An increase in nitrogen levels in the Tukituki River is inevitable under more intensive farming practices proposed for the surrounding area, a Hawke's Bay Regional Council adviser says.
But Ian Millner said demands by environmental groups that nitrate leaching into the river be reduced to the low levels of more than 100 years ago would have a "dire" impact for farmers and growers in the catchment.
The senior land management adviser made the comments during the board of inquiry hearing into the Ruataniwha dam yesterday.
The board is sitting in Hastings to assess an application from the Hawke's Bay Regional Council for permission to build a $265 million dam north-west of Waipawa and Waipukurau, and an associated water distribution scheme on the Ruataniwha Plains.
The aim of the so-called Ruataniwha Water Storage Scheme is to improve water supplies to the drought-prone plains to enable more intensive farming, and therefore more productive use of land in the area.
Environmental groups oppose the scheme, saying increased nutrient run-off, including nitrate leaching, from more intensive farming will further pollute the vulnerable Tukituki River.
In their submissions to the board, the Environmental Defence Society and Fish and Game have called on the board to impose reductions in nitrate leaching rates to levels the council says would take farming back to low stocking levels equivalent to what was practice around 1900 - about three or four cows per hectare.
A lawyer for the Environmental Defence Society, Robert Enright, said the council claim was "a simplification of EDS's position".
"The stocking levels are on the assumption that no steps are taken by the farmer to implement good farming practice," he said. "For example, if you were to apply good farm management practice to the farm you could achieve higher stock levels under EDS's nitrogen limits."
But Mr Millner said he did not accept that proposition.
"In principle I agree with what you say but ... in my opinion I think that would be very difficult to achieve for a significant proportion of this catchment, if not all of this catchment," he said.
"I accept the principle that it is possible to run higher stock levels with very good practice, however, in this case, in this catchment I do not believe that to be realistic."
Mr Millner has been employed by the council since 2008 and his role involves providing advice on land management issues, including erosion, drought resilience, nutrient management and farm planning. He was previously a self-employed farmer on a 340ha hill-country sheep and beef unit in southern Hawke's Bay (outside the Ruataniwha Plains) and is still involved in that business.
He said reducing leaching to the levels proposed by the environmental groups would effectively rule out dairying and commercial vegetable production or any other valuable crop production in the catchment.
"Those numbers [proposed by the groups] are dire in my view."
The point was picked up by Russell Howie, one of the five members of the board of inquiry.
Mr Howie: "So you're really saying to reduce what's there now would have major consequences to the catchment?"
Mr Millner: "I believe so."
Mr Howie: "Even with industry best practice, intensification of farming will increase the current levels of nitrogen in the river?"
Mr Howie: "Yes I think that is a safe assumption."
Mr Millner: "Inevitably?"
Mr Howie: "Yes."
The inquiry continues on Monday and is expected to run into the new year.