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Whenever there is a constantly increasing demand for a finite resource, two things can happen; the price of the resource goes up or government intervention comes into play. In today's socio-economic environment, both of these are happening in certain parts of the world, in regard to irrigation.

While micro-irrigation has been around, in one form or  another, for many years, its acceptance and use have often been a function of  water availability, its cost and government restrictions.

There are a variety of different types of micro-irrigation  systems on the market today, and most can be quite cost-effective in many, but  not all, situations.

Each type has its advantages and disadvantages, depending  on the specific application. Thus, it is important to conduct proper trade  studies to ensure that the optimum system is used for the appropriate  application. While micro-sprays are part of the micro-irrigation arsenal and  play an important role in the irrigation industry, this article will be devoted solely to drip  irrigation systems.

Of the various forms of micro-irrigation, drip irrigation  is the one most widely used because it can save water, reduce the use of  horticultural chemicals, is relatively insensitive to environmental effects, can reduce labor, and often promotes better  plant growth.

Drip Irrigation

Drip irrigation is an effective irrigation system in terms  of water conservation. With drip, water is not wasted by irrigating areas  between plants or due to run-off, excessive evaporation, wind-effects,  overspray, and the like. Where plant spacing is 2.5' on center or greater,  experience has shown that drip irrigation is the most effective system of  choice. This is in comparison to overhead spray systems in terms of water  conservation and installed cost. The further the plant spacing, the more  cost-effective drip irrigation becomes, on a relative basis, when  compared with overhead spray systems.

Drip irrigation systems can be  categorized as either point source or line source dissemination systems.  However, each of these categories has variations within themselves. Other  systems, such as wick irrigation, are currently being evaluated but are not  discussed herein.

Several things are common to  all drip irrigation systems. They all consist of a transport system, usually  hose or pipe, and a water emission device, usually called emitters. In addition,  they all need a relatively fine mesh filtration and some level of pressure  regulation. Most micro-irrigation systems operate at pressures  between 10 psi and 50 psi.

Line Source

Hypothetically, line source  emission would describe a system that seeps water uniformly along the full  length of the line. These are often referred to as soaker or porous hose lines.  They are often made of particulate matter,  such as ground-up tires, that have been compressed into a semi-solid hose-type  material. Some of these have been known to disintegrate over a period of time,  and thus the industry reception has not been overwhelming.

However, recently many types of  point source dissemination systems with pre-installed or pre-molded  dissemination devices have been classified as line source systems. These systems  have pre-installed dissemination devices (emitters or emitter discharge points)  spaced uniformly along the length of a drip line. These devices can be spaced  uniformly at distances ranging from six inches to 48 inches or more.

In a landscaping scenario,  where plants, in any given plot, can vary in size, spacing, and  species, this type of system may not be the preferred system of choice. These  systems can be installed either on or below the surface. If installed below the  surface, they depend on capillary action of the soil to override the forces of  gravity. While these are truly point source systems, they are often considered  as line source systems because in situations where they are used to irrigate  turf or closely spaced ground-cover, they are required to fully "wet" the  soil.

Where these systems have been  used above the surface for the irrigation of uniformly and closely spaced  plants, they have had a fairly good reception and have been quite effective as  an irrigation technique and as a water conservation vehicle.

When used for sub-surface  irrigation, the reception to this system has been mixed. While all irrigation  systems are sensitive to design, installation and soil conditions, sub-surface  types are particularly sensitive to these parameters. Where these systems have  been installed under laboratory or controlled conditions, they have performed  well. In the field, this has not always been the case. For example, if the soil  is too porous, the water may percolate through the soil before it travels  upwards and laterally to the root zone of the plants. On the other hand, if  there isn't enough porosity in the soil, the capillary action of the soil is  minimal.

Another potential problem with  subsurface irrigation revolves around the fact that whenever water is  disseminated under ground, there is a tendency for  a mud slurry to develop at the point of discharge. When the water is turned off,  a slight negative pressure may develop and some of the mud slurry can enter the  drip line from the point of discharge, and clog the system. This is particularly  apt to happen if there is a slight slope along the length of the line (even  though it may be at a constant depth in relation to the surface).

Another problem articulated by  installers and maintenance contractors is that they cannot readily see water  flowing, to determine if the system is working properly. Of course, this is true  of any drip irrigation system. Flow meters will tell them the quantity of water  that is flowing in any given zone, but not where it is flowing in relation to  the length of the line.

Yet another problem voiced by  maintenance contractors is that if there is a line failure or clogged emitter, the only alternative  is to dig up the turf or ground cover and repair the line. Again, the subsurface  application of drip irrigation for turf or ground cover can be extremely labor  intensive. This is because depth of installation, uniformity of depth in  relation to the surface, and uniformity of line spacing are extremely critical  parameters.

Point Source

The point source drip  irrigation systems are classically defined as emitters mounted on water  transport lines. The water would then disseminate at the specific point where  the emitters are placed. An advantage of a point source dissemination system is  that the irrigation can be "customized" for any given landscape scenario. That  is, plants of different sizes and water requirements can all be accommodated in  the same watering zone by varying the number of emitters and/or their flowrate.

Another advantage of a point  source drip irrigation system is that it can be installed after the landscaping  is in place. Installation can be quite easy and, once installed and covered with  mulch or bark chips, it can look quite attractive. Of course, after the plant  material has matured, it will look even more attractive. An additional benefit  of drip irrigation is that the owner doesn't have to cope with overspray and any  associated lawsuits as a result of slips and falls. This is particularly  important for interiorscapes, streetscapes, crib-wall, amusement parks, or any  other application where heavy foot traffic is a factor during the  better part of a day.

Water Transport  Lines

The water transport line can be  either rigid or flexible. Flexible lines are classically made of either PE  (polyethylene) or PVC (polyvinyl chloride). Flexible PE hose requires either  barbed or compression-type connectors. Further, there is no widely recognized  standard in use in the irrigation industry for PE hose.

For example, there are at least  six different sizes of "half-inch" PE hose available in the marketplace, all  classically called "half-inch." Therefore, care must be taken to make sure that  the "half-inch" hose chosen is compatible with the corresponding "half-inch"  fitting. Also, PE does not have a particularly good "bend-radius," and thus  tends to kink during handling and installation. Of course, once having kinked,  the hose has a weak spot at the point of the "kink" and also a reduced  cross-sectional area, which impacts on the pressure drop characteristics of the  line.

PVC flexible hose usually has a  much heavier wall thickness, is considerably more flexible than PE, can be bent  in a fairly tight radius without problems and, if kinked, has a sufficiently  good memory so that it recovers to its original configuration. It is usually  available in standard "IPS" sizes, and glues or solvent welds to regular PVC  pipe fittings. Algaecides can be incorporated  into the basic formulation to generate an "algae-resistant" PVC flexible hose.Rigid pipe lines are usually  made of PVC pipe.

Emitters

Emitters mounted on the line  can either be pressure-compensated or not. Pressure-compensated emitters are  designed to deliver a pre-designated amount of water, independent of the  pressure in the line. That is, the discharge flow stays the same with major  changes in pressure. This feature is particularly important with long lines,  where friction can cause significant pressure changes along its length. This is  also important where elevation changes and associated pressure variations occur  along the length of the line.

Pressure-compensated emitters  assure that every plant along the line gets its predesignated amount of water.  Pressure-compensated emitters are usually available at flow rates of 0.5, 1.0 or  2.0 gallons per hour per outlet.

Some pressure-compensated  emitters are self-flushing or self-cleaning. That is, they spurt out water every  time the system is turned on or off. This offers a great advantage in terms of  minimizing emitter clogging, and in terms of life of the system. However, it may  result in a hydraulic penalty. The consequences of this "penalty" are that the size of  certain system components may have to be increased to compensate for the  additional flow rate associated with the "spurt" or flushing action.

Emitters are available in a  variety of configurations: single outlet, multiple outlet and bubbler type. Each  of these are available in a "barb" configuration, or in a "threaded base"  configuration. The number of outlets in multiple outlet emitters can vary from  two to twelve, and possibly more.

The output of emitters with  "barbed" outlets may be directed to any specific plant, through the use of  micro-tubing, which is oftentimes called "spaghetti." Thus, if the main hose run is  buried beneath the surface of the soil, the outlet can be directed to the  surface by end of the spaghetti helps to  prevent insects from crawling into the tubing and potentially plugging the line.  If the hose line is placed on the surface, and if it's in an area where there is  little or no foot traffic, micro-bubblers may be used, as well as barbed  emitters. Of course, the emitter can be placed at the end of the spaghetti  tubing.

As you can see, there are many  different types of micro-irrigation. Which to choose depends upon the specific  application, taking into consideration the soil, the spacing of the plant  material, and future maintenance.

Editor's Note: Sam Tobey is  president of Salco Products, Inc.