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  Equipment Selection for Micro-Irrigation

David Zoldoske and Kenneth H. Solomon

CATI Publication #880904
© Copyright September 1988, all rights reserved

There are numerous components that go into making up a micro-irrigation system, and of course there are many types and brands to choose from for each component. Here are some helpful hints on how to select the right equipment for your operation.

Micro-irrigation or low volume irrigation systems are designed to supply filtered water directly onto or below the soil surface. The device which delivers the water to the soil can be a drip emitter (so called because it emits water drop by drop) or a small spray device which distributes water over a greater area (10 to 25 feet diameter).

All micro-irrigation systems are comprised of five basic units: pumping unit, control head, main and submain pipes, laterals, and emission devices.

The pumping unit takes water from the source and supplies pressurized water to the control head. Pumps used in micro-irrigation systems are similar to those used in other irrigation methods and include centrifugal, submersible, or turbine pumps. These pumps can be driven by either an electric motor or an internal combustion engine. The most efficiently designed irrigation systems have pumping capacity closely matched to system demand. Selecting a pump to match a particular irrigation requirement is a complex task, and is best done in consultation with pump experts. Check with a reputable irrigation or pump dealer to help engineer the best design.

The control head serves as the irrigation system's policeman, regulating flow, pressure, and filtration. It is also the site for the chemical injection manifold and the water meter. No system should be without a water measuring device. By measuring the system flow rate or total applied water over a period of time, the irrigator ensures not only precise field application, but derives a check on system performance (i.e. If the flow rate is down, check for a clogged filter or emission device, closed or restricted valves, etc. If the flow rate is high check for broken lines or fittings, leaks, etc.).

Filtration is the single most critical area in an irrigation system. If filtration fails, the system fails. In most field situations a combination of filtration units are needed for proper water treatment.

The importance of identifying potential contaminants and their concentrations before deciding on the type of filters to install cannot be over-emphasized.

Contaminants come in a variety of forms, both organic (algae) and inorganic (sand) materials, and vary in concentration depending on the time of year.

Several different types of filters can be used to capture and remove these contaminants from the irrigation water:

Gravity Filters - Used at the pump inlet (non-deep well) to settle out sand.

Wire Screens - Also installed at the inlet but used to trap floating debris.

Centrifugal Action Filter - Installed either at the inlet or outlet. This works by spinning the water, forcing the heavier inorganic particulates to the outside, while the cleaner water is drawn from the inside. These may also be called cyclonic separators, hydrocyclones or sand separators (not to be confused with sand media filters).

Screen Filters - As the name suggests, these cylindrical-shaped filters screen out foreign material. These work best at removing inorganic particulates.

Sand Media Filters - These are a must when dealing with large amounts of organic materials.

With sand media filters, water is passed through a bed of sand, which catches the organic material, and allows clean filtered water to continue into the system. The organic material is removed from the filter by frequently backflushing the sand. If large amounts of organic material are found in the water, some form of chemical treatment (acid or chlorine) may be necessary. Consult a reputable dealer in your area about specific problems that may apply to your conditions.

Pressure taps should be located directly upstream and downstream of the filter. Use a single hand-held gauge to check the pressures to reduce the chance of error in permanently installed gauges. Significant pressure differences indicate clogging or the need for backflushing. Extended operation under these conditions greatly reduces system efficiency.

Water is delivered from the control head and filter to the lateral lines in the field through the main and submain pipelines. These lines are usually installed deep enough to eliminate the chance of damage from cultural practices. Rigid PVC and polyethylene are typical materials used due to their low cost and chemical-resistant qualities.

Lateral lines supply water to the emission devices from the main or submain lines. The lateral line is manufactured from flexible PVC or polyethylene hose and generally ranges in size from 1/2 to 1 inch. The design pressure for the lateral lines varies from 5 to 35 pounds per square inch (psi), depending mostly on emitter characteristics.

Water delivery to the tree or vine is performed by the emission devices. These come in two common forms: drip emitters and micro-sprinkler/sprayer devices. Drip emitters are typically manufactured in three flow rates; 1/2, 1 and 2 gallons per hour (gph). The 1 gph emitter is the most common choice in permanent plantings. However, 1/2 gph emitters are used frequently in sandy ground where water must be distributed over a larger area using more outlets per plant or where tight soils and/or slope cause runoff problems. The 2 gph emitter is used where the soil can accept a higher application rate with good lateral water movement and runoff is not a problem. A system using 2 gph emitters can be more cost effective where conditions allow.

Micro-sprinkler/sprayers are divided into two subgroups: micro-sprinklers, which have moving parts; and micro-sprayers, which have no moving parts. Low volume irrigation systems are commonly designed for one micro-sprinkler/sprayer per tree. The device is located between trees so it wets half the rootzone of two trees. The micro-sprinkler/sprayers have higher flow rates (8 to 25 gph) than drip emitters and wet a larger surface area.

Drip emitters and micro-sprinkler/sprayers come in non-compensating and pressure compensating forms. The non-compensating form is found more frequently in the field. Pressure compensation refers to the ability of the outlet device to deliver a constant flow rate over a range of pressures. The pressure compensating devices are intended for fields with wide fluctuations in elevation, topography, and pressures. While much of the pressure difference can be reduced by "good" hydraulic design, there are instances where compensating devices are a must for a hydraulically uniform and economical system.

Drip emitters are generally fastened directly to the supply tubing or spliced into the tubing by the manufacturer. Micro-sprinkler/sprayers are usually fastened to the supply tube through the use of a barbed fitting and a length of spaghetti tubing (1/8 inch polytube). The supply polytube can either be buried or placed above ground.

The advantages of burying the tubing are many, including protection from the sun, mechanical damage, etc. Buried polytubes work well with micro-sprinklers/sprayers when connected via spaghetti tubing. However, burying drip emitters along with the tubing invites problems with root intrusion and makes emitter inspection more difficult. An irrigation system with buried drip emitters is much less forgiving and requires better management skills than above ground placement.

As a general rule, above ground drip emitters are more difficult to check in the field for plugging due to their greater numbers and smaller wetted pattern. Conversely, the wetted pattern of a micro-sprinkler/sprayer device is easily visible.

The cost of low volume irrigation systems varies widely, depending on the spacing between trees, vines or crop being irrigated, type of filters used, whether the emission devices are drip emitters or micro-spinklers, the competition between local dealers, etc. However, assuming pressurized water is available, a "typical" system using screen and sand filters in sequence, with above ground micro-sprinklers, which is installed by a dealer on a relatively flat field, can cost a grower between $500 to $900 an acre. Drip systems are generally in the same price range. Self installation of a system that has been designed by an expert can reduce that price, but does not allow recourse if there are problems.

If you are comparing bids for irrigation systems from several dealers, be sure to get design specifications for design tolerance and total dynamic head (TDH). Design tolerance is the difference in system pressure found throughout the field and typically ranges from 5 percent to 10 percent with 5 percent representing a more uniform design. TDH indicates the total pressure required to run the system, with the higher value usually requiring a larger pumping plant and higher operating cost. All irrigation designers should be able to provide this information. Though a system with a higher quality design may cost more going in, it will generally be cheaper to operate over the life of the system.