Irrigation Systems and Equipment

A well-designed irrigation system will match water supply and distribution capacity with crop water requirements throughout the season. It is recommended to consult with an irrigation system design specialist before investing money into a new system or equipment upgrades. The information below gives a broad overview of the irrigation equipment used by New England farms. Sprinkler and drip irrigation systems have common components, such as the pumping unit, control head items (e.g., filters, pressure gauges, check-valves), and piping. Differences between systems are just the physical characteristics and the equipment that ultimately applies the water, either sprinklers or emitters.

Pumping Units: The pumping unit, which consists of a pump coupled to a power source, draws water from the supply source and pressurizes it for delivery through the irrigation system. A centrifugal pump or a submersible or deep well turbine pump may be used. Normally, a centrifugal pump cannot be placed more than 20 feet above the water line. Pumps are available in a wide range of flow capacities and delivery pressures. Selecting the correct style of pump and the proper size will depend on site characteristics and the final irrigation system layout. 

Since sprinklers usually have higher discharge and pressure requirements, pumps for sprinkler systems are generally larger than those for drip systems. This is one reason why drip systems are more energy-efficient than sprinkler systems. When growers tie into municipal or household systems, where water is already pressurized, a pumping unit is typically unnecessary. The primary concern is that the supply (volume and flow rate) is large enough. Pumping and pressurizing both surface and ground water can be expensive and should be incorporated into the pricing of the system design. 

Control Head: The control head is the combination of items that control, measure, or treat the water. Control heads can range from simple, manually operated heads with a single valve to quite sophisticated heads with automatic controllers and sensors, water meters, pressure regulators, filters, and nutrient or chemical injection equipment. Control heads for drip systems are often more complex because of the necessary water filtration equipment. Automated equipment can be adapted to some systems to measure soil moisture, start the irrigation system, and send water to the appropriate areas. The more complex and automated the control head and irrigation system, the greater the initial capital investment.

Pressure regulators maintain the desired pressure as the water flows through the system. They are required to supply the appropriate pressure to the distribution system and may be needed to protect filters and other components. They should be sized according to the rate of water flow. 

Backflow preventers or check valves: These devices allow water to flow in one direction only. They are used to prevent water from flowing backward into a water source after the system is shut off. This is especially important to prevent injected fertilizer or pesticides from contaminating water supplies. They are required by law for systems with injectors. In some areas they may be required even if no injector is used. 

Filters: The choice of filter is based on the quality of the water passing through the system and application requirements. For drip irrigation, typical screen mesh size is 150 mesh (100 micron). The filter should be sized for the longest application flow rate needed. Water particulates change through the growing season and filter flow rates are affected with dirtier water and more suspended particulates. It is better to design a system larger than is required, compared to smaller. Screen, disc, and sand filters are appropriate for various irrigation systems.

• Screen or mesh filters are inexpensive and easy to install. Mesh filters work well if there are moderate to low levels of contaminants in the water, such as with a well or public water supply. Screen filters have a limited ability to store contaminants. If the water comes from a river or pond, the screens will probably have to be flushed often. This could result in considerable down time and labor. Mesh screen sizes are between 20 and 200 mesh. The larger the number, the smaller the particle the screen will filter out. The screens are made from stainless steel, nylon, or polyester. For drip tape, follow the mesh size recommendation of the tape manufacturer.
• A disc filter consists of a series of discs that are stacked on top of each other. The discs have microscopic grooves that radiate out from the center and filter out particles. Equivalent mesh sizes are between 40 and 600 mesh. They require less water for cleaning than do sand filters.
• Sand filters are preferred over screen filters if the contaminant load is moderate to heavy. A sand filter can run longer than a screen filter before it needs to be cleaned by back flushing. This results in less down time and labor. The filters can be set up in pairs so that clean water from one filter is used to back flush the other filter. A screen filter can also be used to provide clean water for back flushing a sand filter. The correct filter size is important. Under-sizing will increase pressure loss and there is considerable down time for cleaning. It is better to be too big than too small. The sand used for drip irrigation should be of the correct type, made up of crushed, sharp edged silica or granite. 

Piping: Mainline pipes deliver water from the pump to submains (which are not always needed) and laterals. Mainlines can be metal pipe, PVC pipe, or lay-flat hose. Proper pipe diameter is a function of the pumping rate and distance the water must travel. Larger diameters can move greater volumes and have lower friction losses but are more expensive than smaller pipes. The longer the pipe and the more elbows or junctions, the more pressure loss due to friction. Lateral pipes deliver water from the mainline or submain lines to the sprinkler or emitters. They are of the same 3 general types as the mainline pipes but are usually smaller. Lateral pipe sizes are designed to minimize pressure losses so that sprinkler or emitter discharge at the far end of the lateral stays within 10% of the sprinkler or emitter discharge near the mainline to provide uniform water application. 

Overhead/Sprinkler Systems

The basic types of sprinklers include rotating sprinklers, stationary spray-type nozzles, and perforated pipe. Rotating sprinklers, such as the slowly rotating impact-driven sprinkler, are most common. Fixed spray nozzles are becoming more popular, although they are most widely used in landscaping. Perforated pipe is the simplest sprinkler, consisting of a pipe with numerous holes through which the water sprays. 

Rotating impact sprinklers come in many sizes and variations to meet various design conditions. Some micro-sprinklers operate at pressures as low as 10 psi, while the large gun types require pressures exceeding 80 psi. Sprinkler discharge ranges from a few gallons of water per minute to 1,000 gallons per minute for a big gun. Wetted diameters can range from only a few feet to several hundred feet. When irrigating new plantings, avoid big guns with high pressures. 

When selecting an irrigation system solely to provide water to the plants, use a high discharge rate with large droplets to minimize evaporative loss and the time that foliage is wetted, but do not use such a high rate as to cause puddling in the field. 

Pressure, discharge rate, and wetting diameter are the most significant characteristics of a sprinkler; but nozzle size, jet angle, wind speed and direction, sprinkler overlap, and sprinkler rotation speed are also important. These characteristics determine water application rates, sprinkler spacings, and water droplet sizes. For uniform water application, sprinklers are generally spaced so that 50%–60% of their wetted areas overlap. Annual operating costs of irrigation increase with increasing pressure and discharge requirements. Large gun-type sprinklers will require up to twice as much fuel or electrical energy as smaller sprinklers to apply equal amounts of water.

Drip Irrigation

Drip (or trickle) irrigation is used on a wide range of vegetable crops. Drip irrigation is a method of slowly applying small amounts of water directly to the plant root zone. Water is applied frequently, often daily or several times a week, to maintain favorable soil moisture conditions. The primary advantage of drip irrigation systems is that water use is more efficient than with overhead sprinkler irrigation systems. In many cases, one-half or less of the water applied with sprinkler or surface systems is required with drip systems because there is no evaporation loss from the soil surface. Most of the water conservation from drip irrigation occurs in the early season. The difference in the amount of water used between drip and sprinkler decreases once a full canopy is achieved. 

Drip irrigation systems have several other advantages over sprinkler and surface irrigation systems. Low flow rates and operating pressures are typical for drip systems. These characteristics lead to lower energy and equipment costs. Once in place, drip systems require little labor to operate, can be automatically controlled, and can be managed to precisely apply the amount of water needed by the crop, which also reduces operating costs. With most drip systems, disease and insect damage is reduced because leaves are not moistened by irrigation water. In addition, the areas between rows remain dry, which reduces weed growth and water use, as well as pests and pathogens in these areas of the field. Another advantage is that field management operations can continue during irrigation.

While there are several advantages to drip irrigation in vegetable production, various items also should be considered during design, installation and use. 

Slope: A slope of 2% or less is the ideal for drip irrigation. Many fields in New England have slopes greater than 2%. A difference of 2.3' in elevation will change water pressure by 1 psi, decreasing as elevation increases and vice versa. The length of lateral lines, the pump size, and pressure regulators are chosen based on the slope. It is best to run the rows horizontally across a slope, but if rows must run up and down, it is best to have the water flow downward through the drip tape. This probably requires that water be pumped to the top of the field so it can flow downward from there. With slopes greater than 5%, pressure-compensating drip tape is needed.

Drip tape selection: Drip tape should apply water uniformly throughout the crop root zone. The emitters should be close enough so that there is uniform wetting of the soil. The spacing of the emitters is affected by soil type. Drip tape should have a coefficient of manufacturing variation (CV) number that reveals how much variation in uniformity there is from one emitter to the next. A CV of 0.05 is considered excellent and a CV between 0.05 and 0.1 is acceptable. The rate of water delivery is a function of the size and spacing of the emitters and typically ranges from about 2.5–5.0 gallons per minute per 1,000' of tape (0.25–0.5 gpm per 100'). The choice of tape thickness, measured in mils, is based on how long you want the tape to last and the expected highest water pressure in the lines. The longer the tape is expected to be in the ground, or the higher the pressure in the lines, the thicker the tape should be. Tape thickness is usually between 4 and 10 mil, though thickness of up to 25 mil can be purchased. Tape can be reused for 2–3 years, but the labor costs of retrieving and cleaning the tape usually make this uneconomical for annual vegetable crops.

Drip tape placement: The length of the drip lines is another important consideration. The length is determined by the pump size, the field size(s), and the slope of the land. Any one of these factors will influence wetting uniformity because the emitters will discharge water at different rates if there are changes in pressure along the line. Because of variation in water pressure, tape is rarely laid out longer than a length of 400' on fairly level land and less on slopes. 

The tape should be placed as close to the plant as is practical for the specific crop. This is critical on porous soils, but if necessary, tape can be placed 6”–12" from the plants on soils with good lateral water movement. Tape is often laid between double rows of crops such as peppers. The soil will be wetter on the side of the crop where the tape is and most of the roots will be concentrated there. When possible, the tape should be placed so that the emitters are pointed upward so that any particulate matter will settle away from the emitters after the water stops flowing.

Tape can be placed on top of the soil or a few inches below the surface. If tape is on the surface, it is easy to observe wetting patterns and to make repairs if needed. The disadvantages are that there is greater evaporation in the initial stages of the crop's growth and the tape is more likely to be damaged by production practices, wind, and animals. As tape is heated by the sun during the day, it expands and takes on a snake-like pattern in the row. This is particularly a problem with higher temperatures under plastic. This can be avoided by burying the tape.

Drip Irrigation Maintenance and Challenges: The equipment used in drip systems can present potential problems. Insects, rodents, and people can damage drip irrigation equipment. Pressure regulation is critical, and filtration is required. The drip system, including the pump, headers, filters, and connections, must be checked and ready to operate before planting. Failure to have the system operational could result in costly delays, poor plant survival or irregular stands, and reduced yield. Drip systems cannot be used for frost control. 

Emitter clogging remains the most serious problem with drip irrigation systems. As noted earlier, clogging can be attributed to physical, chemical, or biological contaminants. Filtration and occasional water treatment may be necessary to keep drip systems from clogging. Bacteria can grow inside drip irrigation tubes and form a slime, known as a biofilm, that can clog emitters and can serve as a source of plant and human disease-causing pathogens. Algae present in surface waters and in high iron wells can also clog emitters. Biofilms and algae can be effectively controlled by treating the lines with antimicrobial pesticides. Periodic treatment before clogging develops can keep the system functioning efficiently. The frequency of treatment required depends on the quality of the water source. 

Irrigation water containing high concentrations of iron (greater than 1 ppm) can also result in clogging problems due to the types of bacteria that "feed" on dissolved (ferrous) iron. The bacteria secrete a slime called ochre that may combine with other solid particles in the drip tape and plug emitters. The precipitated (ferric) form of iron, known commonly as rust, can also physically clog emitters. Treating water containing iron with an antimicrobial product (such as chlorine) can oxidize the dissolved iron, causing the element to precipitate so that it can be filtered and removed from the system.

Fertigation 

Fertigation is the injection of soluble fertilizer into irrigation water. Nitrogen (N) and potassium (K) are available in liquid or soluble solid form and can be applied through a drip system. Phosphorus, if needed, is usually broadcast at the beginning of the season. The need for supplemental N can be determined using the Pre-sidedress Soil Nitrate Test (PSNT) as it is with other application methods. Samples for the PSNT should be taken from under the plastic, if used. Use a soil sampler to punch a small hole in the plastic and remove a core of soil, being sure to avoid cutting the irrigation tape.

With conventional topdressing or sidedressing, it is common to apply all the N needed by a crop in one or two applications. With drip irrigation, it is convenient to apply small amounts of N weekly or even daily, which is desirable from a N management standpoint. For example, if you want to apply about 50 lb N per acre, you can inject a little over 7 lb N per acre per week for 7 weeks, or about 1 lb per day if you prefer. Small weekly applications provide for more efficient crop use of N than one or two larger applications. Daily application offers little advantage over weekly application but may be necessary if the injector cannot inject a week's worth of N during the appropriate irrigation run time. To prevent leaching, the irrigation system should not run longer than necessary to effectively wet the root zone of the crop. Before injecting fertilizer, the entire system should be filled with water at full operating pressure. When all the fertilizer has been injected, the system should run long enough to flush all fertilizer from the lines. If fertilizer is left in the lines, clogging may occur due to chemical precipitates or growth of bacterial slimes.

Fertigation and Water Quality Parameters

Various water quality parameters can present challenges for effectively fertigating through drip lines. Certain fertilizer materials may react with chemicals in irrigation water. If the water pH is below 7.0, there is little potential for problems, but at pH 8.0 and above, the risk is high. If the water contains calcium and magnesium at levels above 40–50 ppm, they are likely to react with phosphorus, if it is present in the fertilizer, causing precipitation of phosphates. If fertilizer containing calcium is added to water with concentrations of bicarbonates above 2 meq/liter, calcium carbonate may precipitate. Sulfates in fertilizers can also react with calcium in the water resulting in the precipitation of gypsum. These precipitates can all clog emitters.

Phosphorus- and sulfate-containing fertilizers, if needed, should be applied before planting because leaching is not a concern. Nitrogen is the element that is most appropriate for injection into trickle irrigation water. Calcium nitrate has the potential to cause clogging if the water pH and bicarbonate levels are high, as noted above. If calcium nitrate causes clogging, potassium nitrate or urea can be used as an alternative N source.

Water testing labs can analyze water for pH, calcium, magnesium, and bicarbonates. You can also perform a simple test: Mix fertilizer into a container of irrigation water at the same concentration it will be after injection into the trickle system. Cover the mixture to exclude dust and let it sit for at least the length of time it will be in the system before it reaches the soil. If the water becomes cloudy or a precipitate collects on the bottom of the container, you can expect this to happen in the irrigation system and will likely cause clogging. If it is necessary to lower the water pH, acid can be injected into the irrigation water. This requires special handling precautions and special injection equipment. Be sure to carefully follow directions to avoid personal injury or damage to crops or equipment.