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Water resources - Availability - Victoria

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Basin & Surface Water Management Area: Mitchell River (Vic)

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:

Mean Annual Run-Off (Natural) (ML/yr)	1,100,000
Mean Annual Outflow (Natural) (ML/yr)	1,100,000
In-stream commitment (Total available flow - imported water - sustainable yield) (ML)	1,081,100
Sustainable Yield - Developed Yield (ML)	0
Divertible Yield (ML/yr)	435,000
Developed Yield (ML/yr)	18,900
Sustainable Yeild (ML/yr)	18,900
Yield, Security of Supply
Current Development Category	MEDIUM DEVELOPMENT
Diversion (ML/yr)	11,640
Total Available Water	1,100,000

PLEASE NOTE:

Mean Annual Flow:

The mean annual flow of the Mitchell River Basin was determined using the natural flow series derived at the furthest downstream gauge in the catchment (Station No. 224203). The flows at this site were transposed to the outlet of the river using a relationship derived between the mean annual flow and catchment area of selected gauges in the basin.

Mean Annual Outflow:

The flow in the Mitchell River increases downstream. The flow is therefore greatest at the catchment outlet and represents both the Mean Annual Runoff and the Mean Annual Outflow (for undeveloped conditions). (Refer to the description of the method for determining the MAF).

The mean annual outflow under current development conditions was estimated as the difference between the outflow estimated under natural conditions and the average annual diversions. This assumes negligible river losses.

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 bulk entitlement conversion process, initiated in Victoria in the early 1990s, has involved a comprehensive assessment of the developed water resource. The bulk entitlement volume represents the maximum volume of water that can be diverted at specified locations with current infrastructure in place and with consideration given to the original design intent of the water supply system. The bulk entitlement volumes have been derived using Resource Allocation Models (REALM) of the various supply systems.

For the majority of basins in Victoria the developed resource has been fully allocated. Hence, for these basins the developed yield is equivalent to the average annual allocation.

For the Mitchell River Basin the developed yield (and average annual allocation) has been defined as the average diversion that would occur when the existing entitlements within the basin are fully taken up.

The average regulated diversions have been reported as 90% of the BE caps as these caps reflect an upper limit.

The average unregulated diversions were determined as 90% of sum of the private diversion licence volumes in the basin.

Ninety percent of the licence and entitlement volumes is assumed to represent the likely upper limit to the long term average diversions for these users

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.

For the Mitchell River Basin one potential dam site has been identified providing an increase in system yield of 416 GL.

How has flow regime changed in Mitchell River (Vic)?

Change in Flow Regimes:

A high proportion of the catchments yield is derived from the higher altitude areas in the upper catchment, with the Wonnangatta River contributing approximately 4%, the Wongungarra River contributing 27% and the Dargo River contributing approximately 23% of the annual flow.

The Mean Annual Flow at the confluence of the Wonnangatta and Wongungarra rivers is 337 GL increasing to 859 GL at Glenaladale.

The flow pattern of the streams in the Basin is highly seasonal, with 45% of the annual flow occurring in the spring period. The highest flow period extends from July to October, with 63% of the flow occurring during this time. February is the lowest flow month, with total flows representing 1% of the average annual flow. The highest flow month is September, with total flows representing 18% of the average annual flow.

The streams of the Mitchell River Basin are unregulated, with no major storages to control flows. Total annual diversions from the main tributaries, for both urban and rural use, represent less than 10% of the average summer flows and, overall, have a minor impact on the MAF, but can have a major impact on summer flows.

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 Mitchell River (Vic)

Summary surface water measurement station statistics

Name	Stream gauge ID	Mean annual flow (ML/yr)	Mean annual flow (mm)
Stream guage 224213	224213	no data	no data
Stream guage 224206	224206	346,085	312
Stream guage 224203	224203	890,960	227

Modelled unimpaired stream-flow sites in Mitchell River (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

Table: Summary unimpaired stream flow sites statistics:

Station Name	Mean annual runnoff	Mean annual rainfall (mm)
WONGUNGARRA R AT GUYS	287	1,460
WONNANGATTA R AT WATERFORD	289	1,281

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.