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 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.
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.
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 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.