Introduction

This section presents information about surface water quantity and sustainability. For simplicity of communication, sustainability measures are based on assessing the level of water use and/or allocation with the quantity of water required to fulfil a sustainable flow regime (environmental water provision) - this has been expressed as a sustainable yield.

Vital statistics:

PLEASE NOTE:

Mean Annual Flow:

The management area as defined for the purposes of this Audit has no effective catchment area and therefore there is no significant runoff.

Mean Annual Outflow:

The Mean Annual Outflow (MAO) from the Victorian component of the Murray River to South Australia has been estimated for undeveloped conditions as the difference between the total mean annual outflow from all upstream SWMAs (determined for undeveloped conditions) and the estimated river transmission losses.

i.e. MAO (undeveloped conditions) = Sum of MAO (undeveloped conditions) from all upstream SMWAs river transmission losses.

The total river transmission losses for the reach of the Murray River from Hume Dam to the South Australian border has been determined using a simulation model of the system. The transmission losses have been estimated by the Murray Darling Basin Commission (MDBC) to equal 1835 GL. This includes losses from both the Victorian and NSW share of the flow.

The total estimated loss has been divided between the Victorian portion of the Murray flows and the NSW portion of the Murray flow in accordance with the proportions of the NSW and VIC outflows, as determined for undeveloped flow conditions. The outflow from the Victorian portion represents 57% of the total outflow from Murray River to South Australia. Therefore, losses from the Victorian portion are estimated to equal 1039 GL. This represents about 10% of the Victorian share of the mean annual flow.

The Mean Annual Outflow under current levels of surface water development has been determined as the difference between the Total Available Water and the sum of the diversion and storage and river losses.

i.e. Mean Annual Outflow (developed conditions) = Total Available Water - Diversion storage and river losses.

The total available water within the basin is derived from the outflow of upstream basins to the Murray River and any direct transfers to the River. The outflow comprises controlled releases and spills from storages located in the upstream basins. The average annual release and spill volumes have been determined using the water resource simulation model of the Murray River. Victorias share of the total available water includes the average annual volume diverted from the Snowy River, which is estimated to contribute 285,000 ML.

Under current levels of surface water development the mean annual outflow is 3,874,000 ML, which represents a 60% reduction from natural (or undeveloped) conditions.

Developed Yield:

The developed yield refers to the annual volume of water that is available for diversion at a defined level of reliability, taking account of environmental water requirements.

The Murray Darling Basin Cap arrangements require that the average diversion within the basin does not exceed the 1993/94 levels of development. Inherent in this cap arrangement is the requirement to meet downstream flow needs.

The developed yield for the basin has been limited to the average basin cap for Victoria, which equates to 1639 GL.

In Victoria, the available resource of the Murray River has been fully allocated, hence the developed yield of is equivalent to the reported average allocation and average diversion.

Divertible Yield:

The divertible yield has been defined as the average annual volume of surface water that can be diverted utilising both existing infrastructure and potential infrastructure under the ultimate level of development. In Victoria, potential future dam sites have been identified as part of an earlier study, which considered the options for surface water development in the State. The study was published by the previous Rural Water Commission of Victoria in June 1986 and was titled "Long Run Incremental Cost of Annual Regulated Flow in Victorias River Basins" (Alexander and Haydon 1986). An essential component of the study involved the determination of the incremental costs of future water resource developments in Victoria. A rapid appraisal method for assessing storage yield and construction costs at possible dam sites was developed which allowed selected sites to be ranked on a comparative basis. The construction cost was based on the volume of the dam embankment, catchment area and a parameter obtained from a relationship derived from existing dam embankment construction costs.

The main assumptions adopted in the assessment of potential storage sites and the estimation of yields from storages are listed below:

• Only streams carrying large flows in relation to the total annual basin yield and having a salinity of less than 1600 EC units (1000 mg/l) were investigated for possible storage sites.
• The yield estimation method is based on 95% supply security (a one in twenty year failure to supply unrestricted demands), an urban demand pattern and retention of 20% of storage capacity for dead storage and drought security carryover.

Some factors that are likely to influence actual storage costs and yields were not considered within the scope of the study. For example, costs associated with spillway construction, road construction, and land purchase were not taken into account in the cost equation. Other factors not considered included environmental considerations, variable demands, potential inter- and intra- basin transfers, site geology and water treatment requirements.

For this Audit, the divertible yield was reported as the sum of the developed yield for the SWMA and the incremental yield from the potential storage sites identified in the study noted above.

Dam sites with an effective cost greater than $2000/ML, as determined in 1985 (equivalent to around $3300/ML in 1996),were not included in the estimate of divertible yield as these were considered to be uneconomical.

There are no potential storage sites identified on the Murray River. The divertible yield is therefore equivalent to the developed yield.

How has flow regime changed in Mid-Murray River (Hume to SA Border) (Vic)?

Change in Flow Regimes:

Water diverted from the rivers and streams for the Murray-Darling Basin has grown to over 10,000 GL a year, or 80% of the water that used to flow out of the mouth before irrigation began.

This has caused the bottom end of the Murray to cease flowing for most of the time. Stress on the riverine environment has shown up in rising salinity, more algal blooms, a reduction in native and animal and plant life, and contraction of red gum forests and other wetland habitats.

About 30 weirs and storages have been constructed along the reach of the Murray River that extends to the South Australian Border. The operation of these regulating structures, in addition to the direct diversions from the river and the many storages and diversions from the tributaries, have resulted in major changes to the flow regime. The major storages on the Murray include:

• Hume Dam (3,038,000 megalitres);
• Yarrawonga Weir (120,000 megalitres); and
• a range of weirs and locks including Torumbarry Weir, Euston Weir, Mildura Weir, Wentworth Weir, and the off river storage of Lake Victoria.

The effects of these storages can be summarised as follows:

• Regulation has resulted in a reduction in the magnitude, frequency and duration of high flows leading to reduced inundation of the flood plains.
• The seasonality of the flow range has altered with regulation. During the irrigation season the regulated flow released from Hume Dam to the Murray is in the order of 20 - 25,000 ML/day. In winter, when water is being stored, this flow is reduced to approximately 1200 ML/day, which is just enough to meet the water supply requirements for towns downstream. Under natural conditions, flows peak in spring and then rapidly decrease in late summer and autumn. This effect is most pronounced immediately downstream of the major storages.

A comparison of natural and current flows at Albury, which is located just downstream of Hume Dam and upstream of major diversions, shows that the total flow at Albury has increased. Current flows at Albury are now higher than natural because of the transfer of about 570 GL from the Snowy River via the Snowy Mountains Scheme. The flow has however been redistributed throughout the year with the winter-spring flows being stored in Hume Dam for release to meet irrigation demand during summer and autumn. At the South Australian border, which is downstream of major diversions, the current flows are greatly reduced from their natural state.

Trade and Transfer - a bit of give and take:

Water use efficiency and optimisation strategies within existing infrastructure (eg. water supply efficiency, precision irrigation and scheduling, water recycling, trading and pricing) are part of the modern water resource development planning tool kit. Recognising that water is a finite resource, the States and Territories have developed water allocation systems where security and reliability are assigned to entitlement, trading is provided so water can be moved to high value uses and the choices of individuals are maximised.

Measurement Stations in Mid-Murray River (Hume to SA Border) (Vic)

Summary surface water measurement station statistics

There were no surface water measurement station statistics reported in this area as part of the assessment

Modelled unimpaired stream-flow sites in Mid-Murray River (Hume to SA Border) (Vic)

Over 300 sites across Australia were modelled to predict the unimpaired (natural) stream-flow. The long time series of stream-flow data are important for both research and management of Australia's hydrological and ecological systems. A simple conceptual daily rainfall-runoff model was used to extend the stream-flow data.

The model estimates stream flow from daily runoff and potential evapotranspiration data. The parameters of the model are first calibrated against the available stream-flow data. The optimised parameter values are then used to estimate monthly stream flow from 1901 - 1998.

For further information please refer to : Project Report - Stream Flow Study

There are no stream flow sites for this region.

Further information

• Victoria Water Resources Assessment 2000 Technical Report
• report from the study of streamflow data and modelled streamflow
• Link to data available for download on the:
• extension of unimpaired monthly streamflow data and regionalisation of parameter values to estimate streamflow in ungauged catchments (NLWRA 2000)
• Surface Water Management Areas
• Link to the Map Maker to make a map using this information.