By Daniel Collins
As Sir Geoffrey Palmer once remarked, New Zealand is a pluvial country. It rains a lot. On average, 2.3 metres of water falls across New Zealand each year, or 610,000 million m3 in volumetric terms (about 10 times the volume of Lake Taupo). This is more than most countries, but not all. Values vary from source to source, but according to FAO data the wettest country is Sao Tome and Principe (3.2 m/year), the driest is Egypt (51 mm/year), and our trans-Tasman neighbours receive 534 mm/year.
But these long-term, national averages hide a lot of variability. In recent years, NZ’s annual rainfall has varied from 2 to 2.6 m. What is larger still are the differences among regions: Otago is NZ’s driest region with an average of 1.4 m/year, and the West Coast the wettest with 5.5 m/year. Narrowing our focus even further, the wettest year ever recorded was during 1998 at a location along the Cropp River, West Coast, with a whopping 16.6 m. The driest year 1964 at Alexandra, Otago, with a meagre 212 mm.
The reasons for these patterns, and the variations from year to year, were outlined by James Renwick in the previous article. In short, NZ receives a lot of water from westerly winds crossing the Tasman Sea. As the air masses move over the country they lose moisture as precipitation falls, particularly at high elevations. This means that the Southern Alps receives a lot of rain, as does Mt Taranaki, while areas down-wind of the Alps lie within dry rain shadows, the most pronounced of which are Central Otago and Canterbury’s Mackenzie Basin in the South Island. Far from westerly winds and extra-tropical cyclones, the Tukituki River basin is the North Island’s driest area.
Once the water falls to the Earth, on average over time 70-80% of it will wend its way to the coast, flowing mainly down rivers but also within aquifers or along canals. The other 20-30% will evaporate back into the atmosphere before reaching the coast. Several factors control the partitioning between runoff and evaporation: how hot or windy it is, how steep the terrain is, how much vegetation there is, and the soil type. This is why regions are ranked slightly differently when looking at average annual precipitation than when looking at freshwater flow.
This abundant freshwater flow has also made New Zealand a fluvial country – the landscape is dissected by numerous streams, rivers and even glaciers. The size a river can reach, typically measured by its average flow rate, is the sum of the tributary streams throughout the drainage basin. The Clutha River has the greatest average discharge at the coast (614 m3/s) by virtue of the catchment size (22,000 km2) and because its headwaters are located in the very pluvial Southern Alps. The Waikato River, NZ’s longest, drains a smaller area (13,700 km2), which includes the moderately pluvial Volcanic Plateau, and reaches an average of 400 m3/s at the coast. Perhaps the best way to see the pattern of accumulating river flow is in the adjacent map of NZ’s moderate-to-large rivers. High rainfall but a narrow stretch of land west of the Alps produces many small rivers, while the Clutha and Rakaia start in wet headwaters and then pass through dryer areas (hence fewer small rivers) as they flow to the sea.
A more poetic description of the accumulation of flow would be:
Small streams have big rivers,
Which feed on each small tribut’ry;
And big rivers have greater rivers,
And so on ‘til they reach the sea.
Rivers not only sculpt the landscape (the subject of a later article in the series), but they also provide a diverse range of habitats for our aquatic flora and fauna (the subject of several later articles). Differences in the size, timing of flow, connectivity, and intermittency of each river favour certain assemblages of organisms over others, so that the rivers serve as a bridge between the overarching climatic (and geological) drivers and the ecological outcomes.
Derived from the Latin (aqua) pluvial, rain (water).
FAO vastly underestimates NZ’s rainfall at 1732 mm.
One of the rainiest places in the world.
Derived from the Latin fluvius, river. Interestingly, both pluvial and fluvial are ultimately derived from the same hypothetical and reconstructed Proto-Indo-European root, “pleu-“.
Waikato loosely means ‘the pull of the water’ – the effect the river discharge has on the currents at the mouth.
This verse is modelled, ultimately, on a popular excerpt from Jonathan Swift’s ‘On Poetry, A Rhapsody’.
Dr Daniel Collins is a hydrologist and water resources scientist at NIWA.