Recycled error in Canterbury’s freshwater supply

By Daniel Collins 23/03/2010

How much of New Zealand’s freshwater supply is in Canterbury?

Cantabrians have heard the answer a lot lately, and if the media cycle were to be believed, it’s 70%.

The Creech Report on ECan’s performance said it. The newspaper said it. An ECan councillor said it, but since retracted it after I presented the numbers. A commenter at Kiwiblog said it. And I’m sure a lot of people now believe it.

Unfortunately, they’re all wrong. The best estimate at present, if the water cycle is to be believed, is probably about 10%.

This comes via StatsNZ, and ultimately from NIWA. Freshwater supply – considered only what we can abstract – is defined using the StatsNZ numbers as total inflows to a region minus transfers out and minus evaporation. This gives a mere 10%.

This small number should come as no surprise. Canterbury covers 17% of NZ’s area, is mostly sub-humid, and the highest rainfall rates in NZ are typically on the west side of the Southern Alps, draining to the Tasman Sea.

So where could this “70%” come from?

My first guess was that someone conflated ‘freshwater resource’ with ‘freshwater allocations’, and that the percentage of total NZ water allocations that are allocated in Canterbury was 70%. I’ve said it. A journalist said it. A Press commenter said it.

Unfortunately, we were all wrong. The best estimate at present is probably around 55-58%, at least for consumptive use (i.e., non-hydropower).

55% comes from the 2006 MFE water allocation snapshot. 58% comes from the 2009 Canterbury Water Management Strategy.

So where could this “70%” come from?

The only plausible explanation I can come up with is that 70% is roughly the percentage of NZ’s irrigated area present in Canterbury. The 2006 MFE report states 66%. The Canterbury Water Management Strategy states 70%.

So there you have it. Canterbury has about 10% of New Zealand’s freshwater supply, about 55-58% of NZ’s allocated freshwater (for consumptive use), and about 66-70% of NZ’s consented irrigated area. Canterbury also happens to have allocated a greater portion of its water supply than any other region, at 14%.

It’s ironic that the Creech Report, which was tasked with assessing ECan’s ability to manage freshwater resources, got the amount of freshwater available in Canterbury so wrong. And unfortunate that people probably now think it’s true. The hydromedia cycle obviously needs to be taken in for a tune-up.

Update: March 26, 2010 – If the StatsNZ link slows you down, open the PDF file “Water Physical Stock Account: 1995-2005”, go to tables 4 and 5 on pages 20 and 21, get data as described above from the Canterbury, South Island and North Island columns, do some fancy adding and subtracting, then a bit of division, and hey presto. Round as you see fit.

0 Responses to “Recycled error in Canterbury’s freshwater supply”