Sustainable yield: how much water can we take?
Determining how much water we can sustainably extract requires an understanding of groundwater and surface water systems, and a nationally consistent approach to defining sustainable extraction levels.
Improved water planning and management under the National Water Initiative aims to implement firm pathways and open processes for returning previously over-allocated and/or over-used surface and groundwater systems to environmentally-sustainable levels of extraction.
The environmentally sustainable level of extraction is also referred to as the sustainable yield and is defined in the National Water Initiative as:
The level of water extraction from a particular system which, if exceeded would compromise key environmental assets, or ecosystem functions and the productive base of the resource.
To return our systems to environmentally-sustainable levels of extraction, we first need to know what these levels are for each water management area.
Determining sustainable yields
In the Australian Water Resources 2005 baseline study it was noted that there is no nationally agreed, standardised method for calculating and reporting sustainable yield, despite this being a key recommendation from the Australian Water Resources Assessment 2000.
Further, the study reported that in many areas the states and territories appeared to base their estimates of sustainable yield volumes on the current level of entitlements, rather than on a detailed assessment of resource sustainability.
In its 2007 Biennial Assessment of progress against the National Water Initiative (see also 2009 Biennial Assessment), the National Water Commission also identified a number of problems in the area of environmental sustainability including:
- identifying and addressing over-allocated systems
- defining sustainable levels of extraction
- the quality and adequacy of the science underpinning plans
- transparency of trade-offs between environmental and consumptive uses
- a lack of specificity in the environmental outcomes.
Improving our knowledge
The Australian Water Resources 2005 study noted the need for improved and consistent methodologies for determination of water availability that take in recent climate trends and longer time sequences.
In the 2007 Biennial Assessment, the National Water Commission identified the need for a national understanding of over-allocation that reconciles the varying approaches and terminology used by the states.
The Murray-Darling Basin Sustainable Yields Project being conducted by CSIRO is providing detailed assessments of the water available in all catchments in the Murray-Darling Basin, but this needs to be supplemented by knowledge of environmental water requirements in order to determine sustainable limits.
The COAG Working Group on Water and Climate Change is currently investigating better mechanisms for determining environmentally sustainable limits on surface and groundwater extraction.