Sydney
17.2 River outflow from the region

Outflow from the Sydney region is the streamflow that reaches the sea. The estimated volume for the line item is 7,131,063 ML.

As the Sydney region's river and storage system is very complex there are a number of flows that contribute to this line item.

This line item excludes releases and spills which have been captured downstream, but itemises streamflows which have reached the sea. These streamflows include:

• rule-based environmental flows, other unspecified releases and spills from most downstream storages, and weirs to rivers
• natural runoff from the catchment of rivers downstream of storages and weirs, and streams leading to the sea (less downstream diversions)
• treated water releases from wastewater treatment plants.

Volume of total outflow from the Sydney region to the sea during the 2011–12

	Description	Volume (ML)
	Rule-based environmental flow releases from storages	440,626
add	Unspecified releases from storages	14,615
add	Spills from most downstream storages and weirs	2,224,484
add	Natural runoff from the catchment of rivers downstream of storages and weirs, and unregulated streams leading to the sea	4,430,258
add	Treated water releases from wastewater treatment plants	70,748
less	Downstream diversions	48,443
	Outflow to the sea	7,132,288

 

Volume of rule-based environmental flow releases from storage that reached the sea from storages during 2011–12 year

Authority responsible for environmental flow releases	Storage from which the release was made	Environmental flow release (ML)
Sydney Catchment Authority	Woronora	11,580
	Lake Burragorang (Warragamba Dam)	1,830
	Prospect	Not applicable1
	Lake Yarrunga (Tallowa Dam)	345,263
	Blue Mountains2	Not applicable1
	Pheasants Nest Weir	52,974
	Broughton Pass Weir	28,979
Total		440,626

1 Environmental flows have not been specified from these storages.

2 Blue Mountains storage reservoirs include Medlow Reservoir, Greaves Creek Reservoir, Upper Cascade Reservoir, Middle Cascade Reservoir and Lower Cascade Reservoir.

The previous table shows net environmental flow reaching the sea because the released flows were not diverted to downstream storages/weirs and not used for other purposes. Details of released environmental flows including volumes reached at downstream storages/weirs are available in Water for environmental benefit under Water access and use.

Volume of unspecified releases and spills from storages during the 2011–12 year

Storage	Other releases (ML)1	Spills (ML)	Total (ML)
Lake Burragorang (Warragamba Dam)	472	758,449	758,921
Lake Yarrunga (Tallowa Dam)	14,1432	1,312,535	1,326,678
Blue Mountains	0	1,250	1,250
Pheasants Nest Weir	0	90,000	90,000
Broughton Pass Weir	0	62,250	62,250
Total	14,615	2,224,484	2,239,099

1 Other releases were made to facilitate scheduled deliveries (e.g. to meet minimum discharge required to operate a gate — also known as 'conveyance water'). Fitzroy Falls storage release of 736 ML is not accounted in the table because the flow was received at Lake Yarrunga (Tallowa Dam).

Gauged flows are not available to cover all river reaches downstream of storages and streams leading to the sea. Many of the gauges are tidal and only river height data are available for some of them. Gauged flow data are not available for:

• the Hawkesbury River and the lower Nepean River (except the tributaries shown in the following table)
• northern Sydney rivers
• southern Sydney rivers, except for the Woronora River (at Gauge No. 213211)
• the Illawarra rivers
• downstream river reaches below the gauges shown in the following table.

Natural runoff was therefore estimated from available gauged flows for the rivers listed in the following table, and estimated runoff for other streams.

Total natural runoff is 4,430,258 ML for 2011–12, made up of 1,601,955 ML of gauged flows and 2,828,303 ML of flows below these gauges, as well as in ungauged and tidal streams. This is detailed in the following two tables.

Volume of gauged flows for streams leading to the sea during the 2011–12 year

River	Gauge No.1	Flow volume for 2011–12 (ML)	Comments
Macdonald River at St Albans	212228	111,448	Some data missing which contributes to approximately 33% of entire record.
Colo River at Upper Colo	212290	813,602	Some data missing which contributes to approximately 9% of entire record.
Gross River at Burralow	212291	245,598	Some data missing which contributes to approximately 33% of entire record.
South Creek at Richmond Road	212297	176,525	Some data missing which contributes to approximately 20% of entire record. Excludes 6,089 ML of treated wastewater released from St Marys Water Recycling Plant; this has been accounted for separately.
Woronora River at Needles North Engadine	213211	63,357	Excludes 11,580 ML environmental flow release, which was accounted for separately
Shoalhaven River at Grassy Gully No. 2	215216	191,425	Excludes (from the gauged flow): 14,143 ML diverted by the Sydney Catchment Authority to Shoalhaven City Council pumping at Burrier 345,263 ML environmental flow release and  1,312,535 ML dam spilling from Lake Yarrunga. Excluded items were accounted for separately
Total gauged flow		1,601,955

 1 The Sydney Catchment Authority manages the gauging stations. Volume stated is based on available data only.

Volume of natural streamflow in the river reaches in ungauged and tidal streams during the 2011–12 year

Description	Volume (ML)	Comment
Natural streamflow in the river reaches downstream of the gauges, and in ungauged and tidal streams (before any addition and diversions at further downstream reaches)	2,828,303	Based on the Bureau calculation of line item 9.4 Runoff to surface water

 

Volume of treated water releases from wastewater treatment plants during the 2011–12 year

Description	Volume (ML)
Wastewater releases to the rivers from the area managed by Sydney Water Corporation	67,003
Wastewater releases to the Shoalhaven River below Gauge No. 215216, from the area managed by Shoalhaven City Council	3,745
Total	70,748

 

Volume of deductions from natural river flows for downstream diversions during the 2011–12 year

Description	Volume (ML)	Comments
Riparian right releases	30,699	Based on total estimated riparian use provided by the NSW Office of Water ( 1,830ML released from Lake Burragorang was deducted from the total use)
Stock and domestic, and other lumped access licence releases in the respective river reaches	12,603	As estimated by the NSW Office of Water based on entitlement information available in Water Sharing Plan for the Greater Metropolitan Region Unregulated River Water Sources (2012a)
Sydney Water Corporation diversion from the Hawkesbury River for North Richmond	5,141	Based on the value provided by Sydney Water Corporation
Total	48,443

 

Quantification approaches are provided separately for the following inputs used to calculate outflow to the sea:

• rule-based environmental flow releases
• unspecified releases and spills from storages and weirs to downstream rivers
• natural streamflows in gauged streams
• natural streamflows in ungauged streams
• treated water releases from wastewater treatment plants
• downstream diversions from flow leading to sea.

See line item 17.6 and 17.11.

Various.

See the following:

• line item 17.6 Diversions–other statutory rights
• line item 17.11 Surface water allocation diversion.

See the following:

• line item 17.6 Diversions–other statutory rights
• line item 17.11 Surface water allocation diversion.

See the following:

• line item 17.6 Diversions–other statutory rights
• line item 17.11 Surface water allocation diversion.

Stream gauge data from the Hydstra system.

Sydney Catchment Authority.

River levels are gauged and converted to a flow rate using a rating table. Rating tables are updated by manual stream gauging.

Some of the gauging sites have been linked to the Sydney Catchment Authority's radio telemetry system so that gauging data are readily available. For the other sites, data are downloaded to the authority's Hydstra system in regular intervals from logged data at the site.

Nil.

The volume is based on measured data. Estimated uncertainty based on measurement accuracy and professional judgment on rating tables is +/– 10% to 50%, depending on the site characteristics and flow rate.

The Bureau: National Climate Centre (NCC) daily climate grids (rainfall, temperature and solar radiation); AHGF waterbody feature class; New South Wales Office of Water, GIS data on waterbody and river channel sections; Sydney Catchment Authoirty, GIS data on its operation area; Geoscience Australia, built-up areas and other GIS layers.

The Bureau.

The natural river flow was estimated based on the AWRA-L version 2.0.0 (Van Dijk 2010) streamflow model outputs (see the method described in line item 9.4 Runoff to surface water).

The Bureau calculated natural streamflow for the ungauged and tidal rivers.

See line item 9.4 Runoff to surface water–rivers below the reservoirs.

 

The volume is based on modelled and estimated data. Uncertainty is ungraded.

Daily Return System (DRS) database.

Sydney Catchment Authority.

Known release rates from flow meters, orifice plates, valves with rating tables or measured outlets were used to derive the volume.

Nil.

The volume is based on measured data. Estimated uncertainty based on meter accuracy, and professional judgment on rating tables and site configuration issues is +/– 10%.

See Line item 9.9.

Sydney Water Corporation and Shoalhaven City Council.

See line item 9.9 Discharge from urban water system.

See line item 9.9 Discharge from urban water system.

See line item 9.9 Discharge from urban water system.

This line item corresponds to line item 17.2 River outlow from the region reported in the 2011 Account.