Anthony Fowler: How does Auckland's drought measure up?

A reservoir in the Waitakere Ranges at 42 per cent full in April, 2020. Photo / Sylvie Whinray, File

OPINION

On several occasions this year, statements have appeared about Auckland's "water crisis" being caused by a 200-year drought. Such claims matter because they relate to
responsibility.

After our last water crisis in 1994, the 200-year drought was adopted as Auckland's drought risk threshold. In essence, the city's water supply system is expected to be
able to handle less-severe droughts and Watercare's core responsibility is to incrementally
augment supply to ensure that this is so.

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If the 200-year drought claims are true, we can all just blame the weather and take much of the heat off the politicians and water supply planners. If not, then hard questions need to be asked.

So, are the 200-year drought claims true? I must admit that I was dubious when I first heard them.

First, although it had certainly been very dry since November, I didn't recall a dry winter in 2019, so water storage should have been reasonable going into the summer drawdown period.

Second, I was aware that water supply planning has traditionally focused on droughts of a couple of years duration. A short-duration drought should not be a major concern and I
wondered whether the situation was being used to leverage a stalled application for the next supply increment from the Waikato river.

Central Auckland has a continuous record of monthly rainfall dating back to January 1853, so determining the return period of the 2020 drought is quite feasible.

Anthony Fowler. Photo / Supplied

The first known recording site was in Albert Barracks (now Albert Park), quite close to where the present flagstaff is. In 1868, it was relocated to a ridge in Auckland Domain, behind where the War Memorial Museum was later built. In 1883 it was moved again, this time to the rooftop of the old museum at the northern end of Princes St. The final shift was to Albert Park in 1909.

The three early sites were very exposed compared to Albert Park. Because exposed rain
gauges under-measure rainfall, sometimes by tens of per cent, recorded rainfall in the second half of the 1800s is substantially less than totals recorded later at Albert Park.

This is as expected and, to be useful, the early records need to be scaled up before they can be combined. A simple approach is simply to scale the early data so that the respective site averages agree with Albert Park.

From this we can show six-month and 24-month running sums of central-Auckland rainfall since 1853.

With the caveat that the record prior to 1909 remains rather dodgy, it is clear that total rainfall for the six months ending in April was indeed exceptional. It was the lowest six-month total recorded and a 200-year drought is certainly plausible.

This is what knocked the stuffing out of those dependent on household water tanks and farmers throughout the upper North Island early this year.

For the city's water supply though, it is the 24-month running sum that is more relevant, and here the story that emerges is rather different.

By the end of October, the 24-month total had reduced to 1983mm. This places the 2020 drought among the top eight in 168 years, giving an estimated return period of less than 25 years. This is comparable to 1994, but notably less significant than droughts in the 1910s and 1980s.

Now, admittedly, the drought hasn't ended yet, but claiming a 200-year event is perhaps a tad premature.

Having a water crisis associated with a drought more common than the accepted risk threshold makes me uneasy about Auckland's water future – uneasiness that is aggravated by a couple of other factors.

First, four of the eight droughts mentioned above have occurred in the past 50 years, coinciding with an extended period of lower rainfall.

Second, Auckland has warmed by about 1C over the past 100 years, which has likely increased evaporation. This means that more recent droughts have probably had a bigger impact on reducing runoff into the water reservoirs.

Reduced rainfall and increased evaporation are projected for our region as the world continues to warm. In light of this, a precautionary approach would be to assume that recent drought history may actually signal a significant climate shift in our region.

Hopefully, Watercare will take this into consideration when it next reviews Auckland's water supply and demand projections.

Unfortunately, doing so is likely to make our water bills higher still.

• Anthony Fowler is an associate professor with the School of Environment, University of Auckland.