrigation system is to supply the prescribed amounts of water and chemicals to each plant at frequent intervals and in small volumes. For maximum uniformity the variation in discharge from the various water applicators must be acceptably low. Uniformity of manufacturing is critical when selecting emitters for a microirrigation system. Uniformity is also crucial in evaluating the performance of the system following installation and periodically during the life of the system.
14.6.1 Emitter Discharge
Flow variation among emitters is caused by differences in hydraulic, manufacturing, and field conditions. The discharge (qe) in gal/hr for most emitters can be described by:
qe = Khx (14.3)
where h is the pressure head in feet at the emitter. The emitter discharge coefficient, K, contains the effects of the coefficient of discharge, emitter geometry, and the acceleration of gravity. The value of x, the emitter discharge exponent, characterizes the type and flow regime of the emitter. Orifice-type emitters are fully turbulent and have an emitter discharge exponent of 0.5. With long path emitters, x = 0.5 for those with fully turbulent flow and 1.0 for laminar flow. An x value of less than 0.5 indicates an emitter that compensates for changes in pressure.
To determine K and x for an emitter, the discharge must be measured at two different operating heads (h1 and h2). The x may be determined analytically from:
(14.4)
The value of x calculated from Equation 14.4 is used to calculate K from Equation 14.3.
It is impossible to manufacture any two items exactly alike. Very small variations in emitter passage size, shape, and surface finish can result in variations in discharge. The amount of variation also depends on emitter design, construction materials, and precision during manufacturing.
The coefficient of manufacturing variation for an emitter, v, is a measure of anticipated variations in the discharge for a sample of new emitters. The value of v should be available from the manufacturer. If not available, it can be determined from the discharge data of a sample set of at least 50 emitters operating at a constant reference pressure by:
(14.5)
where: n = number of emitters being tested,
q = discharge rate of an emitter, and
qa= average emitter discharge rate.
| Table 14.3. Classification for manufacturing variation, v, of emitters (Solomon, 1979). | ||
| Classification Rating | Drip & SprayEmitters | Line-Source Tubing |
Excellent | v < 0.05 | v < 0.1 |
Average | 0.05 < v < 0.07 | 0.1 < v < 0.2 |
Marginal | 0.07 < v < 0.11 | - |
Poor | 0.11 < v < 0.15 | 0.2 < v < 0.3 |
Unacceptable | 0.15 < v | 0.3 < v |
For an emitter having a v of 0.06 and a qaof 1 gal/hr, 95% of the emitters will have a discharge rate between 0.88 and 1.12 gal/hr. As a general guide, manufacturing variability can be classified in accordance with Table 14.3. A lower standard is given for line-source tubing because it is difficult to keep both the variation and price low. Line-source tubing is normally used for row crops which are relatively insensitive to moderate variations in discharge among closely spaced outlets.
 |
| Figure 14.11. Discharge variation resulting from pressure changes for emitters having different discharge exponents. (Image courtesy of Keller and Bliesner, 1990, with permission from Blackburn Press.) |
14.6.2 Discharge Versus Pressure
The relationship between pressure head (h) and discharge (qe) is an important characteristic of emitters (Equation 14.3). Figure 14.11 shows this relationship for various types of emitters. The discharge exponent, x, measures the flatness of the relationship between pressure and discharge. It shows clearly the desirability of an emitter that has a low value of x. Emitters that compensate for changes in pressure have the lowest values of x. Compensating emitters have some physical part that responds to pressure to keep discharge constant. Although having the advantage of compensating for changes in pressure, these emitters are prone to material fatigue and temperature change.
On hilly terrain the design of a highly uniform system is constrained by the sensitivity of the flow from emitters because of pressure differences in the laterals from changes in elevation. Pressure compensating emitters and pressure regulated flow in short laterals provide potential solutions. Even on level fields, the lateral length must be kept reasonably short to avoid excessive differences in pressure along the lateral.
For laminar flow emitters with a value of x near 1.0 the percent variation in pressure head results in about the same variation in discharge. Thus, variations in pressure throughout the system with laminar flow emitters should be held within about ±5% of the desired pressure to maintain water applications within acceptable limits.
For turbulent flow emitters, the change in discharge varies with the square root of pressure head; x is near 0.5. Consequently, to double the flow, the pressure must be increased four times. Thus, the pressure head for systems using turbulent flow emitters can vary up to ± 10% of the
| Table 14.4. Characteristics of various types of emitters (Keller and Bliesner, 1990). | |||
| Emitter Type | Discharge Exponent, x | Coefficient of Manufacturing Variation, v | FlushingAbility |
| Orifice | |||
Vortex/orifice | 0.42 | 0.07 | None |
Multiple flexible orifices | 0.7 | 0.05 | Continuous |
Ball & slotted seat | 0.50 | 0.27 | Automatic |
Compensating ball & slotted seat | 0.25 | 0.09 | Automatic |
Capped orifice sprayers | 0.56 | 0.05 | None |
| Long-Path | |||
Small tube | 0.70 | 0.05 | None |
Spiral path | 0.75 | 0.06 | Manual |
Compensating | 0.40 | 0.05 | None |
Compensating | 0.20 | 0.06 | Automatic |
Tortuous | 0.65 | 0.02 | None |
| Short-Path | |||
Groove & flap | 0.33 | 0.02 | Automatic |
Slot & disc | 0.11 | 0.10 | Automatic |
| Line-Source | |||
Porous pipe | 1.0 | 0.40 | None |
Twin chamber | 0.61 | 0.17 | None |
desired pressure without unacceptable variations in water applications.
Flow compensating emitters provide some degree of flow regulation as pressure changes. When x is between 0.2 and 0.35, some regulation is possible and there is still some flexibility for adjusting the discharge rate. Compensating emitters are valuable on hilly sites where it is impractical to design for uniform pressure along the laterals.
Characteristics of various types of emitters are given in Table 14.4. Refer to Figures 14.8 and 14.9 for examples of the types of emitters described in Table 14.4. The exponent, x, for Equation 14.3 is given in Table 14.4 along with typical values for the manufacturers’ coefficient of variation. The remaining column in Table 14.4 indicates the flushing potential built into each type of emitter.
14.6.3 Emission Uniformity
Emission uniformity can be treated like distribution uniformity, DU, in Chapter 5 and is a measure of the uniformity of emissions from all the water applicators within the entire microirrigation system. For field tests,
(14.6)
where DU is the emission uniformity from a field test, %; qLQ is the average discharge for the lowest one-fourth of the field measured emitter discharges (gal/hr); and qa is the average discharge of all the emitters checked in the field (gal/hr).
The efficiency of an irrigation system is the relation between gross irrigation amounts and the net addition of water to the crop root zone. Distribution uniformity and the various sources of water loss that occur during the operation of the system are the two components of microirrigation efficiency. To estimate the distribution uniformity for a proposed design, the following formula was developed (Karmeli and Keller, 1975):
(14.7)
where DU is design emission uniformity in %, v is the coefficient of manufacturing variation (Table 14.3 for typical values) and n is the number of emitters. The ratio qm/qa expresses the relationship between the minimum (qm) and the average (qa) discharges resulting from pressure variations within the system. The factor (1.0 – 1.27 v/vn) adjusts for the additional nonuniformity caused by anticipated manufacturing variations between individual emitters.
14.7 Management
The success of any irrigation system, particularly microirrigation, depends on management. Irrigating by small quantities frequently is quite different from sprinkler and surface irrigation methods where larger, less frequent applications are normal. With microirrigation, precise information on crop water requirements is required to determine the appropriate irrigation amount. Feedback information on soil water or plant water status is frequently used to schedule irrigations for microirrigation systems.
14.7.1 Wetted Area
A major difference among irrigation systems for agronomic crops is the portion of the soil surface wetted. Most microirrigation systems wet only a portion of the cross-sectional area of the soil profile, as depicted in Figure 14.2. The percent of the surface area wetted, Pw, by microirrigation systems compared to the entire cropped area, depends on the volume and rate of discharge at each application point, the spacing of water applicators, and soil type. No best- or minimum-wetted area percentage has been found, but systems having high Pw values provide more stored water, which is a valuable advantage in the event of system failure. A reasonable design objective for widely-spaced crops such as vines, bushes, and trees is to wet between one-third and two-thirds of the soil surface dedicated to each plant. In regions that receive considerable supplemental rainfall, values of Pw less than one-third are acceptable for fine-textured soils. Maintaining Pw below two-thirds for widely-spaced crops maintains dry strips for cultural practices. In closely-spaced row crops with the laterals in every or every other crop row, Pw approaches full coverage.
Spray emitters wet a larger surface area than drip emitters. They are often used on coarse-textured soils where wetting a large surface area would require a large number of drip emitters. Figure 14.2 shows a comparison of wetting profiles for drip and spray emitters.
14.7.2 Salinity
Microirrigation has potential advantages where the soil or irrigation water is saline. The principal advantage is that with microirrigation the water content of the root zone is maintained high and nearly constant. As a result, the salt concentration of the soil solution is low and steady, and thereby not creating as much salt stress as a system where the soil dries between irrigations with congruent significant increases in salt concentration.
A potential disadvantage is the uneven salt distribution in the soil profile. Refer to Figure 14.2 to see comparable salt profiles for various irrigation methods. This uneven distribution can cause problems if the irrigation system fails and the crop roots begin extracting water from areas of high salt concentration, or if salts that are shallow in the profile are flushed into the root zone by rainfall. When used on annual crops, moving the plant row spacing geometry can cause problems if the salts have not been leached.
14.7.3 Water Requirements
The plant canopies of young or widely-spaced crops shade only a portion of the soil surface area and intercept only a portion of the incoming solar radiation. Conventional estimates of water requirements of young crops assume a portion of the applied water will be lost to nonbeneficial consumptive use. This loss is through evaporation from the wetted soil surface or through transpiration from undesirable vegetation. Most microirrigation systems reduce evaporation losses to a minimum, so transpiration by the crop accounts for practically all of the water consumed.
Assuming that evaporation during application is minimal and no upward flow from groundwater into the root zone, the gross irrigation requirement can be expressed as:
dg= ET + dp+ dr– Pe–?S (14.8)
where dg = gross irrigation requirement,
ET = evapotranspiration,
dp = deep percolation,
dr= runoff
Pe = effective precipitation, and
?S = the change in soil water storage.
All terms are normally expressed in units of depth. The volume equivalent for each term in Equation 14.8 is the product of the irrigated area and each term. Thus, if dg was 2 inches and the irrigated area was 5 acres, the volume of water needed would be 10 acre-inch or 271,540 gallons.
One of the objectives of microirrigation is to maintain soil water content constant. If this is achieved, ?S in Equation 14.8 is zero. For well managed microirrigation systems, runoff should be zero. If salinity is not a hazard, then the irrigation requirement does not need to include water for drainage (deep percolation).
Irrigation scheduling involves two primary decisions: when to irrigate (timing) and how much to apply (amount). Microirrigation inherently implies frequent irrigations. Depending on the system and the sophistication of the controls, irrigation frequency can be from once in several days to multiple times every day. Many commercial systems operate daily or every other day. The operational time for a microirrigation system should not exceed 20 hours per day. In case of repair or maintenance requirements, time is required to catch up. This is particularly critical during periods of peak crop water use.
One of the primary design considerations in microirrigation is determining how many emitters are required to meet the irrigation requirement. The number required can be determined by:
(14.9)
where: n = number of emitters,
dg= applied (gross) depth of irrigation required,
A = irrigated area,
qe= emitter discharge, and
t = application time.
For surface and subsurface drip systems the spacing between emitters can be specified to the manufacturer. Typical drip emitter spacings are from one to several feet. The spacing between emitters will depend upon the spacing between laterals, the irrigation requirement, and water availability. For example, if you wish to drip irrigate a field of tomatoes and the rows are 3 feet apart, you might place a lateral along each crop row or midway between adjacent rows. If you placed a lateral in every row and the irrigation requirement was 0.27 inches per day, you wanted to irrigate 1 hour per day, and you chose an emitter with a discharge of 2 gal/hr, then Equation 14.9 could be used. To determine the area to be irrigated by each emitter solve Equation 14.9 for A as illustrated in Example 14.10.
14.8 Summary
Microirrigation delivers low rates of irrigation through a wide variety of available water applicators. Application can be by surface drip, subsurface drip, microspray, or bubbler. Each type is illustrated and described. The components of a typical microirrigation system includes a control station, a mainline, manifolds, and the lateral lines that supply water to the water applicators. The necessity and selection of equipment at the control station are presented along with the procedures to select the appropriate diameter of the various sections of the pipeline. A major concern with microirrigation is the potential for clogging the emitters. The types of filters normally recommended are described and the types of materials that lead to clogging are discussed. The chapter concludes with procedures to determine the proper number of water applications required for various crop conditions.
Questions
1. What environmental and/or economic factors give microirrigation an advantage when selecting an irrigation system?
2. List the four major types of microirrigation systems and discuss scenarios where each type might be used.
3. What is the present land area under microirrigation in the U.S.? How does this compare with the total irrigated area in the U.S.? Do you think microirrigation will increase in the future? Why?
4. List and describe the major components of a microirrigation system.
5. Describe three important potential advantages of microirrigation and situations where these advantages are more likely to occur.
6. Describe three important disadvantages of microirrigation and situations where these advantages are more likely to occur.
7. From a water sample of your choice, evaluate the potential for the water to clog a microirrigation system.
8. You have installed a microirrigation system and during the first two days of irrigation, you note that the pressure gauge before the sand filter remains at 35 psi, but the one at the exit to the filter has dropped from 32 psi to 18 psi. Should you invest in an automatic flushing system for the sand filter?
9. If 500 gal of water are required to fill a microirrigation system, is it cheaper to inject chlorine continuously at 1.0 ppm during an irrigation lasting 4 hours or inserting 10 ppm for the last 20 minutes of the irrigation cycle? Assume the system applies 1,000 gal/hr.
10. Describe why microirrigation lends itself to the best control for irrigation.
11. An emitter from Europe has a discharge rate of 6 L per hour at a head of 10 m. In the laboratory you measure a discharge of 1 gal/hr at a pressure head of 15 ft. What is the discharge rate of this emitter at a head of 20 ft? Is this a pressure compensating emitter?
12. In Example 14.7, if the friction loss in the lateral is 0.01 psi/ft, what will the difference in emitter discharge be between the north and south ends of the field?
13. In Example 14.8 if the header pipe is placed at the north end of the field and the operating pressure remains 15 psi in the header and friction loss is zero, what will be the maximum difference in emitter discharge along the lateral? Which emitter along the lateral will have the highest discharge rate?
References
ASABE Standards (2019). ASAE EP405.1 APR1988 (R2019): Design and installation of microirrigation systems. St. Joseph, MI: ASABE.
Bucks, D. A., Nakayama, F. S., & Gilbert, R. G. (1979). Trickle irrigation water quality and preventive maintenance. Agric. Water Manag, 2, 149-196.
Bucks, D. A., Nakayama, F. S., & Warrick, A. W. (1982). Principles, practices, and potentialities of trickle (drip) irrigation. In D. Hillel (Ed.), Advances in irrigation (Vol. 1, pp. 219-298). New York, NY: Academic Press.
Coelho, E. F., & Or, D. (1997). Applicability of analytical solutions for flow from point sources to drip irrigation management. Soil Sci. Soc. Am. J., 61, 1331-1341.
FAO. (2021). AQUASTAT database. Rome, Italy: Food and Agricultural Organization of the United Nations.
Irrigation Journal. (1974). 1974 Irrigation Survey. Irrig. J., 24(6), 15-22.
Irrigation Journal. (2001). 2000 Annual irrigation survey. Irrig. J., 51(1), 12-30, 40-41.
Karmeli, D., & Keller, J. 1975. Trickle irrigation design. Glendora, CA: Rain Bird Sprinkler Manufacturing Corp.
Keller, J., & Bliesner, R. D. (1990). Sprinkle and trickle irrigation. New York, NY: Van Nostrand Reinhold.
Solomon, K. (1979). Manufacturing variation of trickle emitters. Trans. ASAE, 22(5), 1034-1038, 1043.
Stanhill, G. (1992). Water use efficiency in agriculture. Irrigation in Israel: Past achievements, present challenges and future possibilities. In J. Shalhevet, L. Changming, & X. Yuenian (Eds.), Proc. Binational China-Israel Workshop.
USDA. (2019). 2018 Irrigation and water management survey. Vol. 3. Special Studies, Part 1. AC-17-SS-1. Washington, DC: USDA.