Managing Future Uncertainties Using IWN and The Adaptive Planning Approach

Vijesh Chandra, Executive Advisor – Business Advisory, GHD Limited, 

Florika Patel, Intern Student, GHD Limited

Future uncertainties in climate change impacts, disruptive technology, legislation, cultural
and community expectations and population demographics will impact today’s decision-making on investments in 3-water networks. This paper promotes adoption of Integrated Water Management (IWM) that embraces future uncertainties by exploring the impact of multiple plausible futures and potential shocks and uses an Adaptive Pathways Planning (APP) approach with built-in triggers for decision-making on investments.

Best practice in IWM from Melbourne, Sydney and the USA was applied to a GHD Callaghan
Innovation funded project to assess the effectiveness of rainwater storage solutions for
water supply resilience in Auckland considering future uncertainties.

GHD’s IWM toolkit, a software package for the assessment of integrated water balances
was used for the analysis and modelling of a hypothetical scenario to assess effects of
rainwater re-use on Auckland’s total dam water storage volume. The Toolkit uses an
intuitive graphical interface (MS Visio) to construct alternative water servicing strategies
and perform time-series water balance calculations for multiple combinations of options.
The scenario was that in January 2019, 10% (and 20%) of residential households in
Auckland had access to roof rainwater tanks (plumbed to toilet flushing, washing and
shower). Using actual rainfall series data, the toolkit was used to assess the impact of the
scenario on the total dam storage volume in December 2020 and then in December 2022
if the Auckland drought continued for another 2 years.

Options for 1,000, 2,000 and 5,000 litre tanks were assessed, and it was found that by
December 2020, for the 20% residential uptake, an additional buffer of about 14 million
cubic metres would have been added to the total dam storage volume (Auckland’s average
water use is about 450 million litres per day). It was evident that rainwater tank use during
the rainy season builds the total storage volumes in the dams and provides security of
supply in the dry season. Considering an IWM approach and potential future changes,
additional benefits (e.g. flood reduction, alternative to detention tanks, sustainable practice
promoter) and potential solutions to obstacles (e.g. first flush diverters, UV filters, smart
tanks, slimline tanks, incentives) of city-wide rainwater tank use were explored.

The conclusion was that rainwater tanks and other water source alternatives (e.g. grey
water re-use, stormwater harvesting) could be triggered by future changes in criteria
(economics, resilience-readiness, technological breakthroughs). Therefore, all potential
options can be kept “alive” and an (APP) approach can be used to optimise investment
decision-making as future uncertainties become more certain. Importantly, feasibility of
the options, the triggers and the timelines for planning, design, consenting, and
construction need to be assessed to understand implementation requirements of the
options in time to meet the required level of service.

As the Water Reform takes shape in NZ, we will need detailed analysis, modelling and
feasibility assessments of multiple integrated 3-water strategies and options, that consider
the impact of potential future changes to put us in a better position with respect to longterm resilience planning and optimising investments in 3-water networks.