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Will there be sufficient snowmelt in future to fill $16 billion Lake Onslow 2.0?

View from Brewster Hut, Mount Brewster, Mount Aspiring National Park, New Zealand (Department of Conservation)

First published at

Back in the day, my first real job was as a government scientist in the Water and Soil Division of the Ministry of Works and Development (MWD). We scientists collaborated with dam engineers from Power Division to plan new hydro power developments. The scientists worked on the water supply modelling, environmental, and social impacts of projects, while the engineers worked out how to build a safe dam and deliver power to the national grid.

Now, as everyone knows, hydro power stations need water to generate electricity. The ultimate source of the water supply to Lake Onslow 2.0 will be snowmelt from Mount Brewster and its surrounding mountain terrain and glaciers in Mount Aspiring National Park. Snowmelt water flows from the mountains into Lakes Wakatipu, Wanaka, and Hawea and then down the Clutha/Mata-Au River to Lake Onslow 1.0.

The Clutha/Mata-Au catchment is the biggest in New Zealand. Snow and ice melt comprises at least 75% of its water supply (ORC 2023). We learn from Taylor Bardley 2019 in their paper: A Hydrological Drought Index for the Clutha catchment, New Zealand that:

“It has a large seasonal flow variability with large spring and summer flows and very low flows in winter. ….The mountainous South Island topography to the west means that a large part of the catchment is in a rain shadow and is regularly subject to long periods without any significant precipitation, similar to many other rain shadow catchments globally….The main part of the catchment, from the outflow of the lakes to Roxburgh, is in a rain shadow with rainfall as low as 325 mm per annum [almost desert levels] in the Springvale area near Alexandra…..”

The authors’ upper and mid-Clutha drought index from 1930-2014 shows us that only three years – 1979, 1980, 1983 – were classified as ‘wet’ years. Out of the total 84-year record, 47 years (56%) experienced mild to extreme drought conditions. So we know there is already a drought problem in the catchment, and that it is already highly modified by competing water uses including irrigation takes, rural and domestic water supply takes, and other major hydro dam developments.

In addition, over the past ten years there have been an increasing number of alarming reports about the scale and speed of glacier melt and snow and ice retreat from New Zealand mountains, in common with similar mountain environments around the world. A media article last week reported on NIWA's 2022 preliminary findings that snowmelt and glacier retreat is accelerating in New Zealand. The Brewster Glacier in particular is disappearing fast.

The volume lost from Brewster during 2016-2019 was about equal to the basic drinking water requirements for all New Zealanders for those three years NIWA 2020. That’s a phenomenal amount of water that won’t be coming back. The Brewster Glacier may be gone completely inside the next decade or two. Indeed, based on current trends, almost all of New Zealand's glaciers could be gone by the end of this century. And that means way less water supply for the nation.

So I searched through the public information provided by the Ministry of Business, Innovation and Employment to see if I could find proof that there will be enough future water supply to fill and operate the dam. One would think that this would be the first step with any 21st Century hydro power development, especially one dependent on water supply from disappearing glaciers. All I could find was a presentation outlining project phases. None of the phases appeared to address this fundamental question.

Then I was provided with the 429-page business case for Lake Onslow 2.0 - New Zealand Battery Project: Progressing to the Next Phase (MBIE 31 March 2023). Looking at the Executive Summary, security of future water supply is not mentioned, nor is it included in the 12-point Multicriteria Analysis used for the Treasury’s critical success factors framework. The ‘dry year’ problem discussed briefly does not include a note on existing drought problems nor a trend analysis of future required water supply volumes for the dam.

In fact in the entire 429-page tome, there are only three small sections in Appendix E totalling 246 words that address inflow assumptions and future climate change effects on Lake Onslow 2.0. These sections contain five fundamental errors and omissions:

1) Forecasts for future flows are based on back-casting historical flows from 1932, which means the project proponents assume future flows will remain the same as they were since 1932

2) Climate change is not regarded at all seriously with the comment:

“While the climate probably has changed since the 1930s, and will change further going

forward, we used the full range of inflow sequences back to 1932….”

3) Climate change impacts on flows are predicted using an estimate of a 25% reduction in snowmelt to 2050, when NIWA measurements indicate that a 50% estimate is probably more appropriate. The observed acceleration in climate change impacts on glacier systems reported by NIWA is not mentioned at all

4) Remarkably, large increases in hydropower lake storage levels and spill are projected between 2020 and 2050, 17% at Lake Hawea (Clutha/Mata-Au), which doesn’t make sense when the headwaters – 75% of supply – are melting away at pace

5) No historical flow trend analysis is presented (are river flows decreasing over time?), neither are any snow and ice mass balance calculations (how much water is gone and won’t be coming back?), nor annual hydrological modelling that includes drought indexing and the future demands of other users of water in the catchment

This report would not have passed muster at MWD. These issues need to be urgently addressed before another $70 million is spent on the project. What assurance do tax payers have that Lake Onslow 2.0 won't end up like Lake Mead, built using data from an unusually high flow year, on an assumption that the past predicts the future, ending up with storage levels too low for power generation, or worse, a screamingly expensive 1.5km-long dry hole in the ground by the end of the century?

Where are the numbers to assure ordinary tax payers that Lake Onslow 2.0 won't become a casualty of climate change? The presented modelling work has academic assumptions and real world data gaps that are too broad to be acceptable for a project with this huge price tag. The project proponents need to get out there and actually measure reality.

And where is the forward planning to make sure all our mountain-fed hydro power schemes will be secure throughout the rest of this century? Otherwise as a nation we'd better look to switching to more solar, wind, and geothermal power. After all, $16 billion is enough to retrofit every house in New Zealand with solar panels.

And what about New Zealand's water futures? Who is looking into that? This is something that the Climate Change Commission and Parliamentary Commissioner for the Environment could be tackling on behalf of us all. The trends are obvious and very alarming according to NIWA's glacier monitoring data. New Zealand needs to act now to secure its water futures into the 21st Century.

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