NZ's Big Dry: Five key questions answered

Sue Murphy clutches drought-browned grass from her rural property along the Coatesville-Riverhead Highway in northwest Auckland. Photo / NZ Herald

Scientists are warning that vast swathes of the upper North Island are headed for "permanent wilting point" – and a new record for Auckland's longest dry spell is expected to be set this Saturday. Science reporter Jamie Morton looks at five of the wider questions surrounding New Zealand's big drought.

What's the current situation?

Across most of the upper North Island, conditions are bone-dry – and there's little rain in sight.

The latest observations show severe meteorological drought is widespread across Northland, Auckland and northern Waikato, and new hotspots are emerging in northern Gisborne and northern Canterbury.

Elsewhere – and with the exception of the western and southern South Island – it's unusually dry.

In the most parched places of the upper North, the soil moisture deficit has been off the scale and the ground is so thirsty that what's called "permanent wilting point" is being approached in Northland, Auckland, and Waikato.

That tipping point is effectively the minimum level of water in soil that a plant requires not to wilt or die.

"From Taranaki through Northland, including Waikato and Auckland, as well as parts of the Tasman District in the South Island, soil moisture is down 50mm and would require a month's worth of rain or more to be replenished," Niwa forecaster Ben Noll said.

This Saturday, Auckland is expected to break a record for its longest dry spell – passing a 39-day run recorded when the North Island last saw widespread drought conditions, in 2013.

When Auckland's dry spell reaches 40 days on Saturday, it will become the longest on record 🏜️

The current record for the Greater Auckland area is 39 days, set in 2013. pic.twitter.com/rjpGLuaM2T

— NIWA Weather (@NiwaWeather) February 12, 2020

A dry spell is defined as consecutive days with less than 1 mm of rain and the current record for the greater Auckland area is 39 days.

The region also saw some notable runs in 2007, 1978 and 1964.

Noll said several places across the upper South Island had also experienced record or near-record long dry spells, among them, Takaka (46 days), Blenheim (53 days), Cheviot (49 days), Culverden (45 days), Rangiora (45 days), and Hanmer Forest (40 days) have experienced record or near-record long dry spells.

"The list will expand during February with several North Island sites likely joining."

Is rain coming any time soon?

Short answer: no.

At the beginning of the month, there were hopes a tropical cyclone could bring much-needed moisture down towards New Zealand.

"While cyclone predictions turned out to be accurate - Tropical Cyclone Uesi formed this week - it is looking like the storm could pass near the lower South Island or south of the country early next week, sparing the parched North Island," Noll said.

"There remains at least some hope that some moisture could move toward the North Island between the 20th and the end of the month, but at least until then, rainfall chances appear few and far between.

"It would take several soaking rainfall events to alleviate the extremely dry conditions that are affecting parts of the country."

Source / Niwa

As the hot and dry weather continues Auckland Council has revealed it is looking into employing milk tankers to help fill the water tanks of parched rural residents.

The council estimates there are about 50,000 households on rainwater tanks across the region, putting huge pressures on water cartage services after over a month without any decent rainfall - and none forecast until next week.

However, city residents weren't yet facing water restrictions that had now been imposed in many other places in the North Island – and most heavily in Northland.

What does the dry mean for our farms and forests?

Northland Rural Support Trust co-ordinator Julie Jonker said with no rain on the horizon over the next 10 days, things were looking dire.

The Government has set aside around $80,000 for the trust to run events to help get farmers off-farm, reduce isolation and provide education and technical advice.

While the drought itself would end when soil moisture levels returned to normal, impacts could still continue for some time after that.

Associate Professor Cate Macinnis-Ng, of the University of Auckland, also pointed to the effect that drought could have on other ecosystems.

"Around the world, there are growing records of forest dieback due to increasing frequencies and intensities of drought due to climate change," she said.

Rotorua lawn mowing contractor David May has been receiving many cancellations as the dry weather lingers. Photo / Stephen Parker

"Here in Aotearoa, droughts don't generally cause forest death because our droughts last months rather than years."

But while established forests might be relatively safe from drought impacts, seedlings and restoration plantings were vulnerable as developing root systems of smaller plants did not have access to deep soil water stores.

"If drought continues for extended periods, carbon uptake may be reduced as plant productivity slows," she said.

"Drier plant tissues are also more flammable so we all need to be especially careful with fire.

"The recent forest fires across Australia were exacerbated by drought and severe fire weather across large areas. While we are unlikely to suffer such catastrophic events here, we still need to plan carefully to protect forest and manage our water supply."

How bad did things get the last time around?

Quite bad. The 2012-13 drought covered the entire North Island, along with the west coast of the South Island.

It was one of the most severe droughts to have impacted these areas in at least 40 years, and in some cases more like 70 years.

The lack of rainfall makes for a dusty trail for walkers as the summer-dry bites on the slopes of Maungakiekie / One Tree Hill. Photo / Jason Oxenham

The economic impact of the drought is estimated to be, at a minimum, $1.3 billion; the potential damage from this event was still unclear, with the Government only just having declared a "medium-scale adverse event" in Auckland and Northland.

What's fascinated meteorologists is the striking similarity between the climate patterns then and now.

Just as in 2012-13, a large ridge of persistent high pressure perched to the north of the North Island had blocked any rain-making systems reaching the north.

Meanwhile, in the far south, deluges have wreaked havoc in both summers: floods in Southland this month tipped the region into a state of emergency, and Uesi is forecast to hit the lower south hard again this Sunday.

Over the summer 2012-13, the popular Milford Track was closed several times due to extreme rainfall, while the Department of Conservation also had to shut the Routebourn Track.

Another common factor was an absence of any traditional climate driver in the mix. In both seasons, the tropical Pacific has been in an El Nino-Southern Oscillation (Enso) neutral state - meaning that neither El Nino or La Nina patterns were colouring the weather.

Besides the blocking high to the north, the only other influencers this season have been a positive Indian Ocean Dipole and a negative Southern Annular Mode – both of which have driven Australia's bushfire crisis, and now largely faded.

Is this climate change in action?

While droughts aren't new to New Zealand, it's worth noting that our weather and the climate systems that drive it sit against a background of ongoing warming.

Even under the most optimistic scenarios of climate change, drought will become more frequent - and more severe - over coming decades. Photo / NZ Herald

Average temperatures have risen by about 1C over the past century – and the pace of that change has particularly increased over recent times.

Kiwi scientists have also demonstrated how the 2012-13 drought had been about 20 per cent more likely to occur today than in the late 1800s.

Even under the most optimistic scenarios of climate change, such events would grow more frequent - and more severe - over coming decades.

Victoria University climate scientist Professor James Renwick said there were no indications that climate change would shift the variability of our main climate drivers like La Nina or El Nino – or favour one over the other.

"The jury is still out, but the expectations at the moment are that we carry on much in the same way as we have in the past," he said.

"While, over time, we can expect rainfall extremes to change so it becomes wetter where it's wet and drier where it's dry, that's the situation for the tropics, rather than here in New Zealand."

Our country would likely see the kind of trends that had been evident over the past few years: warmer winters and springs, drier weather in the north, and stronger westerly winds over the South Island.