Australian Natural Resources Atlas

Natural Resource Topics

Water - Appendix 3. Water Availability Assessment Methods

This Appendix provides summary tables and text descriptions of the methods used by the States and Territories to estimate sustainable yield for surface and groundwater resources. Methods vary. Further detail is provided in the National Technical Report and State and Territory Technical Reports accessible through the Australian Natural Resources Atlas.

Table A3. Surface water availability assessment methods.

Surface water: sustainable yield

New South Wales
Sustainable yield Rivers with estimated environmental flow rules in place: current yield.
Environmental water requirement Where environmental flow rules have not been identified: the sustainable yield is the yield determined under the MDBMC Cap. The environmental flow requirement at each reporting location is the total flow in the river at that location for current Water Reform management and infrastructure conditions.
Victoria
Sustainable yield Surface water management areas located in the Murray-Darling Drainage Division: sustainable yield was assumed to be equivalent to the levels of average annual (also equivalent to developed yield) diversions available under the MDBMC cap. Surface water management areas located in southern Victoria: where environmental values could potentially be threatened by further allocations, the sustainable yield was limited to the allocation volume pending the outcome of detailed investigations of environmental water requirements. Remaining surface water management areas: the sustainable yield was assessed such that the degree of change to the natural flow regime is not ?unacceptable? as defined by a rating of 5 for the hydrology sub-index of the Index of Stream Condition.
Environmental water requirement Difference between the total available water and the estimated sustainable yield. The total available water is determined as the sum of the mean annual flow, inflow from upstream catchments and cross catchment transfers that contribute to the available resource in the waterway.
Queensland
Sustainable yield Not determined
Environmental water requirement Not determined
Western Australia
Sustainable yield Yields were reported by considering likely development scenarios and the application of management objective factors (including environmental water provisions) for individual sites.
Environmental water requirement Not reported in technical report
South Australia
Sustainable yield Defined as divertible yield, takes into account the environmental flow requirement.
Environmental water requirement Fifty percent of median annual run-off (relating to farm dam development only).
Tasmania
Sustainable yield Difference between annual median flow and the estimated environmental flow.
Environmental water requirement Annual assessment of sustainable yield: 30% of the annual flow Critical Period assessment of sustainable yield: 40% of the summer flow (December-April) and 20% of the winter flow (May-November).
Northern Territory
Sustainable yield Humid zone: 20% of divertible yield (median annual flow). Arid zone: 5% of divertible yield.
Environmental water requirement Humid zone: 80% of divertible yield median annual flow). Arid zone: 95% of divertible yield.
Australian Capital Territory
Sustainable yield Total water resource less environmental water requirements
Environmental water requirement Environmental flows are based on the ACT environmental flow guidelines.
Water supply catchments. Flows in rivers and streams below the eightieth percentile are environmental flows (protection of low flow). Flows above the eightieth percentile are available for diversion except for spawning flows.
Remaining catchments. Ten percent of the flow volume in events above the eightieth percentile is assumed to be available for diversion. The remaining flow is allocated to the environment (protection of flushing flows).

Table A4. Groundwater availability assessment methods.

Groundwater: sustainable yield

New South Wales
Sustainable yield methodology Based on rainfall recharge, river recharge estimates and any other available information.
Groundwater dependent ecosystems Nominal 30% of annual recharge is assigned to ecosystem maintenance (according to precautionary principle).
Victoria
Sustainable yield methodology Sustainable yield estimates are based on rainfall recharge, throughflow rates, well interference, sea water intrusion, river recharge/discharge, and numerical groundwater modelling (where available).
Groundwater dependent ecosystems Environmental allowances for groundwater dependent ecosystems are made for each groundwater management unit according to conditions in that groundwater management unit. Systems included in the calculation include: river baseflow; wetlands; and marine and estuarine systems (in terms of saltwater intrusion limits only).
Queensland
Sustainable yield methodology Groundwater dependent ecosystems were included. Rainfall recharge, aquifer throughflow rates and extractions were used to determine the sustainable yield (or net recharge to aquifer).
Groundwater dependent ecosystems None in sub-artesian aquifers. In the Great Artesian Basin where artificial ecosystems have developed around mound springs, groundwater dependent ecosystems are considered in the sustainable yield estimate.
In some specific groundwater management units, groundwater dependent ecosystems have been considered, e.g. Sand Islands, Cooloola which is a heritage area. In these areas allowance has been made for cave and aquifer system and fauna.
Western Australia
Sustainable yield methodology Sustainable yield estimates are based on throughflow estimates, chloride analyses, rainfall recharge estimates and land use and determination of impact of land use on recharge.
Groundwater dependent ecosystems Environmental allowance is made for each groundwater management unit according to conditions in that groundwater management unit. Basic allowance is 5% of total recharge. For significant wetlands it is approximately 40% of the total recharge, and for others it varies between 25-70% of total recharge. Groundwater dependent ecosystems allowed for in groundwater management units include terrestrial vegetation, river baseflow (Kimberley, Pilbara, Carnarvon), wetlands, cave and in aquifer systems, fauna (where known) and marine and estuarine systems.
South Australia
Sustainable yield methodology In general no groundwater mining allowed, but there are some exceptions. Sustainable yield estimates are based on groundwater use, water level and salinity information, and recharge analyses. The recharge analyses included rainfall recharge estimates, lateral throughflow, chloride analyses and numerical groundwater modelling.
Groundwater dependent ecosystems Groundwater dependent ecosystems allowed for include maintenance of mound springs in Great Artesian Basin.
Tasmania
Sustainable yield methodology Rainfall recharge method used, with most of State assuming a 3% recharge rate.
Groundwater dependent ecosystems None.
Northern Territory
Sustainable yield methodology Based on rainfall - recharge as estimates. Northern areas: rainfall recharge rates of 0.2-5.0 ML/ha/yr; southern areas: rates of 0.02-2.5 ML/ha/yr were used.
Groundwater dependent ecosystems 50% of annual recharge assigned to groundwater dependent ecosystems
Australian Capital Territory
Sustainable yield methodology Water balance method.
Groundwater dependent ecosystems Nominal 90% of annual recharge due to lack of information on recharge and aquifer yields (using precautionary principle). Includes allowance for in-cave systems and terrestrial vegetation.

State/Territory assessment methods

New South Wales

Lead agency:

Resource Information Unit, Department of Land and Water Conservation, New South Wales

Surface water

Context

The New South Wales Water Reforms, which are being developed and implemented by the New South Wales Government, are aimed at improving water management within the State.

Sustainable water provision estimate: method

To adequately address current water management in New South Wales, the Department of Land and Water Conservation has developed monthly and-more recently-daily simulation models that are capable of modelling water quantity in an integrated manner. The later models operate on a daily time step and are known as integrated quantity/quality models. The Department has integrated quantity/quality models for most of the major regulated surface water management areas in New South Wales and the Barwon-Darling.

Using these hydrologic models, the Department has started to define the volumes of water that can be extracted from each of its regulated surface water management areas. These management practices do not correspond directly to the concept of annual yields, as defined by the Audit. New South Wales has still provided an assessment of the yields, where possible, by attempting to align its current management practices with Audit concepts.

As a result of the Murray-Darling Basin Ministerial Council Cap (Cap) and water reforms, no further headwater storage development or issues of licensed entitlements is proposed for the Murray-Darling Basin. For the purposes of the Audit, the ultimate level of development has therefore been taken to be equivalent to the Cap level of development. New South Wales considers the divertible yield and developed yield to be equivalent. Furthermore, the range of environmental flow rules that have been introduced to most of the regulated surface water management areas within the Murray-Darling Basin result in long-term average diversions (or yield) below the Cap levels. This yield has been taken to represent the current best estimate of sustainable yield. The sustainable yield for those regulated valleys within the basin that do not have environmental flow rules implemented has been assumed to be the Cap yield.

Sustainable yield: assumptions, reliability and errors

Regulated surface water management areas

The integrated quantity/quality model developed by the Department of Land and Water Conservation, was run with Cap or 1993/94 conditions of infrastructure development and 1993/94 management rules in place. The current yield in New South Wales (current = developed = divertible) was determined to be the average annual diversion over the full period of record (generally about 100 years). This annual diversion includes both on-allocation and off-allocation use. This yield may change as models are updated or new information comes to light.

The understanding of the links between flow and sustainability of river ecological systems is also still developing. Ecological sustainability can be affected by factors other than quantity and timing of flow. Therefore, the sustainable yield estimates provided cannot be considered as the amount of water that can be diverted from rivers in perpetuity.

Unregulated surface water management areas

New South Wales is introducing volumetric licences for the management of unregulated valleys and is collecting a range of data in unregulated basins. New South Wales current management practices view the sustainable yield as an allowable daily extraction volume as opposed to an annual yield figure. Future reviews of the performance of the management rules to be adopted in each valley may result in changes to the rules and hence changes to the ?sustainable yield?. Given the limited data, sustainable yield has not been calculated for unregulated basins in New South Wales.

An attempt was made to obtain a categorisation for the catchment, based on the work done for the stressed streams.

A variable P was determined for the total catchment on the basis of the combined stress classification results from the stressed streams analysis:

P = Total area of sub-catchments with high combined stress / Total area of classified sub-catchments in basin

The development category that was assigned to these areas was based on the variable P as defined above, rather than the proportion of use (or allocation) to sustainable yield.

Groundwater

Sustainable yield estimate: method

Assessment was based on the following definition:

Sustainable yield is that proportion of the long-term average annual recharge which can be extracted each year without causing unacceptable impacts on the environment or other groundwater users.

The actual value for proportion stated is not specifically given. This proportion will change according to each situation and is assigned differently to each aquifer system. Recharge calculations with ?sustainability factors? applied to them act as interim sustainable yield figures. These ?sustainability factors? are a proportion of long-term annual average recharge. Sustainability factors are chosen according to level of knowledge of an aquifer system, level of use of that resource, the magnitude of perceived risk to that aquifer system and the environment, and the reliability of recharge to that system. As better understanding is developed, the sustainable yields can be adjusted accordingly.

Sustainable yield: assumptions, reliability and errors

Most groundwater systems have not been modelled. Inputs (or recharge) to the system have generally been kept to rainfall and river components of recharge. Three systems-the Lower Lachlan, the Lower Murrumbidgee, and the Great Artesian Basin-have been handled differently with regard to sustainable yield determinations. This reflects the greater level of knowledge about these systems. ?Throughflow? and ?underflow? have in most cases been omitted from calculations for simplicity and conservatism. Likewise, irrigation ?returns? have not been considered even though in some situations a certain proportion of irrigated water might be expected to access the underlying aquifer.

Two equations were used to estimate recharge. Both have a limited number of terms and allow recharge values to be assigned respectively to those sourced from rainfall and those from rivers.

Rainfall recharge was calculated according to assessed rainfall, area and proportion of rainfall accessing the aquifer. River recharge was estimated using a modified form of the ?Darcy? equation. An additional factor was applied as an ?adjustment? factor intended to reduce the theoretical river recharge and is set as the fraction of the year and/or the fraction of river reach-that is considered as a ?losing stream?. In this way an actual river recharge component is produced.

Once recharge values were estimated, some proportion of that recharge was taken as the sustainable yield. As a ?default?, 70% of average annual rainfall is taken as the proportion that can be extracted from the aquifer annually on a sustainable basis.

Victoria

Lead agency

Department of Natural Resources and Environment

Context

The Victorian Government is committed to striking a balance between satisfying existing demands to urban centres and irrigation industries, and improving the environmental flow regime of rivers.

Surface water

Sustainable water provision estimate: method

The sustainable yield is the estimated maximum volume of water that can be diverted after taking account of in-stream environmental water requirements. It is calculated as a long term average annual volume. While this concept is apparently relatively straightforward as defined, in practice the sustainable yield is very difficult to determine. Once environmental flow requirements at particular points within a surface water management area have been determined, using simulation models it is possible to derive an estimate of the average volumetric environmental allocation and the sustainable yield for the surface water management area. While Victoria has a variety of programs under way aimed at identifying, improving and protecting environmental flow requirements, the necessary investigations take considerable time and resources.

Given the short time frame of the Audit, it was necessary to make some broad assumptions, and use a variety of approaches, to derive estimates of the sustainable yield for surface water management areas in Victoria. Consideration was given to environmental water requirements (known and likely), existing users? rights, and related social and economic impacts.

In summary:

Further details on the methods used to estimate sustainable yield are provided in the State Technical Report.

Sustainable yield: assumptions, reliability and errors

Where sustainable yields have been limited in accordance with the Cap or the current allocations within a surface water management area, it is assumed that the current environmental water provisions represent the volume of water that can currently be made available to the environment after consideration is given to current users? rights and related social and economic impacts.

In the longer term, there may be further scope for improving environmental regimes where necessary (e.g. by providing additional water through improving distribution and water use efficiency). In surface water management areas where a significant portion of the available resource is committed to a downstream surface water management area, there is also potential for trading of entitlements between the two surface water management areas. This will result in a change to both the sustainable yield and the environmental allocation in both surface water management areas. Trade out of a surface water management area would decrease the sustainable yield of that area and a trade of water rights into a surface water management area would increase sustainable yield. However, the sum of the sustainable yields for the two surface water management areas would remain unchanged.

The estimates of sustainable yield made using the Index of Stream Condition hydrology sub-index are considered to be relatively conservative, as the methodology assumes that diversions occur only during the period May to November (i.e. the flow regime for the period December through to April must remain unchanged). However, the approach was found to give inconsistent results across the State and could not be universally applied. The estimates of sustainable yields determined using this approach can therefore only be considered to be interim measures, pending the outcome of detailed environmental flow assessments.

The major limitation associated with the concept of the sustainable yield for a surface water management area is that the assessment is undertaken at the furthest downstream location on rivers/streams within a surface water management area. Therefore the sustainable yield represents an average and does not take into account the impact of diversions on specific river reaches within the catchment. Consequently, where the sustainable yield of a surface water management area is specified as being equal to or greater than the allocated volume, there still could be river reaches within the surface water management area that are over-allocated, potentially overused and therefore stressed. These situations will be identified and addressed in the context of established programs (in particular, the Streamflow Management Plan and Stressed Rivers programs) aimed at addressing the provision of water for the environment. These programs are described in the State Overview Report. Conversely, where the sustainable yield is specified as being equal to the allocated volume, there may still be ?spare? capacity on some river reaches, in the sense that further diversions could occur without stressing the particular river reaches. A further complication is that where surface water management areas are nested (as in the Murray-Darling Basin), a portion of the flows from upstream surface water management areas are often required to meet commitments to downstream surface water management areas. This means that current allocations for use within the upstream surface water management area (and therefore the defined sustainable yields) are relatively low compared to what they would be if resources generated within the upstream surface water management area were to be utilised only within this (upstream) surface water management area.

For the reasons outlined above, the concept of sustainable yield for a surface water management area is not a particularly useful management tool, as proper management requires consideration of the environmental flow requirements for specific river reaches.

Groundwater

Sustainable yield estimate: method

Victoria?s groundwater management regime is based on sustainable development through the establishment of community driven Groundwater Management Plans. The process begins with the identification of groundwater management units which are areas where groundwater development has already occurred or where there is a potential for groundwater development. For these groundwater management units a permissible annual volume for groundwater extraction has been set to reflect the sustainable yield of the aquifer. For the purposes of the Audit, these groundwater management units have been adopted as the basic reporting unit or groundwater management unit.

While a nationally agreed definition of sustainable yield for groundwater systems is now available, there is as yet no agreed methodology for determining sustainable yields. In Victoria, the sustainable yield methodology varies across the State according to the aquifer characteristics being investigated.

In most cases, because of the lack of use data and, in many cases bore hydrograph data, the sustainable yield has been determined as a percentage of rainfall, with adjustments made to take account of environmental requirements to the extent possible given currently available information. Checks on aquifer storage, river recharge/discharge, aquifer throughflow, well interference, sea water intrusion and pressure/head loss are incorporated into the methodology. The most commonly considered issues are baseflow to river systems and the intrusion of sea water. The requirements of groundwater dependent ecosystems have not generally been considered explicitly in this process, as their requirements are as yet poorly understood. However, in setting sustainable yields for groundwater management units, efforts have been made to avoid significant interference with groundwater dependent ecosystems. As the requirements of groundwater dependent ecosystems are evaluated, current government policy will allow for variation of the sustainable yield if the prospect of a detrimental impact emerges (e.g. sea water intrusion, which may result in aquifer salinisation).

Further details on the methods used to estimate sustainable yield are provided in the State Technical Report.

Sustainable yield: assumptions, reliability and errors

The derived estimates of sustainable yield for the groundwater management units are relatively subjective. Until there are hard use data it will not be possible to assess the water balance for groundwater management units, and determine the recharge that provides the basis for sustainable yield. Similarly, the lack of information about the requirements of groundwater dependent ecosystems has meant that some fairly broad assumptions about these requirements have had to be made. Because of these, and other uncertainties such as the impact of climate variability and the likely impacts of plantation forestry on sustainable yields, a conservative approach has been taken in the estimation of sustainable yields for groundwater management units.

When resource commitments in a groundwater management unit reach 70% of the estimated sustainable yield, the area is declared a Groundwater Supply Protection Area, groundwater community management groups are established and more intensive management is triggered. This includes the development of a Groundwater Management Plan for the Groundwater Supply Protection Area, which includes the implementation of more detailed metering of use and monitoring of groundwater levels to allow better determination of the sustainable yield. Review of the sustainable yield in currently over-allocated systems and other highly developed systems is being given a high priority to ensure that they do not become overused, or have adverse impacts on any groundwater dependent ecosystems.

In areas outside of the groundwater management units the sustainable yield estimates are of low reliability, especially in the fractured rock systems. In areas such as the volcanic rises, sustainable yield estimates can be misleading due to the high recharge rates and low aquifer yields and, hence, low extraction capabilities. Care must be taken when determining resource availability in fractured rock systems, as it will not necessarily be the same as the sustainable yield. The salinity of the resource should also be considered at all times when determining groundwater resource availability, particularly in areas outside of the groundwater management units where the resource can be highly saline (>14 000 mg/L) and of little beneficial use.

Queensland

Lead agency

Department of Natural Resources.

Surface water

Context

General acceptance that water is a limited resource has led to the Department of Natural Resources introducing a water allocation and management planning process. The water allocation and management planning process is flexible and a plan will be revised at regular intervals as the needs of the catchment change. All stakeholders are involved in the process.

Water allocation and management planning has a proactive, basin-wide approach and sets it apart from the present licensing system by placing a high priority on community consultation and sustainability of the resource.

Sustainable water provision estimate: method

Water allocation and management planning adopts an integrated approach that is based on the best available ecological, social and economic data, and involves extensive basin-wide hydrologic analysis. It provides the opportunity for local catchment communities to work on draft plans in partnership with agencies, primarily through a Community Reference Panel, but also through public consultation. This ensures the best and fairest mix of present and future uses, while finding a balance with environmental needs in accordance with the principles of sustainable ecological development.

Groundwater

Sustainable yield estimate: method

The sustainable yield figures have been derived using a variety of methods and represent the aquifer yield over a long-term critical period. The figures quoted are the best currently available and were derived from analysing the aquifer response to changes in storage, that is use, recharge, in-flow and outflow. The adopted figure attempted to maximise the water available for use while ensuring that there was no detrimental effect caused to any user or no unacceptable degradation of the resource or the environment.

With the introduction of the Water Allocation Management Program process, sustainable yield is now being defined as the ?groundwater extraction regime?, measured over a specified planning timeframe and that allows acceptable levels of stress and protects the higher value uses associated with the total resource. The estimates of yield results from a negotiation process to ensure that all users have input into the determination of the agreed figure and are aware of the implications.

The definition is framed around an extraction regime, not just an extraction volume. The concept is that a regime is a set of extractions that is defined over time, and that sustainability is measured over a timeframe. Extraction limits may be volumetric, extraction rates are related to maintaining water levels and water quality or a combination of the above.

Western Australia

Lead agency

Water and Rivers Commission

Surface water

Sustainable water provision estimate: method

Estimation of sustainable yields was undertaken as a two-stage process.

The first stage involved accumulating data for the surface water management area to determine the likely development scenario. The second stage involved further analysis of selected sites and adjustment of environmental water provisions accordingly. While it involved subjectivity, the refinements made were based on accumulated regional planning experience, and are reflect realistic outcomes at surface water management area levels.

Sustainable yield: assumptions, reliability and errors

Sustainable yield estimates are indicative of the broad outcomes of a planning process which has approached sustainability through application of precautionary principles for determination of environmental flow provisions.

While calculated at the sub area level, sustainable yield estimates at this level do not have sufficient technical standing to be prescriptive, except in those identified areas, where environmental water provisions have been formally established.

Groundwater

Sustainable yield estimate: method

In the first instance, sustainable groundwater yields were based on results derived from existing groundwater area allocation plans, (water) management plans, or on the outcomes of long-term monitoring of groundwater levels within an aquifer and associated abstraction volumes. This yield includes groundwater from fresh to hypersaline quality. For groundwater management units where detailed studies or long-term monitoring data were not available, the sustainable yield for each was given by the renewable groundwater resource minus an allowance for wetlands and, where appropriate, for sea water intrusion. The renewable groundwater resource was determined from the area of land surface or aquifer multiplied by the mean annual rainfall and the applicable recharge factor for each defined area. Recharge factors for the Perth groundwater division were derived from existing management plans. For the remainder of the State, they were either derived from groundwater investigations or were estimated by reference to other areas and consideration of rainfall, topography and aquifer type.

The total sustainable yield for the State was estimated to be 6304 GL/yr, with 3279 GL/yr available from the sedimentary basins and 3025 GL/yr, from the fractured rock divisions. Across the State, the Audit estimate of sustainable yield is around 14% lower than the Review ?85 estimate of divertible yield. This is largely attributable to the Audit inclusion of environmental water provisions for groundwater dependent ecosystems, but also reflects improvements in data availability and assessments. Changes in land use such as clearing, urbanisation and drainage also affect sustainable yield estimates. For Perth, the Review ?85 estimates were conservative and already gave implicit recognition to an allowance for wetlands which masked the full effect of allocating water to the environment in that division.

The estimates of sustainable yield are generally considered to be conservative but it is important to note, particularly with respect to the fractured rock divisions, that they include resources which Review ?85 classed as minor sources. These minor sources are distributed resources only able to be developed using small bores and spear systems. These systems are extremely important to pastoral supplied, but because of their dispersed nature, they are not reliable nor amenable to high utilisation. The minor sources tend to dominate the sustainable yield assessments in the fractured rock provinces simply because of their areal extent in comparison with free yielding resources. Care should therefore be taken not to assume that there is potential for intensive development from the fractured rock divisions, despite the apparent high availability of water.

South Australia

Lead agency

Department of Water Resources

Surface water

Context

The surface water resources within South Australia are required to be managed so that those who rely on the resource will obtain the best environmental, social and economic gain from them, whilst not compromising the ability of future generations to enjoying the same benefits. The South Australian Water Resources Act 1997 places prime importance on protecting water resources against the detrimental effects of use and development and preserving ecosystems that depend on them.

Sustainable water provision estimate: method

For the Murray River, the sustainable yield has been assumed to equal the extraction defined by the Cap.

For all other areas further analysis was required and needs to take into account emerging factors such as the impact of farm dams. Farm dams were included in the South Australian assessments as they represent the majority of new surface water development in the State. The concept of sustainable yield was difficult to apply to such structures because of the high losses expected due to evaporation, dams may partially empty and refill throughout the year, and the low security of supply at which farm dams operate. Catchments containing large reservoirs were treated the same as those with only farm dam development.

Studies of two South Australian surface water prescribed areas in the Barossa and Clare Valleys indicated that the maximum total volume of farm dam development within a catchment should not exceed 50% of the natural median annual run-off. The studies showed that 30-50% of the annual volume captured in a farm dam could be diverted on a reasonably consistent basis, but that divertible yield ultimately depended on annual rainfall, the amount of unregulated area in the catchment, dam size and design, all of which may vary significantly. The remaining 50-70% of dam capacity was lost as evaporation, recharge/leakage or could be accounted as carry over volume and unfilled storage.

Sustainable yield from surface water management areas was therefore calculated as 50% of the water captured or 25% of the median annual run-off from the surface water management area. This figure has become widely accepted in controlling farm dam development and represents a reasonable estimation of expected rates of supply.

Ephemeral streams require further detailed investigation. A number of studies targeted at assessing the environmental flow requirements of ephemeral streams under the maximum permitted levels of farm dam development (50% median natural run-off) have been initiated. Preliminary results based on limited data indicate that while such levels reduce the mean annual flow in the order of 20%, environmental flow requirements are maintained.

Monitoring procedures are being developed to enable environmental flows to be more accurately assessed across all key South Australian surface water resources.

Groundwater

Sustainable yield estimate: method

The following general definition for sustainable yield has been used:

The groundwater extraction regime, measured over a specified planning timeframe that allows acceptable levels of stress and protects the higher-value uses associated with the total resource. The sustainable yield is determined by the rate at which groundwater can be pumped without causing long-term decline of potentiometric surface (or watertable) or undesirable effects-such as salinity increases. This may mean extraction rates less than recharge as sustainability from the salinity view point may be considerably less than sustainability from the hydraulic perspective.

For sedimentary aquifers where abstraction data exists, sustainable yield has been determined using water level, salinity and metered use records in combination with recharge analysis involving rainfall recharge estimates, lateral throughflow estimation, chloride analysis, and numerical groundwater modelling.

Very little is known about how water is stored and transported in fractured rock aquifers and there are no reliable methods for estimating sustainable yield from them. Fractured rock aquifers are characterised by high spatial variability in hydraulic conductivity, making traditional hydraulic methods for estimating groundwater flow difficult to apply. Specific yield may also be extremely variable and difficult to measure, making groundwater recharge estimation from hydrographs unreliable. Sufficiently accurate data on aquifer thickness or porosity is generally not possible for reliable determination of aquifer storage. Numerical values given for sustainable yield in these units are an estimate.

In 15 groundwater management unit/unincorporated areas, sustainable yield investigations have not been conducted, and the numerical value given for sustainable yield has been based on estimated abstractions or educated guesses.

Mallee: Mining of groundwater has been included in the sustainable yield estimate for the unconfined aquifer of the Mallee. The existing permissible annual volume for the Mallee is based on components of recharge, lateral throughflow and groundwater ?mining?. The resultant drawdown is 5 cm/year averaged over the whole region. The current controlled mining policy for irrigation extraction is forecast to deplete the resource by up to 15% over the next 300 years.

Great Artesian Basin: Ecosystems are included in the sustainable yield estimation for the Great Artesian Basin. In the two formally managed areas within the Great Artesian Basin-Curdimurka and Muloorina Prescribed Well Areas-groundwater extraction is subject to restrictions including drawdown limits, which ensure the protection of ecologically significant mound springs nearby.

As the environmental requirements become clearer in other areas, greater emphasis will be placed on sustainable yields-particularly to ensure the maintenance of stream baseflow. For example, in the Mt Lofty Ranges the sustainable yield has been set at 75% of the recharge, to account for environmental flows in streams.

Tasmania

Lead agency

Department of Primary Industries, Water and Environment

Surface water

Sustainable water provision estimate: method

Sustainable yield was calculated by taking the difference between the annual median flow and the estimated environmental flow.

Reliability and errors

The environmental flow method used is broadly based on the Montana Method (Tennant 1976) and is only being used as an expedient means of assessing environmental water requirements for the Audit. The estimates should not be taken to represent true environmental water requirements.

Groundwater

Sustainable yield estimate: method

The sustainable yield for each of the groundwater management units and unincorporated areas was set at the average annual recharge to the aquifer. For all groundwater management units and unincorporated areas, the estimation of recharge was based on a percentage of the area-weighted average rainfall volume falling within the area. For 12 out of 14 groundwater management units and all three unincorporated areas within Tasmania, recharge was assigned as 3% of rainfall. The remaining two groundwater management units were on beach sand deposits, where the annual recharge was estimated at 30% of rainfall.

Northern Territory

Lead agency

Department of Lands, Planning and Environment

Surface water

Sustainable water provision estimate: method

The paucity of scientific knowledge on water requirements of ecosystems across the humid, arid and semi-arid regions of the Northern Territory is acknowledged.

For this assessment the Northern Territory has been divided into two zones: the humid zone (Northern Territory portions of the Timor Sea and Gulf of Carpentaria drainage divisions) and the arid zone (Northern Territory portions of the Western Plateau and Lake Eyre drainage divisions). Sustainable surface water provision is directly linked to these zones. At any location along a stream in the humid zone, the amount of surface water extraction has been limited to a maximum of 20%. For streams in the arid zone, the maximum extraction percentage is 5%.

Sustainable yield: assumptions, reliability and errors

In the absence of any scientific basis, environmental flow requirement is assumed to be 80% of the flow in rivers of the humid zone and 95% in rivers of the arid zone. The sustainable yield is therefore estimated as 20% of the flow in rivers of the humid zone, and 5% in rivers of the arid zone.

Currently the surface water requirements of the environment can not be determined. Also only limited flow data exists for the rivers of the Northern Territory. The reliability of the sustainable yield estimate is low.

Groundwater

Sustainable yield estimate: method

The Northern Territory has adopted two approaches for sustainable yield estimation-one each for Groundwater Management Units and Unincorporated Areas.

For Groundwater Management Units, sustainable yield has been defined as the groundwater extraction regime, measured over a specified planning timeframe which allows acceptable levels of stress and protects dependent economic, social, and environmental values.

For unincorporated areas the sustainable yield has been defined as 50% of the average annual aquifer recharge. Effectively this is stating that of the available groundwater resources for the greater part of the Northern Territory, 50% is required to be allocated to sustain groundwater dependent ecosystems.

Generally to determine aquifer recharge, the Northern Territory was subdivided into four zones based on the likely dominant mechanism of recharge. Within the zones the probable recharge rates (ML/ha/year) ranged from 0.2 to 5 ML/Ha/year in the northern most zone to 0.02 to 2.5 ML/ha/year in the southern most zone. The range of recharge mechanisms and rates combine to reflect relatively higher annual recharge in the north to lower and infrequent recharge in the south. The rate applied to the groundwater management units and unincorporated areas was based upon the level of understanding of the groundwater management unit/unincorporated area.

Sustainable yield: assumptions, reliability and errors

For unincorporated areas, 50% of average annual aquifer recharge is allocated to sustain groundwater dependent ecosystems. Subjectively assigned recharge values to the four zones based on likely recharge mechanisms. The recharge mechanisms are broadly based on rainfall pattern of the Northern Territory from the northern top end to the southern desert area.

Without further scientific knowledge, the groundwater requirements of the environment can not be determined. Also few aquifers in the Northern Territory have sufficient data to enable their sustainable yield to be accurately estimated. Reliability is low.

Australian Capital Territory

Lead agency

Environment Protection Unit, Environment ACT

Context

The Australian Capital Territory Government controls most water resources in the Australian Capital Territory. Water use is dominated by urban water supply, which represents over 90% of the total licensed volume. The remainder is represented by irrigation and rural use.

Surface water

Sustainable water provision estimate: method

The sustainable yield of the Australian Capital Territory Management Area was determined by the subtraction of environmental water requirements from the total water resource.

Environmental flows were determined using the Environmental Flow Guidelines. These refer to the flow regime necessary to sustain habitats, encourage spawning and migration, enable the processes on which succession and biodiversity depend, and maintain the desired nutrient structure within lakes, streams, wetlands and riparian areas.

For water supply catchments, the Environmental Flow Guidelines give priority to water supply needs, which could intrude on environmental flows during dry periods. For those catchments, flows in rivers and streams below the eightieth percentile flows are protected from abstraction. All the water above the eightieth percentile is available for use, except for spawning flows. In the remaining catchments, 10% of the water above the eightieth percentile is available for use.

Sustainable yield: assumptions, reliability and errors

The precise determination of environmental flows depends on availability, reliability and relevance of data. The percentile flow is based on ?time weighted? recorded flows and gauged data with an available record period of at least 10 years. Analysis assumes relatively undeveloped conditions with no significant dams or other flow modifications, and the characteristics of catchment hydrology approximating natural conditions.

Improved knowledge is required on the impact of increasing groundwater abstraction on river and streamflow (baseflows) and the flow requirements of local streams. Techniques also need to be developed to assess the effectiveness of environmental flows.

Groundwater

Sustainable yield estimate: method

Sustainable yield estimates were based on a water balance method. This included provision for evapotranspiration, aquifer throughflow, leakage from one aquifer to the other, and surface water - groundwater interactions. The results were checked against rainfall recharge estimates. In line with the precautionary principle a cautious approach was taken in the determination of environmental flow requirements. Groundwater abstraction was limited to 10% of average annual recharge to ensure supply for groundwater dependent ecosystems.

Sustainable yield: assumptions, reliability and errors

Groundwater information is very limited and is based largely on modelling of groundwater and surface water interaction. Sustainable yield estimates are of low reliability because most of the aquifers are contained in fractured rock systems where it is difficult to quantify the resource. This is due to high recharge rates and low aquifer yields (low storage capacity and low extractive ability).

The water balance method also has a low level of accuracy, due to the difficulty in estimating evapotranspiration and aquifer flows in fractured rock systems.

In the Australian Capital Territory, a linear relationship between annual rainfall and rates of recharge has been assumed. Verification of this relationship is required through comparison with other methods. The rainfall-recharge equation could be used as a comparative tool to determine aquifer performance and increase reliability of the sustainable yield estimates.

Table of Contents for the Australian Water Resources Assessment 2000

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