Working towards efficient irrigation

To save water, money and energy, a management programme is essential

On the spot: the hose reel remains at the heart of most irrigation systems in the UK. Photo: Briggs Irrigation
On the spot: the hose reel remains at the heart of most irrigation systems in the UK. Photo: Briggs Irrigation

Water has been described as the most important input for field crops. A lack of water can affect yield and quality, and without the right amount crops may fail altogether.

Irrespective of whatever climate change may bring, the water deficit in the southern part of the UK currently comes to around 300-350mm - this being the difference between average summer rain and what plants will generally consume through evapotranspiration. That average deficit translates to around 750,000 gallons of water (or 3,500,000 litres) on a hectare of planting through the summer.

Of course, there are considerable variations between wet and dry summers, but the difference in value between a vegetable crop that is 95 per cent on specification and high yielding, and one that is inconsistent and patchy, is so great that few growers, especially those in the East and South, conduct their businesses without access to irrigation.

Today, therefore, the big question is not so much "Should I invest in irrigation?" but rather "How can I bring my system up to date and improve its efficiency?" Efficiency is important if for no other reason than that it is necessary to demonstrate it as part of the abstraction renewal process.

Scheduling programmes

To strive for efficiency sounds like an eminently desirable objective, but it can mean different things to different growers depending on their individual circumstances, crops and the type of irrigation system they use.

Upgrading water pumps may bring a saving in energy costs; comparing and exchanging booms with drip irrigation or rainguns can bring about savings in water use or, alternatively, reduce capital expenditure. But to use water, money and energy efficiently, a scheduling programme is required and, to achieve quality crops, a uniformity of water delivery is usually essential.

There is one forum in which machinery manufacturers, dealers, consultants, academics and growers meet to exchange thinking on irrigation, and that is the UK Irrigation Association.

The diagram above, which is taken from the association's guide to irrigation efficiency, illustrates how optimising equipment alone is not enough to guarantee efficiency; scheduling and management are also required. However, the first step in improving efficiency is to take an in-depth look at the system already in place.

Rainguns and economics

Rainguns have always been the mainstay of irrigation systems in England and Wales. They have long been considered versatile, robust and easy to manage but they do have a reputation in some quarters for being wasteful with water. When well managed, that reputation can be refuted.

Figures from the UK Irrigation Association suggest the typical capital cost for a hose reel fitted with a raingun is about £1,000 per hectare.

Conversely, the "in-field" cost of a fully automated trickle irrigation system for potatoes - including filtration, fertiliser injector, remote control and disposable lateral tapes installed in each ridge - is estimated at £2,000-£3,000 per hectare.

However, rainguns are labour- and energy-intensive when compared to trickle-fed systems. They operate at much higher pressures and require input to move them around the field. The typical energy cost of applying water on potatoes through a hose reel and raingun is currently £0.36 per cubic metre compared with only £0.06 per cubic metre for trickle systems. According to the UK Irrigation Association, this demonstrates just how important it is to include ongoing costs and not to just rely on capital costs when comparing systems.

Rainguns remain the favoured system in the UK, not least because of lower capital cost and flexibility. Companies such as RST Irrigation, in Brandon, Suffolk, and Wright Rain, in Ringwood, Hampshire, are well established in the UK as suppliers of this type of equipment. RST supplies in excess of 35km of the familiar aluminium pipes annually, but year by year, hose reels increase in popularity, size and robustness.

Trickle irrigation systems are well established in the soft fruit, orchard, salad and glasshouse sectors. This so-called "drip system" is sometimes hailed as the future of irrigation because it saves energy by reducing pressures from six to 1.5 bar. Others claim that it reduces water use, increases yield and improves crop quality.

Of these claims, the last is perhaps one reason why many of the growers who have switched to this method are now producing high-quality premium-grade produce that meets supermarket requirements.

When installing a trickle irrigation system a consultant can help to ensure the balance of all the components. Among the specialist companies in this field are: Fieldwater, in Ulcombe, Kent; Hortech Solutions, in Ledbury, Herefordshire; and New Leaf Irrigation, in Hopton, Norfolk.

The third major sector in field irrigation is the growing use of booms and field-set sprinklers. These seem ideal for crops where the process of applying water needs to be visible and accurate.

As with trickle irrigation, energy requirements are lower than a raingun, with pumping pressures at three to four bar. Advantages include the ability to have a uniform application with fine sprays to minimise soil damage to delicate crops and reduce soil run-off. To replace a raingun with a boom irrigating a 72m-wide strip would cost an extra £10,000-£12,000.

Director Adrian Colwill of Briggs Irrigation, a major player in this field, stresses the importance of uniformity in the end product, and, he claims, a switch to booms is a popular way of bringing about more effective irrigation without changing everything in the system.

Pumping the volume

With the increasing complexity of irrigation systems and the need to balance all the components in the mix, many growers are turning to consultants to find efficient ways to fulfil their needs.

Speaking at this month's irrigation seminar in Peterborough, Simon Heelas, the proprietor of Powerflow Services, in Llanbadoc Usk, Gwent, illustrated how his expertise in irrigation systems had benefited from his understanding of pumps and pumping systems.

"Pumps are sometimes the forgotten end of the system," says Heelas. "Before looking at innovations such as variable-speed technology, it is essential to select the correct pump or pumps."

Pumps must work at the efficient part of their performance curve if energy savings are to made. To achieve this, Heelas determines the maximum and minimum flow-rate required. Sometimes this shows that two, three or even more (electric) pumps will be more efficient than one, thanks to their ability to accommodate a wide range of flow-rates.

"A successful pumping system is the bringing together of many elements, not least pumps but valves, controls and manifolds, each with the appropriate sizing and selection," says Heelas.

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