Water supply under pressure

Northland Regional Council hydrology monitoring officer Marcus Schlesier gauging flow rates in the Kerikeri area on Thursday. Picture / NRC

With no prospects of significant rain on the horizon, local authorities are keeping a wary eye on an increasingly parched Northland, parts of which experienced some of the driest conditions in decades last year.

Colin Dall, the Northland Regional Council's group manager regulatory services, said a number of rivers in the region had fallen below designated 'minimum flow' levels designed to restrict the amount of water that could be taken to protect their ecology.

"Things are changing as staff carry out manual gauging to confirm low flows in different catchments, but at this point we've got about half a dozen consent holders who have ceased their takes in accordance with their resource consent conditions (mainly in the Mid and Far North), and the council has also had to impose restrictions on some other users," he said.

District councils taking water for public supply and farmers irrigating pasture were among the biggest users among the several hundred people or organisations with consents to take water, including some major users who were permitted to take hundreds of thousands of litres daily, but hundreds of others were taking relatively small amounts without the need for a consent.

Mr Dall said the council had been advising people for some time not to waste water and to make sure they had taken sensible precautions for dry conditions, especially those relying on their own supply via tanks. It had also contacted some of its more vulnerable water take consent holders/water users, urging them to conserve water where possible.

With summer now in full swing, and no significant rain forecast, members of the council's hydrology team were currently gauging some of the more critical rivers in the region to ensure they had the most up-to-date data and figures to work with.

"Preliminary information suggests a number of rivers are already below 'minimum flow' levels, and although this isn't unusual over a dry summer, this wouldn't usually happen until February or March, and is linked to last year's drier conditions," he said.

A dry 2019, unusually low flows for this time of year and associated reduced oxygen levels were leading to stagnation and blackish-looking water in some rivers, which was often incorrectly confused with sewage discharges.

The regional council was also receiving numerous calls from members of the public reporting potentially unauthorised and/or excessive water use, each of which was being investigated to ensure compliance with regional rules.

Regional river and rainfall data were available on the council's website (www.nrc.govt.nz/riversandrain), while water restrictions imposed by the district councils could be found at www.bewaterwise.org.nz

Meanwhile, Mr Dall said last year had been confirmed as one of the driest on record in many parts of Northland, and in one case the second-driest, where records stretched back more than a century.

"Just 1100mm of rain was recorded in the Puhipuhi area, north of Whangārei, last year, a little over half the average 2000mm annual rainfall, making it the second-driest year there since 1914," he said.

The Kaitaia area had recorded the driest year since records began in 1949 (791mm compared to an average 1350mm; the Northland Age recorded 1015.5mm, compared to its 90-year average of 1353mm), and Kerikeri's 1165mm was the third-driest since records began in 1945, significantly under its 1490mm average.

Whangārei too had had a very dry year, the 837mm recorded at its airport the driest since records began in 1943 (compared to a yearly average of 1364mm).

Ngunguru had received 967mm (yearly average 1790mm), and the Brynderwyn area, south of Whangārei, 976mm (yearly average 1390mm), the lowest rainfall in both areas since records began in the 1980s.

Mr Dall said that with such a large rainfall deficit, it could take the region overall a number of years to recover, even once summer was over and the rain came again.

Low river flows and higher water temperatures over summer is putting river ecology under stress, and could result in more prolific growths of algae and low oxygen levels. Lakes and rivers could also experience blooms/excessive growths in cyanobacteria which could produce toxins.

There had been few reports in Northland of such growths producing toxins and affecting water use, but people should always be wary of swimming in and using water from lakes and rivers that had cyanobacteria blooms for drinking.

■ More information on algal blooms was available at www.nrc.govt.nz/naturalphenomena