Drip Irrigation in Pecans (B 936) University of Georgia Extension Research conducted on drip-irrigated pecans in Georgia over the past several years has shown that drip irrigation is highly beneficial even in wet years. The objective of drip irrigation is to supply each plant with sufficient soil moisture to meet transpiration demands. Drip irrigation offers unique agronomic, agrotechnical and economic advantages for the efficient use of water. 2017-05-23 11:35:46.92 2006-06-02 14:26:44.0 Drip Irrigation in Pecans | Publications | UGA Extension Skip to content

Drip Irrigation in Pecans (B 936)

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Pecan tree

Kerry Harrison,
Biological and Agricultural Engineering Department


Research conducted on drip-irrigated pecans in Georgia over the past several years has shown that drip irrigation is highly beneficial even in wet years. This has been confirmed by growers who use it and by the number of systems being installed each year.

Drip irrigation is the frequent, slow application of water to the soil through mechanical devices or holes called emitters (drippers or applicators) located along the water delivery line. This eliminates spraying or running water down furrows and supplies filtered water under low pressure directly onto or into the soil. Water is carried through a pipe network to each plant. Emitters dissipate the pressure in the pipe distribution network by means of either a small-diameter orifice or long flow path, thereby decreasing the water pressure to allow discharge at low volumes of water per hour. After leaving the emitter, the water is distributed by its normal movement through the soil profile. Therefore, the area that can be wetted from each emitter is limited by the water’s horizontal movement in the soil.

The objective of drip irrigation is to supply each plant with sufficient soil moisture to meet transpiration demands. Drip irrigation offers unique agronomic, agrotechnical and economic advantages for the efficient use of water.

Advantages of Drip Irrigation

The main advantages of drip irrigation are:

  1. It allows maximum beneficial use of available water supplies by controlling water flow to allow maximum crop yields with the greatest economy in water use.
  2. Evaporation losses are minimized since water is discharged at or below ground level.
  3. Pressure requirements are low resulting in lower operating costs.
  4. Labor requirements are usually lower than with most other types of irrigation.
  5. Irrigation can be applied during mechanical operations.
  6. Fertilizers and other chemicals can be applied through the system.
  7. Plant protection from diseases and insects is improved by not wetting plant leaves.
  8. Reducing the wetted area limits weed growth and restricts populations of potential pest hosts.
  9. Effect of wind on the wetting pattern is nil, which allows round-the-clock irrigation with lower flow rates and a resultant reduction in pipe sizes and easier adaption to existing water supplies.
  10. Most systems display the well-known advantages of permanency.
  11. Systems are readily adapted to automatic controls.
  12. Drip irrigation provides improved infiltration in soils with low intake capacity.
  13. It allows satisfactory use of more saline water.

Disadvantages of Drip Irrigation

Although drip irrigation offers several advantages for the grower, the system has disadvantages, problems and limitations.

  1. The water supply must be free of soil particles to function properly. An adequate and dependable filtering system is difficult to provide.
  2. Emitter clogging can result from poor water filtration, algae, bacteria, sulfur, iron and calcium in the water. Non-uniformity of water discharged from the emitters causes additional complications.
  3. On sandy soils, drip irrigation does not provide adequate water distribution. The water does not tend to move laterally; therefore, insufficient root volume is wetted causing high water use and leaching of nutrients.
  4. Mice and other animals sometimes chew on the flexible plastic pipes, causing considerable damage.

Components of Drip Irrigation Systems

A drip irrigation system consists of emitters, lateral lines, main lines, filters, control valves and, as in all systems, a pumping plant and water source. The pumping plant and water source are usually the most expensive items. The purpose of this publication, however, is to answer the questions asked most frequently by growers, not justify the economics.

Emitters

The emitter controls the flow from the lateral line into the soil. Emitters range from simple porous wall pipe (line source) to complicated mechanical passageway (point source) units (Figure 1). The emitter decreases the pressure (reduces the head) from the lateral line to the soil. This may be done by small holes, long passageways, vortex chambers or other mechanical means. The pressure of some emitters may be regulated by changing length of cross section of passageways, size of orifice or flow patterns.

Emitters can be placed on the soil surface (Figure 2), or they may be buried at shallow depths for protection. Rates of flow from point source emitters are usually fixed from 1/2 gallon per hour up to 60 gph (microsprinklers). However, rates on some emitters can be manually adjusted. Emitters used in pecan orchards typically have an output rate of 2 gallons per hour. The emitters are connected to or are a part of the lateral line and can be either self-cleaning or manually cleaned.

Figure 1. Typical emitters for drip irrigation of pecans.Figure 1. Typical emitters for drip irrigation of pecans. Counterclockwise from upper left: Pressure compensating self-flushing emitter; Pressure compensating emitter; Mechanical passageway, in-line, non-flushing emitter; Mechanical passageway manual cleaning emitter.
Figure 2. Installing an emitter on an above-ground lateral line.Figure 2. Installing an emitter on an above-ground lateral line.

Lateral Lines

The lateral lines are small in diameter (3/8 to 3/4 inch), and placed one or two per tree row (Figure 3). Whether you use one line or two depends on the cost and system design. Some systems must have two lines per tree row to achieve adequate distribution of water around the tree. The lines are manufactured from polyethylene (PE) material not black polyvinylchloride (PVC). The PE material is more resistant to sunlight than the black PVC. The PE material is also used because of its high strength and impact resistance properties. The lines can be installed on the ground surface or buried below ground surface depending on system design and owner preference. Buried systems are usually installed 4 to 6 inches below ground surface.

Figure 3. Lateral line used in drip irrigation.Figure 3a. Lateral line used in drip irrigation.
Figure 3. Lateral line used in drip irrigation.Figure 3b. Lateral line used in drip irrigation.

Main Lines

The main lines carry water to the lateral lines from the pump. They are made of plastic and are buried (Figure 4). Their size depends on the required water flow to the laterals. In most cases, the sizing of the main line is the most difficult task. Pressure (friction) losses must be kept to a minimum if uniform distribution of water is to occur.

Figure 4. Installing PVC mainline.Figure 4a. Installing PVC mainline.
Figure 4. Installing PVC mainline.Figure 4b. Installing PVC mainline.

Control Valves

Mechanical pressure regulators (made of either brass or plastic) may be required to maintain the system near the design pressure. Pressure required for different emitters ranges from 2 or 3 psi (pounds per square inch) to 30 or 40 psi. Other control valves that may be required include on-off valves to control the flow of water from one zone to another zone and valves to “flush” the ends of PVC lines where sediment may collect. Figures 5-8 illustrate the various types of control stations that may be encountered in the field.

Figure 5. Control station consisting of one solenoid valve for automatic on-off control.Figure 5a. Control station consisting of one solenoid valve for automatic on-off control.
Figure 5. Control station consisting of one solenoid valve for automatic on-off control.Figure 5b. Control station consisting of one solenoid valve for automatic on-off control.
Figure 6. Control stationFigure 6. Control station with (from left) manual on-off valve, filter, pressure regulator, secondary manual on/off valve and pressure gauge.
Figure 7. Control station with flush valve (left) and pressure relief valve (right).Figure 7. Control station with flush valve (left) and pressure relief valve (right).

Figure 8. Control station with manual on-off valve (left valve), injection port (center valve) and kinetic air relief valve (right valve).Figure 8. Control station with manual on-off valve (left valve), injection port (center valve) and kinetic air relief valve (right valve).

Filters and Screens

Most water must be cleaner than drinking water to be used in drip irrigation. To ensure clean water, various types of sand or cartridge filters and screens of 80 to 200 mesh are used, individually or in combination. The sand filter can have manual or automatic backflushing devices for cleaning. The cartridge filter is changed when dirty, and screens are normally cleaned by hand. Figure 9 shows the simple screen filter installation.

Figure 9. Simple screen filter installation.Figure 9a. Simple screen filter installation.
Figure 9. Simple screen filter installation.Figure 9b. Simple screen filter installation.

Design and Installation of System Components

A primary objective of good drip irrigation system design and management is to provide sufficient system flow capacity to adequately irrigate the least-watered tree without overwatering any trees. Uniformity of application depends on the uniformity of emitter discharge. Nonuniform discharge is caused by clogging, pressure differences in the system and manufacturing tolerances.

Emitters

The appropriate emitters to use will depend on topography and the crop to be irrigated. If the topography is uneven, extra precautions will need to be taken in the design. The emitter may need to be a pressure-regulating type emitter; the system may have to be designed so the lateral lines are on the contour, or the operating pressure will have to be high enough so the differential pressures (pressure differences between emitters) will be small or negligible. In orchards, the tree spacing, the number of emitters per tree (wetted area under the tree) and emitter discharge will determine the amount of water delivered per tree and the flow necessary in the lateral line.

Research indicates that the minimum water requirement for pecans in Georgia is 2,400 gallons per acre per day. This amount of water will keep plants alive but may not be adequate for significant yields. It can be as high as 5,000 gallons per acre per day for some orchards to maintain significantly high yields. As the plants per acre increase (spacing gets closer) the amount of water to use per day generally increases. You may need to adjust this rate to fit your situation and equipment limitations. Use the following equation to help you determine certain parameters for your orchard:

E = V/(NxFxH)

Where:
E = the number of emitters installed per tree
V = volume of water applied in gallons per acre per day
N = number of trees per acre
F = emitter flow rate in gallons per hour
H = hours per day that the system will be operated

For a given orchard, the number of trees per acre (N) is fixed. The output of the emitters (F) is most commonly 2 gph but can be other values. The volume of water applied per day (V) will vary during the year but should be high enough to satisfy your needs. For this example it will be 3,600 gallons per acre per day. Therefore, for a given orchard, only two values (E and H) must be determined.

Example:

Use pecans spaced 60 feet x 60 feet and a 2 gph emitter. For this situation N = 12.1 trees/acre, and V = 3600 gal./acre/day, therefore,

E = 3600/(12.1 x 2 x H)

By inserting values for H and solving for E, the following table can be developed:

Option # E — emitters per tree H — hrs of operation/day
1 8 18.6
2 10 14.9
3 12 12.4
4 14 10.6
5 16 9.3

Drip irrigation systems should not operate more than 15 hours per day on any one zone. Watering longer than this could saturate the soil profile and deplete available soil air to the roots. The preferred operation time is 12 hours or less per zone per day. Also, the total wetted area (number of emitters) under the tree should be adequate to wet the majority of the root system. Therefore, options 2, 3, 4 and 5 above are acceptable for this example.

If the system is designed to operate in halves, then it must operate no more than 12 hours per zone per 24 hours (i.e., half of total operating time per zone). The condition of no more than 12 hours operation time rules out option 2 and leaves option 3 as a marginal choice. This leaves options 4 and 5 as acceptable. The decision of which option to select depends on the initial cost of the system. Obviously, the more emitters you install per tree the more expensive the system will be. Also, the more emitters you install the larger the pump and well have to be, adding to the initial cost. The most economical option is option 4. With this option each zone will be watered about 12 hours per day to receive slightly less than maximum design water. For a system operating in halves, this is a total operating time of 24 hours per day. This situation does not allow for any downtime and maintenance during peak water needs but does allow for some catch-up capacity during other time of the year.

Placing the emitters around the tree becomes the next concern. You can place the emitters along one side of the tree or both sides of the tree. Factors affecting this decision are adequate water distribution, economics and system design. A compromise will usually have to be made. That is, the least expensive system (installed on one side) may not yield adequate water distribution and have unacceptable design parameters, while a system designed for precise water application per tree will be the most expensive. You may have to install emitters on both sides of the tree to satisfy the water distribution demands and maintain acceptable design parameters. Generally, if the tree spacing is narrow (fewer than 50 feet) one emitter line is adequate; for wider spacings, two emitter lines per tree row are usually desirable.

Some emitters are manufactured as an integral part of the tubing and are factory-installed at specific intervals. These are called in-line emitters. It is better to install this type emitter entirely above ground.

The other type of emitter is inserted into the tubing at the desired locations by the installer. These are called point-source emitters. They may also be installed above ground (Figure 10), but more often they are installed 4 to 6 inches below the surface (Figure 11). If emitters are installed underground, the water should be ported to the surface using small diameter tubing that extends from the emitter to 1 or 2 inches above the ground surface (Figure 12). This allows the operator to visually check the system operation and to periodically determine the emitter output rate. Porting also helps prevent the emitters from being clogged by dirt.

Figure 10. Above ground installation of emitter and lateral line.Figure 10a. Above ground installation of emitter and lateral line.
Figure 10. Above ground installation of emitter and lateral line.Figure 10b. Above ground installation of emitter and lateral line.
Figure 10. Above ground installation of emitter and lateral line.Figure 10c. Above ground installation of emitter and lateral line.
Figure 11. Above ground installation (left) and same emitter installed below ground with emitter "ported" to surface (right).Figure 11. Above ground installation (left) and same emitter installed below ground with emitter "ported" to surface (right).
Figure 12. Below ground installation of lateral line and above ground installation of emitter.Figure 12. Below ground installation of lateral line and above ground installation of emitter.

A special type of emitter sometimes used in drip irrigation is called a Microjet® (Figure 13). These emitters spray water over a larger area than that normally wet by emitters mentioned previously. They are especially useful in sandy soil types where lateral movement of water is limited. The Microjet® spreads the water over the area then it is allowed to penetrate the soil profile. The two main disadvantages of the Microjet® are 1) it is an above ground obstacle subject to damage from mechanical harvesting equipment and, 2) evaporative losses have to be accounted for just as in sprinkler irrigation. The design and installation of components for a Microjet® system are identical to those used with other drip emitters.

Figure 13. Microjet installation.Figure 13a. Microjet® installation.
Figure 13. Microjet installation.Figure 13b. Microjet® installation.

Lateral Lines

The most frequently asked questions about lateral lines are “How far from the tree should they be buried?” and “How long can they be?” How far you can run the lateral line from the main line depends more on design than anything else. It is true that, for the same conditions (emitters per tree, slope, etc.), two lines can be run farther than one line. In most cases one line can be run up to 300 feet and two lines run up to 600 feet. Use these numbers as a guide. They are not considered the “rule” in design because other factors must be considered. Have a competent irrigation designer evaluate the maximum length of lateral lines.

Use a common-sense approach when deciding how far away from the tree to install the lateral line. How close to the tree the line is installed depends on the equipment used and the age of the tree. You do not want to get too close and damage essential brace roots on mature trees. The equipment used to install the tubing is usually mounted on the rear of a tractor; however, it is sometimes mounted on the front and rear (Figure 14). The wheel spacing on the tractor will, therefore, limit how close you can get. Should the trees be young trees, which means that brace roots are of no concern, you should still consider where the feeder roots will be in years to come. For these reasons, the lateral lines are usually installed no closer to the tree than 4 feet.

Figure 14. Installing lateral line using subsoil plow.Figure 14a. Installing lateral line using subsoil plow — rear mounted equipment.
Figure 14. Installing lateral line using subsoil plow.Figure 14b. Installing lateral line using subsoil plow — both front and rear mounted equipment.

How far away the tubing is installed from the tree depends on management practices. Most orchards have a herbicide strip, and the lateral lines are placed in the outer edge of this strip. The farther the lateral lines are from the tree, the wider the herbicide strip must be. You could be spraying as much as two-thirds of your orchard floor with herbicide, which can be very expensive. Also, at some distance from the tree you reach a point where lateral lines are usually installed no more than 10 feet from the tree. In most orchards the distance of the lateral lines from the tree row will be 6 to 8 feet (Figures 14 and 15).

Figure 15. Finished installation of lateral lines.Figure 15a. Finished installation of lateral lines.
Figure 15. Finished installation of lateral lines.Figure 15b. Finished installation of lateral lines.
Figure 15. Finished installation of lateral lines.Figure 15c. Finished installation of lateral lines.

Mainlines

Mainlines are usually rigid PVC pipe. In a small system only one size pipe or pressure-rated pipe may be used, while in larger systems several sizes and pressure ratings of pipe may be used. Because of its rigid construction, the pipe must be installed deep enough to withstand heavy surface loads such as sprayers. It must also be below the “frost-line” so the stress of freezing and thawing water will not burst the pipe. The frost-line in Georgia is shallower than the depth required for withstanding heavy surface loads.

The minimum required depth of cover for PVC pipelines is as follows:

Pipe Size
(in.)
Minimum Depth of Cover
(in.)
1/2 - 2 1/2 18
3-4 24
4 or more 30

Several methods exist for connecting the mainline to the lateral lines. Since the mainline is usually deeper than the lateral line, either the mainline must be brought up to the lateral line or the lateral line taken down to the mainline. It is usually more economical and easier to take the lateral line down to the mainline (Figure 16).

Figure 16. Connecting l  
 <p class= Status and Revision History
In review Feb 20, 2009
Published on May 14, 2009
Reviewed on Feb 16, 2012