Low flows: A new challenge looms for people and businesses reliant on water from Bay of Plenty streams

Monday, 5 July 2021

If you live in the city of Tauranga, your drinking water comes from streams. If you work in the multimillion-dollar Bay of Plenty horticulture industry, there’s a good chance you need stream water too. People across this region rely on these waterways every day, so what happens when the flow goes slow? SCOTT YEOMAN reports.

Flooding has always been Simon Short’s biggest worry.

For more than 20 years, he has worked alongside the Ngongotahā Stream in the Bay of Plenty and has witnessed, time and time again, its waters rapidly rise – and then spill over.

Sometimes with devastating consequences.

“The volatility of it, and the speed of its volatility, just makes it a live beast,” Short says.

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But now, it’s not just flooding that Short needs to worry about.

It’s also the exact opposite.

The Ngongotahā Stream this year dropped to its lowest ever recorded flow since monitoring began in 1975.

Thanks to a sustained period of lower than normal rainfall, the stream’s flow rate has been steadily falling since the summer of 2017/2018.

And the Ngongotahā is not alone.

Consistently low stream flows have been recorded across the region, and scientists have also reported declining groundwater levels in aquifers.

Flow levels in the two streams that feed Tauranga’s water supply – including drinking water for its roughly 140,000 residents – remain lower than normal, despite the city having just had its longest water restrictions in recent history.

“Uncharted territory,” is how Steve Pickles describes it.

Pickles is the Bay of Plenty Regional Council’s water shortage event manager – a newly created role.

It was created last year around the same time the regional council introduced, for the first time, a “standard operating procedure” for managing and responding to a water shortage event.

There are three alert levels to this new procedure – Level 1: normal water availability, Level 2: impending water shortage, and Level 3: water shortage event.

Most of the Bay of Plenty has so far remained in Level 1, but in January this year about 60,000 hectares of the region moved to Level 2.

This area, called the Rotorua Focus Zone, has only just moved back to Level 1 this week, after a decent amount of rainfall in June stabilised the declining stream flows. But the region is not out of the woods yet. There is still a lot of ground to make up.

Parts of the Bay of Plenty are still in long-term rainfall deficit, Pickles says, and streams like the Ngongotahā still have lower base flows than at this time last year.

“While small rain events and the cooler winter months provide some respite, it’s not solving the underlying lack of rain over the longer term,” he says.

“If we don’t get enough rainfall this winter, our aquifers, streams and rivers won’t have a chance to recharge themselves before we head into another summer.”

And if that happens, Pickles says, we could be starting 2022 “from quite a gnarly place”.

“Looking forward, how concerned am I? I suppose if we don’t get the sort of quantities to bring us up to what would be typically a normal season of rainfall … we could be heading into the spring and summer of 2021/22 in quite a bad situation for some areas, and what that could mean is that we would have to move into Level 3.”

It’s not that the streams are dropping in height necessarily, it’s that they are slowing down.

Pickles says the lowering flows may not be easily visible to the naked eye due to it being a gradual decline, and the result of lowering flows may actually affect stream velocity and channel shape, water level height, or a combination of all.

The Ngongotahā, for example, is still flowing at about 1000 litres per second – definitely not a trickle. But, when comparing that to the 2400 litres per second it was flowing at in July 2017, you can see a change.

Stream flows also naturally fluctuate up and down over time, so it is the base flow that the scientists are looking at.

Summer is when Simon Short is at his busiest.

One of the main draw cards at his adventure tourism park, Velocity Valley, is a jet boat sprint course that is filled up with water from the Ngongotahā Stream.

The stream falls within the Rotorua Focus Zone.

“We’ve never had to worry about too little water, we’ve always worried about too much water,” Short says, standing on the bank of the Ngongotahā Stream on a crisp, sunny winter morning, the stream flowing calmly below.

“Three years ago, the big flood that took Ngongotahā out was …”

Short raises his arm above his head, before continuing.

“… approximately here. The water level was about up here, which obviously caused massive damage to our business.”

In April 2018, the Ngongotahā Stream reached its highest recorded level and burst its banks in several places during a 100-year flood event. A state of emergency was declared, homes and businesses were inundated and evacuated, and there was serious damage to roads and bridges.

Residents watched as the floodwaters rose over their sections and then into their houses in a matter of minutes. There was little to no time to save belongings, cars and furniture were swamped, and people had to swim or hold on to fences to avoid being swept away.

Thirty-eight houses were later recorded as insanitary and uninhabitable.

At Velocity Valley, Short says there was more than $125,000 worth of damage.

He says they cleaned up and carried on, moving equipment a little higher.

“We built this thing right beside the river and down very low to the river, so we take it on the chin that we made that decision to set up here.”

But then, a few months ago, just as things were returning to normal after the Covid-19 tourism pinch, Short started to become aware of the other potential risk looming over his business.

Right now he fills up his jet boat sprint course at a rate of 10 litres per second. He is consented to take 20 litres per second from the Ngongotahā Stream, so he’s only using half of what he’s allowed.

“It’s all we need,” he says.

However, if the Rotorua Focus Zone was ever to move to Level 3, and if the regional council was forced to restrict Short’s water take, it could potentially spell trouble.

“In the middle of the summer time, when it’s our only opportunity to make money, with the domestic market being our only source of income, that would be a big issue. That would seriously hurt,” he says.

“It will be manageable, as long as they [the regional council] don’t bring it down to a massive level. If they, you know, quarter what we’re taking at the moment, it could be a problem.”

And that is what Pickles is trying to avoid.

As the regional council’s water shortage event manager, Pickles has been travelling around the region, meeting with consent holders like Short to let them know conditions are “trending in a concerning direction”.

He also wants to warn them about the worst case scenario (water restrictions), and try to find ways to make their water extraction more efficient and sustainable.

“When taking water from streams and rivers, the important thing for that is the rate at which they take the water out, because that affects the stream flow. It doesn’t really matter what volume they’re taking, to some degree.”

For example, to lower its extraction rate, Short says Velocity Valley might start taking 5 litres per second for 16 hours, instead of 10 litres per second for eight hours. It will be the same volume of water, just at a lower rate.

Meanwhile, other consent holders, like orchards for example, might look at things like storage – slowly taking water from a stream when conditions allow and when there is plentiful supply, storing it in a large tank or a pond, and then using that water wisely over the drier summer months.

Most of the water volume taken from streams in the Rotorua Focus Zone is for municipal water supply, the next highest use is for horticulture.

Region wide, it’s the same. When looking at surface water overall (streams, rivers, lakes), municipalities are consented to take the most, and horticulture (irrigation and frost protection) is consented to take the next highest.

About two-thirds (70 per cent) of all 1300 water resource consents in the Bay of Plenty (taking from surface and ground waters) are for horticultural use.

The kiwifruit harvest has just been completed.

Kiwifruit is New Zealand’s largest horticultural export with more than 80 per cent of the fruit coming from the Bay of Plenty, according to New Zealand Kiwifruit Growers Incorporated (NZKGI).

Last year, the industry contributed $152 million to the region.

NZKGI chief executive Colin Bond says soil moisture levels have been well below normal in the Bay of Plenty and the taste and size of kiwifruit can be compromised for those who do not irrigate.

There are eight kiwifruit orchards in the Rotorua Focus Zone, he says.

While there was no change to water takes under Level 2, the regional council increased its monitoring. Under Level 3 conditions, growers could be required to cease or reduce their water take, however this will be dependent on the regional council and how and where it applies restrictions.

“Ceasing or reducing water intake to irrigate kiwifruit orchards could affect the fruit’s size and taste, and it is important this doesn’t occur in order to retain the fruit’s high value.”

Bond says NZKGI works closely with the regional council on water-related matters to provide consistent messaging and accurate information.

He says the kiwifruit industry has a water strategy that recommends growers install water storage on their orchards as a priority to combat dry periods.

Meanwhile, in the Bay of Plenty’s biggest city, Tauranga, a ban on the use of irrigation systems and sprinklers, which was put in place just before Christmas last year, has only just been lifted.

Tauranga gets its water from two streams – the Tautau and the Waiorohi.

While flow levels in those two streams are now stable, they still remain lower than normal, according to the council’s City Waters director Stephen Burton.

“Should we experience another winter that’s drier than usual, we will start next spring on the back foot making early water restrictions likely,” Burton says.

“Water is a finite resource, of which low stream levels are a stark reminder.”

In December 2017, Tauranga imposed water restrictions for the first time in 17 years.

There have been restrictions every summer since then. This year, the restrictions stretched through autumn.

“City-wide sprinkler restrictions have never been necessary beyond the first week of April,” Burton says.

“The length of this year’s water restrictions reflects the knock-on effect of three unusually dry summers for the Bay of Plenty which has had an impact on soil moisture and stream levels.”

Residents in Tauranga are still being asked to use water wisely and an education campaign will be launched later in the year, encouraging people to make every day a water conservation day.

This is a necessary shift in mindset, according to Burton.

Tauranga uses an average of 43.7 million litres of water per day and in summer this can rise to 58 million litres per day.

Tauranga City Council is currently building the Waiāri Water Supply Scheme, due to be completed late next year, to help meet the future water supply needs of the growing city.

This involves developing a new water supply intake from the Waiāri Stream, a water processing plant on No1 Road, Te Puke and an underground water pipeline from the plant to Pāpāmoa.

A lot of streams in the Bay of Plenty, including the Ngongotahā, are fed by freshwater springs.

Levels have also been dropping in some of the groundwater aquifers of those springs.

“The reduced rainfall over the last few years could be showing up now as a decrease in spring flows,” Pickles says. “So the aquifer levels are dropping, and so there’s less water popping out as springs into the heads of those streams.”

He says it can take quite a while for rainwater that falls on the ground to percolate through to an aquifer, and so there can also be a delay in the effect of drought on decreasing water levels.

Pickles says streams do flood from time to time, and then they get reduced flows, “that’s the natural process of a stream”.

“The situation we’re seeing now, though, and we have to be aware of, is that streams are at their lowest for an extended period of time. That’s a little bit of an uncharted territory for us, this is only the second year of having those sorts of situations occurring since our records began.”

He says low stream flows can adversely affect stream health.

“If a stream dries up or heats up too much, it becomes a less desirable habitat for aquatic species and also affects the cultural and amenity values of the stream.

“Water users can also struggle to pump water through their infrastructure,” he says.

Pickles says as our climate continues to change, the National Institute of Water and Atmospheric Research (Niwa) is predicting more extended periods of warmer days for the Bay of Plenty, interspersed with periods of intense rainfall.

“As a result, we’re likely to experience drought conditions more frequently, which will have implications for pasture, crop and animal health.”

The health of the Ngongotahā Stream is an ongoing concern for local hapū Ngāti Tura and Ngāti Te Ngākau.

“Our concerns are that our awa is no longer a source of kōura harvesting and a playground for our tamariki,” spokesman Joe Edwards says.

“It is not only the low flow, but E coli levels, that threatens the wellbeing of our hapū,” Edwards says.

“Until we have a greater understanding of what is going on in the upper reaches, our response to solutions is limited.”

The regional council says there are only two consented water takes from the Ngongotahā Stream, one for Velocity Valley and another for a trout hatchery.

For Short at Velocity Valley, the low stream flow is just another obstacle to overcome.

“It’s never ending, and I’ve just developed tough skin and just deal with each situation at a time.”

He says he is committed to doing what he can to help the Ngongotahā Stream, which he says is “a beautiful thing”.

“Living next to it and watching it every day for 20 years really shows you what nature can throw at you, whether it be a drought or a flood,” he says.

“It’s from one extreme to the other.”