Sustainable Yield

Many countries (e.g. the USA and India) have widely adopted the concept of 'safe yield' (i.e. annual recharge) as a sustainable extraction limit. In many instances, this adoption is necessitated by high levels of groundwater development, but it has limited ability to account for hydraulic connectivity between water resources and environmental dependencies (Custodio, 2002; see also Llamas and Garrido, Chapter 13, this volume).

Australia's relatively recent development of groundwater allows a more conservative approach to sustainable yield. As any significant development of an aquifer will alter the water balance and have some impact, 'sustainability' must be interpreted as 'social acceptability of impacts' (Herczeg and Leaney, 2002). The central role of community in defining sustainable yield was noted by ARMCANZ (1996a):

As any definition of sustainable yield embraces a range of technical as well as social, environmental and economic factors, it is necessary for considerable community input to make judgement of what is sustainable.

The NGC (2004) agreed the following definition of sustainable groundwater yield ('sustainable yield'):

The groundwater extraction regime, measured over a specified planning timeframe, that allows acceptable levels of stress and protects dependent economic, social, and environmental values.

In adopting this definition, the NGC requested it be used with the explanatory notes provided, abridged in Box 15.2.

While implication within this definition and accompanying explanatory notes is to adopt a conservative approach to sustainable yield, the definition has been designed to allow for groundwater 'mining'. The willingness of the states to accept 'mining' - 'the exploitation of groundwater at a rate that is much greater than recharge' (Custodio, 2002) - as 'sustainable' has resulted in differences in the application of sustainable yield across the states. SA, in particular, accepts the notion of controlled depletion on the basis that (assuming no groundwater-dependent ecosystems) the groundwater is of no benefit if unused.

Box 15.2. Explanatory notes to accompany the nationally accepted definition of sustainable groundwater yield. (From NGC, 2004.)

Extraction regime

It is recognized that sustainable groundwater yield should be expressed in the form of an extraction regime, not just an extraction volume. The concept is that a regime is a set of management practices that are defined within a specified time (or planning period) and space. Extraction limits may be expressed in volumetric quantity terms and may further specify the extraction or withdrawal regime by way of accounting rules and/or rates of extraction over a given period and/or impact, water level or quality trigger rules. The limits may be probabilistic and/or conditional.

An oft-used means of defining the extraction regime has been by way of a maximum volume that may be taken in any single year. In some cases, where drawdown beyond the rate of recharge may be acceptable, it may be only for a specified period, after which time the rate may be less than the rate of recharge to compensate. In some cases and under specific circumstances (e.g. high or low rainfall years), the amount of water that may be taken may be greater or lesser than the longer-term value, and the conditions for this can be specified.

Acceptable levels of stress

The approach recognizes that any extraction of groundwater will result in some level of stress or impact on the total system, including groundwater-dependent ecosystems. The concept of acceptable levels of stress as the determining factor for sustainable yield embodies recognition of the need for trade-offs to determine what is acceptable. How trade-offs are made is a case- and site-specific issue and a matter for the individual states to administer.

The definition should be applied in recognition of the total system. That is, it should recognize the interactions between aquifers and between surface and groundwater systems and associated water dependent ecosystems.

In calculating sustainable yield, a precautionary approach must be taken with estimates being lower where there is limited knowledge. Application of the calculated sustainable yield as a limit on extractions must be applied through a process of adaptive management involving monitoring impacts of extraction. Sustainable yields should be regularly reassessed and may be adjusted in accordance with a specified planning framework to take account of any new information, including improved valuations of dependent ecosystems.

Storage depletion

The approach recognizes that extraction of groundwater over any time frame will result in some depletion of groundwater storage (reflected in a lowering of water levels or potentiometric head). It also recognizes that extracting groundwater in a way that results in any unacceptable depletion of storage lies outside the definition of sustainable groundwater yield.

Where depletion is expected to continue beyond the specified planning time frame, an assessment needs to be made of the likely acceptability of that continuation and whether intervention action might be necessary to reduce extraction. If intervention is likely to be necessary, planning for that action should be undertaken so that it can be implemented at the end of the specified time frame.

Major considerations in determining the acceptability of any specific level of storage depletion should be 'intergenerational equity', and a balance between

Box 15.2. Continued environmental matters identified in the National Principles for Provision of Water for Ecosystems and social and economic values.

Protecting dependent economic, social and environmental values

The definition recognizes that groundwater resources have multiple values, some of which are extractive while others are in situ (e.g. associated water-dependent ecosystems) and all have a legitimate claim on the water resource.

The national definition of sustainable yield does not identify a standard planning time frame. The cumulative nature of extraction impacts and temporal response of aquifers can make the planning time frame a critical component of groundwater planning. These attributes of groundwater make sustainable yield estimations particularly subject to changes in social values and technical knowledge (see case study 2). Community understanding of groundwater availability can be difficult to progress with regard to the differences between the amount of water stored in an aquifer and the rate of recharge of that storage.

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