Auckland normally gets most of its water from dams in the Waitakere and Hunua Ranges, but it's coming to rely on the Waikato River to get through the drought crisis. Bernard Orsman looks at the role dams play in the city's water supply.
Every year the skies open and dump 1.2 metres of rain on Auckland, some of which ends up in 12 dams operated by Watercare.
The dams supply about 80 per cent of Auckland's water, but during the current drought crisis the city has come to depend on the Waikato River for more of its water.
The Waikato River became an even more important water source two weeks ago when an agreement in principle was reached to secure an extra 50 million litres a day for Auckland before next winter.
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The last dam built in Auckland was the huge Mangatangi Dam in the Hunua Ranges, which supplies 26 per cent of the city's water. It was completed in 1977 when the city's population was 750,000.
The population has more than doubled to 1.6 million so why hasn't Watercare built more dams to capture more of the rain that falls over the 1000sq km of the Super City?
"We haven't got any plans to build any more dams," says Watercare's media liaison adviser Maxine Clayton.
When Watercare put in a resource consent to increase its daily allowance of 150 million litres a day by a further 200 million litres a day in 2013, it considered more than 90 options, including new dams, she said.
A new Lower Mangatawhiri Dam in the Hunua Ranges and a dam at Campbell Rd in Riverhead were considered, said Clayton, but rejected on the basis of cost and technical difficulties.
The Waikato River was the preferred option because, unlike dams, it is not dependent on rainfall, she said.
Not everyone agrees with Watercare's reliance on the Waikato River, including the Waikato River Authority.
Authority iwi co-chair Roger Pikia said the river is paying a bigger price for Auckland's drought - the worst in the city's history.
Pikia said Auckland took 17 per cent of its water needs from the river last year and is currently taking 40 per cent, saying it is not sustainable for more and more water to be going out of the Waikato catchment.
"This is not a Waikato versus Auckland issue, this is a Waikato River issue," Pikia said.
Waikato Regional Council chairman Russ Rimington has said Watercare had "dropped the ball with its strategic planning" and should look at other options, such as rainwater tanks and enlarging dams.
Watercare and Auckland Council have shown little interest in following cities like Sydney, which have made rainwater tanks compulsory in most new builds.
On the back of a warning from MetService that the city's water shortage has been pushed to "critical", the council last month announced it was scrapping resource consent fees for suburban rainwater tanks and considering making water tanks a requirement for new developments.
Planner and former Auckland Regional councillor Dr Joel Cayford said Auckland's dams not only provide better quality water than the Waikato River, but a cheaper source.
"Auckland's best raw water sources are the dams in the Waitakere and the Hunuas which are bush-clad catchment and produce water that mainly needs sand filtration and disinfection, and which can flow under gravity to where it is needed," he said.
Cayford said raw water from the Waikato is much more expensive to treat and pump to Auckland because it contains organochlorine pesticide farm chemicals and microbial contaminants, including cryptosporidium, from dairy herd sewage.
Watercare operates five dams in the Hunua Ranges, five dams in the Waitakere Ranges and two small dams at Helensville - the Mangakura Dam and Sandhills Weir.
In normal times, the dams account for about 80 per cent of the city's water supply with the Waikato River providing about 17 per cent and the Onehunga Aquifer 3 per cent. All up, the city uses about 160 million cubic metres of water a year.
Auckland's water use is growing at about 3 per cent a year.
As well as the dams, the water arrives at taps in homes and businesses via 15 'A' grade water treatment plants, more than 9000km of pipes, 85 reservoirs and 94 pump stations.
The Hunua dams - Mangatangi, Upper Mangatawhiri, Cosseys, Wairoa and Hays Creek - provide about 60 per cent of the city's water.
After Hays Creek dam was mothballed in 2005 because the Papakura water treatment plant needed significant upgrade to meet New Zealand water drinking standards, Watercare is rushing it back into service at a cost of $57 million.
Last week, work began to clear the old Papakura water treatment plant to build a new modular treatment plant to process water from Hays Creek.
Clayton said Watercare has continued to maintain Hays Creek since 2005, saying it has provided a nearby quarry and local farmers with water.
Once it is up and running by December, Hays Creek will provide 6 million litres of water, increasing to 18 million litres a day next year.
The Waitakere dams - Waitakere, Upper Nihotupu, Lower Nihotupu, Upper Huia and Lower Huia - provide about 20 per cent of the city's water.
Clayton said, unlike a hydro dam, Watercare's dams are designed to store water and the capacity to spill when full.
"The western dams are at a higher risk of spilling due to the relatively large catchment area, compared to the volume of water they can store. In a typical year the volume of water withdrawn from these lakes is equivalent to 2 to 5 times their storage," she said.
When the Upper Nihotupu and Upper Huia dams spill, the water flows to the lower dams.
Clayton said none of the Waitakere dams have spilled this year. As of today, the Waitakere and Upper Nihotupu Dams are 100 per cent full and the Upper Huia dam is 85 per cent full.
Throughout the year, the Waitakere dams are drawn down to low levels in preparation for rainfall. This year, they were drawn down until they were less than 30 per cent full.
The Hunua and Waitakere Ranges received about 1.8m of rain a year, 50 per cent higher than the citywide average of 1.2m.
Recent rainfall has been higher in the Waitakere Ranges compared to the Hunua Ranges.
Watercare is working to optimise the delivery and treatment of water from the dams in the Waitakere Ranges. The biggest project is replacing the Huia water treatment plant to lift production from 110 million litres of water a day to 140 million litres a day.
Watercare is also replacing a major water main to West Auckland with a new water main to West Auckland and West Harbour to meet growth in the northwest.
Clayton said Watercare operates the dams and other water sources as one "virtual" source to increase the volume of water available.
Water from the Hunua Ranges and the Ardmore water treatment plant goes to the city, North Shore and West Auckland when the western dams are low.
The Waitakere dams and the Huia water treatment plant supply south and east Auckland.
The other two dams - Mangakura and Sandhills Weir - are relatively small and provide water to Helensville and Parakai.