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Title: Assessment of environmental water requirements for the proposed Basin Plan: Wimmera River Terminal Wetlands
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Assessment of Wimmera River Terminal Wetlands environmental water requirements
The Water Act 2007 (Cwlth) established the Murray‐Darling Basin Authority (MDBA) and tasked it with the preparation of a Basin Plan to provide for the integrated management of the Basin’s water resources. One of the key requirements of the Basin Plan is to establish environmentally sustainable limits on the quantities of surface water that may be taken for consumptive use, termed Sustainable Diversion Limits (SDLs). SDLs are the maximum long‐term annual average volumes of water that can be taken from the Basin and they must represent an Environmentally Sustainable Level of Take (ESLT).
The method used to determine the ESLT is described in detail within ‘The proposed “environmentally sustainable level of take” for surface water of the Murray-Darling Basin: Method and Outcomes,’ (MDBA 2011). A summary of the main steps undertaken to determine the ESLT is presented in Figure 1. The assessment of environmental water requirements including specification of site-specific flow indicators at a subset of hydrologic indicator sites (Step 3 of the overall ESLT method) is the focus of this document.
The work described herein is the MDBA’s current understanding of the environmental water requirements of Wimmera River Terminal Wetlands. It is not expected that the environmental water requirements assessments will remain static, rather it is intended that they will evolve over time in response to new knowledge or implementation of environmental watering actions. Within this context, feedback is sought on the material presented within this document whether that be as part of the formal draft Basin Plan consultation phase or during the environmental watering implementation phase within the framework of the Environmental Watering Plan.
1.1.Method to determine site-specific flow indicators
Assessment of environmental water requirements for different elements of the flow regime using the hydrologic indicator site approach is one of the key lines of evidence that has informed the proposed SDLs. Effort focussed on regions and parts of the flow regime with greatest sensitivity to the scale of reduction in diversions necessary to achieve environmental objectives, an ESLT and a healthy working Basin.
Within the overall framework of the ESLT method (Figure 1) the MDBA used an iterative process to assess environmental water requirements and develop site-specific flow indicators.
The hydrologic indicator site approach uses detailed eco-hydrological assessment of environmental water requirements for a subset of the key environmental assets and key ecosystem functions across the Basin. Effort focused on high flow (freshes, bankfull flows and overbank flows) requirements reflecting the prioritisation of effort on parts of the flow regime that are most sensitive to the determination of the ESLT and SDLs. The Wimmera River Terminal Wetlands is one of the key environmental assets where a detailed assessment of environmental water requirements was undertaken.
Figure 1: Outline of method used to determine an Environmentally Sustainable Level of Take (Source: MDBA 2011).
Detailed environmental water requirement assessments lead to the specification of site-specific flow indicators to achieve site-specific ecological targets. Flow indicators were expressed at a hydrologic indicator site or sites. Environmental water requirements specified at hydrologic indicator sites are intended to represent the broader environmental flow needs of river valleys or reaches and thus the needs of a broader suite of ecological assets and functions.
This report provides a description of the detailed eco-hydrological assessment of environmental water requirements for the Wimmera River Terminal Wetlands including information supporting the development of site-specific flow indicators for the site (with reference to water levels at Lake Hindmarsh and Lake Albacutya). More information on how the site-specific flow indicators for Wimmera River Terminal Wetlands were used within the Basin-wide modelling process to inform the ESLT (i.e. Step 5 and 6 in Figure 1) can be found in the report ‘Hydrologic modelling to inform the proposed Basin Plan: Methods and results’ (MDBA 2012).
A description of the detailed eco-hydrological assessments of environmental water requirements for other indicator sites are described in other documents in the series ‘Assessment of environmental water requirements for the proposed Basin Plan’.