Water availability in the Kakanui and Manuherikia catchments could be threatened if highland snow tussock grasslands are replaced by pasture or forestry, a new Department of Conservation report says.
While studies had previously been done relating to snow tussock on Otago's Te Papanui Conservation Park, the report by Doc and Environmental Associates Ltd's Tom Heller focused on the impact on water users, Doc Otago Resource Management Act planner Bruce Hill says.
Kakanui and Manuherikia catchments were chosen because their community's economic welfare was closely tied to the availability of water, they contained threatened fish species and had large areas of tussock grasslands in their headwaters. They were also facing increasing pressure from higher-altitude farm development, exotic forestry and wilding pines.
The modelling showed minimum flows in both catchments would be reduced if their tussock headlands were converted, even by a small amount, to forest or pasture, with forestry having the greater impact, he said.
It would mean more days when the mean river flow was below the regulated minimum flow, restricting the amount of water available for farmers and communities.
"How these effects might specifically affect existing or future users is uncertain, but the effects appear significant if a further 10% of the Kakanui catchment or 20% of the Manuherikia catchment were to be converted to to forest," the report said.
Other unintended effects might become apparent with reduced river flows, which were known to negatively affect water quality and recreational values.
Mr Hill hoped the data in the study which had been sent out to farmers, irrigators, councils and interest groups would help make the community more aware of what nature provided.
The report might also be used in Doc's Resource Management Act advocacy and in tenure review discussions.
Wilding pines were a threat to catchments such as the Kakanui and the issue of who would pay for their control could be informed by the report, he said. The methodology used in the report would also be available for communities interested in assessing similar impacts from the loss of tussock.
