Growing Magazine - July, 2014

IRRIGATION & WATER MANAGEMENT

Is Overhead Irrigation a System of the Past?

By Nicole Alvarez


New sprinklers only use around 0.5 to 20 gallons of water per minute and 10 to 30 psi to operate.
Photos courtesy of Senninger Irrigation.

Drip irrigation systems may be highly efficient in water use, but new overhead sprinklers are almost as water and energy-efficient.

Most growers have strong opinions about drip and overhead irrigation systems. Some say that sprinklers could never be as efficient as drip. Others say that drip is incredibly expensive or that it is only good for some fields and crops.

In reality, both overhead and drip are extremely efficient irrigation systems. Choosing one over the other boils down to priorities and farm management practices.

In recent years, drip irrigation systems have gained a strong fan base due to their low flow rates and ability to save water. Advocates like the Maine Organic Farmers and Gardeners Association claim that a correctly installed drip system can use 50 to 80 percent less water than sprinklers. This makes it an attractive option for growers in locations with water restrictions or those who simply want to go green.

Drip irrigation does save water and energy. Still, growers looking for versatility, lower overhead costs and high yields may be better off sticking with overhead irrigation.

Dispelling the myths

Irrigation efficiency is about more than just pumping the smallest possible amount of water. A system's efficiency is determined by its performance and distribution uniformity, along with the crop's response to irrigation.

Efficiency is the reason many growers have become big fans of drip. Drip systems irrigate with flow rates around 0.5 to 2 gallons per hour and normally operate at pressures ranging from 10 to 30 psi. Drip puts water directly over or near the roots, so well-designed systems can boast distribution uniformity of 90 to 95 percent.

However, sprinkler systems can be almost as water and energy-efficient as drip systems.

Most fruit and vegetable growers are familiar with impact sprinklers. Compared to drip systems, these sprinklers normally require higher flows and pressures to operate, and wind easily affects their spray patterns. They also offer less uniformity. This makes drip an obvious choice for anyone looking to save on water and energy costs.

In reality, sprinkler systems use anywhere from 0.5 to 20 gallons of water per minute. Moreover, they only need 10 to 30 psi to operate, depending on the model selected. Well-designed systems with models made for wind and evaporation resistance can also irrigate with 90 to 95 percent uniformity.



Growers can use overhead irrigation systems for germination, frost control, fertigation and chemigation.

A side-by-side comparison of water use, energy use and distribution uniformity for drip and overhead irrigation would reveal that both systems are very efficient. Drip narrowly beats overhead systems in water savings when you take into consideration the amount of time each system has to operate.

That slight increase in water savings makes the switch to drip seem logical enough. However, a closer analysis shows that despite the benefits of drip irrigation, these systems often come with hefty equipment costs and limited applications.

System versatility

Irrigation systems are vital for crop germination and are frequently used to distribute fertilizers or chemicals. In colder climates, overhead systems are also used for frost protection.

Both drip and overhead systems can be used for fertigation or chemigation. However, drip can limit the options when it comes to product selection.

Growers can use foliar or soil-applied fertilizers with sprinklers. Foliar fertilizers are ideal for correcting nutrient deficiencies and reducing postharvest disorders. Foliar products take advantage of the leaves' effectiveness in transporting nutrients.

Certain soil conditions, such as high pH, low pH, drought, excessive moisture or cool temperatures, can make nutrient uptake through roots difficult. This means soil-applied fertilizers can keep crops healthy if the pH isn't limiting nutrient availability.

Overhead systems also provide better pesticide options. With sprinklers, growers can throw chemicals on the underside of leaves, where insects and their eggs may hide. This helps prevent plant diseases and fungal problems caused by common pests.

When it comes to germination, overhead sprinklers apply water over the entire soil surface in a pattern similar to rainfall. The rain-like pattern makes sprinklers suitable for germination and irrigation, which is why sprinklers are used throughout the growing season. Drip systems place water near the drip line and are not as effective at germinating small seeds. This is especially true for sandy soils, since water may not be able to disperse horizontally and encourage root expansion.

Many growers with drip systems rely on other means of wetting the soil before they attempt germination. Nature doesn't always cooperate, so growers may use overhead systems to imitate rainfall. If they decide to continue with drip for germination, they must pay attention to soil preparation, drip tape placement and irrigation scheduling.

Perhaps the biggest benefit of overhead systems is their ability to protect crops from frost damage in cold climates.

Sprinklers can protect crops from frost damage if evaporation is low but dew points are high in climates where temperatures drop to around 32 degrees Fahrenheit. They spray a consistent, uniform layer of water that freezes and crystallizes over plants. The freezing process releases about 80 calories of heat for every 0.03 ounce of water that freezes. The ice encases the plant and partially isolates it from the harsh temperatures.

Sprinklers are more economical than heaters, with some sprinklers generating 2.5 million kilocalories of heat per hectare per hour with only 5 kilowatts of electric power.

Less complicated farm management at a lower cost

After switching to drip irrigation, growers initially use less water, save money and get similar yields as they did with an overhead system. Once that first year passes, the grower realizes that the system is complex and high-maintenance. Replicating the first year's results requires several hours of labor and a keen eye for detail.

Drip systems need to function efficiently throughout the entire growing season. Any failure at a critical point in the production cycle can cause severe crop losses. Unfortunately, system failures are often the result of inadequate maintenance.

Drip systems need constant filter monitoring and frequent flushes or replacements to prevent plugging. Emitter orifices range from 0.2 to 2 millimeters and are easily plugged by algae, fertilizer deposits, and minerals like calcium and iron.

The system's filter should be flushed daily to prevent plugging, and an inspection of the entire system should be conducted weekly. Without proper filtration, water pressures will eventually crack the plugged lines. Chemicals also need to be incorporated into the grower's management strategy to dissolve mineral concentrations that can plug emitters.

Overhead systems require less filtration, since sprinklers have larger orifice sizes. This makes them suitable for growers who get irrigation water from lakes and rivers. The visible application pattern also makes it easier for growers to identify potential system issues before they adversely affect crop growth.

Drip systems are sensitive and more prone to damage than overhead systems. Rodents, weed cutters, laborers, mechanical harvesting equipment and even the sun can damage drip systems.

Overhead systems use PVC lines, steel piping and corrosion-resistant thermoplastics. Solid set installations have been known to last anywhere from 10 to 30 years. The underground tubing and piping of permanent solid set systems generally last around 30 years in the field, while risers and overhead sprinklers can last 10 to 15 years. Solid set systems also bring the added benefits of high automation and low labor requirements, adaptation to the rotation of crops, and recovery irrigation.

Minimal investments for a strong ROI

For large-scale commercial growers, overhead irrigation is more economically feasible.

Drip systems typically cost $500 to $1,200 or more per acre. Part of the total cost is capital investment, and another part is the annual cost of disposable components. In some fields, growers may need to replace sections of their drip tubing every season. This adds to the cost of removal and disposal or recycling. The numbers do not include the additional cost of frost protection or germination systems.

The cost of a solid set system depends on whether it's a permanent installation or a portable system. Permanent systems can be costly to install, since laterals need to be placed underground, with only sprinklers and risers above the ground. However, since the system is not moved around the field, labor costs are minimal.

Portable systems are less expensive, but they come with higher labor requirements. These systems use a lateral pipeline with sprinklers installed at regular intervals. When irrigation is complete in one area, the lateral line needs to be disassembled and moved to the next position.

While installation of a solid set system is not always cheap, one of the main advantages is the need for less tubing and equipment. In most solid set systems, sprinklers can be spaced 6 to 18 meters apart and still provide uniform application. Drip tubing needs to be set up around 1 meter to 0.5 meter apart.

In larger fields, a substantial amount of drip tubing is needed to irrigate crops, and the tubing may only last a few seasons. According to an article in California Agriculture, the cost of the drip tubing and emission devices is approximately 25 percent of the initial expense. However, these components have the shortest durability.

Breaking down the benefits

Is overhead irrigation a system of the past? Certainly not. The benefits of drip irrigation are many, and it certainly has its place in many fields, but it cannot completely replace overhead irrigation.

An irrigation system needs to be cost-effective, and it needs to be designed for each grower's goals, crops, management style and soil. Growers need to determine what's important to them and their business, whether it's lower labor costs or lower water use, before moving forward with any system.

It's important to remember that no field is suited for every type of irrigation, and most growers have different expectations for their systems. Sprinklers cannot irrigate every crop, and drip is not a one-size-fits-all solution. Each system has advantages and disadvantages.

Overhead irrigation is a versatile and efficient system. A comparison of its benefits and disadvantages should help any grower quickly realize where their priorities lie:

Advantages

  • Visible confirmation of system uniformity
  • Larger area of coverage
  • Long product life (often over 10 years)
  • Used for germination, fertigation, chemigation and frost control
  • Short irrigation intervals
  • Lower maintenance costs

Disadvantages

  • Higher potential for evaporation and wind drift issues compared to drip
  • Waters both crops and weeds
  • Cannot be used on crops susceptible to foliar diseases
  • Higher potential for runoff and erosion compared to drip

The author is a technical writer for Senninger Irrigation in Clermont, Florida.