Canterbury irrigation scheme benefiting region already

Stage one of the scheme is already irrigating about 20,000ha of farmland north of the Rakaia River, with water taken from an intake on the Rakaia about 8km downstream from the gorge.

Stage two, broadly serving the area between stage one and the Waimakariri River, recently got the go-ahead, with unanimous approval from at least 200 shareholders at a special meeting.

Crombie says stage two is now at the advanced design stage. Construction contracts are expected to be signed soon, with some site works possible before Christmas and construction likely to begin soon after. This will see another 20,000ha of the Canterbury plains irrigated and it is expected to be complete by this time in 2018.

The smaller, physically separate, Sheffield scheme also got approval to proceed. Irrigating 4250ha with water taken from the Waimakariri, it also should be complete this time next year.

Crombie says the scheme replaces unsustainable groundwater abstraction with sustainable surface water. Stage one, operating for just one season, has already reduced groundwater abstraction by about 75%.

Despite a continuing drought, groundwater bores in the stage one area have maintained their levels, while bores in the stage two area have continued to decline.

Some farmers in that area have been unable to start irrigating this season, he says.

“So they’re on restrictions from day one. We expect this will get better. So once we have a more normal year, we’ll expect to see these groundwater levels rise.

“We’ve reduced the groundwater take by about 20 million cubic metres, a lot of water.”

He adds that replacing groundwater with surface water irrigation also benefits farmers in the scheme area who choose not to sign up with CPWL but continue to draw off their own bores.

“Some people have taken that choice. Over time, who knows?” Crombie says. “I suspect society’s going to say we don’t want that groundwater taken, and those people may have to reconsider. If there’s a better option, why wouldn’t they use it?”

For Crombie, who was a founding trustee when the scheme was launched in 2003, the stage two go-ahead is the culmination of years of hard work.

“There’s always been a way forward, but at times it’s been challenging in terms of funding or uptake and commercial realities. There have been a lot of hurdles to get over, but I’ve never once felt this wasn’t achievable.”

The scheme had been contentious, particularly the original idea of a storage dam fed from the Waimakariri River, flooding a large part of the Waianiwaniwa Valley in the Malvern Hills. In the face of strong opposition that concept was dropped in 2009.

Construction began in 2014, stage one consisting of a 17km above-ground canal heading northeast from an intake structure on the Rakaia River, and feeding 130km of buried distribution pipes.

Stage two has also been much re-thought. Instead of the stage one canal being extended all the way to the Waimakariri, with water being taken from both rivers and able to flow in either direction, the stage two area will be serviced solely by Rakaia water taken from the end of the existing canal.

It will also be entirely underground, consisting of about 21km of 2.5-2.6m diameter glass-reinforced polymer main trunk pipe feeding a further 184km of HDPE pipes to the individual farms.

Crombie says it will be $50 million cheaper and much less invasive than the original stage two proposal.

“That’s a major refinement and it’s come about for two reasons. First, the amount of water required has reduced over the years with better irrigation practices. Second, the materials... have come on the market since the concept was developed.”

Another key to its success is a 2012 agreement with Trustpower -- operators of the Lake Coleridge hydro dam -- to take water from Lake Coleridge at times of low flow in the Rakaia. No special infrastructure is needed, since Trustpower simply spills water into the Rakaia as required.

“If we want water tomorrow, they’ll release it at midnight tonight and it’ll be down [at the scheme intake] by tomorrow morning,” Crombie explains.

He says the scheme was taking water from the Rakaia catchment under strict conditions: no one can take water from the river when flows go below its limit, but then the scheme is able to draw off Lake Coleridge.

“There’s 300 million cubic metres of water in Lake Coleridge. Our total storage demand is about 100 million cubic metres so we’re not taking even a third of the water in the lake and the lake’s being refilled all the time, obviously,” he adds. “There’s something like 700 million cubic metres going through that lake every year. It’s a big bucket.”