Planning for regional growth
As part of the process of maintaining an adequate water supply for the Hutt, Porirua, Upper Hutt and Wellington city councils into the future, Greater Wellington Regional Council has recently increased the water storage capacity of the Stuart Macaskill Lakes at Te Marua and is investigating additional water source and storage options.
Our aim is to have a water supply system that results in a very low risk of water shortage. We use a security of supply design standard of not more than a 2% annual probability of shortage, which is regarded as best practice for our combination of run-of-river, aquifer and storage sources.
Infrastructure is planned and developed to provide additional water to maintain the 2% standard for a growing population and any changes in the average level of water consumption per person (includes residential and commercial).
We use a water modelling programme developed by NIWA that utilises climate, population and water use data, to assess future water demand scenarios. These inputs are reviewed and updated at regular intervals as modelling capabilities develop, population projections change and water use trends emerge.
Our water modelling programme was updated in 2013. This update took into account the consistent reduction in water use per person that has been seen over the last few years and an improved understanding of the behaviour of the Waiwhetu Aquifer.
This means that the sustainable population (the population that our present network can reliably supply with enough water in most years) has risen – from 414,000 to 447,000 people. Based on Statistics NZ 2012 medium-growth projections (the latest available), a population of 447,000 for the four cities in our supply area would be reached about 2035. Given this projection, the likely date for developing a new bulk water source is now 2035 – 15 years later than previously estimated.
While the currently-projected 15 year deferral of the need for a new bulk water source is welcome, we have continued to work on options for the additional sources of water so that we are fully prepared should this outlook change.
In 2012, GWRC decided to pursue a flexible and incremental approach to developing a new water source. This approach resulted in the purchase of land at Kaitoke that has the potential to hold three water storage lakes of differing sizes (and costs).
The Whakatikei dam remains an important option for future long-term water supply development, however, the proposed storage lakes in Kaitoke are likely to be built first as they provide more choice regarding the scale, environmental impact and cost of the next stage of development, whenever that is needed.
Reducing demand for water “per person” (by methods such as reduced leakage from city water reticulation networks, increased use of water-efficient appliances and fittings or household water metering and associated conservation education) at a rate equivalent to population growth would help to defer the need to build a new water source.
We are working with the region's city councils to develop strategies that include elements of water demand management, for public consultation. However, our planning approach is conservative and does not rely on the reduction of per capita water use in the future.