Irrigation Notes


California State University, Fresno, California 93740-0018


January 1988

 

Irrigation System Selection*

By Kenneth H. Solomon

INTRODUCTION

It is of course necessary to choose an irrigation system before design, equipment specification and installation can proceed. To do a proper job of system selection, one must give careful consideration to both the environment in which the irrigation system must function, and to the capabilities and limitations of all potential irrigation system alternatives. The intent of this paper is to summarize such considerations for the most common types of irrigation systems.

FACTORS BEARING ON IRRIGATION SYSTEM SELECTION

The following outline lists a number of factors of the environment which will have a bearing on the evaluation of irrigation system alternates and the selection of a particular system. Not all points will be equally significant in each case, but the outline can serve as a useful checklist to prevent overlooking important factors.

A. Physical Considerations

1. Crops & Cultural Practices

2. Soils

a. Texture, Depth & Uniformity

b. Intake Rate & Erosion Potential

c. Salinity & Internal Drainage

d. Bearing Strength

3. Topography - Slope & Irregularity

4. Water Supply

a. Source & Delivery Schedule

b. Quantity Available & Reliability

d. Water Quality - Chemical and Suspended Solids

5. Climate

6. Land Value and Availability

7. Boundary Constraints and Obstructions

8. Flood Hazard

9. Water Table

10. Pests

11. Energy Availability and Reliability

B. Economic Considerations

1. Capital Investment Required

2. Credit Availability & Interest Rate

3. Equipment Life & Annualized Cost

4. Costs & Inflation

a. Energy, Operation & Maintenance

b. Labor (Various Skill Levels)

c. Supervision & Management

5. Cash Flow

6. Efficiency Factors

C. Social Considerations

1. Legal and Political Issues

2. Local Cooperation and Support

3. Availability and Reliability of Labor

4. Skill and Knowledge Level of Labor

5. Local and Governmental Expectations

6. Level of Automatic Control Desired

7. Potential for Damage by Vandalism

8. Health Issues

The remaining sections will present the more common types of irrigation systems, along with discussions of the particular capabilities and limitations of each. Labor, management, energy and economic factors relevant to each system type are briefly addressed.

SURFACE IRRIGATION

TYPES OF SURFACE IRRIGATION SYSTEMS

Basin Irrigation

In basin irrigation, water is applied to a completely level (sometimes called "dead-level") area enclosed by dikes or borders. This method of irrigation is used successfully for both field and row crops. The floor of the basin may be flat, ridged or shaped into beds, depending on crop and cultural practices. Basins need not be rectangular or straight sided, and the border dikes may or may not be permanent. This irrigation technique is also called by a variety of other names: check flooding; level borders; check irrigation; check-basin irrigation; dead-level irrigation; and level-basin irrigation.

Basin size is limited by available water stream size, topography, soil factors, and degree of leveling required. Basin may be quite small or as large as 40 acres or so. Level basins simplify water management, since the irrigator need only supply a specified volume of water to the field. With adequate stream size, the water will spread quickly over the field, minimizing non-uniformities in inundation time. Basin irrigation is most effective on uniform soils, precisely leveled, when large stream sizes (relative to basin area) are available. High efficiencies are possible with low labor requirements.

Border Strip Irrigation

Border strip irrigation uses land formed into strips, level across the narrow dimension but sloping along the long dimension, and bounded by ridges or borders. Water is turned into the upper end of the border strip, and advances down the strip. After a time, the water is turned off, and a recession front, where standing water has soaked into the soil, moves down the strip. High irrigation efficiencies are possible with this method of irrigation, but are rarely obtained in practice, due to the difficulty of balancing the advance and recession phases of water application.

Border strip irrigation is one of the most complicated of all irrigation methods. The primary design factors are border length and slope, stream size per unit width of border, planned soil moisture deficiency at the time of irrigation, soil intake rate, and degree of flow retardance by the crop as the water flow down the strip. However, because of the large variations in field conditions that occur during the season, the irrigator can have as great an effect on irrigation efficiency as the system designer.

Furrow Irrigation

Furrows are sloping channels formed in the soil. Infiltration occurs laterally and vertically through the wetted perimeter of the furrow. Systems may be designed with a variety of shapes and spacings. Optimal furrow lengths are primarily controlled by intake rates and stream size. The intake rates in furrows may be quite variable, even when soils are uniform, due to cultural practices. The intake rate of a new furrow will be greater than a furrow that has been irrigated, and wheel row furrows can have greatly reduced infiltration rates due to compaction. Because of the many design and management controllable parameters, furrow irrigation systems can be utilized in many situations, within the limits of soil uniformity and topography. With runoff return flow systems, furrow irrigation can be a highly uniform and efficient method of applying water. However, the uniformity and efficiency are highly dependent on proper management, so mismanagement can severely degrade system performance.

CAPABILITIES AND LIMITATIONS

Crop

Some form of surface irrigation is adaptable to most any crop. Basin and border strip irrigation have been successfully used on a wide variety of crops. Furrow irrigation is less well adapted to field crops if cultural practices require travel across the furrows. Basin and border strip irrigations flood the soil surface, and will cause some soils to form a crust, which may inhibit the sprouting of seeds.

Soils and Topography

Surface irrigation systems perform better when soils are uniform, since the soil controls the intake of water. For basin irrigation, basin size should be appropriate for soil texture and infiltration rate. Basin lengths should be limited to 330 feet on very coarse textured soils, but may reach 1320 feet on other soils. Furrow irrigation is possible with all types of soils, but extremely high or low intake rate soils require excessive labor or capital cost adjustments that are seldom economical. Uniform, mild slopes are best adapted to surface irrigation. Undulating topography and shallow soils do not respond well to grading to a plane. Steep slopes and irregular topography increase the cost of land leveling and reduce basin or border size. Deep cuts may expose areas of nonproductive soils, requiring special fertility management. Erosion control measures may be required if large stream sizes are used. In areas of high intensity rainfall and low intake rate soils, surface drainage should be considered with basin irrigation, to reduce damage due to untimely inundation.

Water Quantity and Quality

It is important that irrigation stream size be properly matched to basin or border size for uniform irrigation. Since intake rates for border and furrow systems may vary during the season, it helpful if the water supply rate can be varied from one irrigation to the next. Border and furrow systems are not suitable for leaching of salts for soil reclamation, since the water cannot be held on the soil for any length of time. The basin method, however, is ideal for this purpose. Under normal operating conditions, leaching fractions adequate for salinity control can be maintained with basin, border or furrow irrigation.

Efficiencies

High irrigation efficiencies are possible with all surface irrigation methods, but is far more easy to obtain these potential efficiencies with the basin method. Design efficiencies for basin systems should be high, perhaps 80-90%, for all but very high intake rate soils. Reasonable efficiencies for border strip irrigation are from 70 to 85%, and are 65 to 75% for furrow irrigation. With either the border of furrow methods, runoff return flow systems may be needed to achieve high water use efficiencies.

The system designer and operator can control many of the factors affecting irrigation efficiency, but the potential uniformity of water application with surface irrigation is limited by the variability of soil properties (primarily infiltration rate) throughout the field. Field studies indicate that even for relatively uniform soils, there may be a distribution uniformity of infiltration rates of only about 80%. It has been suggested that surface irrigation uniformity estimates based on infiltration time differences may need to be decreased by 5 to 10% to account for soil variability.

LABOR AND ENERGY CONSIDERATIONS

Basin irrigation involves the least labor of the surface methods, particularly if the system is automated. Border and furrow systems may also be automated to some degree to reduce labor requirements. The complicated "art" of border irrigation (and to a lesser extent furrow irrigation) requires skilled irrigators to obtain high efficiencies. The labor skill needed for setting border or furrow flows can be decreased with higher cost equipment. The setting of siphons or slide openings to obtain the desired flow rate is a required skill, but one that can be learned. With surface irrigation, little or no energy is required to distribute the water throughout the field, but some energy may be extended in bringing the water to the field, especially when water is pumped from groundwater. In some instances these energy costs can be substantial, particularly with low water use efficiencies. Some labor and energy will be necessary for land grading and preparation.

ECONOMIC FACTORS

A major cost in surface irrigation is that of land grading or leveling. The cost is directly related to the volume of earth that must be moved, the area to be finished, and the length and size of farm canals. Typical earth moving volumes are on the order of 420 cubic yards per acre, but have on occasion exceeded 1300 cubic yards per acre. Volumes greater than 800 cubic yards per acre are generally considered excessive, suggesting a design review may be needed. Typical earth moving costs (1986) are US $0.50 per cubic yard. For basin irrigation, final finishing with laser controlled drag scrapers after major earth moving will cost around US $45 per acre. Touchup leveling (at about $20 per acre) may be required every 2 to 3 years, although some farmers choose to touchup each year.

Ditch construction can cost from US $1 per foot for earthen-lined ditches to US $15 per foot for large concrete-lined ditches. Buried low pressure plastic or concrete pipelines for low flows can cost about double the cost of concrete-lined ditches, and may cost 5 to 10 times as much for higher flows. They are generally uneconomical on flat terrain where pumping is not required. They may be desirable on steeper slopes (over 1%).

A reservoir for short-term storage of water may be advisable to permit use of a large stream size accumulated from a smaller steady flow. A medium-sized compacted earth reservoir capable of storing 24 hours water volume would cost about US $100 per acre for a small (to 40 acre) farm. For larger farms, the cost can drop to about US $50 per acre. A lined reservoir may cost twice to five times as much.

SPRINKLER IRRIGATION

In sprinkler irrigation, water is delivered through a pressurized pipe network to sprinklers, nozzles or jets which spray the water into the air, to fall to the soil in an artificial "rain". The basic components of any sprinkler systems are: a water source, a pump to pressurize the water, a pipe network to distribute the water throughout the field, sprinklers to spray the water over the ground, and valves to control the flow of water. The sprinklers, when properly spaced, give a relatively uniform application of water over the irrigated area. Sprinkler systems are usually (there are some exceptions) designed to apply water at a lower rate than the soil infiltration rate, so that the amount of water infiltrated at any point depends upon the application rate and time of application, but not the soil infiltration rate.

TYPES OF SPRINKLER SYSTEMS

There are many types of sprinkler devices and sprinkler systems available today. While a description of all the possibilities is beyond the scope of this article, a discussion of the more common types will be instructive.

Hand-Move or Portable Sprinkler System

These systems employ a lateral pipeline with sprinklers installed at regular intervals. The lateral pipe is often made of aluminum, with 20, 30, or 40 foot sections, and special quick-coupling connections at each pipe joint. The sprinkler is installed on a pipe riser so that it may operate above the crop being grown (in orchards, the riser may be short, so that the sprinklers operate under the tree canopy). The risers are connected to the lateral at the pipe coupling, with the length of pipe section chosen to correspond to the desired sprinkler spacing. The sprinkler lateral is placed in one location and operated until the desired water application has been made. Then the lateral line is disassembled and moved to the next position to be irrigated. This type of sprinkler system has a low initial cost, but a high labor requirement. It can be used on most crops, though with some, such as corn, the laterals become difficult to move as the crop reaches maturity. On bare "sticky" soils, moving the lateral lines is very difficult, and an extra line (a "dry" line) is used.

Side Roll System

This system is a variation on the hand-moved lateral sprinkler line. The lateral line is mounted on wheels, with the pipe forming the axle (specially strengthened pipe and couplers are used). The wheel height is selected so that the axle clears the crop as it is moved. A drive unit, usually an air-cooled gasoline-powered engine located near the center of the lateral, is used to move the system from one irrigation position to another by rolling the wheels.

Traveling Gun System

This system utilizes a high volume, high pressure sprinkler ("gun") mounted on a trailer, with water being supplied through a flexible hose or from an open ditch along which the trailer passes. The gun may be operated in a stationary position for the desired time, and then moved to the next location. However, the most common use is as a continuous move system, where the gun sprinkles as it moves. The trailer may be moved through the field by a winch and cable, or it may be pulled along as the hose is wound up on a reel at the edge of the field. The gun used is usually a part-circle sprinkler, operating through 80 to 90% of the circle for best uniformity, and allowing the trailer to move ahead on dry ground. These systems can be used on most crops, though due to the large droplets and high application rates produced, they are best suited to coarse soils having high intake rates and to crops providing good ground cover.

Center Pivot and Linear Move Systems

The center pivot system consists of a single sprinkler lateral supported by a series of towers. The towers are self-propelled so that the lateral rotates around a pivot point in the center of the irrigated area. The time for the system to revolve through one complete circle can range from a half a day to many days. The longer the lateral, the faster the end of the lateral travels and the larger the area irrigated by the end section. Thus, the water application rate must increase with distance from the pivot to deliver an even application amount. The high application rate at the outer end of the system may cause runoff on some soils. A variety of sprinkler products have been developed specifically for use on these machines to better match water requirements, water application rates and soil characteristics. Since the center pivot irrigates a circle, it leaves the corners of the field unirrigated (unless additions of special equipment are made to the system). Center pivots are capable of irrigating most field crops, but have on occasion been used on tree and vine crops.

Linear move systems are similar to center pivot systems in construction, except that neither end of the lateral pipeline is fixed. The whole line moves down the field in a direction perpendicular to the lateral. Water delivery to the continuously moving lateral is by flexible hose or open ditch pickup. The system is designed to irrigate rectangular fields free of tall obstructions. Both the center pivot and the linear move systems are capable of very high efficiency water application. They require high capital investments, but have low irrigation labor requirements.

LEPA Systems

Low Energy Precision Application (LEPA) systems are similar to linear move irrigation systems, but are different enough to deserve separate mention of their own. The lateral line is equipped with drop tubes and very low pressure orifice emission devices discharging water just above the ground surface into furrows. This distribution system is often combined with micro-basin land preparation for improved runoff control (and to retain rainfall which might fall during the season). High efficiency irrigation is possible, but requires either very high soil intake rates or adequate surface storage in the furrow micro-basins to prevent runoff or non-uniformity along a furrow.

Solid Set and Permanent Systems

Solid set systems are similar in concept to the hand-move lateral sprinkler system, except that enough laterals are placed in the field that it is not necessary to move pipe during the season. The laterals are controlled by valves which direct the water into the laterals irrigating at any particular moment. The pipe laterals for the solid set system are moved into the field at the beginning of the season (after planting and perhaps the first cultivation), and are not removed until the end of the irrigation season (prior to harvest). The solid set system utilizes labor available at the beginning and ends of the irrigation season, but minimizes labor needs during the irrigation season. A permanent system is a solid set system where the main supply lines and the sprinkler laterals are buried and left in place permanently (this is usually done with PVC plastic pipe).

CAPABILITIES AND LIMITATIONS

Crops, Soils, and Topography

Nearly all crops can be irrigated with some type of sprinkler system, though the characteristics of the crop, especially the height, must be considered in system selection. Sprinklers are sometimes used to germinate seed and establish ground cover for crops like lettuce, alfalfa, and sod. The light frequent applications that are desirable for this purpose are easily achieved with some sprinkler systems. Most soils can be irrigated with the sprinkler method, although soils with an intake rate below 0.2 inches per hour may require special measures. Sprinklers are applicable to soils that are too shallow to permit surface shaping or too variable for efficient surface irrigation. In general, sprinklers can be used on any topography that can be farmed. Land leveling is not normally required.

Water Quantity and Quality

Leaching salts from the soil for reclamation can be done with sprinklers using much less water than is required by flooding methods (although a longer time is required to accomplish the reclamation). This can be particularly important in areas with a high water table. A disadvantage of sprinkler irrigation is that many crops (citrus, for example) are sensitive to foliar damage when sprinkled with saline waters.

Efficiencies

Attainable irrigation efficiencies for different sprinkler systems are given in Table 1.

Table 1. Attainable Sprinkler Irrigation Efficiencies

System Type

Efficiency

Hand-Move or Portable

65-75%

Side Roll

65-75%

Traveling Gun

60-70%

Center Pivot

75-90%

Linear Move

75-90%

Solid Set or Permanent

70-80%

LEPA

80-95%

LABOR AND ENERGY CONSIDERATIONS

Labor requirements vary depending on the degree of automation and mechanization of the equipment used. Hand-move systems require the least degree of skill, but the greatest amount of labor. At the other extreme, center pivot, linear move and LEPA systems require considerable skill in operation and maintenance, but the overall amount of labor needed is low. Energy consumption relates to operating pressure requirements, which vary considerably among sprinkler systems. At the extremes, the LEPA systems may require only 15 PSI or so, while the traveling gun system may require 100 PSI or more. Other systems may use 30 to 60 PSI, depending on design of the sprinklers and nozzles chosen.

Table 2. Sprinkler Irrigation System Costs



System Type

Field

Size

(acres)

Capital

Cost

($/acre)

Energy

Use

(kwh/ac-in)

Labor

Required

(hrs/ac-in)

Maintenance

Cost Factor*

(%)

Hand-Move or Portable

160

180-270

9-22

0.17

2

Side Roll

160

385-445

9-22

0.12

2

Traveling Gun

80

385-485

36-50

0.07

6

Center Pivot:

Without Corner System

With Corner System

 

135-200

150

 

285-445

385-485

 

9-24

10-25

 

0.01

0.01

 

5

6

Linear Move (Ditch Fed)

320

445-525

9-24

0.02

6

Linear Move (Hose Fed)

320

650-830

13-27

0.02

6

Solid Set

160

1100-1300

9-22

0.10

2

Permanent

160

930-1400

9-22

0.01

1

* Annual maintenance costs are expressed as a percentage of the system capital cost.

ECONOMIC FACTORS

Table 2 summarizes cost factors for sprinkler irrigation systems. Capital costs depend on the type of system and size of the irrigated area. The investment costs given were typical for 1986, and assume that water is available at ground level at the side of the field, and include mainline and pumping plant.

Energy costs are highly variable from place to place. The energy requirements listed in Table 2 may be used to estimate costs by applying the locally appropriate unit energy cost. A pump efficiency of 75% has been assumed. The energy figures cited are in terms of kilowatt hours per acre-inch (gross) of water applied.

Operating labor costs vary by system type and local costs for labor. The Table 2 gives typical values for labor hours required per acre inch (gross) of irrigation water applied.

Maintenance costs are difficult to predict, but the data in Table 2 may be used as an approximate guide. The annual maintenance cost is estimated by multiplying the initial capital cost of the system by the tabulated percentage factor.

TRICKLE IRRIGATION

Trickle irrigation is the slow, frequent application of water to the soil though emitters placed along a water delivery line. The term trickle irrigation is general, and includes several more specific methods. Drip irrigation applies the water through small emitters to the soil surface, usually at or near the plant to be irrigated. Subsurface irrigation is the application of water below the soil surface. Emitter discharge rates for drip and subsurface irrigation are generally less than 12 liters per hour. Bubbler irrigation is the application of a small stream of water to the soil surface. The applicator discharge rate (up to 250 liters per hour) exceeds the soil's infiltration rate, so the water ponds on the soil surface. A small basin is used to control the distribution of water. Micro-spray irrigation applies water to the soil surface by a small spray or mist. Discharge rates are usually less than 120 liters per hour.

CAPABILITIES AND LIMITATIONS

Crops, Soil, and Topography

Trickle irrigation is best suited for tree, vine, and row crops. The main limitation is the cost of the system, which can be quite high for closely-spaced crops. Complete cover crops, such as grains or pasture cannot be economically irrigated with trickle systems. Trickle irrigation is suitable for most soils, with only the extremes causing any special concern. On very fine textured soils, trickle application rates may cause ponding, with potential runoff, erosion and aeration problems. On very coarse textured soils, lateral movement of water under the applicators will be limited, so more emission outlets per plant may be required to wet the desired root area. With proper design, using pressure compensating emitters and pressure regulators if required, trickle irrigation can be adapted to virtually any topography. In some areas, trickle irrigation is successfully practiced on such steep slopes that cultivation becomes the limiting factor.

Water Quantity and Quality

Trickle irrigation uses a slower rate of water application over a longer period of time than other irrigation methods. The most economical design would have water flowing into the farm area throughout most of the day, every day, during peak use periods. If water is not available on a continuous basis, on-farm water storage may be necessary. Trickle irrigation can be used successfully with waters of some salinity, although some special cautions are needed. Salts will tend to concentrate at the perimeter of the wetted soil volume. If too much time passes between irrigations, the movement of soil water may reverse itself, brining salts back into the root zone. Salts concentrating on the surface around the edge of the surface wetted area can be a hazard should a light rain occur. Such a rain can move the salts down into the root zone, without applying enough water to leach the salts through and below the root zone. When rain falls after a period of salt accumulation, irrigation should continue as normal until about 50 mm of rain have fallen to prevent salt damage. In arid regions where annual rainfall is insufficient (less than 300 to 400 mm) to leach the salts, artificial leaching may be necessary from time to time, requiring the use of a supplemental sprinkler or surface irrigation system.

Though a form of pressurized irrigation, trickle is a low pressure, low flow rate method. These conditions require small flow channel openings in the emission devices, which are prone to plugging. The sensitivity of emitters to plugging varies with design, but virtually all emitters will require some degree of water treatment in agricultural situations. Cyclonic separators and screen filters are used to remove inorganic particles from the irrigation water, and media filters are used to remove organic contaminants. Chemical treatment of the water may also be required to control biological activity in the water, to adjust pH, or to prevent chemical precipitation which could plug emitters. Proper design and care of the water treatment system is vital to the successful use of trickle irrigation.

Efficiencies

Properly designed and maintained trickle systems are capable of high efficiencies. Design efficiencies should be on the order of 90 to 95%. With reasonable care and maintenance, field efficiencies in the range of 80 to 90% may be expected. Where plugging is a problem, or emitter performance is highly variable, field efficiencies may be as low as 60%. A large field study in California found field measured trickle system efficiencies averaged 80%.

LABOR AND ENERGY CONSIDERATIONS

Due to their low flow characteristics, trickle irrigation systems usually have few subunits, and are designed for long irrigation times. The systems are easily operated manually, but can also be fully automated. Thus, the major labor requirement is for system maintenance and inspection. The amount of maintenance labor required is related to the sensitivity of the emitters to plugging and the quality of the irrigation water. In a vineyard situation, one irrigator can inspect and maintain about 50 acres per day.

Trickle irrigation systems generally use less energy than other forms of pressurized irrigation systems. The emission devices usually operate at pressures ranging from 5 to 25 PSI. Additional pressure is required to compensate for pressure losses through the control head (filters and control valves) and the pipe network. System pressures range from about 30 PSI (small systems on flat terrain) to 60 PSI (larger systems on undulating terrain).

ECONOMIC FACTORS

Trickle systems costs can vary greatly, depending on crop (plant, and therefore, emitter and hose spacings) and type of hose employed (permanent or "disposable" thin-walled tubing). Trickle costs will be the lowest for widely-spaced orchard crops, perhaps $900/acre. For closer-spaced vines, the costs increase to about $1400/ac. For closely-spaced vegetable crops (tomatoes, etc.), trickle systems with retrievable laterals can cost from $1200 to $2000/acre. For vegetable systems using disposable laterals, system costs may range from $750 to $1200/acre, with an additional $140 to $180 /acre being spent annually for the disposable lateral lines. These cost figures are for high quality systems and include pumps, filters, controls, mainlines, manifolds and emitters. In situations where more basic pump, filtration and control equipment will suffice, costs may be 20 to 25% lower than the figures cited.

Typical operation and maintenance costs for trickle irrigation systems vary greatly depending on local circumstances and irrigation efficiencies achieved. One approach is to estimate operation and maintenance costs ($ per acre per year) as a percentage of the initial capital cost, as shown in Table 3.

Table 3. Annual Operation and Maintenance Costs for Trickle Systems as a Percentage of Initial Capital Cost

Expense Category

Percentage

Labor

1.5

Power*

3-7

Water*

4-6

Maintenance

3

Taxes and Insurance

2

*Depends on system efficiency

Other approaches to figuring operating costs are based on estimates of energy and labor requirements. An energy requirement of 7.3 to 14.6 kwh per acre inch of water applied (gross) may be used for trickle systems. A corresponding estimate for labor required is 0.04 hours per acre inch of water applied (gross).

ACKNOWLEDGMENT

This paper has drawn heavily upon a draft of the manual Selection of Irrigation Methods for Agriculture being prepared by the On-Farm Irrigation Committee of the Irrigation and Drainage Division of the American Society of Civil Engineers (ASCE). (Other sources used in the preparation of this article or that would be helpful to the read interested in further information are listed below under References.) Most of the cost figures included here are taken directly from the draft manual. The manual also discusses types of irrigation systems other than those common ones covered in this brief article. Interested readers are encouraged to contact the ASCE office (Address: 345 E 47th Street, New York, NY 10017-2398. Telephone: 212-705-7496) to inquire about receiving a copy of this manual when it is completed. Committee members who have helped with the preparation of this manual are:

Carl L. Anderson Allie W. Blair

Ronald D. Bliesner Albert J. Clemmens

Glenn L. Dobbs Marshall J. English

Allan D. Halderman DeLynn R. Hay

John D. Hedlund John L. Merriam

John A. Replogle Len J. Ring

Martin L. Soffran Kenneth H. Solomon

Robert E. Walker Ivan A. Walter

John E. Welton Mulluneh Yitayew

REFERENCES

Hanson BR. 1987. Irrigation. Soil and Water, Fall 1987, No. 72, pp. 1, 3-12.

Jensen ME (editor). 1980. Design and Operation of Farm Irrigation Systems. American Society of Agricultural Engineers, St. Joseph, MI, 829 p.

Keller J. 1976. Irrigation Scheduling and Efficiency. Proceedings, Rain Bird Seminars Re-lating to Irrigation Decision Making, Rain Bird, Glendora, CA, pp 85-95.

Microirrigation Committee, Soil and Water Division, ASAE. EP-405 Design, Installation and Performance of Trickle Irrigation Systems. In: RH Hahn and EE Rosentreter (editors), Standards 1987, 34th Edition, ASAE, St. Joseph, MI, pp 522-525.

Nakayama FS and Bucks DA (editors). 1986. Trickle Irrigation for Crop Production: Design, Operation and Management. Elsevier Science Publishers BV, Amsterdam, The Netherlands, 383 p.

On-Farm Irrigation Committee, Irrigation and Drainage Division, ASCE. 1987. Selection of Irrigation Methods for Agriculture (June 22, 1987 Draft Version). ASCE, New York, 95 p.

Turner JH and Anderson CL. 1980. Planning for An Irrigation System. American Association for Vocational Instructional Materials, Athens, GA, 120 p.

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