How much water does NZ have?

By Waiology 08/06/2011


By Daniel Collins

The amount of water that falls on New Zealand each year is about 560,000 million m3. Lumped together as ‘precipitation’, this is mostly rain, but also snow, hail and even some graupel. That’s enough water to cover the country 2.1 metres deep, or to fill Lake Taupo over 9 times. More than most countries, but not all.

How much of this precipitation makes its way to the sea, instead of evaporating along the way, is less clear. This number is important because it tells us about how much freshwater is available to the environment and us. The most commonly cited report to date, released by Statistics NZ, puts this at about 79%, but there is good reason to believe this estimate to be on the high side. A study published in 1972 reported 72%. Unpublished results from another in 2006 reported 66%. And the Statistics NZ analysis is being updated at this very moment — we’ll let you know the results when they’re available.

And just as New Zealand has more or less water than other countries, the same goes for our regions. Check out the figures below. According to the StatsNZ report again, the West Coast is by far the wettest, on account of the Southern Alps blocking the mean westerly airflow, followed by Southland. Otago is the driest, with its heart sheltered from high rainfall. If you spread West Coast’s precipitation evenly over its surface, you’d be swimming in water 5.5 metres deep. For Otago, you’d just be standing in water 1.1 m deep. Because some of this water evaporates, the equivalent depth of freshwater spread over the regions is lower — 5 m for the West Coast and 80 cm for Otago.

Now remember that these are estimates. We can’t measure all the rainfall everywhere or gauge every stream — there’s not enough money and we don’t really need to. Instead, we design monitoring networks to capture the most important information, make informed judgments based on what data we do collect, and rely on physical principles to fill in the gaps. As more data are collected, and as the science advances, these estimates get refined. A lot of what Waiology will cover is how we refine these estimates and what we find out.

NZWaterByRegion

Update 22/11/2011: Updated analysis is available here.


0 Responses to “How much water does NZ have?”

  • How is depth of preciptitation measured? I thought it would just be the annual rainfall in metres?

  • You’re right, Jeremy, I do mean the depth of rainfall that falls annually. The local measurements from rain and snow gauges are interpolated across the region to estimate a total volume, then that volume is divided by the area of the region to get an equivalent depth in metres. I’ll change the label to read ‘metres/year’. [DC]

  • ok. it does appear to create some unexpected results. The Malborough figure seems high given the annual rainfall at Blenheim, or does this include some mountain guages? When I think of Malborough I think of the dry hills and versus lush Auckland (or is it that Auckland should be higher up on the table, nearer to the Waikato, particularly if you take into the >2000mm rainfalls on the Waitakere and Hunua ranges). And surprised by the Southland figure, particularly when I look at the annual rainfall totals for Gore and Invercargill.

  • Marlborough and Southland do indeed have their wetter, more mountainous parts. It’s just that their main population centres and roads don’t go that way. I’ll write up a post going into more detail soon, but in the mean time check out the rainfall map here. And, just so it’s clear to everyone where the regions actually are, that map is here. [DC]

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  • […] 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 […]

  • The estimate of rain and snow above is 560,000 million cubic metres, but an updated estimate using more data is 610,000 x million cubic metres – just over 10 times the volume of Lake Taupo. [DC]

    • That’s a tricky one, nationally. I don’t think that number is readily available, though rain day maps have been generated in the past. A handy summary of 2011’s climate can be found here: http://www.niwa.co.nz/annual-climate-summary-2011. And if you want data for specific locations, you can go here: http://cliflo.niwa.co.nz/. But taking a causal non-scientific poll of some colleagues, it is probably raining somewhere in NZ in at least 1 in 3 days, possibly up to 3 in 4 given the effects of the Southern Alps, but they’re just quick guesses I’m afraid.