Irrigation and Technology Assessment (B 1275) University of Georgia Extension The Greenhouse*A*Syst series of publications is a confidential self-assessment program you can use to evaluate your greenhouse business for risks associated with water management issues. Armed with facts and figures, you will then be able to reevaluate your management strategies and determine ways to conserve water and minimize those risks. By following the guidelines, you will be able to establish a formal company-wide water conservation plan. Implementing this plan will facilitate more efficient use of resources and impart significant savings in water use, fertilizer and pesticides. 2017-01-05 15:15:27.84 2006-06-02 14:35:46.0 Greenhouse*A*Syst Series: Irrigation and Technology Assessment | Publications | UGA Extension Skip to content

Irrigation and Technology Assessment (B 1275)

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University of Georgia Greenhouse*A*Syst Publication Series: A Program Designed to Assess and Manage Issues Involving Our Natural Resources and Environment

Home*A*Syst is a national program cooperatively supported by the USDA Cooperative State Research, Education and Extension Service (CSREES), USDA Natural Resources Conservation Service (NRCS), and U.S. Environmental Protection Agency (EPA).

This publication follows the Farm*A*Syst/Home*A*Syst grower self-assessment model of dividing farming management into a series of issues, dividing each issue into categories, including educational materials, and following up the self-assessment with the development of action plans to address the key areas of concern. Universities that have *A*syst publication series include Oklahoma, Kansas, Texas, and Wisconsin. New series have recently been successfully developed at major universities including Orchard*A*Syst, and Food *A*Syst.

The Greenhouse*A*Syst publication Series has been developed to assist greenhouse owners with the task of assessing three management issues: Water management, Environmental Risk and Business Profitability. To date, 6 publications in this 12-part series are being reviewed and 6 more are being developed.

The Greenhouse*A*Syst series of publications is a confidential self-assessment program you can use to evaluate your greenhouse business for risks associated with water management issues. Armed with facts and figures, you will then be able to reevaluate your management strategies and determine ways to conserve water and minimize those risks. By following the guidelines, you will be able to establish a formal company-wide water conservation plan. Implementing this plan will facilitate more efficient use of resources and impart significant savings in water use, fertilizer and pesticides.

This bulletin will also help you establish a water conservation document you may find useful if and when state or local water authorities develop policies or implement water restrictions. Most water authorities are favorably impressed with businesses that have developed water conservation plans.

Greenhouse*A*Syst risk assessment consists of a series of questions that will walk you through the considerations to be taken into account while evaluating your business. In order to gain the full benefit of the Greenhouse*A*Syst program, we recommend that you use all 12 publications in the series in the following order.

Risk Area

Greenhouse*A*Syst Publication

Suggested Order

Water Source and Expansion

Available

1

Delivery and Technology

Available

2

Water Management

In production

3

Water Quality Assessment

In production

4

Water Recycling/Pollution Prevention

In production

5

Water Regulations/Company Policy

In production

6

Fertility Management

In development

7

Operation Safety and Biosecurity

In development

8

Shipping, Transportation, Material Handling

In development

9

Greenhouse Energy Utilization

In development

10

Time and Labor Management

In development

11

Greenhouse Maintenance

In development

12

Irrigation and Technology Assessment

Publication #2 in the Series

Paul A. Thomas, Extension Horticulturist
Forrest E. Stegelin, Extension Economist
Rose Mary Seymour, Pollution Prevention, Biological & Agricultural Engineering
Bodie V. Pennisi, Extension Horticulturist

What Can This Bulletin Series Do for Me?

One of the most effective ways to reduce cost in a greenhouse operation is to automate the activities that occur on a regular basis. In most greenhouse operations, irrigation is a daily activity and a major source of labor costs. Manual watering not only costs much more, it also is generally wasteful. An automated system using modern irrigation technology is not only more efficient at getting water to the plant it also saves many, many hours in basic labor. However, the benefit most owners fail to realize is the reduction in management time devoted to irrigation. This section will help you assess the true cost of water related activities in your facility and assist you in developing a plan to upgrade your irrigation technology, and your management strategy. The overall savings and water conservation should become a major improvement to your companies overall effectiveness.

The goal of this section is to help you formulate an accurate assessment of your current technology and potential efficiencies gained by upgrading.

How Much Is Water Actually Costing You?

There are many factors to consider besides just the base rate a municipality is charging you. If you own your own well, the water you extract from the ground has a cost. You must consider your fuel or electric rates for pump operation, labor costs of water application, equipment depreciation and replacement, and equipment maintenance costs. There are a great many hidden considerations here.

Do you know how much the application of water is costing you per hour?

The number of gallons used per month, divided by the dollars of labor used for watering during that time will give you a good assessment. You could then look at the irrigated square feet, apply that cost to specific crops, or as an annual per sq ft expense to plan for.

Do you plan pot-filling operations with a water reservoir level in mind?

Keeping at least a half-inch reservoir reduces water spillage and need for repeat irrigation. Allowing for a water reservoir reduces labor and soil waste.

Have you designed the layout of your production system with water use efficiency in mind?

Adjusting bench width, pipe diameter, water pressure or other improvements could make it easier to water and even improve water use.

How often do you inspect your water delivery system for needed repairs?

Systems should be thoroughly inspected at least once each year.

Do you have in place a technology that reduces or eliminates off-target water use?

This may include flood floors, drip tubes, ebb-and-flow benches, etc. If not, have you considered the cost of such technology in relation to labor savings over a 10-year period?

Most new technologies pay for themselves in 2 to 3 years due to saving from high labor costs.

Delivery and Irrigation Systems

Water delivery systems begin with a pump or a public water meter. For the public water supply source, the water supplier will specify your water meter according to your water needs and usage. If the water supply is on-site, a pump must be selected to get the water to the irrigation system at an optimum pressure for the system operation.

Selecting the Correct Pump

The information needed to select the correct pump for a use is as follows:

  • Water Requirement
  • Capacity of Water Source
  • Suction Head
  • Elevation Head
  • Irrigation System Pressure Requirement (including friction losses in pipes and fittings)
  • Well Diameter (for groundwater)
  • Power Available

Determine the water requirement as a flow rate and volume per day. The water supply must be able to supply both. For a large pond, this is usually not a problem. For wells, the well and aquifer have a limit on their production capacity, which is usually determined when the well is drilled and finished. This well production capacity must be greater than the flow rate for the operation’s water requirement or another well or water source will be needed to meet the peak demand.

For pumps located above the water level, the suction head is the vertical distance from the pump elevation to the water surface elevation. With wells, if the well has a lower yield rate than the pumping rate of the pump, the pump capacity will be reduced due to increasing suction head. Pump capacity must be matched to well yield. The well yield should be determined when the well drilling has been completed. The suction head is also critical to the design of a pump for extracting surface water. If the suction head for surface water pumping is too high, meaning the pump is too far above the water surface, the pump cannot pull the water adequately to pressurize the pump or move the water to get the desirable flow rate.

The elevation head is the vertical distance that a pump must lift water. For pumps in wells, the elevation head is the vertical distance to the highest elevation of the irrigation system from the water table surface in the well when the pump is running. For surface water pumps, the elevation head is the vertical distance between the center-line of the impeller to the highest elevation of the irrigation system.

Water coming out of the pump must be pressurized to overcome friction losses in the distribution lines and meet the irrigation system pressure requirement. If the pump goes directly into a pressure tank, then the pump must pressurize the water to some amount greater than the high pressure switch of the tank. Every sprinkler or emitter has an optimum operating pressure, and the pump imparts pressure to the water to reach the required operating pressure. The horsepower of the pump is directly related to the irrigation system pressure of the water. Thus, a drip irrigation system that requires less operating pressure will require a pump with less horsepower than a sprinkler system requiring more pressure to operate. Also, certain kinds of pumps are more appropriate for increasing pressure while other pumps are more appropriate for increasing flow rate. This is why centrifugal pumps are more desirable for certain situations and turbine pumps are more desirable for other situations. Table 1 gives some general indication of the advantages and disadvantages of different kinds of pumps.

Table 1. Characteristics of different pumps (adapted from Aldrich and Bartok, 1994).

Pump Type

Typical Suction Head
(ft)

Typical Total Head
(ft)

Remarks

Centrifugal

15

230

Advantages: Reliable, good service life; will pump water containing sand

Disadvantages: Loses prime easily; capacity decreases as suction head increases

Jet

85

162

Advantages: Few moving parts; high capacity at low head

Disadvantages: Damaged by sand or silt in water; capacity decreases with service time

Submersible

 

>1,000

Advantages: Easy to frost-proof; high capacities and efficiencies

Disadvantages: Damaged by sand or silt; repair requires pulling from well

Deep well turbine

 

>1,000

Advantages: Easy to frost-proof; high capacities and efficiencies

Disadvantages: Needs straight well casing; repair requires pulling from well

* Total head is the suction head, the elevation head and the irrigation system pressure requirement added together.

For electrically powered pumps, the site of the pump must have adequate power available, including adequate wiring, fuses and circuit breakers to allow continuous and safe pumping over time. Thermal overload protection is usually a component of the electronics of the pump. Table 2 provides the wire size and fuse rating for various Size 60 cycle AC motors, both single and three phase circuits.

Table 2. Wire size and fuse ratings for single-phase 60 cycle AC motors (adapted from Aldrich and Bartok, NRAES-33, 1994.

115V Circuit

230V Circuit

Motor Size (HP)

Fuse Size (amps)

Wire Size

Motor Size (HP)

Fuse Size (amps)

Wire Length of Run

50 ft

100 ft

150 ft

200 ft

50 ft

100 ft

150 ft

200 ft

¼

15

14

12

10

8

¼

15

14

14

14

14

20

14

12

8

8

15

14

14

14

14

½

25

12

10

8

6

½

15

14

14

14

12

¾

30

12

10

6

6

¾

15

14

14

12

12

 

 

 

 

 

 

1

20

14

14

12

10

 

 

 

 

 

 

25

14

12

10

10

 

 

 

 

 

 

2

30

14

12

10

8

 

 

 

 

 

 

3

45

10

10

8

8

 

 

 

 

 

 

5

70

8

8

6

6

The above values are based on 2% voltage drop in the wire and 125% of the name-plate current (in amps) wire carrying capacity.

The power requirement for selecting the correct pump is based on its flow rate and total pressure head required for the system. The flow rate is provided by the manufacturer’ specifications and determined by the well head size. Along with the flow rate, the total pressure head must be calculated. The total pressure head is the sum of four components of head.

  1. suction head.
  2. elevation head.
  3. irrigation system pressure requirement.
  4. friction loss head from distribution pipes.

When the term head is used, the units of measure are feet. Feet of head can be converted to pounds per square inch of pressure (psi) by dividing the feet of head by 2.3 (the conversion is 2.3 ft/psi). For example 23 feet of head is equivalent to 10 psi.

To determine the horsepower requirement of a pump, the total pressure head (psi) is multiplied by the flow rate (gpm) and divided by a conver-sion factor of 3960 (to convert feet of head and gpm to horsepower). This horsepower value is called the water horsepower. It is a measure of the actual energy required to provide adequate pressure and flow rate. The water horsepower is divided by the pump efficiency to size the pump. Pump efficiency is provided by the manufacturer of the pump and depends on the make, model and type of pump desired.

To determine the size motor and electric power requirement, the horsepower calculated for the pump sizing is divided by the motor efficiency to get the horsepower of the motor, which is called the brake horsepower. Most small electric motors have an efficiency of about 90 percent. Other kinds of power supply will have different efficiency values.

Example Pump Selection:

Given: Water is to be pumped at 30 gpm from a pond to a greenhouse 200 feet away (length of delivery line). The pump will be powered by an electric motor. The operating pressure requirement for the irrigation system is 45 psi. The minimum elevation of the pond water surface is about 45 feet below the greenhouse bench elevations. The pump is located 15 feet elevation above the minimum elevation of the pond water surface. The suction line is 42 feet of 2-inch PVC pipe, and the pipe from the pump to the greenhouse is 1½-inch PVC pipe. The intake line has a jet screen in the pond. Friction losses of this system consist of the loss through the intake screen, the suction line losses and delivery line losses as given here:

Jet Screen losses – 10 ft
Suction Line Losses – 0.7 ft
Delivery Line Losses – 12 ft

The losses given above are calculated from friction loss tables for the pipe or given from manufacturing specifications for the jet screen.

The components of the total pressure head requirement are calculated in feet of head:

Suction head = 15 feet (from elevation difference of pump and minimum water surface level
Elevation head = 45 - 15 = 30 ft (elevation diffe-rence from pump to greenhouse bench elevation)
Irrigation System Pressure Requirement converted to Head = 45 psi x 2.3 ft/psi = 103.5 ft
Total Friction Losses = 10 + 0.7 + 12 = 22.7 ft

Total Head Required = 15 + 30 +103.5 + 22.7 = 171.2 ft

Water Horsepower

=

GPM x Total Head Required

=

30 gpm x 171.2 ft

=

1.3 hp

3960

3960

The pump chosen should have an efficiency of at least 0.55. If we assume the pump efficiency is 0.55, the pump horsepower required is 1.3 hp /0.55 = 2.4 hp

With the pump horsepower and flow rate requirement, the correct pump can be selected from manufacturer pump curves or tables that provide the head, flow rate and efficiency ratings for different pump sizes.

The last step is to determine the motor horsepower required to power the above specified pump. For this step, the pump horsepower is divided by the motor efficiency to get the required horsepower for the motor. Assuming an electric motor efficiency of 90 percent:

2.4/0.90 = 2.6 hp

This would indicate that a 2.5 horsepower motor would be undersize, so a 3 horsepower pump would be chosen.

Pressure Tanks

Smaller watering systems not on a municipal water supply may need a pressure tank to supply the needed pressure to the irrigation system. A pressure tank is placed between a pump and the point of use of a water system to allow the water to become pressurized in the tank. The pump forces water into the tank, compressing the air in the tank. As the air compresses, the air and water pressure in the tank increases. Tanks have a pressure switch that controls the range of pressure that occurs within the tank. For a greenhouse system, the typical pressure ranges are 30-50 psi or 40-60 psi. The pump will start when the lower pressure is reached in the tank and run until the pressure in the tank reaches the upper pressure value, when the pump will turn off. The proper setting for the tank depends on the irrigation system operating pressure requirements.

The size of a pressure tank depends on the pump size. In most tanks, only 20-40 percent of the volume actually holds water; the rest of the volume is filled with the pressurized air. The pressure tank size should be 10 times the pumping rate in gpm.

Water Supply Protection and Backflow Prevention

When you remove water from any kind of water supply without proper water supply protection equipment, you run the risk of contaminating the water supply with pathogens or chemicals that you use. Cleanup of contaminated water is often expensive and can be avoided by proper protection equipment. Redundancy, or back up protection devices for water supply protection, is recommended so the failure of one component does not mean instant contamination.

Water supply protection equipment is designed to prevent back-siphoning of water into the source once it has been removed from the source. Back siphoning can occur when pressures change quickly within a distribution system, causing the water to move differently from the intended direction. For example, if a pump for a pressurized system stops running, the pressure from the downstream water will push that water back through the pump and into the water supply if there is no backflow prevention equipment to prevent this. Backflow equipment must be rated for the operating pressures of the system. There should be water protection equipment immediately downstream of any pump and upstream of valves, irrigation or injection system components. For municipally supplied water, there should be backflow prevention devices immediately downstream from the water meter.

While public water systems usually will specify what water supply protection equipment must be used when connecting to the supply, utilities do not set requirements for backflow prevention for on-site water supplies. Any time there is a connection linking your water source to another system operating at a higher pressure, such as a fertilizer injector, there is a danger of backflow into the water source. The rules of the Georgia Department of Agriculture, Prevention of Ground and Surface Water Contamination, Chapter 40-23-2, require all irrigation systems designed or used for application of fertilizer or chemicals other than pesticides must be equipped with backflow prevention equipment consisting of a functional check valve, low pressure drain and vacuum/air relief valve. Pesticides labeled to be applied through irrigation systems will have water supply protection guidance on their labels. Pesticides not labeled for use through irrigation systems should not be applied through watering systems at all.

Anti-siphon devices should also be placed just upstream of any faucet with hose connection. These devices prevent back-siphoning from submerged hoses that could contaminate the water supply. The anti-siphon devices are required by law for any water system where the water is considered potable; so, if you are not using potable water and you are pumping from surface waters or ground water, you do not need to have the anti-siphon devices at hose connections, but you must put up signs declaring that the water from the faucet is not potable to prevent anyone from drinking the water.

Maintaining an air gap of at least 8 inches between the hose outlet and the water level where water is being directed is an alternative way to prevent backsiphoning from hoses into the supply pipeline system.

Why Back-Flow Prevention Is Essential

Why do you need back-flow prevention? If you intentionally or unintentionally cause contamination of your local water supply by failing to have a back-flow preventer, the fines and resulting lawsuits could quickly put you out of business. Florida, a state that takes this issue very seriously, has 36 case histories you can review, on the University of Florida website at http://www.treeo.ufl.edu/backflow/casehist.html

Using the Watering System to Apply Chemicals and Fertilizers

Many operations use the watering system to apply fertilizers, chemicals and sometimes certain pesticides. Drip irrigation systems will also inject chemicals to unclog emitters and clean out the system at times. In addition, chemicals to neutralize or acidify water applied to crops may be injected into the water.

When any chemical is mixed into the water lines, the water supply must be protected by some kind of backflow prevention device. Also, a pesticide must be labeled for application through irrigation systems to be legally applied through an irrigation system. The pesticide label will provide instructions on the water supply protection requirements to inject that pesticide into an irrigation system. For any injection of chemicals into the water supply, the chemical supply line must have an anti-back flow injection valve that will not allow water to flow into the chemical tank or container if for some reason the injector device fails.

There are two basic kinds of equipment for injecting chemicals and fertilizers into a watering system. They are venturi metering devices and positive displacement pumps. Either one of these injectors can be adjusted to change the mix ratio of chemical to water. The venturi device will vary its rate when pressure in the water supply changes, but the positive displacement pumps do not vary in their rate with flow rate or pressure changes in the water supply.

The two styles of positive displacement pumps typically used are piston injectors or diaphragm injectors. The diaphragm injectors are only for low rate chemical injections, while a wider range of injection rates is possible with piston injectors. The larger piston sizes inject larger rates of chemicals.

Injector units are usually rated in gallons per hour (GPH). Typically, for either type of injector, the range of injection rates varies from a tenth of the nominal injection rate to the nominal rate as the maximum. For example, a 10 GPH injector will have an injector rate range of approximately 1 GPH to 10 GPH. To change the injection rate with piston injectors, the injection device must be turned off. Diaphragm injection pumps can be adjusted while running. For chemical applications where the rate of injection does not vary from day to day, a piston displacement pump is suitable; but if rates of injection are variable from day to day, the diaphragm injector will be easier to manage.

Important characteristics and components to consider in choosing a good injection device are.

  • Accuracy of calibration of + 0.5 percent
  • Calibration tube included
  • Adjustable while running
  • Durable, non-corrosive components – stainless steel balls and Niton seals
  • Chemical tank agitation
  • Access for repairs to equipment
  • Appropriate size for chemical tanks.

Good management practices to ensure the life and accuracy of injection equipment include cleaning the system when injection is complete. Flush the injection system with clean water after all of the chemical has been injected to prevent accumulation of precipitates and long-term contamination of the equipment. After the chemical injection is complete, continue to run water through the irrigation system to clean the chemical out of the system as much as possible when this will not defeat the purpose of the injection. The injection system should be frequently monitored while operating to observe that the chemical is moving out at a steady rate into the irrigation system. For the chemical concentration to be accurate in the irrigation water, the irrigation system operations should be tested to measure flow rate or a flow meter should be in place to make sure flow rates are consistent during the injection. Any non-uniformity in the water application results in a similar non-uniformity of chemical application to the plant materials. Any off-target water application has an associated off-target (or waste) of chemical application. Test the injection system for accurate calibration anytime the rate of chemical application is changed.

Overhead Sprinkler Irrigation System Design, Operation and Maintenance

The first key to efficient irrigation is a good system design. This involves choosing an adequate pump size, making sure the water pressure is adequate for the entire irrigation distribution system, economic pipe sizing for the distribution system, and the appropriate application device (sprinkler, emitter or microspray). Pump sizing and water pressure requirements have been discussed previously. This section will consider pipe sizing and application device choices.

Irrigation that uses sprinklers requires

Status and Revision History
Published on Jan 15, 2006
Re-published on Feb 27, 2009
Reviewed on Feb 14, 2012
Reviewed on Jan 5, 2017