Increasing rates of water use in New Zealand are causing changes to the magnitude, duration, timing, rates of change and predictability of fluxes of water in rivers, wetlands, and groundwater systems.
In turn, these changes affect important physical processes such as sediment erosion transport, and biological processes such as of the growth and removal of algae and aquatic plants, and cueing spawning and other life-history stages.
The National Policy Statement for Freshwater Management states that regional councils must include rules in regional plans that set environmental flows and levels for each Freshwater Management Unit (FMU), and may set different flows and levels for different parts of individual FMUs. Environmental flows must be set at levels that achieve the intended outcomes for values relating to the FMU or relevant part of the FMU and all applicable long-term visions. These outcomes and long-term visions can be set and adapted over time to take a phased approach to achieving the desired results. Rules in regional plans that seek to protect environmental flows may take different forms but often comprise a minimum flow and a total allocation of water abstraction.
The EFlows Explorer webtool was designed to aid understanding of how minimum flow and total allocation can be set for all river reaches across a catchment or region, by demonstrating how they interact with reliability of water supply and using environmental metrics to represent losses in total area of aquatic habitat.
Six key variables are considered within the webtool:
- minimum flow: the river flow below which all abstraction must cease;
- total allocation: the maximum rate of water abstracted from surface or river-connected groundwater summed across upstream abstractions;
- management flow: the minimum flow plus the total allocation;
- partial reliability: the percent of time for which some abstraction may occur because river flow is greater than the minimum flow;
- full reliability: the percent of time for which the total allocation may be abstracted, which also equates to percent of time river flow is greater than the management flow;
- width at minimum flow: the wetted width at the minimum flow as a percent of wetted width at a specified reference flow (width at minimum flow/width at reference flow)*100.
Setting input values for various combinations of two input variables allows the other four variables to be calculated. For example, setting minimum flow and total allocation allows calculation of management flow, partial reliability, full reliability, and width at minimum flow. Some variables can be specified in a variety of ways. For example, minimum flow can be specified as a percentage of mean annual low flow (MALF) or in cumecs (m3/s).
This tool can provide managers and decision makers with well-characterised options for sustainable management of New Zealand's freshwater resources by demonstrating and quantifying links between water resource use and environmental consequences.
Northland Regional Council is currently using the outputs from eFlows Explorer to understand the implications of a possible change to how they set their environmental flow limits and allocation in their regional plan. The results from EFlows Explorer will help them to understand the implications of this change on reliability of supply and habitat availability for key fish species.
The eFlows Explorer webtool can be accessed here: https://shiny.niwa.co.nz/eflowsexplorer/
To view a webinar demonstration of the tool being used visit: https://niwa.co.nz/freshwater-and-estuaries/research-projects/eflows-ex…
Caption: The eFlows Explorer tool displaying a scenario demonstrating the variables of minimum flow and total allocation.
