Irrigation efficiency

Water scarcity is a growing concern in the U.S. and beyond and is projected to become more severe. Competition for water exists among agricultural users, municipalities and industrial plants and is fueled by increases in population, urbanization of rural areas, increases in agricultural irrigation, climate change and saltwater intrusion. Currently, many sources of freshwater in the U.S. are at risk of depletion. Careful use of freshwater is becoming increasingly important as a result of this trend.

In this article we present ways to improve irrigation efficiency and suggest cultural practices that may reduce water use.

Irrigation Distribution Uniformity (DU)

If an irrigation system is properly designed, maintained and operated, all plants within a zone should receive nearly the same amount of water. If water distribution is not uniform, it could lead to:

• Lack of uniformity in plant growth
• Increased pumping cost due to basing irrigation on the driest plants within a zone (see scheduling coefficient section)
• Overwatering plants that are in “wetter” areas of a zone.

Measuring DU

It is best to test DU when there are no plants in the plot, but if the plot already has plants in it, place catch cans just above the plant canopy.

1. Place 24 buckets or other impermeable catch cans in a uniform grid pattern inside the zone.
2. Run a typical irrigation cycle.
3. Record the amount of water in each catch can and list from lowest to highest.
4. Calculate the average volume of water from all 24 catch cans.
5. Calculate the average of lower 25 percent catch cans.
6. Divide the average of lower 25 percent by the overall average (step 4) and multiply by 100 to produce percent uniformity.

A minimum of 24 catch cans is recommended, but 16 have been used successfully in nurseries. In general, more catch cans will better characterize the DU. Using a multiple of four is essential.

For micro-irrigation systems, place emitters directly into collection containers.

DU is affected by:

System design

• Consult an engineer to ensure that the system is designed properly from the beginning
• Design system to accommodate the flow rates of the sum of all emitters/nozzles that will be operated at one time
• Ensure that proper sprinkler overlap is occurring
• Use nozzles that create matched precipitation within a zone
• Operate within the proper pressure range for the emitters/nozzles – Pipe diameter, length and slope affect pressure
• Height of nozzles should be above plant canopy
• Replace old parts with identical parts
• Make sure all risers are perpendicular to the ground. Stake with rebar if necessary
• Ensure all nozzles are moving at the same speed

Emitters/nozzles

• Ensure that all emitters/nozzles (and pipes) are free of debris
• Use the proper emitters/nozzles for the current system and ensure that they are working properly
• If emitters/nozzles are worn out, replace them with identical emitters/ nozzle (same manufacturer and model, not just same type of emitter/nozzle)

Wind

• Wind can cause overhead irrigation water to be redirected from the plot
• Consider adding a windbreak to minimize wind and improve irrigation uniformity
• Do not conduct DU test if wind speed is 5 mph or greater

Pressure

• Pressure is lost due to friction and is a function of flow rate, pipe diameter and distance
• Bigger pipes cause less friction at a given flow rate, thus less water pressure is lost on the way to the emitter/nozzle
• Valves, elbows, tees and any other similar changes in the pipe cause additional friction loss
• Generally, the further the emitter/nozzle is from the water source, the larger the pipe needs to be
• Pressure may need to be adjusted to increase uniformity

– If pressure is too low, emitter/nozzle heads may need to be changed or the number of emitters/ nozzles reduced

– Consider irrigating fewer zones at the same time

– If pressure is too high, pressure regulators and pressure reducers can help control and reduce pressure, respectively

Matched precipitation rate

To avoid watering roadways and other areas without plants, nozzles in the middle of a plot should be 360° nozzles, edges need 180° nozzles and corners need 90° nozzles. To maintain application rate uniformity, the flow rate of 180° and 90° sprinklers should be half and a quarter of the 360°, respectively. Selecting nozzles that will provide the same application rate (or precipitation rate) across a zone creates a matched precipitation rate. Using a 360° that applies 5 gallons per hour (gph) and a 90° that provides 5 gph will greatly decrease uniformity and will cause the area irrigated by the 90° to be more heavily irrigated while other areas will be underirrigated.

Maximizing efficiency with a single row of sprinklers

A single line of sprinklers will normally result in a lower DU than a grid or offset pattern, but is sometimes the only viable option in an overwintering house or narrow zone between overwintering houses. If full-circle (360°) nozzles are used in a single line, a general rule of thumb is the width of the production area watered by each line should be equal to or less than twice 40 percent of the nozzle output radius.

Following this general guideline will increase application uniformity by limiting the area in which plants are placed but it will also waste production space and water that falls in outlying areas. Sprinkler spacing within the line should be equal to the output radius or closer if located in a windy site.

Most irrigation companies have software that will calculate the DU based on riser spacing and sprinkler type, making it easy to select the ideal riser spacing for a given sprinkler and maximize use of irrigated space.

For example, using a 2009 impact sprinkler in a 200 feet x 50 feet area spacing, a single line of sprinklers with a radius of 34 feet yields a DU of 55 percent. Doubling the number of sprinklers for a spacing of 17 feet apart yields a DU of 78 percent.

This example does not hold true for all sprinkler types. The pattern of spray is very important in determining the best spacing for optimal DU.

Application rate

It is important to know how much water an irrigation system applies during an irrigation event. If plants in a plot only require 0.5 inches of water a day but the irrigation system uniformly applies 2 inches of water, current irrigation duration or run time is too long. Also see scheduling coefficient section.

It is easy to determine how much water an irrigation system applies to a plot while testing for distribution uniformity:

1. Place 24 straight-sided buckets or other straight-sided impermeable catch cans in a uniform grid pattern inside the irrigation zone. Catch cans that are too heavy to blow over work best
2. Operate irrigation for a typical irrigation cycle.
3. Measure the amount of water in each catch can using a ruler in inches and record.
4. Calculate the average of all 24 catch cans.
5. Divide the amount of time the irrigation ran (in minutes) by 60 minutes.
6. Divide the average of all 24 catch cans (step 4) by the number from step 5 to get inches applied per hour.

If too much water is being applied, decrease the amount of time the irrigation runs. If too little water is being applied, increase the amount of time irrigation runs.

Scheduling coefficient

A scheduling coefficient (SC) can be used to adjust irrigation run time to the driest portion of the zone.

It is the ratio of the average application rate for the whole zone compared to the contiguous area with the lowest application rate. The lower the necessary scheduling coefficient, the better. The ideal scheduling coefficient is 1.0.

Example:

• If the average application rate is 0.8 inch per hour and the lowest application rate is 0.6 inches per hour, the SC is 1.3 (0.8 ÷ 0.6)
• If plants within the plot need 1 inch of water per day, the irrigation must be operated for 30 percent longer in order to ensure that plants in the driest portion of the plot receive an inch of water 8 1 inch ÷ 0.8 in/hr × 1.3 × 60 min/hr = 97.5 minutes of run time

Cultural practices for water reduction

There are many strategies that can improve irrigation efficiency and decrease water use even before the irrigation system is turned on. These include grouping plants according to water needs, adjusting plant spacing and selecting the proper substrate.

Plant water requirements

Different plant species use different amounts of water. Grouping plants that have similar water needs into different irrigation zones and irrigating accordingly is one way to conserve water.

• Smaller containers dry out more quickly and need to be watered more often than larger containers
• Larger plants generally require more water per irrigation event than smaller plants; however, larger containers can retain more water between irrigation events
• Solid-walled containers require less water, while porous containers such as root pruning containers or those made with porous materials often require more water and/or more frequent irrigation

– This is due to water evaporating from the sides of the container

• Container color influences container temperature, which affects evaporation from the substrate

– As a result, dark colored containers require more water than light colored containers

• Plants with thick, waxy leaves lose less water and therefore do not need to be irrigated as often as plants with leaves that are not waxy
• Shaded plants do not require as much water as plants in full sun, which transpire more and lose more water to evaporation from the substrate
• Newly planted liners need more frequent irrigation events than well-rooted plants that can access more of the container volume for water
• Vase-shaped canopies have a high capture factor and funnel water into the container, increasing interception efficiency and requiring less irrigation run time than umbrella-shaped or spreading canopies which may deflect water away from the container

– Umbrella-shaped canopies with smaller leaves may deflect less water than those with larger leaves

Container spacing (overhead irrigation only)

Container spacing is a major factor in using water applied by overhead irrigation systems efficiently; proper spacing optimizes plant growth and the production area.

Too much space between containers decreases the number of plants that can be grown in a given block and increases the amount of water that is wasted.

• In general, the further apart containers are spaced, the more irrigation water that lands between containers and is wasted

– Surface area covered by containers is 91 percent at best and drops drastically as containers are spaced further apart

– In most cases, 50-75 percent of overhead irrigation does not contact the substrate surface and instead falls between containers

– Evapotranspiration rate also increases as container spacing is increased due to:

– Increased air circulation between containers

– Increased sunlight penetration to container sidewalls

– When spaced so that branches overlap, vase-shaped plants intercept water that would otherwise be intercepted by neighboring plants. Therefore, closely spaced vase-shaped plants may need longer irrigation events relative to those at wider spacing because each plant may not receive the same amount of water as when spaced farther apart

• Consider planting into the final container size and placing pots with no space in-between (pot-to-pot) until plant canopies begin to overlap to maximize water intercepted and retained from overhead irrigation and rainfall

– This can reduce labor needed to space smaller containers multiple times

– Until plants establish, the substrate will stay very moist, decreasing irrigation application amount even further

– Plant species must be tolerant of moist root conditions

• Placing containers in an offset (rather than square) pattern will enable more of a plot’s surface to be covered by containers

– An offset pattern increases irrigation efficiency by 5 to 10% when compared with a square pattern

• Growers using overhead irrigation for containers larger than a 7-gallon should consider switching to micro-irrigation as the interception efficiency of watering these widely-spaced containers drops below 25 percent

Substrate selection

Different substrates have different physical properties that influence the water holding capacity of the substrate and the portion of stored water that is available to plants.

• Particle size affects both total water holding capacity and the available water of a substrate

– Small particles = small pore space

– Small pore spaces increase water retention and decrease aeration

– Large particles = large pore spaces

– Large pore spaces allow water to drain, decreasing water holding capacity and increasing aeration

• As organic matter decomposes, particle size decreases
• 100 percent pine bark has relatively low moisture retention and requires more irrigation events

– If pine bark dries out too much it becomes hydrophobic, making it hard to rewet

– Adding peat increases the amount of water a pine bark-based substrate can hold

– Adding sand does not appreciably change the total amount of water a container can hold, but it increases the portion of water that is available to plants compared with 100 percent pine bark (1/2 in. screened)

• Even when substrate composition seems identical from one shipment to another, physical properties may vary due to differences in particle size
• Increasing the water holding capacity or the ratio of available water to unavailable water can decrease irrigation frequency

– Overirrigating when using highly moisture retentive substrates can create an environment favorable to pathogens that cause root rot

Author’s note: Excerpted from “Nursery Irrigation: A Guide for Reducing Risk and Improving Production,” a University of Tennessee publication. To view the entire publication, go to https://extension.tennessee.edu/publications/documents/pb1836.pdf.

Amy Fulcher is associate professor for Ornamental Plant Production and Landscape Management, University of Tennessee, afulcher@utk.edu; Whitney Yeary is an extension assistant, Plant Sciences, UT; and Brian Leib is associate professor, Biosystems Engineering and Soil Science, University of Tennessee.