North Otago top for snow-melt

Irrigators, power companies and mountaineers could all be affected if climate change leads to changes in snow levels.

New research from the National Institute of Water and Atmospheric Research (Niwa) reveals that snow melting off the Southern Alps has a greater impact on North Otago waterways than anywhere else in the South Island.

An assessment of 20 years of daily temperature and precipitation data from the Virtual Climate Station showed that of the large rivers that reach the sea, the Waitaki had the highest snow-melt contribution with 12%, while Lake Pukaki had the highest snow-melt inflow of any South Island hydro lake, with 18%.

Niwa hydrology scientist Dr Tim Kerr said the findings would have important consequences for water management, and civil defence.

''Increased flow from snow-melt in summer is beneficial for irrigation, but decreased flow in winter, when snow builds up, is problematic for hydro-electricity.''

Knowledge of how snow-melt might affect the size of a flood was also an important consideration for building structures near rivers, or when forecasting flood events, he said.

''Add into the mix the potential for climate warming to change these seasonal and flooding effects as less snow and more rain falls, and you can see that it is worth having an idea where snow-melt is important.''

Overall, it is estimated 3.4%, or 1 litre in every 30, of the South Island's river flow comes from snow-melt, and New Zealand Mountain Safety Council alpine and avalanche programme manager Andrew Hobman said that also meant trampers and climbers should be wary of making river crossings.

''Rivers are definitely going to be up at the moment and they are going to drop and rise throughout the day as that melt happens.

''It's important to understand before you go on a tramp or trek that you can get a significant change from crossing a river in the morning, or at dawn, to when you are are coming back again in late afternoon - that river might have risen quite a lot through just daily melt.''

He said it was also important to be wary of avalanches, which could be caused by snow-melt, and warned people to stay off unsupported snow if they were trekking above the bush line.

He advised people to watch out for warning signs such as a sudden end to vegetation, which could be in indicator they were entering an avalanche zone, if they could see snow above them.

''It's a good time to remember that 55% of all fatalities are climbers, and the majority of those fatalities occur in the summer months.''