Groundwater Management Unit: Mid Murrumbidgee Alluvium
The groundwater resource characteristics for the Mid Murrumbidgee Alluvium groundwater managment unit are presented below. This includes technical detail on aquifer properties and water level change for key monitoring bores.
|Total Water Allocated||50,823 ML/yr|
|Total Water Used||16,034 ML/yr|
|Average Salinity||500 mg/L|
|Sustainable Yield||89,000 ML/yr|
|Depth to top of aquifer||8 m|
The shallow aquifer, to a depth generally of 30 40m, is referred to as the Shepparton Formation and while mainly clayey and silty it does have some very coarse polymict gravel lenses. These have water under essentially unconfined conditions, and have been used since the early development of the region as a source for stock and domestic supplies and for limited irrigation. The deeper aquifer, referred to as the Calivil Formation, is characterised by clean white quartz sand and fine gravel layers which have excellent water transmitting capacity. They are the source for the large yields being used for town, city and regional water supplies. The alluvial aquifer system in this valley is marked by good hydraulic connection between the river, the shallow aquifer and the deep aquifer. The aquifer compensates for excessive groundwater pumpage by inducing additional infiltration from the river. Water levels in the aquifer adjust rapidly to a new equilibrium between infitrating river water and groundwater pumpage, so that long term declining trends do not appear in the bore hydrographs. This has been demonstrated by a review of the groundwater conditions in the Gumly Gumly area (upstream of Wagga Wagga) after 20 years of pumping for a regional water supply scheme (Woolley 1992). The other side of the management coin is that river flow is decreased by increasing the rate of groundwater pumping, an aspect which must be considered in averall water management in the valley.
Method used for determining sustainable yield:
One of the most basic pieces of data required for sensible management of a resource is the quantity of input to a system or recharge. The sustainable yield of an aquifer is almost always considerably less than recharge because of the provision for the environmental needs. Nevertheless, any sustainable yield study will always involve the determination of recharge as a first necessary step. The following working definition has been adopted: 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 proportion is not specified in this definition. This proportion will change according to each situation and is assigned differently to each aquifer system. An interim sustainable yield figure has been derived for NSW aquifers by applying a sustainability factor to estimated recharge. These sustainability factors are a proportion of long term annual average recharge. An arbitrary default factor of 70% (ie 30% of recharge is reserved for environmental needs) has been adopted for most cases to date, but as better understanding of groundwater systems is developed, the sustainable yields can be adjusted accordingly. The initial figures are intended to be conservative while bearing in mind that it is most often easier to subsequently adjust Sustainable yield values upward rather than downward. Sustainability factors offer protection to the integrity of the groundwater system itself and ultimately all groundwater users including the environment and ensure that neither temporary nor permanent damage to the aquifer system results from overuse. Sustainable yield values can - and indeed will - change over time as our technical understanding of the dynamics of individual groundwater systems is enhanced as a result of more rigorous investigation and in response to changes in natural and socio-economic realities. In short, this is a commencement of a continuous process of periodic review and adjustment of sustainable yield estimates. It follows therefore that a set of sustainable yield figures will reflect a level of understanding that exists at a point in time. Groundwater management committees may change the sustainable yield factor to suit local conditions. High levels of accuracy in determining sustainable yield require a degree of rigorous study that would take years if not decades to achieve. As many NSW systems are either over- allocated or nearly so, it is not practical nor is it in the communitys best interests to wait those decades before adopting allocation ceilings that are technically highly accurate. In short, at this stage a very high degree of accuracy is not required to commence management consistent with the philosophy of sustainability. Nevertheless, the approach applied has generated a set of figures that have been produced as a synthesis of knowledge accumulated to the present and have been adjusted according to good hydrogeological common sense and an understanding of local issues. Additionally, the approach has been conservative in the interest of resource protection but tempered with compromise recognising the need to preserve current development and acknowledging the importance of encouraging continued development where appropriate to do so. Where rigorous numerical models have been developed and have resulted in the generation of acceptable recharge figures for an aquifer system, these values have been adopted as acceptable for use in sustainable yield determinations. In some cases systems that are similar to a modelled system have had recharge determined empirically using the modelled system as a reference. Most systems however, have not been modelled. In those cases, inputs (or recharge) to the system have generally been kept to rainfall and river components of recharge. Throughflow and underflow have in most cases been omitted from calculations in the interest of both 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:
- Rainfall sourced and;
- River sourced.
Rainfall recharge was calculated according to assessed rainfall, area and assumed proportion of rainfall accessing the aquifer. River recharge was estimated using an equation, which is a modified form of the Darcy equation that is used in the assessment of river recharge in the Modflow software package that models groundwater flow. The result is a theoretical contribution of the river to the recharge. An additional factor was applied to this result as an adjustment factor intended to reduce the theoretical river recharge and is set as a) the fraction of the year and/or b). fraction of river reach - that is considered as a loosing stream. In this way an actual river recharge component is produced. There is a strong subjective character to the results achieved by the above method which has been applied to all GMUs across NSW, but they have been made with common sense and with hydrogeological principles in mind. They are therefore valid within the needs of the present situation.
The sustainable yield for this GMU has been assessed at 89000ML/y, allowing for an environmental factor of 30% (ie sustainable yield is assessed as 70% of long term average annual recharge.) The recharge from the River is assessed at about 85% of the total, and this is consistent with conclusions drawn from analysis of long term pumping from the regional water supply scheme bores at Gumly Gumly.
Assumptions used for allocating development categories:
The classification of the GMU as Category 2 for allocations is based on preliminary a estimates of sustainable yield, and on allocation data compiled by regional staff. It is regarded as a reliable estimate for the GMU as a whole, not likely to be changed because of lack of precision in either allocation or sustainable yield estimate. The classification as Category 2 for usage is based on the same estimate for sustainable yield, and on estimates of usage, based on data held in the DLWC regional office and on data supplied by the water supply authorities. It is regarded as reasonably reliable. and is towards the upper limit for this classification. More precise figures for sustainable yield and/or usage might change the classifiaction to Category 2. Note the classification of the GMU as a whole indicates that the level of both allocation and usage is well within sustainable limits. Much of the pumpage for municipal and regional water supply purposes, however, is concentrated in a section of the valley between Pomingalarna (just downstrem of Wagga Wagga) and Oura (upstream of Wagga Wagga). This section, examined alone, is probably Category 3 or 4 for both allocation and abstraction.
Technical information on the key groundwater bores and monitoring stations is presented below, including hydrographs where available. Link to a discussion on groundwater levels and trends at a State level as it relates to dryland salinity.
|Bore ID||Start record||End record||Depth of bore (m)||Reduced level (m)|
The following groundwater management units also occur in Lachlan Province:
- Alexandra (VIC)
- Araluen Alluvium and Weathered Granite (NSW)
- Ascot (VIC)
- Bell Valley Alluvium (NSW)
- Belubula River Alluvium (NSW)
- Billabong Creek Alluvium (NSW)
- Bullarook (VIC)
- Bungaree (VIC)
- Cudgegong Valley Alluvium (NSW)
- Glengower (VIC)
- King Lake (VIC)
- Lancefield (VIC)
- Lower Macquarie Alluvium (NSW)
- Mid and Upper Murrumbidgee Catchment Fractured (NSW)
- Molong Limestone (NSW)
- Moolort (VIC)
- Mudgee Limestone (NSW)
- Murrumbidgee (ACT)
- Namadgi (ACT)
- Orange Basalt (NSW)
- Queanbeyan and Molonglo (ACT)
- Spring Hill Groundwater Supply Protection Area (VIC)
- Tourello (VIC)
- Unincorporated Area - Lachlan (VIC)
- Unincorporated Area - Lachlan Fold Belt Province (NSW)
- Upper Lachlan Alluvium (NSW)
- Upper Macquarie Alluvium (NSW)
- Wandin Yallock (VIC)
- Young Granite (NSW)
- New South Wales Water Resources Assessment 2000 Report
- New South Wales Water Resources Assessment 2000 Technical Report
- For more information about water and other natural resource issues link to www.dlwc.nsw.gov.au
- Link to data available for download on the Groundwater management units and provinces - ARC/INFO export
- Link to Map maker to make a map using this information.
Links to an another web site
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