Shoring up Norfolk Island’s water supply
The tiny volcanic outcrop of Norfolk Island, 1500 kilometres southeast of Brisbane, is one of Australia’s most remote territories.
Its 1700 or so residents – whose homes are scattered across the island’s 36 square kilometres of green farmland and pockets of sub-tropical forest – have no access to reticulated water. There are no large dams, and most of the permanent streams dried up decades ago.
Instead, 95 per cent of the island’s households rely on rainwater tanks, with some households and farms also using bores, old hand-dug wells, or a combination of tanks and bores, to meet their water needs.
More than just a dry spell
It’s a way of life that’s become second nature to those island families who have lived there for decades, even generations, says second-generation islander, Merv Buffet, whose grandfather arrived from Pitcairn Island in the nineteenth century.
“Until 30 years ago, if anybody wanted to extract water from the ground, you did it from hand-dug wells, which are up to a 125-foot (38-metre) deep, using a hand windlass [instead of an electric pump].
“You only used what water you needed, because it was too laborious to go back again and retrieve more.”
Conserving water has enabled islanders to weather inevitable dry years. In recent decades, however, dry spells in this part of the Pacific have become more frequent and severe, a trend projected to continue according to global climate models.
In the summer of 2019-2020, household tanks across the island ran dry. Bores and wells used to provide rainwater tank ‘top ups’ during dry spells could no longer supply enough water to replenish them.
At the request of Emergency Management Norfolk Island, the Australian Government sent an Australian Defence Force (ADF) team to set up a water purification and desalination plant at Cascade Bay.
The solution saved the day, and the rains returned soon after.
But for the local community and the government, the crisis highlighted the urgent need to find out what was happening to the island’s water, and how the community could plan for a more secure water future in a drier climate.
Assessing water above and below ground
A year earlier, in February 2019, a Water Emergency Workshop was held on the island with representatives from the council and community, the Australian Department of Infrastructure, Transport, Regional Development and Communications, CSIRO, the Bureau of Meteorology, the ADF and Emergency Management Australia.
A key problem identified at that workshop was the lack of data on the island’s hydrology and groundwater. Soon after, the government engaged a CSIRO team, led by CSIRO hydrologist Dr Cuan Petheram, to work with the local community in undertaking the Norfolk Island Water Resource Assessment (known as NIWRA).
Most bores on the island are on private land, largely for private use. After getting permission from landowners involved in the assessment, the team began the task of collecting data from across the island.
Using a range of techniques, they measured rates of water infiltration to the soil, surface runoff, groundwater storage, transpiration rates from different vegetation types, water quality, and water storage and use in participating households.
Mapping the changing hydrology of Norfolk Island
Despite the lack of historical data on the island’s hydrology, the NIWRA team did manage to locate an old groundwater map from the 1970s. By digitising the information and combining it with the new data, they were able to model broader groundwater changes across the island over the past 50 years.
Their results showed that, while mean annual rainfall on Norfolk Island had fallen by 11 per cent since 1970, the reduction in streamflows and groundwater recharge – the amount of surface runoff that finds its way into underground aquifers – was even greater.
“For every 1 per cent reduction in long-term rainfall, there was a 2 per cent reduction in groundwater recharge and a 5 per cent reduction in streamflow,” says Dr Petheram.
The NIWRA findings also showed that the volume of rock saturated by water in aquifers had decreased by about 20 per cent over 50 years, while groundwater levels had fallen between 5 and 20 metres, and 35 per cent of the island’s bores and wells had dried up.
In short, says Dr Petheram, the long-term changes in rainfall have resulted in a profound change to the hydrology of Norfolk Island.
“The 12 months of data is just a snapshot, a point in time,” says Dr Petheram.
“Water authorities on the mainland are basing infrastructure investment decisions on many decades of baseline information. So, we’ll continue to collect baseline information on the island’s hydrology because it will be needed for future planning.”
Options to improve water resilience
As part of the NIWRA work, the CSIRO team was asked to carry out an options analysis of potential initiatives that could improve the island’s capacity to withstand extended dry periods.
“Our approach was strictly focused on identifying comparative water yield, cost, benefits and barriers for each of the options,” says Dr Petheram.
“At the end of the day, decisions about what to do next will need to be made by the community, the Norfolk Island Regional Council and the Commonwealth of Australia, in light of their values, needs and responsibilities.”
The options identified in the study include:
- extending and upgrading guttering to direct more runoff from existing rooftops into tanks
- establishing ‘cluster-scale’ roof rainwater harvesting from adjacent rooftops
- setting up standalone ‘tank farms’ to collect and store water
- increasing rainwater tank storage capacity
- building strategically located earth-bank dams in natural gullies
- extending existing bores or drilling new ones into the deep, fractured, unweathered basalt
- managed aquifer recharge, where water is banked in natural underground aquifers until needed
- managing vegetation cover (for example, removing ‘thirsty’ deep-rooted, woody weeds)
- installing a desalination plant.
“What happens next will depend on community values, the level of risk the community is prepared to accept, and what resources are available to construct and maintain infrastructure,” says Dr Petheram.
“A robust emergency water supply strategy would encompass a suite of options, implemented over different timeframes, providing redundancy and cost-effectiveness.”
Acid soils and water quality next up
While the Australian Government is continuing to fund the NIWRA monitoring program, Dr Petheram’s team will also undertake related studies over the next two years.
One of these will focus on the island’s acid sulfate soils, which occur in many of the island’s drainage lines, and acidify and change soil properties as they dry out. This poses problems for new reservoirs, heritage structures and downstream environments.
Another study aims to better understand the relative impacts of the island’s deep-rooted woody weeds and Norfolk Island pines on groundwater.
The team will also focus on water quality, particularly around Emily Bay, home to one of the world’s most southerly coral reefs.
Council taking action
At the same time, the council is planning to increase the island’s water storage capacity and reduce its reliance on groundwater, says Peter (PJ) Wilson, team leader for waste and environment at the Norfolk Island Regional Council.
“The NIWRA studies have confirmed what many locals have seen for a long time, and that is the concerning drop in the island’s groundwater,” he says.
“Council is in the process of strengthening planning requirements to ensure more rainwater storage is installed for new developments.”
Other plans include treatment of wastewater for reuse in agriculture, a rainwater harvesting project utilising council buildings.