Low flows in the Hakataramea River, which anglers fear are destroying its international reputation for fishing, were not caused by irrigation but by natural changes in weather, Dunedin-based hydrologist Dave Stewart said on Thursday.
He was giving evidence at an Environment Canterbury hearing into resource consent applications to take more water for irrigation in the Hakataramea Valley.
The Central South Island Fish and Game Council had already said irrigation was causing the river to dry out in places - and more would make that worse.
Omarama fishing guide Wayne Grafton predicted the Hakataramea River's reputation could be killed by more irrigation.
But Mr Stewart researched flows in the river and changes in weather patterns and concluded irrigation was not to blame.
Irrigation only worsened the situations at low flows.
All except one of the new applications proposed to harvest water during high and flood flows in the catchment and that would have no impact on low flows in the Hakataramea River.
Mr Stewart said water resources in the catchment were influenced by more than irrigation.
Weather had been drier in recent years than in previous periods.
Over the past 50 years, rainfall at one site had decreased by about 40mm a year.
"What causes the low flow periods is a function of rainfall and weather, not increased irrigation," he said.
Environmental engineer Lynn Torgerson said it was difficult to determine the causes of water quality problems in the Hakataramea River.
While overland and subsurface water flows contributed, there were other sources, such as stock and wind-blown soil.
A Hakataramea catchment monitoring plan was proposed to determine whether new irrigation was causing or contributing to changes in the river.
It would establish a baseline before irrigation was expanded, looking at land, surface water and groundwater and other factors affecting water quality.
Once the new consents were exercised, ongoing monitoring would be carried out to determine whether there were any changes.
Each farm would have a farm management plan, audited and updated regularly, as a condition on any consents.
