Model scope

The FWMT is designed to help Healthy Waters optimise its capital and operational investments to deliver the greatest value-for-money water management outcomes for Auckland ratepayers.

The FWMT will allow Healthy Waters to model the movement of water and operationally-relevant contaminants through rural and urban catchments, identify critical contaminant sources, estimate the loads and concentrations of contaminants in receiving environments, infer their likely impacts on ecosystem health, simulate the effectiveness of different management options for mitigating those impacts, estimate the cost of achieving desired management outcomes, and determine where those costs fall.

This information will guide Healthy Waters as it develops action plans for achieving water quality targets in Auckland’s rural and urban catchments and in linking operational, asset management, and investment decisions for the NDC.

Operationally-relevant contaminants

The primary purpose of the FWMT is to aid stormwater management and so must simulate behaviour of contaminants that are “operationally-relevant” to integrated catchment management, and specified in the regional stormwater NDC.

Operationally-relevant contaminants:

• have a direct and measurable effect on water quality, causing changes in processes and/or contaminants underpinning waterway health (e.g., discharged by activity, concentrated through activity effects on flow regime, altered by changes in stream form and condition).
• have a direct and measurable response to management actions (e.g., contaminant form, mass, flow, instream physicochemistry).
• are supported by reliable observed datasets and well understood theory of pressure-response (e.g., guiding process-based configuration).
• have national or regional guidance underpinning state assessment (e.g., National Objective Framework, Australia and New Zealand Standards, AUP Standards, Taumata Arowai guidance).

The operationally-relevant contaminants modelled by FWMT Stage 1 span:

1. Instream – Nitrogen (total nitrogen, total oxidised nitrogen, total ammoniacal nitrogen, dissolved inorganic nitrogen), Phosphorus (total phosphorus, dissolved reactive phosphorus), Copper (total), Zinc (total), Sediment (total suspended solids, sand, silt and clay proportion), and Faecal Indicator Bacteria (E.coli).
2. In-lake (by water column depth) – Nitrogen (total nitrogen, dissolved inorganic nitrogen, nitrate-nitrogen, ammoniacal-nitrogen), Phosphorus (total phosphorus, phosphate-phosphorus), Dissolved Oxygen (DO), Chlorophyll-a (Chl-a) and Secchi-depth (SD) including amalgamation to Trophic Level Index (TLI) 3 and 4 score.

FWMT Stage 1 produces hydrology [top panel] and contaminant concentration [bottom panel] time-series over a baseline period [2013-17] in 5,465 sub-catchments and across various operationally relevant contaminants. Example is total suspended solids concentration. Modelled output [Version 1.2] in orange and monthly State of Environment sampling in blue diamonds.

Core models

The accounting framework (FWMT) comprises two linked process-based, continuous-simulation open-source models, developed by the US-EPA:

• Loading Simulation Programme in C++ (LSPC), which is used to simulate hydrology, sediment erosion and transport, and water quality processes at a sub-catchment scale, and
• System for Stormwater Treatment and Analysis (SUSTAIN), which is used to predict outcomes under a range of stormwater management/intervention scenarios, and estimate the costs and benefits of different management options to identify the optimal mix of actions for achieving specified contaminant objectives (including nature based solutions).

LSPC represents watersheds via a link-node stream network that integrates sub-catchments of varying HRU extent, topography and climate conditions. LSPC simulates land and stream dynamics. LSPC has been coupled to ensemble lake biophysical process models in the Lake Management Tool (LMT – in development).

SUSTAIN optimises intervention choices for contaminant impact and cost. SUSTAIN can simulate millions of alternative intervention choices on a sub-catchment basis (type, extent, mix) for cost and reduction in HRU yields to identify “best” solutions for design storm conditions (least cost, greatest reduction in contaminant yield). SUSTAIN is able to then generate longer-term continuous outputs for any mix of “best” solutions (over multiple years and broader climatic conditions).

 FWMT core models (LSPC, SUSTAIN) have evolved from a multidecadal investment programme in open sourced modelling by the US-EPA.

FWMT Stage 1 operates LSPC and SUSTAIN on 15-minute time-steps across 5,465 sub-catchments spanning ~490,000 Ha of the Auckland region and inclusive of 106 activity types (urban and rural HRUs) and 100+ uniquely designed and costed management interventions – simulating all operationally relevant contaminant responses.

FWMT Stage 1 is “regionalised” with all HRU, reach and intervention processes consistent across the Auckland region. Note outputs are not then identical in sub-catchments; differing HRU extents and types, stream types and intervention choices generate diverse outputs across sub-catchments.

 FWMT Stage 1 simulates millions of alternative feasible interventions at sub-catchment scale, identifying the best combinations for improved water quality at least costs [on lifecycle basis and over varying climatic conditions].

Supporting models

The FWMT requires additional supporting models to operate, including:

• Lifecycle cost models for all urban and rural interventions – spanning both devices () and underlying source controls (green streets, road sweeping, management practices) for a variety of design-specifications and discount rate (50-year)
• Greenhouse gas emission lifecycle models for all urban and rural interventions – as above for CO2eq emissions profiles of interventions (50-year)
• Geospatial feasibility (opportunity) models for limited urban and rural interventions – FME models for determining feasible footprint (treatment) area and upstream drainage (treated) of interventions (including existing stormwater network devices)

All supporting models are intellectual property of Auckland Council (see “Resources”). Requests to modify or adopt models for other purposes are encouraged and can be supported by the Healthy Waters department (fwmt@aucklandcouncil.govt.nz).

Intervention lifecycle models for urban and rural interventions are used in the FWMT Stage 1 to optimise for 50-year discounted (2-6%) and standardised devices (size, design) and source controls.

Intervention lifecycle models have been developed for urban and rural interventions of varying design (low, medium, high cost components) and standardised size, demonstrating efficiencies simply in ensuring minimum device sizes are set sufficiently high.

Model applications

The management tool programme is operational and purposely for asset/network/catchment management decision-making. Application to regulatory decisions is not in-scope for FWMT Stage 1.

The programme offers regionwide transparent, objective data on water quality for decision-making and reporting of:

• Baseline water quality – variation in hydrology and contaminant regimes, regionwide over the 2003-2017 period in all sub-catchments; (Completed)
• Scenario water quality – forecast outcomes for development (maximum permitted development as circa 2050) and climate change (ensemble regional climate model [RCM] downscaled forecasts of shared socioeconomic pathways [SSP]); (Ongoing)
• Action plans for water quality – testing and identifying optimal, feasible interventions to improve operational contaminant effects on water quality (from mountains to sea, integrated). FWMT Stage 1 is limited to action plans for baseline water quality. (Ongoing) 

Freshwater Management Tool model schema for optimised baseline (action plan) application.

Note: FWMT is also undergoing development to couple directly with ensemble lake process-models (Lake Management Tool) including DY-CD, GLM-AED and GOTM-PCLake, and with instream macroinvertebrate extirpation models (SSDM) to expand applications.

Lake Management Tool model schema for baseline and scenario applications – utilising FWMT for catchment modelling.

Process configuration

Activities (land use, wastewater network discharges) are represented as Hydrological Response Units (HRU) or point-source discharges (wastewater). HRU’s have regionalised process parameters for hydrological flow (runoff, interflow, active groundwater) and contaminants (build-up, detachment, wash-off, soil loss). HRU’s span differences of soil, slope, land cover and use (impact). 

Hydrologic response units (HRU) are regionally parameterised in FWMT Stage 1 for hydrological and contaminant processes, with regionwide mapping for baseline and scenario applications.

Waterways are simulated using regionalised parameters for contaminant fractionation (from total to dissolved, reduced and particulate), sediment dynamics (deposition, resuspension, scour/bank erosion) and broader physicochemical processes including nutrient uptake.

FWMT Stage 1 uses a static HRU and waterway configuration for sub-catchments over baseline, scenario and intervention applications. Meteorological conditions vary on 15-minute basis at sub-catchment level. Combined, simulating dynamic steady state outputs to sub-catchment with underlying activity yields, loads and intervention impacts (cost, load reduction).

Hydrologic response units (HRU) once mapped and parameterised generate instream water quality time-series to grade state.

Model Outputs

FWMT Stage 1 outputs include geospatial, time-series and integrated summary information on HRU contaminant yields and loads to water, instream flow , intervention opportunity, treatment and lifecycle costs for contaminant objective:

• Baseline and scenario outputs – activity contaminant yield and load, instream concentration and load including from instream erosion (continuous and annualised);
• Intervention optimisation – intervention opportunities (location, footprint, drainage area) and best solutions including the optimal interventions to deliver water quality improvements.

Action plans are produced by the FWMT Stage 1 using intervention optimisation for baseline state (v1.2).  Each resolves the least cost mixes of feasible, scaled and integrated actions necessary to change water quality for some critical freshwater objective.

Action plans are limited to what is currently feasible to both implement and model robustly, resolved in each of 5,465 sub-catchments, for all operationally relevant contaminants and of the least-cost (optimal) improvement in freshwater state (95th% numeric attribute).

Combined, over 110 interventions, across 106 activities under 5-years of baseline weather and returned as 100 incrementally better solutions in 5,465 sub-catchments for each of the graded forms of nutrient, metal, sediment and E.coli modelled by FWMT Stage 1 – with corresponding spatial information on uptake, cost, treatment and efficiency.

Water quality information is available regionwide to sub-catchment including contaminant source information resolved to hydrological response units.

Intervention modelling is supported by detailed feasibility mapping and local information on footprint area, offtake and treated area.

Optimised intervention modelling determines best solutions from feasible options in millions of alternative action plans. All action plans produced include geospatial and time-series data to aid implementation. Best solutions cost less for outcome.