NZ Farmer - Lake Opuha works around the clock to keep up with irrigation demand

Written by Pat Deavoll

South Canterbury's Lake Opuha is getting a workout this summer with an "almost unprecedented" swing in December weather ramping up irrigation demand.

Opuha Water Limited executive officer Tony McCormick said the shift from extremely wet to extremely dry had resulted in a rapid increase in irrigation from "virtually zero" in early November to nearly 100 per cent four weeks later.

"The sudden dry conditions affected lake storage and required special efforts from our ops team to get the scheme up to full capacity in a short time," he said. 

"Combined with the rapid rise in releases to meet irrigation and river flow demands, the lake level started to drop at quite a dramatic rate at the same time."

McCormick said he was concerned enough to convene a meeting with the Opuha Environmental Flow Release Advisory Group to see if there were grounds for some constraints on water use or river flow. 

"The group heard from us that we were experiencing very high loss rates in the Levels scheme, but we felt that we were starting to get on top of those.

"We also explained that with the late and rapid start to the irrigation season many of our shareholders were struggling to catch up or keep up with their irrigation."

"It was agreed by the group that a precautionary reduction in the minimum flow was the best option if constraints were considered necessary."

Rain last week gave lake (about 15 kilometres north of Fairlie) storage levels a slight boost, but irrigation demand had not backed off, McCormick said.

Rainfall was patchy around the scheme, but Fox Peak - in the lake catchment - received more than 50 millimetres which pushed up the inflows. However, they were dropping rapidly.

"While the lake level is now at the historical average for January, it is still early in the month, so I am expecting it will end up below.

"The recent rain has been very welcome, but we will continue to monitor the storage situation very closely."

"The season is shaping up to be one where we will need to keep a very close eye on water use, lake storage conditions and outlook. Certainly, [there is] no room for complacency by any of us on water use," McCormick said.

McCormack said the Levels scheme was experiencing high loss rates. 

"There was about 10 days when we were struggling to get sufficient water around the scheme due to high losses over several specific sections.

"The situation has eased back now, and we believe the losses were the result of a combination of circumstances but most especially the rapid start-up.

"In a normal season there is a steady ramp-up of flows but this one it went from virtually zero to full bore in a week, and so we had races full very quickly."

In 2014 the Levels Plain board made a policy decision that all flood flow irrigation be phased out over a three year period. 

McCormick said there were less than 10 properties still operating on flood irrigation. As well as being recognised as an inefficient method of watering pasture, the cost of providing water under this regime had become prohibitive.

Most of the remaining flood flow properties were small landholdings where the cost of converting to a spray system would be disproportionately expensive - both in the amount of water used and the operator time.

"There needs to be acceptance by the landowners that continuing with flood irrigation is no longer an acceptable option, both within our co-operative and in the new regulatory and social environment," McCormick said.