In any given year, around 500 billion cubic metres of fresh water falls as precipitation throughout New Zealand. New Zealand receives most of its precipitation as rain, and the rest as snow, plus occasional hail and sleet.
While New Zealand has abundant rainfall, it is not always in the places it is needed for irrigation, power generation, or domestic supply.
The Southern Alps have some of the highest annual rainfalls in the world – on average, more than 10,000 millimetres each year. This is caused by the moist airflow from the Tasman Sea being blocked by the mountains.
The Southern Alps cause a rain-shadow effect to their east. Remarkably, annual rainfall in Central Otago, within 100 kilometres of the alps, can be as low as 400 millimetres per year.
North Island rainfall tends to be around 1,000 millimetres per year, with less in the eastern regions, and more in the west and north. Because the North Island is less mountainous than the South Island, rainfall levels tend to be more even across the island.
Snow and ice
Snowfall occurs mainly in the alpine areas, but occasionally down to sea level. Most seasonal snow in New Zealand is concentrated in the Southern Alps and on the volcanic cones of the North Island. In winter, up to 35% of the South Island is snow-covered. The permanent snowline is above 1,500–2,200 metres.
In alpine areas, river flows reduce in winter when snow falls, and increase during spring and summer thaws. Many hydroelectric schemes rely on snowmelt to fill storage lakes.
A glacier is a mass of snow and ice. New Zealand has about 3,140 glaciers – only 18 are in the North Island, and all of those are on Mt Ruapehu. The Tasman Glacier near Aoraki/Mt Cook is New Zealand’s largest, with an area of nearly 10,000 hectares. Almost 50% of all New Zealand’s ice is contained in the 10 largest glaciers.
Fluctuations in glacier ice provide an excellent tool for measuring climate variability, as changes in glacier mass directly reflect the climate in the preceding year.
New Zealand glaciers, like those worldwide, have been shrinking since the late 1800s. Around 25% of New Zealand’s ice cover has been lost over the last 150 years.