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Alabama Agricultural Irrigation Information Network

System Design

Larry M. Curtis, Biosystems Engineer

Ted W. Tyson, Biosystems Engineer

Planning and design of an irrigation system involves data gathering and analysis, followed by a specification process in which system layout and components are selected in accordance with fundamental design criteria. The object is to develop a system that will:

1. Provide sufficient supplemental water throughout the growing season;

2. Deliver the quantity of water required by the crop during its period of peak water use;

3. Deliver this water uniformly to each plant; and

4. Insure that an appropriate portion of each plant's root volume is wetted so that the plant can make efficient use of the water delivered.

Further, a good plan will accomplish this four-fold purpose efficiently in terms of system cost, operating cost and water and energy conservation.

The design criteria are interrelated, so that each design decision must be considered with respect to its effects on each of the other criteria. System design is therefore a complex process usually involving several stages of preliminary design work before arriving at a plan which optimizes all criteria for the particular situation. This publication explains the basic principles and techniques used. Professional help is usually needed for developing a detailed design for an irrigation system. Such professional help is available from a variety of sources, including manufacturers and suppliers of irrigation equipment, private consultants, and public employees such as specialists with the Cooperative Extension Service and the Soil Conservation Service.

The following general procedures are essential in developing a plan for an irrigation system. While details of procedure may vary depending on the particular situation, information related to each of these steps is always necessary. Figure 1 shows the layout and basic components of a typical micro-irrigation system and summarizes the design criteria that should be met in developing a system plan. More information on specific aspects of micro-irrigation systems can be found in other publications in this handbook.

Site Analysis

A first step in development of an irrigation plan is site analysis, which provides information essential for system design. Site factors include the soil type, the overall field area and topography (changes in elevation), possible water sources (surface or well) and distance to surface sources. Also, the distance from the water source to electric power and the availability of 3-phase power should be observed. A visit with the local electric power supplier may be necessary to determine rates, special costs or conditions.

Site study is best carried out with a particular crop or crops in mind. Site analysis for specific crops should include determination of field layouts, row location, direction and length, and determination of cultural practices such as use of plastic mulch, weed control techniques, etc.

In addition to on-site observations, data can be gathered from topographical maps, aerial photographs, soil surveys, and other information available from the County FSA and NRCS offices. The Alabama Geological Survey is helpful in determining potential for ground (well) water development. The water source must be tested for water quality and evaluated in terms of quantities available for both seasonal and peak daily needs. If site analysis indicates that an existing water supply such as a stream, lake, or well would not be sufficient for a proposed crop (as explained below), locations for a well or impoundment of adequate size should be evaluated.

Figure 1. Components of a Typical Micro-Irrigation System.

 

Water Quantity and Flow Rate Determination

Each crop to be irrigated has its own water requirements. A micro-irrigation system must be planned keeping in mind both the overall quantity of water needed for a growing season or year and quantities needed for application per day. Based on the crop water requirements, the design process includes:

  • evaluation and, if needed, development of the water source to ensure availability of an adequate seasonal supply of water, and
  • determination of the flow rate (and thus the system design parameters) needed to apply this water to the crop as needed.

Water Supply

Factors to be considered in evaluating the water source differ depending on whether water will be pumped directly from a stream or well, or drawn from a surface reservoir.

If water is to be pumped directly from a flowing stream or from a well with no storage, the critical factor usually is the flow rate available at the time of the crop's peak daily water need. If a well or stream will reliably supply the peak daily water demand, the water source almost always will be able to meet the crop needs at other times in the growing season when daily water needs are less. Thus the overall quantity of water available for the growing season or year will be adequate. When stream sources are used, maintenance of flow for downstream users, water quality and aquatic habitat should also be considered.

If water is to be pumped from a pond or lake, peak daily needs can be assumed to be met, and the designer must evaluate the total quantity of water available over the season or year. This calculation will include determination of both the reservoir volume and recharge amounts supplied by streams or wells flowing into the reservoir. The reservoir or the combination of reservoir and water flowing into the reservoir must be sufficient to supply the overall water needs of the crop in whatever worst-case drought year is anticipated.

If site analysis indicates the available water source would not be adequate, the system design must include a plan for development of a sufficient water supply. In any case, of course, the water demand criteria depend on the crop to be grown, discussed in detail below.

Crop Water Requirements

Loss of available water from the root zone of a crop occurs through a process called evapotranspiration (ET). This is a combination of evaporation from the soil surface and transpiration from the leaf surfaces, In the transpiration process, water moves from the soil into the roots, up through the plant stalk or stems into the leaves, and exits to the atmosphere as water vapor through microscopic holes in the leaves. As plants increase in size and cover or shade most of the soil surface almost all of the available water removed from the soil is removed through the transpiration process. Evaporation losses are significant only when the soil surface is wet immediately after a rain or irrigation.

Peak evapotranspirafion-the peak water requirement-depends on the stage of maturity of the crop and on weather factors. Peak water needs normally develop when the crop has maximum vegetative cover and growth, especially if this occurs during the hottest, driest part of the season. Different plant species may use different amounts of water when experiencing similar weather and soil moisture conditions because of characteristics such as leaf area, waxy or non-waxy leaves, etc.

Peak daily ET rates for given crops are usually calculated as an average of the daily water use over the week or month of highest seasonal water need. Typical average peak ET rates for a hot, humid climate such as Alabama's range between .2 and .3 inches per day (5,431 to 8,147 gallons per acre per day). ET data, along with other considerations, provide a basis for establishing the minimum daily design flow requirements for an irrigation system, as explained below.

Minimum Per Day System Design Requirements

The minimum water delivery capacity (volume in gallons per day) the system must deliver is determined by considering the average peak water requirement (ET) of the crop, the acreage (or number of plants) to be irrigated, and the efficiency of the system.

The minimum capacity for the system should be determined based on the peak water needs of the mature crop. This is true even though for many crops (especially tree fruits such as apples, pecans and peaches) the maximum water needs will not occur until some years have passed. Although this minimum requirement should be determined initially, it may be decided for economic reasons to design the system initially to operate at less than this capacity, with the long range plan of expanding the system to meet the maximum water needs as the crop reaches maturity.

Another factor that must be considered in determining minimurn system capacity is application efficiency-the ratio of water actually reaching and used by plants vs the amount of water pumped. Generally, micro-irrigation systems are 80-90% efficient, depending on effectiveness of design, installation and operation, and on type of emitter used. With other factors being equal, drip tape and microsprayer/sprinkler emitters usually are in the 80-85% efficiency range, while drip emitters are in the 85-90% range. To determine minimum pumping capacity, the quantity of water required by the crop each day is divided by the anticipated efficiency to get the design amount that should be supplied each day. Conservatively, .85 can be used in most design applications.

In system design for tree, bush and vine crops, daily peak water needs usually are first determined for an individual plant. This daily peak need divided by the system efficiency gives the design peak need for the plant. Peak daily use for the entire orchard is then found by multiplying the design peak plant need by the total number of plants in the orchard.

For some crops in the Southeast peak per acre design requirements have been established through research and grower experience. This is often reported in gallons per acre per day. For example, 3600 gallons per acre per day has been established as a design criterion for peaches on loam or clay soil types. Peak requirements for other crops are reported in other publications in this handbook.

Minimum System Flow Rate

After the daily crop design requirement is determined, the system flow rate needed basically depends on the number of hours the system will be operated per day. The flow rate determined in this way may need to be adjusted by consideration of operational factors as well as the characteristics of the emitters selected (explained in the following section). However, the basic minimum flow rate requirement must first be calculated by dividing the daily design requirement by the maximum number of hours of operation per day. In order to allow time for maintenance, breakdowns, etc., micro-irrigation systems are usually designed to apply the minimurn design requirement in 20 hours per day or less.

For Example:

Peaches - 20 Acres

Design Requirement: 3600 gallons/acre/day

(actual flow---efficiency included)

Hours irrigation per day: 20

Tree spacing 15' X 20'

Trees per acre = 145

Gallons/tree/day = 3600 ÷:145 = 24.8

Peak Volume Requirement:

20 acres X 145 trees/acres X 24.8 gal/day/tree = 72000 gal/day

Flow Rate  =   72000 gallons/day     =    60 gpm

                 20 hours/day X 60 min/hr

The system must be designed, therefore, to pump at least 60 gallons per minute. 

In practice, irrigating the entire 20 acres for 20 hours would not be done, since this long an irrigation period would probably result in soil saturation, with possible impairment of root growth. Maximum micro-irrigation set time varies according to soil type, with an upper limit of about 15 hours.

For this reason (along with other design considerations explained below), fields or orchards are usually divided so that water is delivered in sequence to at least two zones and often more than two zones. Each zone is irrigated for a certain number of hours, shut off and the next zone turned on, etc., until the entire area is irrigated. Zoning does not affect the system flow rate requirement as determined above. For example, if the 20-acre orchard is divided into two 10-acre zones, each will require 36,000 gallons per day(half of 72,000) and will be irrigated for 10 hours per day, giving the same required flow rate:

     36000 gal/day    =   60gpm

10 hrs/day X 60 min/hr

Emitter Selection and Zone Design

After the daily peak requirement per plant and the minimum flow rate for the orchard or field are determined, component selection, beginning with the type of emmitter (drip emitter, micro- sprayer, etc.) can begin. It is at this stage also that determination of zones to be used is made.

Two criteria must be used in selecting emitters:

1 . Type and number of emitters chosen must wet adequate root volume.

2. Flow rates of emitters must supply the peak daily needs when operated on a schedule allowing sufficient off time so that soil saturation and consequent root impairment are avoided.

Figure 2. Typical Wetted Zones of Alabama Clay, Loam, and Sandy Soils.

 

Wetted Area Determination

The first criterion, wetting a sufficient portion of the plant's root volume, must be achieved for a plant to efficiently utilize the water that is delivered. When drip devices are used, the area or volume of the soil that is wetted depends on the soil type. Typical wetted zones of sandy, loam and clay soils are illustrated in Figure 2. Generally, at least 25 percent and up to 60 percent of the plant root zone should be wetted to get the maximum benefit from micro-irrigation. On sandy soils, micro- spray or sprinlder emitters rather than drip emitters may be needed to wet sufficient root volume.

While published guidelines for drip emitter wetted diameters on typical sand, loam and clay soils can be used, it is much better to field test by actually wetting one or more selected typical locations in the field. This can be done easily by setting up a small portable pump (or connecting to a house or barn water system) and running a suitable line with drip emitters attached to the selected sites (adjust pressure according to emitters used). 

On sandy soils, the maximum wetted width may be reached in as little as two hours, but on clay soils this may take 12 to 15 hours (caution: do not allow surface runoff to occur). For safety, the emitter should be operuted the same number of hours that is planned for the zone set time, according to soil type. Measurement of wetted diameter is done by trenching across the visible wetted edge to see how far from the emitter water has moved. The wetted area will be wider beneath the sod surface, but it is impossible to know how far it extends without digging. See Figure 3.

The number of emitters required at each plant or tree to wet sufficient root zone can be calculated by the formula:

minimum

number of              plant           row             % of area to

emitters              spacing   X    width      X      be wetted

                                 area wetted by each emitter

This will give the number of emitters per plant. If the number is a fraction, the next larger number should be used. For example:

                    15'X 20'spacing X 50% wetted area     =       150

                       80 sq ft wetted by each emitter                 80

In this case, therefore, two emitters must be used to wet sufficient root zone.

Emitter Flow Rate Determination

The wetted-area criterion establishes the mininium number of emitters needed per plant. The second criterion, meeting the required peak daily water need within an appropriate operational time, is next addressed by choosing an appropriate combination of number of emitters and emitter flow rates.

At this point, the design process involves testing various combinations for optimum "fit" of design criteria, including not only meeting water requirements but keeping capital and operating costs of the system as low as possible. One prime requirement is to keep system flow rate as close to the basic design rate as possible, since higher flow rates require larger pumps, pipes, filters and other components. And it is important to realize that the number and flow rates of emitters will affect system flow rate.

For example (again using the peach illustration), if the peak daily water requirement is 24.8 gallons per tree and we must have two emitters per tree to satisfy wetted-area needs, choosing two 1-gph emitters will result in an operating time of 12.4 hours:

hours of operation perday  =  peak daily water requirement per tree

                                               number of emitters X flow rate

                                                       per tree          per emitter

                                                       24.8 gallons/tree/day     =     12.4 hours

                                                   2 emitters X 1 gal/hr/emitter 

This operating time is acceptable, and irrigating the entire 20 acres in 12.4 hours would offer the advantage of simplicity of operation, with one on-off cycle per day. However, the system flow rate will have to be increased well above the minimum requirement:

                                       20 acres x 145 trees per acre x 2 emitters per tree X 1 gph per emitter  = 96.7 gpm

                                                                               60 min/hr

Zone Determination

Although other aspects of the above design would be acceptable, a flow rate over half again higher than the basic design minimum (96.7 gpm. vs 60 gpm) would be unacceptable because of the larger pump and piping that would have to be used. A better alternative is to use two 2-gal/ hour emitters per tree. Since doubling the emitter flow rate cuts the operating time in half, operating time (in periods of maximum water use) will be 6.2 hours:

                                  24.8 gal/tree               =   6.2 hours

                   2 emitters/tree X 2 gph/emitter

With this plan, the most efficient approach is to divide the field into three zones. Total time to apply this amount would be 6.2 hours/zone X 3 zones = 18.6 hours.

Each irrigation zone will be 6.67 acres (20÷3), and the system flow rate required can be determined as follows:

                6.67 acres X 145 trees per acre X 2 emitters per tree X 2 gph per emitter  =   64.5 gpm

                                                            60 min/hour

Since the system flow rate will not have to be raised significantly above the 60 gpm basic minimum calculated earher to satisfy total per day needs, this design keeps cost of pipe, pump and other components at a minimum and reasonably satisfies all other design criteria.

Water Distribution and Control

Establishing adequate water distribution and control includes making proper design choices for water distribution lines, emitters, pressure regulation, filtration, injection of fertilizer or other chemicals, and pumping. in making these choices, the designer must keep in mind the capability of the system not only to deliver the overall quantity of water needed but to distribute it in a uniform manner.

Emmission Uniformity

Uniformity of water application is a major design factor requiring close attention. For example, a poorly-designed system may deliver the required quantity of water to a field but distribute that water such that some plants receive much more water than others. To avoid this kind of problem the desired degree of water application uniformity must be established and used as a design criterion

One way to express uniformity of application is by a value called "Emission Uniformity." This is a number expressed as a percent that indicates the uniformity of flow in an irrigation zone. Perfect uniformity cannot be attained. However, emission uniformities of 80% for row crops on flat ground and 90% or above for orchards can be achieved and are recommended. By predetermining and using the desired emission uniformity as a design criterion in the planning process, the designer can insure that the system or zones in a system have an emission uniformity equal to or higher than the design value.

Figure 4. Typical Emitter Pressure/Flow Curves.

Lack of uniformity may be caused by variability in the emitters used or by variations in pressure at different points in the system. The emitters have a built in variability that exists because no two emitters can be manufactured exactly alike. Manufacturers usually publish the coefficient of variation (Cv) for each of their products, and the system designer must take account of this source of variability. Where more than one emitter is used per plant, as in orchards, the effect of this manufacturers' variability decreases because the emitter variations at each tree tend to average out.

Factors which can cause pressure variations in a new system are, chiefly, elevation changes in the field and friction losses in lines. Pressure will drop where lines go uphill, and longer lines will have greater pressure loss due to friction than will shorter fines. Where pressure differentials are unavoidable, pressure regulation devices aid in achieving emission uniformity. Essentially, the lowest pressure level in the zone or system becomes the design pressure, and pressure regulating devices keep pressures in other parts from exceeding the design pressure level.

On relatively flat ground pressure regulation may be at one point only (the water entry point to the zone); or, where the manifold has considerable elevation change along its length, at each lateral line.

The system designer should also be aware of emitter characteristics in designing for emission uniformity. While some emitters have direct and proportional change in flow rate in response to changes in pressure, others have practically no change in emission under different pressures. The measure of an emitter's pressure sensitivity is called the "emitter exponent," ranging from zero to one. An exponent of 1.0 means that the emitter has a directly proportional flow increase as pressure increases. An exponent of 0.0 means that the emitter output does not vary at all as pressure changes (a pressure-compensating emitter). An emitter exponent somewhere in between, such as 0.5, means that as pressure increases (or decreases), flow also increases (or decreases), but less than the percentage change in pressure. Typical emitter response to pressure change is illustrated in Figure 4 for emitters with exponents of zero, 0.5, and 1.0.

Where extreme and numerous changes in elevation along distribution lines occur (and especially along laterals), pressure regulation devices may not be sufficient to control pressure variations. In such a situation, pressure-compensating emitters (those with lower exponents) can be used to insure that emission uniformity is maintained. Most manufacturers provide pressure/flow response curves for each of their products, which a designer should consult in selecting emitters. Drip tape products usually are not pressure compensating, and if drip tape is used the rows should be as level as possible, and may have to be placed on the contour. "In-line" drip tubing with factory-installed pressurecompensating emitters is available for use on row crops and may be selected when elevation changes along the rows make tape products impractical. Emitter flow rates are also subject to change due to variations in temperature. Designers should request information from manufacturers if significant temperature variation is anticipated.

Pressure-Compensating vs Non-Compensating Emitters in Design

Two types of emitters are available: pressure-compensating or non-compensating. Non-compensating emitters have a predetermined standard flow rate at a specific operating pressure. For example, typical 1 and 2-gph emitters may deliver the stated flow rates at 15 psi pressure but will have different flow rates at different pressures. Pressure-compensating emitters do not change flow rates significantly at different pressures. Thus pressure-compensating emitters, while more expensive, are useful in keeping system water application uniform in situations where pressure variations would otherwise cause unacceptable variations in amounts of water applied per plant (as explained in the section on Emission Uniformity).

Manufacturers publish information indicating the flow rates of non-compensating emitters over a range of pressures. This gives the designer the flexibility to plan a system with emitters that actually operate at 1.1 gph, 1.2 gph, or some other flow rate, depending on the operating pressure chosen (see Figure 4A.)

For example, the peach orchard design can be modified by reducing the design pressure slightly to reduce the emitter flow rate from 2 to 1.86 gph. This has the effect of increasing the operating time to 6.67 hours. The peak daily need requirement is still met:

6.67 hrs X 2 emitters/tree X 1.86 gph/emitter = 24.8 gal/tree

With three zones, total operating time will be 20 hours (6.67 X 3). And system flow rate comes down from 64.5 gph to 60 gph:

6.67acres x 145 trees per acre X 2 emitters per tree x 1.86 gph per emitter     = 60 gpm

                                        60 min/hour

Thus all design criteria are optimized.

Distribution Line

Lateral lines placed down each tree or plant row carry the emitters, either built-in or attached, which deliver water to each plant. Several factors determine the characteristics of this line. For example, a typical lateral line size is 0.620 inches inside diameter. For a given diameter lateral, the length of fine possible will depend On the number of emitters along the line, the flow rate of each emitter, the spacing of each emitter and the type of emitter. Most manufacturers provide tables or charts that allow selection of lateral fines based On flow rate, spacing etc. These lines must be selected to conform to the emission uniformity requirements of the zone. As a rule of thumb, 50% of the pressure variation allowable in a zone is used in the lateral fine, with the rest being used in the manifold.

In addition, the topography or slope along a fine is a critical factor in determining the length of the line. For example if a manifold must be placed on a slope so that the laterals run uphill and downhill instead of on a level, the pressure variation that would otherwise result can be minimized by making the downhill laterals longer than those going uphill. The gain in pressure due to the downhill slope will be offset by the larger friction loss in the longer downhill laterals, and the loss in pressure due to the uphill slope will be offset by the smaller friction loss in the shorter uphill laterals.

The manifold delivering water to the laterals must be designed so the entire zone (laterals and manifold) meets emission uniformity requirements. Usually, about 50% of the allowable pressure variation in the zone is used as pressure loss in the manifold. The flow rate toward the end of a manifold is much less than that at the center or origin, since fewer laterals are left to be supplied. This means that friction will be less, so that in larger systems with long manifolds it may be economical to reduce the pipe size in stages along the manifold as the flow rate decreases. Again, the designer must take into account the pressure variation that can be tolerated along the manifold. Figure 5 shows the steps required in choosing distribution lines to maintain emission uniformity, beginning with selection of emitters.

Submain and main lines are the primary arteries that move water from the pump to the sub-units or zones of an irrigation system. Generally these lines, which are often buried PVC pipe, carry the full pumping capacity of the system. They should be selected to insure that the friction loss is within acceptable limits (see pressure loss table for PVC pipe in the Appendix of this handbook), and that the velocity of water in the lines is less than 5 feet per second. Minimizing friction loss reduces the horsepower and energy requirements of the system. The low water velocity protects piping and other components against water-hammer and other destructive effects. Other components such as air, vacuum and pressure relief valves are often included the pipeline design to protect the pipe system from water hammer, air entrapment or vacuum conditions.

Control Station

The final portion of the irrigation design process consists of selecting components of the control station. The components are illustrated in Figure 1. One critical aspect of the control station is selection of a suitable filtration system, an essential component of all micro-irrigation systems. Because Micro-irrigation passes water through extremely small outlets or orifices, the likelihood that orifices will plug is high unless water is well filtered and/or treated. The most common filtration components used are media filters (sand, crushed granite, etc.) in conjunction with screen filters. Another filtration device sometimes used is a sand separator, which removes sand particles before water enters the media or screen filters. Because they work by centrifugal force, to function properly sand separators must be selected based on the system flow rate.

The filters must be selected to handle the flow rate and water quality determined for the system. Filters must be cleaned periodically and can be provided with manual, semi-autornatic or automatic flush controls. Tables and charts are available from manufacturers showing rated flow and other characteristics for filter selection.

It is sometimes desirable to install screen filters at each submain or at each lateral as a backup or final filter component. See Figure 1. These filters are helpful in avoiding problems caused by sediments or other contaminants that pass the main filtration system or that enter the system due to breaks, repairs, etc.

The control station also should have components for backfow prevention to prevent water that is contaminated in any way from flowing back into the water supply. This is of particular concern when any type of chemical is injected into the irrgation water.

Another component at the control station is a chemigation or injection station, consisting of some type of injection device, a supply tank and injection ports for placing fertilizers, chlorine or other chemicals in the irrigation water. Injection stations should be installed between backflow prevention equipment and the main filters. At least two right angles in 25 feet of pipeline should separate the injection point and the main filters. This arrangement gives adequate time for any chemical reactions so that if precipitates (solid particles) result from injection they can be caught by the filters.

A variety of pressure gauges and other control devices, as indicated in Figure 1, also must be selected that are compatible with the pipe size and flow requirements of the system. Regular monitoring of accurate pressure gauges will detect many kinds of problems that may occur in a microsystem.

A flow meter should be installed on every micro-irrigation system. Although not necessary for operation, a flow meter is one of the best tools available for monitoring the system. Decreases in flow can indicate system plugging. Increases may indicate leaks or broken lines. Propeller-type flow meters are more accurate and therefore more useful than pitot-type meters, especially for needed procedures like chlorination. Cumulative flow amounts, which propeller meters show, can help in irrigation scheduling.

When a micro- system is automated, a controller that opens and doses valves to each zone is used. This controller must be selected based on the number of valves and other components that are to be controlled. For example, the controller may start and stop the pump, back-flush the filters, and operate the fertilizer injection system as well as open and close valves in the field.

Finally, an essential component of every irrigation system is the pump that provides the water and pressure to operate the system. The pump must be selected to deliver the required quantity (design flow rate) of water. In addtion, the pump must be capable of delivering this water, design pressure to every zone. This pressure is the sum of:

1. the pressure required to overcome the elevation change from the water supply level to the highest point in the field to be irrigated, plus

2. the pressure required to overcome friction in the submain and main line, fittings, filters and all other components, plus

3. the pressure required to operate the zone (that is, deliver the design flow rate).

All three of these factors must be taken into consideration in determining pump size and horsepower. For a typical installation using emitters designed to operate at 20 psi, pressure at the pump may need to be 50 psi, with 30 psi required to lift water to field level and overcome friction (items 1 & 2 above).

Pump horsepower requirements are directly proportional to both flow rate and pressure requirements. Since the horsepower requirement largely determines both initial costs of pumping units and system operating costs, keeping flow rate as low as possible and minimizing friction losses are significant design criteria

Design steps for minimum emission uniformity EU of 90% for a peach orchard on level site, with 15'X 20'spacing, 25 trees per row, 36 rows per zone, row length 375 feet, two 2-gph emitters per tree.

1. Emitter selection: 2 gph at 20 psi, exponent = 0.75, Cv = 0.05

Allowable minimum emitter flow for 90% uniformity: 1.88 gph

      (from standard emission uniformity formula)

Allowable minimum pressure for emitter flow rate of 1.88 gph: 18.5 psi

      (from emitter manufacturer's literature)

Allowable pressure variation in zone for 90% uniformity: 3.75 psi

      (from standard design formula)

2. Lateral selection: 0.620 inch ID polyethylene tubing, length 187.5 ft ea

Friction loss in laterals: .37 psi (from standard design tables)

3. Manifold selection: 2 inch class 160 PVC (one size, not reducing), 360 ft long from origin at submain

Friction loss in manifold: .65 psi (from standard design tables)

4. Calculated total system pressure variation:

0.65 psi friction loss in manifold

0.37 psi friction loss in lateral

0.0 psi loss from elevation changes (level site)

1.02 psi total pressure loss

Since 1.02 pressure variation is well under the 3.75 psi allowable pressure variation for 90% uniformity, system will conservatively meet design uniformity objective.

Note: pressure and flow rate values shown in figure are approximate.

Figure 5. Example of Design for Emissions Uniformity.

 References

American Society of Agricultural Engineers. 1987. Design, installation, and performance of trickle irrigation systems. ASAE Engineering Practice ASAE EP405, pp. 522-525.

Boswell, M.J. 1990. Micro-Irrigation Design Manual. James Hardie Irrigation Co., El Cajon CA.

Fry, AW No date. Drip Irrigation Systems. Rain Bird AgriProducts Division, Glendora CA.

Karmeli, D. and J. Keller. 1975. Trickle Irrigation Design. Rain Bird Sprinkler Mfg. Co., Glendora CA.

Keller, J. and R.D. Bliesner. 1990. Sprinkle and Trickle Irrigation. New York: Van Nostrand Reinhold.

Nakayama, F.S. and D.A. Bucks. 1986. Trickle Irrigation for Crop Production: Design, Operation and Management. New York: Elsevier Science Publishers. Publications included in the Alabama Micro-irrigation Handbook present up-to-date information on practical irrigation methods that save energy and water while protecting water quality. For more information, see your county Extension agent. 

Publication No.

Micro-Irrigation

ANR-645

Date Here

Oct. 1998

Larry M. Curtis, Biosystems Engineer, Professor, Biosystems Engineering, and

Ted W. Tyson, Biosystems Engineer, Associate Professor, Biosystems Engineering.

Issued in furtherance of Cooperative Extension work in agriculture and home economics, Acts of May 8 and June 30, 1914, and other related acts, in cooperation with the U.S. Department of Agriculture. The Alabama Cooperative Extension System (Alabama A&M University and Auburn University) offers educational programs, materials, and equal opportunity employment to all people without regard to race, color, national origin, religion, sex, age, veteran status, or disability.

This document is author-produced (unedited).