New Zealand’s rivers are running dramatically different to how they did a century ago – and climate change may partly explain a drying trend observed over summer seasons.
New Zealand’s rivers are running dramatically different to how they did a century ago – and climate change may partly explain a drying trend observed over summer seasons.
New Zealand’s rivers are running dramatically different to how they did a century ago – and climate change may partly explain a drying trend observed over summer seasons.
A just-published study – and the first of its kind in New Zealand - found marked changes in average summer and winter flows in our rivers over the last 50 to 90 years.
“Typical summer river flows have significantly decreased for much of the North Island, with the exception of Manawatu and Wellington,” the study’s lead author Laura Queen said.
In winter, meanwhile, average river flows had decreased in the upper North Island - but increased much on the West Coast of the South Island, and in Fiordland and Southland.
“The trends we found are the result of variations in the climate, whether it’s natural climate cycles such as the El Niño and La Niña oscillation, or human-caused climate change,” said Queen, a Victoria University PhD candidate based at Niwa.
“Our next task is to identify the extent to which each is contributing to these trends.”
Queen said human-induced climate change was causing temperatures and atmospheric moisture to rise and wind circulation patterns to shift across New Zealand - with potentially major knock-on effects for river flow.
“Even in places where average river flows are decreasing, such as the upper North Island in summer, scientists expect we will still see increases in extreme rainfall and shorter duration flash flooding—just as we’ve seen this summer,” she said.
“In the South Island, changes to the strength of the southern storm track have increased rainfall on the West Coast and that affects river flow in the region.
“We’ve also seen increasing winter river flows in the snow-fed catchments in the South Island. This could be caused by changing wind circulation patterns that are delivering more rain, as well as by rising temperatures turning snowfall into rainfall.”
The study, published in the Journal of Hydrometeorology, was based on long-term data from 53 sites around the country, from the Awanui catchment in Northland to the Pomahaka in south Otago.
South Otago's Pomahaka River. Photo / Helena de Reus
It looked at rivers in wilderness areas where water flows were not affected by irrigation, hydroelectric dams or land use changes such as deforestation.
Focusing on these remote rivers meant the researchers could isolate the impacts of climate on river flow, Queen said.
“Instead of looking at rivers individually, we grouped them and analysed how they’d changed over time.
“Trends that are hard to see in a single river become clear when you look at data for a group of rivers over a period of at least 50 years.”
Regional trends identified in the study will have implications for the management of water resources, she says.
“Increasing winter river flow in the west South Island may affect hydroelectric dam management.
“Meanwhile, decreasing winter and summer flow in the upper North Island has potential impacts for water availability in New Zealand’s most densely populated area.”
The study noted that anthropogenic climate change was affecting rivers worldwide, threatening water availability and altering the risk of natural hazards.
“Understanding the pattern of regional streamflow trends can help to inform region-specific policies to mitigate and adapt to any negative impacts on society and the environment,” it said.
The study follows several major recent reports focused on climate change, and which noted widespread impacts on waterways.
Modelling has already suggested river flows are likely to increase on the west coast of the South Island and in rivers that drain the eastern side of the Southern Alps.
They’re also predicted to decrease on the east coast of the North and South Islands, and in Waikato and Northland, although these projections were less certain.
