Auckland’s land use drives a range of hydrological contaminant responses being simulated by the Freshwater Management Tool.
Auckland Council is responsible for the management of freshwater quality and quantity in the Auckland region. To manage these responsibilities holistically, in a prioritised manner and in accordance with requirements set out in the National Policy Statement for Freshwater Management (NPS-FM), Auckland Council’s Healthy Waters department and its partners, Morphum Environmental and Paradigm Environmental are building a freshwater quality and quantity accounting tool that will facilitate transparent and informed freshwater resource management. This comprehensive model is based on an open-source software platform and will support regional decision-making and community engagement. The Freshwater Management Tool (FWMT), is being designed to perform two key functions that cover the entire region:
Generate ‘current state’ scenario assessments for the region (concentration, load) for flow and contaminants.
Generate cost and effect of selected optimised interventions for various ’future state’ scenarios, to inform decision-making.
FWMT as an accounting system
The first function of the FWMT is the Current State Model, or ‘accounting system’. Developed using hydrological modelling software, the model is based on continuous, process-based simulation of the response of catchments parameterised by steady state slope, soil, land cover and land use to 15 years of rainfall data. The model is calibrated to monitored stream data allowing extrapolation through simulation of the current state of:
Flow conditions for watershed runoff.
Contaminant build-up/wash-off (Sediment, Zinc, Copper, Nitrate, Ammonia, Phosphorous and Escherichia coli) at the sub-catchment level.
Mixing, routing and contaminant processing through the stream and pipe networks.
Downstream loading to the marine receiving environment.
The accounting system produces time series for over 5,500 sub-catchments in the Auckland region and 3,300km of stream network at 15-minute time steps, across a multi-year climate time series. This allows post-processing to determine various metrics flexible to evolving reporting and regulatory requirements.
FWMT as a decision support tool
The second function of the FWMT is the Future State Decision Support Tool, to answer the “what if” questions that are required to achieve positive environmental outcomes. A Stormwater Management Model simulates the processes of load reduction, retention, detention and contaminant removal from structural and non-structural interventions in the modelled catchments. This will be run for predicted future development, and software optimisation engines will select the most cost-effective combinations of interventions for each sub-catchment to achieve varying attribute states at specified nodes in the stream network. This optimisation will form part of the robust evidence base for setting objectives, policies and rules in regional planning instruments, as well as helping to define the costs involved to achieve these objectives. The decision-support tool’s key strength and point-of-difference is that scenario costs and effects will be optimised and quantified to frame limit-setting discussions.
Future directions
The FWMT is the first regional, continuous simulation, process-based freshwater model in New Zealand. It features region-wide parameters across over 5,500 sub-catchments that will support Auckland Council’s implementation of the NPS-FM. Once implemented, the integrated FWMT will inform regulatory and community-based decision-making, as the Council and wider community seek to effect Te Mana o te Wai. Looking to the future, the FWMT could assist climate change decision-making conversations in the Auckland Region, with the scope to develop various climate change scenarios. Optimisation of strategies for wider outcomes could also help implement broader sustainability objectives through freshwater management. The FWMT’s strengths as a flexible and adaptable accounting and decision-support tool are likely to assist a variety of Auckland Council’s evolving needs, for managing freshwater quality and quantity under the NPS-FM.
Dissolved inorganic nitrogen for stream segments and failing upstream areas in Manukau Harbour.
Dissolved reactive phosphorus for stream segments and failing upstream areas in Manukau Harbour.
The FWMT will be used to optimise cost and benefit of interventions across the region, such as constructed treatment wetlands pictured: Brylee Reserve Wetland