Types of irrigation

Ditch (furrow) irrigation

The plants are grown in somewhat raised beds or listed rows, and the water is distributed throughout the field via canals, unlined ditches, or furrows, between the rows or beds. Depending on economic and physical factors such as the size of the field, the types of technology available, and the cost of manpower, the ditches can be dug with hand tools, turned with a plow pulled by an animal or tractor, or precisely fashioned using laser-guided instruments. Water can be transported to the furrows via rigid gated plastic or aluminum pipe, layflat plastic with holes punched at each furrow, concrete or plastic lined ditches, or unlined ditches. Where ditches are used, siphon tubes are generally used to move water from the main ditch to the furrow. When pipes are used, water flow can be controlled by turning it on or off at the local source or by using automatic or manually controlled gates to shunt it from one set of ditches to another. Unless the field is small or very level, parts of it may suffer from water-logging while other parts may be too dry. Depending on heat, wind, and soil permeability, much water may be lost before it can benefit the plants. Automatic valves, also known as surge valves, can increase the efficiency of furrow irrigation because they alternately wet the furrows and allow the soil infiltration rate to slow prior to using the furrow for actual irrigation.

Once common in the U.S., many ditch irrigation systems have been replaced because of high labor costs and increasing demands on water resources. Furrow irrigation also has a tendency to raise the water table in some areas and cause soil salination, requiring drainage. These types of systems are still common in other parts of the world.

Terracing

Large steps are cut into hillsides and supported by stone or concrete walls. The level parts are used as garden plots or small fields. As water flows down the hillside it is channelled to each plot (probably most often by ditch irrigation). Terracing is usually very labor-intensive, since the fields are small and access to them may be steep and narrow (so it's hard to mechanize the work). In addition, the walls need constant maintenance, especially in rainy climates. However, terracing does allow steep mountainsides to be used to grow plants (although it may be more cost effective to use them only for animal pasturage).

Overhead (sprinkler) irrigation

In overhead or sprinkler irrigation, water is piped to one or more central locations within the field and distributed by overhead high-pressure sprinklers or guns or by lower-pressure sprays. A system utilizing sprinklers, sprays, or guns mounted overhead on permanently installed risers is often referred to as a solid-set irrigation system. Some sprinklers can also be hidden below ground level, if aesthetics is a concern, and pop up in response to increased water pressure. This type of system is commonly used in lawns, golf courses, cemeteries, parks, and other turf areas.

Sprinklers that spray in a fixed pattern are generally called sprays or spray heads. Sprays are not usually designed to operate at pressures above 30 psi (200 kPa), due to misting problems that may develop, while higher pressure sprinklers that rotate are usually called rotors. Rotors are usually driven by a ball drive, gear drive, or impact mechanism. Rotors can be designed to rotate in a full or partial circle. Guns are similar to rotors, except that they generally operate at very high pressures of 40 to 130 psi (275 to 900 kPa) and flows of 50 to 1200 gal/min (3 to 76 L/s), usually with nozzle diameters in the range of 0.5 to 1.9 inches (10 to 50 mm). Guns are used not only for irrigation, but also for industrial applications such as dust suppression and logging.

Sprinklers may also be mounted on movable platforms connected to the water source by a hose. At the high-tech end, computerized, automatically moving wheeled systems may irrigate large areas unattended. At the low end, such as in a small greenhouse or landscape, a person may be watering each plant individually with a hose end sprinkler or even a watering can.

One drawback of overhead irrigation is that much water can be lost because of high winds or evaporation, and irrigating the entire field uniformly can be difficult or tedious if the system is not properly designed. Water remaining on plants' leaves may promote fungal and other diseases. If fertilizers are included in the irrigation water, plant leaves can be burned, especially on hot, sunny days.

Overhead irrigation is generally the best solution for watering lawns and golf courses, although drip irrigation is gaining in popularity in some lawn applications. (See also center pivot irrigation.)

Manually assembled systems of piping that are broken down to permit tillage and harvesting are sometimes called "hand set" or "hand move pipe". These are also commonly used on athletic fields where permanently installed sprinklers or outlets are not desired or where lower initial costs are a factor.

Many types of sprinklers are available for use in the landscape. Permanent systems with pop-up spray heads are most common. They are installed underground and rise above the ground surface to operate. Some are designed for use in turf (those having 2- to 3- inch pop-up height); others are designed for use in beds of taller plants (with a 6- to 12-inch pop-up height). Some sprinkler heads are designed for watering small, irregularly shaped areas. These typically have a radius of 15 feet or less. Others, like rotary sprinkler heads, wet a radius of 20 to 50 feet and are used to irrigate large areas. Most sprinklers are available with either full-circle or part-circle patterns, and most have an adjustable radius for watering irregular areas.

Proper Design

The installation of an efficient sprinkler system begins with good design. The system must apply water uniformly over the desired area with a minimum of overspray into adjacent areas. Choosing the appropriate sprinkler for a given area is important, but the location and spacing of sprinklers is equally important. Placing part-circle sprinklers along the boundaries of the irrigated area allows uniform watering along the edges while avoiding wasteful overspray onto buildings, paved areas, and other adjacent areas.

Proper spacing of sprinklers is critical in achieving uniform water application. Sprinklers spaced too far apart will waste water by applying too much water in some areas and not enough in others. On the other hand, spacing sprinklers closer than required increases the cost of the system and wastes water. In general, the spacing between sprinklers should be about 50 to 60 percent of the wetted diameter. For example, sprinklers with a wetted diameter of 80 feet should be spaced 40 to 50 feet apart.

When part-circle sprinklers are used on the same zone with full-circle sprinklers, the sprinklers should be carefully selected to achieve a matched precipitator rate. A half-circle sprinkler waters only half as much area as a full-circle sprinkler; therefore it should discharge only half as much water. If a full-circle sprinkler discharges 6 gallons per minute, then a half-circle sprinkler should deliver 3 gallons per minute and a quarter-circle sprinkler 1l/2 gallons per minute. Most manufacturers offer sprinklers with matched precipitation rate (MPR) nozzles.

One other important aspect of proper design is pipe sizing. Selection of pipe sizes should be based on the flow rate through the pipe. If pipes are too small, excessive pressure losses occur. This causes some sprinklers to apply more water than others and results in non-uniform application and wasted water. Additional information on pipe sizing and irrigation system design is provided in design manuals avail- able from sprinkler manufacturers.

Check the Application Rate of Your Sprinkler Systems

Application rate is the rate at which a sprinkler system applies water to the soil surface, measured in inches per hour. If application rates exceed the infiltration capacity of the soil, then runoff occurs. Problems with runoff are more likely to occur in clay soils or compacted soils that have a lower intake capacity than sandy soils.

Rotary sprinklers usually have application rates of 1/4 to l/2 inch per hour and rarely cause runoff. Spray heads, on the other hand, typically have application rates between 1 and 2 inches per hour and may cause runoff on clay soils, especially on slopes greater than l0 percent. If runoff occurs, apply only half the total amount of water, then turn the system off for an hour or two to let the water soak in before applying the remainder of the water. Many irrigation controllers can be programmed to cycle irrigation applications.

The application rate of a sprinkler system can be determined by placing three or four rain gauges at random locations in an irrigated area for a predetermined length of time, usually 1 hour. Figure 1 illustrates gauge placement. By knowing the application rates of your sprinkler system, you can operate the system to apply a given amount of water and avoid wasting water. The average water level within the gauges is a measure of the output of the system in inches per hour (if the test was conducted over a 1-hour period). Repeat this procedure in each sprinkler zone, particularly if different types of sprinklers are used on different zones.

Adjust Sprinkler Heads as Needed

Improper adjustment of sprinkler heads not only wastes water but may also damage buildings or cause accidents if the water is allowed to spray onto buildings, streets, or sidewalks. Carefully adjust the radius and arc of part-circle sprinklers to prevent undesirable overspray. Check the system several times during the year to ensure proper adjustment.

 

Center pivot irrigation

Center pivot irrigation is a form of overhead irrigation consisting of several segments of pipe (usually galvanized steel or aluminum) joined together and supported by trusses, mounted on wheeled towers with sprinklers positioned along its length. The system moves in a circular pattern and is fed with water from the pivot point at the center of the arc. These systems are common in parts of the United States where terrain is flat. Most center pivot systems now have drops hanging from a u-shaped pipe called a gooseneck attached at the top of the pipe with sprinkler heads that are positioned a few feet (at most) above the crop, thus limiting evaporative losses. Drops can also be used with drag hoses or bubblers that deposit the water directly on the ground between crops. The crops are planted in a circle to conform to the center pivot. This type of system is known as LEPA (Low Energy Precision Application).

Pivot irrigation in progressOriginally, most center pivots were water powered. These were replaced by hydraulic systems (T-L) and electric motor driven systems (Lindsay, Reinke, Valley). Most systems today are driven by an electric motor mounted at each tower.

Center pivot equipment can also be configured to move in a straight line, where the water is pulled from a central ditch. In this scenario, the system is called a linear move irrigation system.

Lateral move irrigation

A series of pipes, each with a wheel of about 1.5 m diameter permanently affixed to its midpoint and sprinklers along its length, are coupled together at one edge of a field. Water is supplied at one end using a large hose. After sufficient water has been applied, the hose is removed and the remaining assembly rotated either by hand or with a purpose-built mechanism, so that the sprinklers move 10m across the field. The hose is reconnected. The process is repeated until the opposite edge of the field is reached.

This system is less expensive to install than a center pivot, but much more labor intensive to operate, and it is limited in the amount of water it can carry. Most systems utilize 4 or 5 inch diameter aluminum pipe. One feature of a lateral move system is that it consists of sections that can be easily disconnected. They are most often used for small or oddly-shaped fields, such as those found in hilly or mountainous regions, or in regions where labor is inexpensive.

Drip, or trickle, irrigation

Water is delivered at or near the root zone of plants, drop by drop. This type of system can be the most water-efficient method of irrigation, if managed properly, since evaporation and runoff are minimized. In modern agriculture, drip irrigation is often combined with plastic mulch, further reducing evaporation, and being also the means of delivery of fertilizer. The process is known as fertigation.

Deep percolation, where water moves below the root zone, can occur if a drip system is operated for too long of a duration. Drip irrigation methods range from very high-tech and computerized to low-tech and relatively labor-intensive. Lower water pressures are usually needed than for most other types of systems, with the exception of low energy center pivot systems and surface irrigation systems, and the distribution can be adjusted for uniformity throughout a field or for precise water delivery to individual plants in a landscape containing a mix of plant species. Although it is difficult to regulate pressure on steep slopes, the field does not have to be level. High-tech solutions involve precisely calibrated emitters located along lines of tubing that extend from a computerized set of valves. Both pressure regulation and filtration to remove particles are important. The tubes are usually black (or buried under soil or mulch) to prevent the growth of algae. But drip irrigation can also be as low-tech as a porous clay vessel sunk into the soil and occasionally filled from a hose or bucket. Subsurface drip irrigation has been used successfully on lawns, but it is more expensive than a more traditional sprinkler system. Surface drip systems are not cost-effective (or esthetically pleasing) for lawns and golf courses.

Drip irrigation is recommended for use on trees, shrubs, and flowers in the high- and moderate-water-use zones of the landscape to maximize efficiency. Several types of drip irrigation systems can be adapted to suit a variety of applications, from watering individual trees and shrubs to beds of annuals, herbaceous perennials, ground covers, or mixed borders.

Parts of a Drip System

In a drip system, water is distributed to the plants through small, flexible 3/8- to 3/4-inch-diameter plastic pipes and emitters or by perforated or porous pipe.

Emitters may be purchased separately from the tubing and placed in the line wherever watering is desired. Another option is to purchase drip tubing with emitters already installed at the factory, usually spaced 12 to 24 inches apart. Most emitters will discharge water at a rate of 1/2, 1, or 2 gallons per hour at a pressure of about 20 pounds per square inch (psi).

Perforated or porous pipe discharges water along its entire length to wet a continuous strip. By spacing pipes 12 to 18 inches apart, it is possible to wet a solid area. It is a good system for closely spaced plantings of annuals, herbaceous perennials, or ground covers.

Most drip systems include polyvinylchloride (PVC) pipe for the main lines and polyethylene (PE) tubing for distribution lines. Polyethylene tubing is flexible, easy to cut, and can be connected without glue or clamps. Emitters are installed by punching a hole in the polyethylene tubing and snapping the emitters into place.

A drip system must have a main valve to turn it on and off. This may be an automatic electric valve connected to a controller or a manual gate valve. You can also connect the drip lines directly to an outside faucet. However, when connecting the system directly to the faucet, use an automated timer to turn the system off after a preset length of time. Otherwise, you may forget and leave the system on for several days.

Two other necessary components of a drip system are a filter and a pressure regulator (Figure 2). A drip system uses small passageways to control the rate of water application so even tiny particles suspended in the water may cause clogging. To prevent clogging, use a screen filter with a 150- to 200-mesh screen. These components are usually installed below ground in a valve box.

Most drip systems are designed to operate at a pressure of about 20 psi. In comparison, household water pressure typically ranges from 40 to 100 psi. A pressure regulator installed immediately after the filter in the main irrigation line reduces the pressure in the line and helps to ensure efficient system operation.

Which Drip System Is Best?

Because so many different types of drip irrigation components are available, choosing the best system for a particular application is often difficult. The best advice is to keep your system as simple as possible and try to wet only those areas where water can be taken up by the roots of the desired plants.

For trees and shrubs, it is generally best to use a system that allows you to insert emitters wherever water is needed. The appropriate number of emitters per plant and flow rate per emitter depend on the size and type of plant. Generally, the larger the plant, the more water it requires. Table 1 lists plant heights and the number of emitters needed to deliver adequate water.

During very dry weather, an emitter system needs to run about three times per week for 4 hours each time to meet the optimum water needs of the plants. Keep in mind that some species require more water than others. Consider this when installing emitters.

For watering annuals, perennials, and ground covers, it is usually necessary to irrigate a solid area. This can be done using emitter lines with emitters spaced every 12 to 18 inches. By placing emitter lines 12 to 18 inches apart, a uniform wetting pattern can be achieved. Perforated or porous pipe spaced every 12 to 18 inches apart can also be used. In sandy soils, the lines will need to be closer together than in finer-textured clay soils. In annual flower beds, the drip lines can be laid aside during bed preparation and replaced afterward.

 

Subirrigation

Used in commercial greenhouse production, usually for potted plants, water is delivered from below, absorbed upwards, and the excess collected for recycling. Typically, a solution of water and nutrients floods a container or flows through a trough for a short period of time, 10-20 minutes, and is then pumped back into a holding tank for reuse. Subirrigation requires fairly sophisticated, expensive equipment and management. Advantages are water and nutrient conservation, and labor-saving through lowered system maintenance and automation. It is similar in principle and action to subsurface drip irrigation. The same concept of subsurface flooding and drainage is also being experimented with as an outdoor subirrigation method.

 

Hand Watering

Hand watering is not just for newly planted ornamental plants. It is also an effective and efficient way of applying water to selected plants that show signs of stress during dry periods. The direct application of water to the base of the plant, provided it is applied slowly enough to be absorbed by the soil, uses less water and is more efficient than sprinkler irrigation. To avoid runoff when using a hand-held hose, use a nozzle that divides the spray into rain-size droplets. Some nozzles have built-in spray pattern adjustments.

When watering by hand, apply about 5 gallons of water per 10 square feet, which is approximately the amount of water delivered by a % -inch garden hose operating for 1 minute at medium pressure. Watering small shrubs (less than 4 feet in height) for 1 minute with the hand-held hose should suffice. Larger shrubs (4 feet and up) will require slightly more water. Increase the watering time by 15 seconds for each foot of height over 4 feet. For large trees apply about 6 to 7 gallons for each 10 square feet of canopy area. For best results, check the output of your faucet by determining the number of seconds it takes to fill a 1-gallon jug and then estimating output per 60 seconds. If runoff occurs before you have applied the correct amount of water, move on to another spot and come back after the water has soaked in.