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Chemigation
Dean E. Eisenhauer, Derrel L. Martin, Derek M. Heeren, Glenn J. Hoffman
Pages 321-338 (doi: 10.13031/ISM.2021.15) in Irrigation Systems Management. ,
Abstract. See https://www.asabe.org/ISM for a PDF file of this entire textbook at no cost.
15.1 Introduction
Chemigation is the application of chemicals with irrigation systems by injecting chemical solution into the irrigation water stream. The advantages and disadvantages of chemigation are important factors to be considered. All components of the chemigation system must be made of non-corrosive materials. The prevention of any chemicals from entering the water supply is crucial and the practice of chemigation is regulated by federal, state, and local agencies. An important component of a chemigation system is the injection device to assure accurate chemical application. As precision agriculture increases in popularity, the injection devices will become more sophisticated to account for spatial differences of chemical needs within a field.
In chemigation the term “chemicals” usually refers to fertilizers and pesticides with pesticides being inclusive of herbicides, insecticides, fungicides, nematicides, rodenticides, etc. According to data from USDA (2019), of the more than 25 million acres of irrigated field corn, vegetables, cotton, and orchards irrigated in the U.S., fertigation (application of fertilizers) was practiced on 32% of the irrigated area and pesticides were applied with chemigation on over 10% of the area. In some locations the term chemigation also includes the application of chemicals that are necessary for irrigation system maintenance, such as chlorination and acid treatment of microirrigation systems for preventing plugging of emitters by algae, slimes, and chemical precipitates (Chapter 14).
15.1.1 Advantages of Chemigation
The advantages of using chemigation can include better uniformity of chemical application, more timely application of chemicals, effective chemical incorporation, reduction in the number of field operations and the associated soil compaction and crop damage, improved efficacy of pesticides, and reduced environmental contamination (Threadgill et al.,1990 and van der Gulik et al., 2007). Timely application of soluble fertilizers using chemigation can reduce leaching losses on sandy soils, especially during years with greater than normal precipitation (Watts and Martin, 1981). Chemigation allows for the application of nitrogen at times that better match the time of crop uptake especially in taller crops such as field corn. Also, under the tall crop conditions, chemigation is often a viable alternative to aerial application of pesticides. During pest outbreaks timeliness of pesticide application can be critical, and rather than waiting for a commercial aerial sprayer irrigators can take advantage of using their irrigation system for the application. Even when irrigators could use their own field equipment to apply fertilizers or pesticides, using the irrigation system helps minimize field operations.
Precision agriculture depends on variable application of chemicals within a field to better match spatial differences in crop or pest control needs. Variable rate irrigation, discussed in Chapters 6 and 13, makes chemigation a viable application method for precision agriculture. Lo et al. (2019) documented how variable-rate chemigation is compatible with the needs in precision agriculture on a field scale.
15.1.2 Disadvantages of Chemigation
Water resource contamination, application onto non-target areas, increased risk of human exposure to chemicals, and limitations of the chemical products to be applied are some of the potential disadvantages of chemigation. Often with chemigation the chemical is mixed with the irrigation water through injection into the water stream. This poses an environmental risk when the irrigation system shuts off creating the potential of the chemical in the irrigation pipelines and chemical supply tank to backflow into the water source. In Section 15.3 we discuss methods to reduce the risk of this contamination.
In irrigated areas the public and field workers often become accustomed to working with and around irrigation systems and may regard the irrigation water as fresh water that they might consider safe for drinking or other uses and not know that hazardous chemicals might be mixed in the water. Also, with chemigation there is the potential risk of water contamination due to drift, runoff, or application of chemical onto non-target areas. An example of the latter case is a sprinkler system that applies water onto a stream or irrigation ditch that traverses the field.
As discussed by Threadgill et al. (1990) the chemical compatibility to chemigation must be considered. For example, by federal law in the U.S., pesticide labels must specifically state that it is legal to apply the chemical with an irrigation system. Also, there are fertilizers or pesticides that may not be good choices for chemigation if there is potential for precipitation of solids in the water when mixed with the chemical.
15.2 Chemical Injection System
15.2.1 Chemical Injection Pumps
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| Figure 15.1. Chemigation system (drawing on left modified from Eisenhauer and Hay, 1989). |
To inject chemical solutions into a pressurized irrigation water stream requires the following equipment components: An injection pump, a chemical supply tank, injection tubing and associated valves, and calibration devices (Figure 15.1). There are three main types of pumps that are commonly used for chemical injection: piston, diaphragm, and venturi injectors (Figures 15.2 and 15.3). Piston and diaphragm pumps are classified as positive displacement pumps where pump discharge is not greatly influenced by the level of pressure in the irrigation
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(b) |
| Figure 15.3. Venturi injectors (a) with valve in line to create pressure differential and (b) with booster pump to create pressure differential. Images courtesy of Mazzei Injector Company. |
(a)  (b)  |
| Figure 15.2. Positive displacement injection pumps: (a) piston, (b) diaphragm. |
pipeline. The lower pressure in the throat of venturi injectors leads to the chemical solution being inducted into the water stream. The required pressure differential across the venturi makes the injection rate sensitive to the irrigation system pressure. In-line venturi injectors can be used for smaller irrigation pipelines while for larger systems, a by-pass line equipped with pressure reducing valves on the irrigation pipeline or an auxiliary booster pump in the by-pass line are necessary to create the required pressure differential for the venturi to function properly (Figure 15.3). Tests conducted by Kranz et al. (1996) found that regardless of the type of injection device, on-site calibration is necessary under the inlet and outlet pressure conditions at the site. Thus, the chemigation application system should be equipped with a calibration device, usually a calibration tube plumbed at the tank outlet.
For smallholder farmers who use gravity water supply systems such as those shown in Figure 14.7 an injection pump is not necessary. In this case the chemical can be mixed with the irrigation water in the supply reservoir and distributed with the irrigation system. Essential characteristics of the injection pump include material compatibility with the chemical being injected, flow adjustment capability within the range of its maximum capacity, and metering accuracy. Additional desirable characteristics may include flow proportional pump controllers and adaptability to precision agriculture. With flow proportional control the chemical injection rate can be made proportional to the flow rate in the irrigation pipeline. This is especially useful for chemical application accuracy when the land area irrigated per unit time varies, e.g, center pivot systems equipped with end guns and swing-boom corner water systems. Eisenhauer and Bockstadter (1990) found that without flow proportional injection rates, chemical application rate errors can exceed 20% with these scenarios. This error can be reduced to 4% or less when using flow proportional injection. Flow proportional injection also allows for variable rate chemical application when used with variable rate irrigation systems for either sector control or zone control (Lo et al., 2018). Injection pumping systems are available for sector control variable rate chemigation and do not require simultaneous variable rate irrigation.
There are many options for powering the chemical injection pump including belt connection to the power shaft of internal combustion engines (Figure 15.2a), electric motors (Figure 15.2b), and oil hydraulic motors. It is desirable that these power sources are connected to the irrigation power sources in such a way that if the irrigation system shuts off, the injection pump will shut off simultaneously, a one-way interlock. This prevents concentrated chemical from continuing to be pumped into the irrigation pipeline. Also, it is desirable, especially for continuously moving irrigation systems, to have the irrigation system shut off in the event that the injection system shuts off inadvertently. This two-way interlock will prevent untreated areas in the field.
15.2.2 Tanks and Chemical Injection Tubing
The chemical supply tank and chemical injection tubing, associated fittings, and all backflow and safety devices should be made of non-corrosive and chemically resistant materials. In addition, it is important that plastic tanks be made of sunlight resistant materials since it is common for them to be exposed to sunlight for long periods of time. Tank failure and tubing failure can result in a spill of concentrated chemicals resulting in expensive chemical losses and significant soil contamination near the injection site. Depending on the chemical and tank volume secondary containment may be required by regulations.
A common feature of chemical supply tanks is an agitator for mixing purposes. To avoid the accumulation of precipitates in the irrigation system the compatibility of the chemical with the irrigation water should always be evaluated before injecting the chemical. A simple “jar test” can be conducted. In a clear glass jar mix the chemical with the irrigation water to a concentration slightly higher than the planned concentration to be applied. After allowing the jar to sit undisturbed for 24 hours examine the contents for cloudiness, scums, and sediments, indicators of potential chemical precipitation problems.
15.3 Backflow Prevention and Other Safety Devices
As with all chemical applications in agriculture, there is always a concern about the potential for environmental contamination and for worker safety. With chemigation a primary concern is chemical contamination of the irrigation water source due to backflow of the water-chemical mixture in the irrigation pipeline and/or the flow of concentrated chemical from the supply tank to the water source. Another important matter is soil contamination with concentrated chemical in the injection area. Flow of chemical to the water source is not an issue as long as the irrigation system is operating since the flow direction is away from the water source. When the irrigation water flow stops there is potential for backflow.
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| Figure 15.4. Soil and water source contamination scenarios with chemigation systems (modified from Eisenhauer and Hay, 1989). |
In Figure 15.4 several soil and water source contamination possibilities that occur with chemigation are illustrated. In the first scenario (Figure 15.4a) the injection system could shut off unexpectedly while the irrigation pump continues to operate, causing water backflow through the chemical injection system and an overflow of the supply tank. This can lead to soil contamination near the injection site with subsequent potential for leaching to the groundwater, overland runoff of chemical, or flow of chemical to groundwater via the gravel pack of a well. Another possible occurrence is the flow from both the irrigation water supply and the injection system stopping resulting in backflow of the water-chemical mixture to the water source (Figure 15.4b). The most environmentally hazardous scenarios are when concentrated chemical is allowed to flow directly to the irrigation water source. This can occur by gravity-driven flow from the supply tank to the water source (Figure 15.4c) when both the irrigation water flow stops and the injection pump stops. Probably the worst case scenerio occurs when the irrigation water flow stops but the injection pump continues to operate (Figure 15.4d). Eisenhauer et al. (1988) found that there was over 400 times as much pesticide in a full supply tank than was present in the water-chemical mixture in a 130-acre center pivot irrigation system lateral. This not only illustrated the environmental value but the monetary incentive of retaining the concentrated chemical in the tank.
The risk of contaminating the water source and soil near the injection site can be minimized by using the proper backflow prevention and chemigation safety devices (Eisenhauer and Hay, 1989, Kranz et al., 2015, and Threadgill et al., 1990). These devices will be discussed in the following sections.
15.3.1 Irrigation Pipeline Backflow Prevention Devices
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| Figure 15.5. Chemigation check valve assembly. (Bottom image courtesy of Kranz et al., 2016.) |
Backflow prevention in the irrigation pipeline reduces the risk of direct chemical contamination of the water source caused by the scenarios illustrated in Figures 15.4 b, c, and d. The chemigation check valve assembly (CCVA, Figure 15.5) is the most common method of backflow protection on irrigation pipelines that are connected to privately owned wells or single function irrigation water supply districts that are not used as a potable water supply. This is not an acceptable device for irrigation pipelines that are directly connected to public water supply distribution systems. The CCVA is designed for both backpressure and backsiphonage conditions. The check valve in the CCVA is usually an internal spring-loaded valve with a swing gate that is fitted with a resilient-gasket. This valve closes automatically when the irrigation water flow stops. The location of chemical injection must be downstream of the CCVA. Usually incorporated in the CCVA are a vacuum relief valve, a low pressure drain and an inspection port, all located upstream of the check valve. When irrigation water flow stops the check valve closes preventing backflow of the water-chemical mixture in the irrigation pipeline (Figure 15.6a). The vacuum relief valve allows air into the system preventing the creation of a vacuum that could lead to siphoning. In the event that the check valve fails, the low pressure drain can discharge small leakage rates away from the water source (Figure 15.6b). The inspection port (Figure 15.7) can be used by the irrigator or regulatory agency personnel to visually check for leakage from the check valve.
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| Figure 15.6. (a) Backflow prevention operations of CCVA check valve and (b) low pressure drain valve. (Images courtesy of DeLynn Hay, Nebraska Extension.) |
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| Figure 15.7. Inspection for check valve leakage in CCVA. |
An air gap is an acceptable method and alternative to the CCVA. An air gap is created by discharging the irrigation water supply into a tank, reservoir, or farm irrigation ditch in a manner such that there is a free atmospheric vertical separation between the discharge from the water supply pipeline and the water surface in the reservoir (Figure 15.8). The recommended air gap vertical distance is two times the inside diameter of the supply pipeline with a minimum distance of 1 in. (AWWA, 2015). The chemical is then either mixed in the water in the reservoir such as would be possible in the smallholder system illustrated in Figure 14.7 or injected into the water downstream of the reservoir or into the farm irrigation ditch. Depending on the reservoir elevation an irrigation pump may be necessary downstream of the reservoir.
Recommendations for backflow prevention when irrigation systems are connected to a public water supply system are presented by AWWA (2015). In general irrigation connections are considered a high hazard by AWWA (2015) which recommends four acceptable methods for backflow prevention: an air gap (discussed above), reduced-pressure zone backflow prevention assembly, a pressure vacuum breaker assembly, or an atmospheric vacuum breaker assembly.
The reduced-pressure zone (RPZ) backflow prevention assembly (Figure 15.9a) consists of two independently-acting internally-loaded check valves in series with one another and a differential pressure relief valve located in the chamber between the check valves and lower in elevation than the upstream check valve. The RPZ device is capable of preventing backflow in the event of either a backpressure or backsiphonage conditions. The loading or opening pressure of the upstream check valve (minimum 3 psi) creates a differential pressure across the valve. The relief valve opening pressure (minimum 2 psi) is less than the differential pressure created across the first valve. In a backsiphonage condition the lower pressure upstream of the first check valve will cause the relief valve to open to the atmosphere and prevent backflow to the water source even in the event of failure of the second (downstream) check valve. The relief valve will also open in a backpressure condition if the second check valve fails and allows leakage of pressure into the middle chamber.
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| Figure 15.8. Air gap separation backflow prevention. |
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| Figure 15.9. Backflow prevention assemblies, (a) reduced-pressure zone and (b) pressure vacuum breaker. |
The pressure vacuum breaker (PVB) assemblyis the third alternative for preventing backflow in a high hazard environment (Figure 15.9b). It is only applicable in a backsiphonage condition. The assembly includes an internally loaded check valve and an internally loaded air-inlet vacuum relief valve that opens to the atmosphere. The PVB must be positioned so that the elevation of the discharge pipe is a minimum of 12 inches higher than the elevation of the highest irrigation outlet. Like the PVB the atmospheric vacuum breaker (AVB) can also be used under high hazard backsiphonage conditions, but it has more limited application in irrigation systems because shutoff valves downstream of the AVB are not allowed. Under normal irrigation flow conditions, the poppet in the AVB seals on the air-inlet seat. Under a back-siphonage condition the poppet opens and drops to seal on the check valve seat. The poppet is not spring loaded so if a closed downstream valve causes the poppet to be seated at the air inlet for long periods of time there is a risk for it to stick to the seat and not open when backsiphonage occurs.
Public suppliers of potable water and local plumbing codes almost invariably have their own requirements and specifications for backflow prevention which are usually based on recommendations by the American Water Works Association and the Foundation for Cross-Connection Control and Hydraulic Research at the University of Southern California.
Chemical Injection Pipeline Safety Devices
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| Figure 15.10. Chemical injection line check valve. (Image b courtesy of DeLynn Hay, Nebraska Extension.) |
As discussed above it is imperative to stop the flow of concentrated chemical from the supply tank when the irrigation water flow stops. The one-way interlock between the irrigation water supply and the injection device, as discussed in Section 15.2.1, stops the injection pump when the water flow stops preventing the situation illustrated in Figure 15.4d. But the interlock will not stop the flow caused by gravity illustrated in Figure 15.4c. A chemical injection line check valve (Figure 15.10) will help prevent this flow. The check valve is internally loaded, usually by spring, so that it has an opening pressure of 10 psi or greater. At 10 psi the valve would block flow until 23.1 feet of water head is exceeded. The chemical injection line check valve will also stop the backflow through the injection system in the event that the injection system stops but the irrigation water continues to flow (Figure 15.4a). The chemical injection line check value is usually an integral part of the injection port with the discharge end near the center of the irrigation pipeline as would be the case for the valve shown in Figure 15.10a.
Whenever possible it is helpful to place the point of chemical injection at an elevation higher than the maximum liquid level in the supply tank Figure (15.11b). This will provide more protection against flow caused by gravity. If this is not possible another technique for additional protection is to create a vertical pipe loop with a vacuum relief valve at the apex. The apex must be at an elevation higher (minimum 12 in) than the maximum elevation of the liquid level in the supply tank (Figure 15.11a). The vacuum relief valve will break the siphon and stop the flow from the tank.
Another valve that is useful on the injection tubing is a bleed valve (Figure 15.10a). Upon system shutdown, pressure is usually locked into the tubing between the injection pump and the chemigation line check valve. The bleed valve can relieve this pressure before the tubing is disconnected preventing the operator from being sprayed with concentrated chemical. The bleed valve can also be helpful for removing air from the injection line when priming the system.
For further safety, a normally-closed solenoid valve on the inlet side of the injection pump can be electronically interlocked with the injection pump power supply to provide a positive shut off on the chemical injection line if the injection pump stops. This valve is sometimes included in regulatory requirements. Another device that is sometimes required by regulation is a flow sensor positioned in the injection line just upstream of the injection port. The flow sensor safeguards against continued operation if there is a rupture of the injection line, injection pump failure, loss of prime, or the injection port is plugged.
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| Figure 15.11. Chemical injection line options for providing additional protection from chemical flow due to gravity from the chemical supply tank into the irrigation pipeline. (Modified from Eisenhauer and Hay, 1989.) |
15.3.3 Irrigation Pipeline Low Pressure Switch
A low pressure switch on the irrigation pipeline will shut the irrigation system and injection system off if the system pressure drops below a critical point. One potential advantage of chemigation is the uniformity of chemical application which is dependent on the irrigation application uniformity. If the pressure is too low the irrigation uniformity is compromised making it important to stop the application.
15.3.4 Other Safety Items and Considerations
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| Figure 15.12. Field posted for chemigation. |
A strainer on the inlet side of the injection device is essential to prevent foreign materials from clogging or fouling the injection pump, chemical injection line check valve, or other injection system safety equipment. For public and applicator safety posting of fields (Figure 15.12) can be helpful and may be required by regulations. To avoid complacency it is usually recommended/regulated that the signs not be permanent but have specific times of posting prior to chemigation and following the event, for example a maximum of 48 hours before application and 48 hours after the pesticide re-entry period.
Chemigation applicators should follow safety procedures that are common to all chemical application systems such as wearing the appropriate protective clothing (gloves, goggle, rain gear, etc) and adhering to re-entry periods that may be specified on a pesticide label. A fresh water faucet located upstream of the chemical injection port and preferably upstream of the CCVA is advised for washing as needed.
15.3.5 Federal, State, and Local Regulations
In the United States the practice of chemigation is regulated by federal, state, and local government agencies. The intent of the regulations is to reduce the risk of environmental contamination and to protect worker and public safety. Essentially all of the regulations contain some if not all of the backflow prevention and safety equipment discussed above and it is common that the regulations will reference the Engineering Practice, ASAE EP 409.1 Safety Devices for Chemigation (ASABE, 2018).
At the federal level the application of pesticides is regulated by the U.S. Environmental Protection Agency (EPA) through the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA). FIFRA requires that pesticides be applied according to the product label. Fertigation is not regulated by FIFRA. The Label Improvement Program, U.S. EPA Pesticide Registration (PR) Notice 87-1 and an updated list of alternatives provides pesticide manufacturers with generic statements that they can incorporate on the label of their product. For example the label will state whether or not the product can be applied using chemigation and if so what specific safety requirements must be followed. Are there field posting requirements? Is chemigation limited to specific types of irrigation systems, such as sprinklers? What are the specific backflow and safety equipment requirements? For products labeled for chemigation it is common to see the requirement of a CCVA, a chemical injection line check valve, a normally-closed solenoid valve on the inlet side of the injection pump, a one-way system interlock, irrigation system low pressure shutoff switch, and field posting. An example of an acceptable alternative device is to substitute the normally-closed solenoid valve with a chemical injection line check that has an opening pressure of 10 psi or higher. If the label allows for application of a pesticide using water from a public water supply it is likely to state that either an approved air gap or RPZ are required and that specific buffer distances from public places must be followed.
Many states in the U.S. have chemigation regulations and usually these apply to both fertilizers and pesticides. The state requirements can be more extensive than federal requirements but not less. An example is that some states require two CCVA assemblies placed in series if pesticides are to be applied. States usually provide lists of approved CCVA’s that are commercially available. Permitting of chemigation sites is often required and the equipment at permitted sites may be regularly inspected for working performance. Chemigation applicator training and competency testing is sometimes required by states. Other items in some state regulations include accident reporting, secondary containment of the supply tank, and specific pre and post time requirements of posting fields.
Local government subdivisions may also have their own more chemigation regulations. For public water supply systems such as municipalities, the local plumbing code usually has specific regulations for connected irrigation systems.
15.4 Management of Chemigation Systems
Proper management of chemigation systems is necessary to apply chemicals uniformly in the correct amount and to ensure personal and public safety. If insecticides are being applied it is important to read and follow the product label for proper use and application and personal safety recommendations. Avoiding chemigation when wind speeds are high will help with uniform sprinkler applications and avoid drift onto non-target areas such as roads and areas with high public use. Two key management requirements are calibration of the injection system and flushing the injection and irrigation systems following chemigation events.
15.4.1 Injection Rates and Calibration of Injection Devices
The injection device must be calibrated so that the correct amount of chemical is applied per unit of land area. Kranz and Eisenhauer (1996) tested chemical injection pumps and found that even with positive displacement packed-piston and diaphragm pumps there is some sensitivity of pump discharge to irrigation pipeline pressure. Thus all types of injection devices should be calibrated in the field with the irrigation system operating at normal pressure.
For stationary irrigation systems e.g. surface irrigation systems, set-type sprinkler systems and microirrgation systems, the rate of chemical injection is calculated by:
(15.1)
where: qi = injection rate of solution (gal/h),
Gp = amount of chemical solution to apply (gal/ac),
As = area of the irrigation set or zone (ac), and
ti = total time of injection during the irrigation set (h).
With set-type systems the time period for injection does not have to be equal to the set-time or zone run time; it can be equal to or less than the set-time. For certain chemicals such as insecticides or fungicides it may be advantageous to inject near the end of the set-time so more chemical remains on the plant leaves following the event. In other cases it may be advantageous to apply the chemicals at the beginning in the event and then flush the irrigation system at the end of the event. Some controllers can be programmed so that the injection pump is shut off near the end of the event for flushing purposes. In Example 15.1 we illustrate the use of Equation 15.1 for a subsurface drip irrigation system.
For continuously moving irrigation systems the injection rate is dependent upon the rate of land being irrigated per unit time as follows:
qi = GpRi (15.2)
where Ri= rate of land area irrigated per unit of time (ac/h).
For travelers and lateral move systems Ri is the area of the irrigated field divided by the total time of irrigation. For travelers the area and time can be for an individual set. Ri varies with the lateral pipeline position with center pivots that are equipped with end guns or swing-boom corner watering systems which leads to error in chemical application when using constant injection rates and the angular speed remains constant. Eisenhauer and Bockstadter (1990) found that for a typical 1/4-section center pivot equipped with an end gun for irrigating a portion of each of the four corners, the average absolute injection rate error was 7.5% when Ri was calculated based on the total irrigated area and the total time to make a revolution and the injection rate and pivot speed were constant. The chemical would be under applied in the corners when the end gun is operating and over applied whenever the end gun is off. As discussed in Section15.2.1, using a flow proportional injection system is one way of reducing this chemical application rate error. Reducing the speed of the center pivot when the end gun is operating using an auxiliary timer or sector control variable rate center pivot is another way to reduce this error. Equation 15.2 is applied in Example 15.4.
Calibration of an injection pump is usually done with a calibration tube, a clear plastic tube with volume gradations marked on the outside. The tube is plumbed at the outlet of the chemical supply tank and using valves it can be isolated from the tank so that the liquid flowing into the injection device is only from the calibration tube. The calibration process is illustrated in Example 15.1.
15.4.2 Flushing the Injection and Irrigation System
At the end of a chemigation event it is important that the chemical injection pump and the injection line tubing and associated valves and the irrigation pipeline be flushed free of chemicals using fresh water. Flushing the injection system reduces the chance that chemical precipitates will foul the equipment components during future chemigation events. The irrigation pipeline system including the laterals should be flushed to prevent unexpected exposure of field workers to chemicals remaining in the line and to properly distribute the chemical in the field. The time required to flush a mainline is equal to the pipeline length divided by the mean velocity according to the following equation:
(15.3)
where: Tm = flushing time of a mainline (min),
ID = inside diameter of the pipe (in),
Lm = the length of the mainline (ft), and
Q = irrigation system flow rate (gpm).
Equation 15.3 can also be used for manifolds and submains by solving it for each segment of the manifold.
Flushing time of a lateral is different than the mainline. DeTar (1983) provides the following equation for calculation of flushing times in laterals that have outlets with equal discharge:
(15.4)
where: TL= flushing time in a lateral (min),
q = outlet discharge (gpm),
Ll = lateral length (ft), and
S = outlet spacing (ft).
The ratio Ll/S is the number of outlets, N, on the lateral and can be substituted into Equation 15.4 accordingly. Equation 15.4 is valid for laterals that have 10 outlets or more. Equation 15.4 can also be used for manifolds that have more than 10 laterals attached but the flow rate of each lateral must be the same. Figure 15.13 was developed using Equation 15.4.
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| Figure 15.13. Travel or flush times in irrigation laterals, (a) aluminum sprinkler laterals, (b) drip irrigation tubing. |
In Example 15.3 the three hours of flushing time is more than adequate since only 85 minutes total is necessary. The main point is that the time allowed for flushing must be equal to or greater that the total flushing time that was calculated. The travel time of chemical from the injection device to the most distant emitter equals the total flushing time. So the flushing time is important for the proper amount of chemical to be distributed in each zone. In Example 15.3 each zone will receive 4 hours of injected chemical but each emitter will only have discharged the correct amount of chemical when flushing is complete. It is important that the irrigation system be primed with water prior to injecting chemicals so that the priming time is not part of the injection time. Example 15.3 illustrates that there is flexibility in managing chemigation with set-type systems. The solution given in Example 15.3 is only one of many acceptable management approaches.
For systems like center pivots, the discharge on the lateral varies by distance along the lateral as discussed in Chapter 13. Buttermore and Eisenhauer (1989) developed the following flushing time equation for the center pivot lateral problem:
(15.5)
The symbols Ll, S, and Q have the same meanings as in Equations 15.3 and 15.4. Equation 15.5 only applies to the case when an end gun is not operating and the diameter of the lateral is constant along its length. Buttermore and Eisenhauer (1989) presented equations that apply for the end gun condition and for the case where laterals have multiple diameters along their length. In general operating an end gun will result in shorter flush times than when not operating. The application of Equations 15.2 and 15.5 are illustrated in Example 15.4.
In Example 15.4 Riwas based on the total area irrigated in the field and the total time it takes to irrigate the field and the calculated injection rate was based on this average irrigation rate. In this example if an end gun operated in the four corners the Riwould be about 5.45 ac/h when the end gun is off and 6.05 ac/h when it is on. Thus with constant injection rate and constant angular speed of the pivot chemical application rate errors would be at least 5 to 6 % with the chemical application being too high when the end gun is off and too low when it is on.
15.5 Summary
Chemigation is the practice of applying chemicals with irrigation systems. The chemicals, which include fertilizers, pesticides, and system maintenance chemicals, are mixed with the irrigation water for application. The chemicals are often injected into the irrigation water stream with a pump, but in small-scale systems chemicals can be mixed with the water in the water supply tank. Chemigation offers many potential advantages to irrigators including more timely application of chemicals and reduced field operations. However the risk of contaminating the water supply is a concern because of the potential of backflow of chemical when the irrigation system shuts off. Backflow prevention equipment and safety devices are necessary to reduce the risk of contamination and the requirements for this equipment is often regulated by federal, state, and local agencies. Calibration of the injection system is essential for accurate chemical application. An important management practice is to adequately flush the injection system and the entire irrigation system following chemical application.
Questions
1. Define chemigation.
2. Locate the label of a herbicide and a insecticide that are approved for chemigation. Are the backflow prevention equipment requirements the same on each label? Can the products be legally applied in irrigation systems that are connected to a public water supply system?
3. Explain what it means to have a flow proportional injection system and under what conditions or situations would it be useful to use a flow proportional injection system.
4. What does the term positive displacement pump mean?
5. List three potential advantages and three possible disadvantages of the practice of chemigation.
6. List the components of a chemigation check valve assembly and explain their function.
7. What are the two functions of the chemical injection line check valve?
8. An injection rate of 1.5 gal/h is required to apply the desired amount of a pesticide by chemigation. If the injector pump has a maximum capacity of 2 gal/h, what is the estimated correct percent setting of the device? At this setting the liquid level in a calibration tube is timed to check the flow rate of the injector under field operating conditions. 500 ml of chemical is pumped in 4 min and 54 sec (4 min:54 sec). Determine the actual injection rate in gal/h. Does the injection device setting need to be increased or decreased to match the desired 1.5 gal/h injection rate?
9. A 122-ac center pivot will be used to apply 28% UAN nitrogen solution (3 lb of N per gal). The planned application is 25 lb/ac of nitrogen. The timer on the pivot is set at 35% which will result in a making a revolution in 60 hours. Determine the required injection rate in gal/h.
10. A herbicide is applied using chemigation with a stationary sprinkler system. The system has 3-inch diameter aluminum irrigation laterals which are 1000 feet long with 3-gpm sprinklers spaced at 30-foot intervals. How long will it take to flush the laterals with fresh water after chemical injection has stopped?
References
ASABE Standards. (2018). ASAE EP409.1 MAR 1989 (R2018): Safety devices for chemigation. St. Joseph, MI: ASABE.
AWWA. (2015). Backflow prevention and cross-connection control, recommended practices. Manual of water supply practices M14 (4th ed.). American Water Works Association.
Buttermore, G. W., & Eisenhauer, D. E. (1989). Calculation of chemical flushing times in center pivot irrigation systems. Trans. ASAE, 32(4), 1193-1196.
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