1 August 1997

Achieving an even wetting

DO your sums carefully before installing alternative irrigation systems to replace or back-up rainguns. That was the thrust of a MAFF-funded irrigation demonstration in Notts last week.

Pressure to use water more efficiently is prompting more growers to consider relatively expensive alternatives to conventional hosereel rain guns in a bid to improve the uniformity of application, say staff at ADAS Gleadthorpe.

Raingun spread patterns are notoriously vulnerable to disruption in windy weather. Hosereel and boom systems, drip tapes and pipes, and fixed low maintenance sprinklers all offer benefits – but at a price.

Their key advantage, according to Gleadthorpes Simon Groves, is more even application. As well as using water more economically they can also lead to more uniform produce to meet supermarket demand.

A badly set raingun working in high wind can produce a spread pattern which is just 40% uniform. Correct nozzle size and pressure, night use when winds are generally lighter, and narrower lane spacing can improve that to 80-85% uniformity.

But other options can go even further, says ADAS. A relatively small step is to replace the gun with a boom. That can raise uniformity of application to over 90%.

Modern, easily folded clip-on booms are simple to operate, says John Bailey, senior ADAS mechanisation officer. "Its a one-man job with the better booms." The extra capital cost is small compared with sprinklers and pipes or tapes (see table).

Interest in trickle irrigation through tapes or pipes has rocketed in recent years on the back of design improvements and the fact that an abstraction licences is not required, says Gleadthorpe-based machinery officer Bill Basford.

Correctly positioned they ensure more water gets to the rooting zone, giving well over 90% uniform application. They also operate at much lower pressures than rain guns and need less pump energy, he notes.

"There is also the possibility of less evaporation, so there may be water savings. But that is unproven in UK conditions. We want to do more studies."

Tapes and pipes offer dramatic in-season labour savings over hose-reel systems, ADAS adds. But installation and removal costs can be considerable, with 10 man hours/ha for each operation reported. In a wet season the expense may be wasted if irrigation is not needed.

Practical problems also remain, not least the retrieval of pipe work at the end of the season. Keeping the more fragile tapes in good condition to ensure re-use and contain overall costs can be tricky.

ADAS has built a prototype, hydraulically-powered machine to work ahead of a potato harvester on flailed or well desiccated crops.

Whether to bury pipes and tapes is uncertain. Burying anchors them and gets water directly to the rooting zone, which should reduce evaporation. It also avoids chewing damage from wildlife. "The drip holes make perfect nipple drinkers," Mr Basford notes. But retrieval is more complicated.n

Hes got it taped… trickle tape and pipes can do much to improve irrigation efficiency (inset). But watch the cost, warns ADASs Simon Groves.

This fixed sprinkler system from the US, the Nelson R-2000, uses 63mm (2.5in) diameter alkathene feeders with snap-in nozzle risers every 12m (39ft). It produced 22% more high grade parsnips than a hosereel at an ADAS-monitored site last year, says importer David Yates.

RAINGUN ALTERNATIVES

&#8226 Better water use.

&#8226 More uniform produce.

&#8226 Lower energy needs.

&#8226 Equipment costs higher.

&#8226 Labour implications.


ADAS GUIDELINES

working pressurelifeannual cost

(bar)(yrs)(£/ha)

hosereel & raingun6.5-8.510128

hosereel & boom4-510185

fixed sprinkler3.58407

drip tape0.6-21792

drip tape0.6-23298

drip pipe0.6-28626