How much water do we use?

By Waiology 21/10/2011 5

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.

5 Responses to “How much water do we use?”

  • Hi Daniel – Good to see you’ve had a go at breaking down the stats to give a more meaningful picture – but unfortunately regional picture is about as relevant as national one! Need to go to a catchment basis to give something of relevance. You also need to be aware allocation comaprisons between regions are meaningless in NZ as every region allocates water differently (rates, annual/monthly/fortnightly/weekly volumes, and then reliability factor and technical efficiency % if any………….). There are even inconsistencies within regions. Finally the allocation picture is very much a meaningless measure as it’s the actual use that counts…… and we’ll start getting a good handle on that by 2016……. Always happy to talk if you’ve questions.

  • Hi Andrew. Those are of course good points you raise, and they’re on Waiology’s radar, but I’d say you make the data out a tad worse than they actually are. A regional picture is tremendously more informative than a national one: it tells us that some regions are consented to take more water than others (how is not particularly relevant to a basic water balance, though important for other questions); and it tells us that water is not allocated in proportion to the amount of water available. This is important to appreciate, lest we become inclined to use national generalisations to make local decisions, at whatever scale. As for allocation vs use… indeed usage data would be great, and the Aqualinc report offers an early estimate of this (more in a later post), but allocation data still tell us about the demand for water. In a variable and uncertain environment/economy, a priori demand (consented takes) are likely always to outstrip eventual use, and both planners and users are better off if they understood how. [DC]

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