Effective irrigation systems – horses for courses

Key factors determine the application, irrigation efficiency and economics of cost-effective sprinkler systems, whether hand-movable, solid set or centre pivot.

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The most suitable and cost-effective irrigation system for a specific application is determined by the size of the land to be irrigated, the crop, the water source, energy and labour. Three broad categories are compared here.

READ: Irrigation breakthrough: cutting water and energy use in orchards

Irrigation system breakdown

Solid set versus quick-coupler or dragline
According to the ARC Agricultural Engineering Institute, the capital cost of a quick-coupling movable pipe sprinkler irrigation system is between R10 000/ha and R12 000/ha, and between R11 000/ha and R13 000/ ha for a dragline irrigation system, depending on sprinkler spacing, application rate and cycle time.

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The same source quotes the price of a solid set system as R24 000/ha to R26 000/ha. However, skilful system design and the correct component choice can keep the price down without sacrificing efficiency.

  • Wear, tear and system maintenance

Wear and tear is higher in a system where pipes, sprinklers and ancillary components are moved by hand than in a solid set system. With any portable system, maintenance, repair and component replacement from wear and tear is a significant expense. What is more, faulty components are often not identified until they malfunction or leak badly.

A part of or the entire system may then have to be shut down until it can be repaired or replaced. This wastes valuable production time and can affect application uniformity and irrigation scheduling.

  • System cost and size

In both movable and solid set systems, the cost is roughly proportional to its size. Thus, cost per hectare tends to remain constant. All other factors being equal, the capital investment of a solid set system is generally higher than that of any other type of irrigation system, to some extent offsetting its other advantages.

  • Labour

Movable pipe and dragline irrigation systems are subject to specific labour requirements, bringing their own set of problems.
Today, the cost of labour is too high for workers to be used optimally to move pipes and ancillary components in portable and dragline systems. Overtime payment for after-hours work also applies. In addition, strikes and stayaways can cripple hand-portable irrigation.

A solid set system is far less labour-intensive. Human intervention is largely confined to opening and shutting taps according to a specific routine. In the event of labour unrest, the farmer can take care of the routine work himself.
Solid set systems also lend themselves to being computer-controlled.

  • More reliable, accurate and flexible irrigation scheduling

When water availability is limited or restricted, irrigation scheduling and efficient water use become critical. For example, irrigating at night or in the cool of the early morning reduces evaporation.

Annual electricity cost on a crop such as sugarcane may amount to R3 200/ ha in some parts of the country. Irrigating during low-demand times, when lower electricity rates apply, can reduce the cost dramatically. Unfortunately, these times may also coincide with periods when overtime rates apply.

  • Crop-friendly and worker-friendly irrigation system

Some of the disadvantages of a hand-portable system include trampled crops and soil compaction in the land, which to some extent offset its low capital cost. Wildlife in sugarcane, such as bushpigs, leopards, pythons and mambas, can be dangerous for workers moving pipes. In extreme cases, this may lead to workers understandably refusing to go into the lands, or to surreptitiously altering irrigation schedule.

  • A land on fire?

Arson is a problem in some parts of the country. If a land is on fire, the particular block may need to be irrigated immediately to help contain the blaze. With a dragline or movable pipe system, not only is there a danger to the workers, but moving it is often impossible as the sprinklers and pipes may be in another part of the land, or labour might not be available.

A centre pivot may be in a different sector and impossible to move to the fire in time. Should the blaze get out of hand, the system can be badly damaged or even destroyed. Only with a solid set system are the sprinklers always available where needed; all it takes is to open the right tap.

  • Saving on labour

A solid set or a centre pivot system is far less labour- intensive than either a dragline or a movable pipe system. This is reflected in higher efficiency in the production records of a large farming enterprise near Malelane in the Mpumalanga Lowveld The business’s centre pivot systems give 10% higher yields than those of the dragline systems.

For this reason, and because of its low labour requirements, either a centre pivot or a permanent set system seems to be the logical option when planning a new irrigation system or upgrading/replacing an old one.

Centre pivot or solid set?
Centre pivot and solid set overhead irrigation systems have important advantages over hand-portable irrigation systems, whether movable pipe or dragline. Their very low labour requirements are mostly limited to opening and closing taps and switching the entire system on according to a schedule. These tasks also lend themselves perfectly to computer control.

A well-designed centre pivot system on an optimally sized and shaped land can be highly efficient in terms of application uniformity. The size and shape is, however, crucial: circular, with a radius of 282m and an area of 25ha is optimal. Because the towers in a centre pivot system are a fixed distance apart, the size of the irrigated area can only change incrementally.

Starting with a very small system, a centre pivot system’s cost/ha irrigated decreases as its size increases towards the optimum. After that, it still decreases but the efficiency declines as the rate or application (mm/minute) around the outer edge starts exceeding the infiltration rate of water into the soil.

As a centre pivot land is circular, it leaves the four corners of a square land unirrigated, although together they represent 22% of its total area. Ingenious systems with mobile boom sprayers at the outer tips of the centre pivot have been devised to irrigate these corners, with variable results and degrees of reliability, as well as greatly increased cost.

In contrast, the system cost of a well-designed solid set system is roughly proportional to its size, the cost/ha tending to remain constant. This has an advantage in that an irregularly-shaped land can be optimally utilised, with little wasted or unirrigated land.

Centre pivot systems also have other problems. To operate, a centre pivot system requires its full quota of water flow for the entire cycle, which may be a problem during drought periods. Scheduling flexibility is low, making it practically impossible to use the system for evaporative cooling during extremely hot, dry and windy days when crops are severely stressed.

While it is less of a problem for a homogeneous crop, this inflexibility is a serious obstacle where various crops with differing water requirements, or the same crop of different ages, are planted in different sections under the centre pivot. It is also difficult to accommodate erratic rainfall in centre pivot scheduling.

By contrast, scheduling flexibility is one of the most important advantages of a solid set system. It is practical to irrigate selected parts of the land when only a portion of the normal irrigation water allowance is available. A solid set system also allows a farmer to carry out quick evaporative crop cooling during extremely hot weather, as well as frost protection.

A centre pivot system does not even approach this flexibility. As centre pivot systems are constructed of metal, corrosion is an ever-present problem. If the system is used for fertigation or chemigation, special treatment of the metal components in contact with the water is necessary, increasing the cost considerably.

A centre pivot system, let alone its copper cable, is an ideal source of relatively easily accessible ‘scrap’ metal for well-organised scavengers. Being mechanical in nature, centre pivot systems suffer wear and tear and therefore require far more maintenance than solid set systems.

In addition, the access road and tracks for the wheels also need maintenance. The land needed for the access road, tower and tracks for the system’s wheels is permanently lost to cultivation, reducing the total growing area by 2%, added to the 22% already lost to the corners of a square land. The access roads and wheel tracks are also prone to puddling, waterlogging and erosion.

Centre pivot flexibility
In contrast to a centre pivot system, where the above-ground structure can be moved out of the way for unhampered access for soil cultivation and crop production operations, the riser pipes of a solid set system are fixed. These hamper access for large tractors and implements.

There is a solution, however: a quick coupling-decoupling inverted T-adaptor fitted to the lateral lines below the surface enables the riser pipes to be speedily removed or replaced. Whereas centre pivot systems are typically limited to a circular shape, and reasonably level land, a solid set system is highly flexible in terms of the size, shape and contours of the land to be irrigated, A breakthrough development by Nelspruit- based company, Floppy Sprinkler, does away with the need to seasonally remove or replace stand pipes.

Its revolutionary overhead system places the sprinklers on their HDPE tubing lines attached to tensioned steel cable high enough above the ground for the highest tractor or implement to work underneath, unhindered by infrastructure. By eliminating the need for stand pipes and leaving the land surface clean and uncluttered, this system combines the best of all worlds.

Sprinkler costs
Sprinkler costs could vary depending on each specific farming operation. When calculating costs for a planned irrigation system, the following expenses should also be considered:

  • Cost of a replacement kit (every 60 months);
  • Labour for servicing;
  • Temporary replacement unit for one taken to the workshop for servicing;
  • Loss of sprinkler efficiency due factors such as wear, jamming, broken springs, blockages and leaks;
  • Higher electricity bills due to high system pressure requirements;
  • Evaporative loss because of mist formation;
  • Loss of application uniformity due to wind drift.