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 Unpublished results from another in 2006 reported 66%.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.
Update 22/11/2011: Updated analysis is available here.