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. Drip (or trickle) irrigation is used on a wide range of vegetable crops. 

 

Drip Irrigation System: Benefits, Operations And Maintenance

The primary advantage of drip irrigation systems is that water use is more efficient than with sprinkler or surface 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. In addition, substances applied through the drip irrigation system, such as pesticides and fertilizers, are conserved along with water.

 

BENEFITS OF DRIP IRRIGATION SYSTEM

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 apply precisely 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.

 

DISADVANTAGES OF DRIP IRRIGATION SYSTEM

There are also potential problems with drip irrigation systems. Most drip irrigation systems require a higher level of management than other irrigation systems.

 

Moisture dispersal throughout the soil is limited, and usually a smaller soil water reserve is available to plants. Under these conditions, the potential to stress plants is greater than with other types of irrigation systems. Drip systems must be carefully managed to avoid localized moisture stress.

 

The equipment used in drip systems also presents potential problems and drawbacks. Drip irrigation equipment can be damaged by insects, rodents, and laborers.

 

Pressure regulation and filtration require equipment not commonly found on sprinkler or surface systems. The drip system, including 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.

 

Calculating the length of time required to apply a specific depth of water with a drip irrigation system is more difficult than with sprinkler systems.

 

Drip systems add additional cost for processing vegetables, are not adapted to drilled crops such as peas and, therefore, may not be economical for these crops

 

Drip irrigation is especially effective when used with plastic film or organic mulches. Unlike sprinkler systems, trickle systems apply water to only a small portion (mulched) of the total crop acreage.  Usually, a fair assumption to make is that the mulched width approximates the extent of the plant root zone and should be used to calculate system run times for most vegetables.

 

Operational Issues of Drip Irrigation Systems

Water is carried through plastic tubing and distributed along the tubing through orifices or devices called emitters. The emitters dissipate the pressure from the system by forcing the water exiting from an emitter through orifices, tortuous flow paths, pressure reducing flow paths, or long low paths, thus allowing a limited flow of water to be discharged.

The pressure-reducing flow path also allows the emitter diameter to remain relatively large, allowing particles that could clog an emitter to be discharged.

 

Insect damage to thin-walled polyethylene drip tubing or “tape” is a major problem. Ants, wireworms, earwigs, mole crickets, field crickets, grubs and other insects typically damage drip tape by chewing holes through the side walls. This damage destroys the integrity of the tape, resulting in small to massive leaks that may result in poor moisture distribution and soil erosion.

Other types of drip tape damage may be mistaken for insects. For example, rats, mice, gophers and birds can chew, gnaw or peck holes in thin walled polyethylene tapes. Damaged tape should be inspected under magnification to provide clues to the source prior to taking action to remediate the responsible agent.

 

Although modern emitter design reduces the potential for trapping small particles, emitter clogging remains the most serious problem with trickle irrigation systems. Clogging can be attributed to physical, chemical, or biological contaminants. Filtration and occasional water treatment may both be necessary to keep trickle systems from clogging.

 

Bacteria can grow inside trickle irrigation tubes and form a slime that can clog emitters. Algae present in surface waters can also clog emitters. Bacteria and algae can be effectively controlled by chlorination of the trickle system.

 

Periodic treatment before clogging develops can keep the system functioning efficiently. The frequency of treatment depends on the quality of the water source. Generally, two or three treatments per season is adequate.

 

Irrigation water containing high concentrations of iron (greater than 1 ppm) can also result in clogging problems due to 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 trickle tubing and plug emitters.

The precipitated (ferric) form of iron, known commonly as rust, can also physically clog emitters. Treating water containing iron with chlorine will oxidize the dissolved iron, causing the element to precipitate so that it can be filtered and removed from the system.

 

DRIP IRRIGATION SYSTEM MAINTENANCE AND TREATMENT

Chlorine treatment should take place upstream of filters in order to remove the precipitated iron and microorganisms from the system. Take care when adding chlorine to drip irrigation systems, however, since concentration at or above 30 ppm (parts per million) can be toxic to growing plants.

 

Chlorine is available in either gas, liquid, or solid forms. Chlorine gas is extremely dangerous and not recommended for agricultural purposes. Solid chlorine is available as granules or tablets containing 65 to 70% calcium hypochlorite. Liquid chlorine is available in many forms, including laundry bleach and postharvest wash materials. Liquid forms typically contain between 5% and 15% sodium hypochlorite.

 

Use chlorine only if the product is labeled for use in irrigation systems.

Because chlorination is most effective at pH 6.5 to 7.5, some commercial chlorination equipment also injects buffers to maintain optimum pH for effective kill of microorganisms. This type of equipment is expensive but more effective than simply injecting sodium hypochlorite solution.

 

The rate of chlorine injection required is dependent on the number of microorganisms, the amount of iron in the water source, and the method of treatment being used.

 

For managing dissolved iron and microbes in the water source, one of the following basic strategies is suggested as a starting point:

 

Drip Irrigation System Iron Treatment

Inject liquid sodium hypochlorite continuously at a rate of 1 ppm (parts per million) for each 1 ppm of iron in irrigation water. In most cases, 3 to 5 ppm is sufficient.

 

Drip Irrigation Treatment for Bacteria and Algae

Inject liquid sodium hypochlorite continuously at a rate of 5 to 10 ppm (parts per million) where the biological load is high.

 

Inject 10 to 20 ppm during the last 30 minutes of each irrigation cycle.

 

Inject 50 ppm during the last 30 minutes of irrigation cycles one to two times each month.

 

Super chlorinate (inject at a rate of 200 to 500 ppm) once per month for the length of time required to fill the entire system with this solution and shut down the system. After 24 hours, open the laterals and flush the lines.

 

Chlorine can be injected using many types of fertilizer/pesticide injectors, including positive displacement injection pumps. These types of pumps are powered by gasoline or electric motors and include piston, diaphragm, gear or lobe, and roller (or peristaltic) types.

The injection rate for positive displacement injection pumps can be calculated from the following equation:

 

Injection rate of chlorine solution in liters per hour =

[(0.006) x (desired chlorine concentration in ppm) x (irrigation liters per minute)] / % chlorine in bleach or concentrate

 

As an example, assume household bleach (5.25% sodium hypochlorite) is being used as a chlorine solution, that a treatment level of 5 ppm of chlorine is desired, and that the trickle system has a 757 liters per minute flow rate.

 

Injection rate of chlorine solution in gallons per hour =

[(0.006) x (5 ppm) x (757 liters/minute)] / 5.25% = 4.315 liters chorine per hour

Proportional injectors are also commonly used to inject chlorine. Proportional injectors are powered by the water pressure of the irrigation system and inject materials at a rate which is proportional to the irrigation system flow rate or system pressure. Injection rates are often adjustable and are usually specified as ratios, percentages, or ppm.

 

Injection rate of chlorine solution in ppm concentrate=

[(100) x (desired chlorine concentration in ppm)] / % chlorine in bleach or concentrate

As an example, assume postharvest wash material (12.5% sodium hypochlorite) is being used as a chlorine solution and that a treatment level of 10 ppm of chlorine is desired.

 

Injection rate of chlorine solution in ppm concentrate = [(100) x (10 ppm)] / 12.5% = 80 ppm

 

It is important to note that both liquid and solid forms of chlorine will cause water pH to rise. This is critical because chlorine (sodium hypochlorite) is most effective in water at pH 6.5-7.5. If water pH is above 7.5, it must be reduced to 6.5-7.5 for chlorine injection to be effective as a disinfectant.

 

Important Notes

1. Approved backflow control valves and interlocks must be used in the injection system to prevent contamination of the water source. This is an absolute requirement if a public water source is used.

 

2. Chlorine concentrations above 30 ppm may cause phytotoxicity.