By Daniel Collins

One of the arguments being used at the moment to promote water storage and irrigation schemes is that much of the water that falls on New Zealand flows to the sea, not to the farm. Conor English, CEO of Federated Farmers, wrote in an opinion piece earlier this year:

“It’s not that New Zealand is running out of water, it’s that water is running out of New Zealand.”

As it turns out, about 80% of the water that falls on New Zealand flows out to sea, the rest evaporates back into the atmosphere.

As for what we use, Lachlan McKenzie, previously from Federated Farmers, is cited as saying that 97% of New Zealand’s water flows wastefully to the sea (audio). Similarly, in a Q&A file released in May, the Government states that 2% of our freshwater resource is used.

McKenzie’s 3% used and the Government’s 2% are basically the same. They ultimately come from two sources: the 2006 Statistics NZ report mentioned previously, and a report on water allocation by Aqualinc Research Ltd. The numbers refer to the fraction of total annual water supply that can be abstracted legally, as an average for New Zealand as a whole. (It’s important to remember that water doesn’t need to be abstracted to be useful or beneficial, even for commercial purposes, but that’s a discussion for another time.)

According to the Aqualinc report, nearly 27,000 x 106 m3 of water may be abstracted from rivers and aquifers each year. That’s almost half the volume of Lake Taupo*. Of this, 16,000 x 106 m3 is for the Manapouri hydropower scheme that takes water from the Waiau River and discharges it directly to the sea. If we’re just thinking about water that can be consumed for irrigation, drinking and so on, then we’re left with 11,000 x 106 m3. Using the Statistics NZ data (either 2006 or 2011 reports), this is 2% of New Zealand’s annual freshwater supply.

But this is a national and annual average, which is adequate for a big-picture view but irrelevant for practical purposes. It implicitly assumes that the water is equally available everywhere, all the time. It is not. The water in Southland is of no use to Canterbury or Hawke’s Bay, and unless we build pipes or tunnels across the Southern Alps, nor is the West Coast’s. The water in winter is of no use in summer unless we build reservoirs, hence the drive for more water storage. (Expect more on the hydrology of water storage in the future.)

For a more useful description of the annual water used in each region, check out the figures below. Omitting Southland’s hydropower, most of the water allocated for consumptive purposes is in Canterbury, followed by Otago. Canterbury’s allocation is 5,000 x 106 m3/yr — nearly half of New Zealand’s total. As a fraction of each region’s annual water supply, Canterbury and Otago are also on top, at 8.3% and 7.7% respectively. Cool and wet Southland and West Coast come in at the bottom with 0.2% each, as does less developed Gisborne.

It’s easy to see how the regional picture is a lot different from the national picture, but in terms of how we use our water, this is really only part of the jigsaw puzzle. Stay tuned for more pieces.

Water Allocation

* The volume of Lake Taupo is about 59 km3 or 59,000 x 106 m3. 27,000 x 106 m3/yr is about 21 Olympic swimming pools worth of water per minute.