Sydney
17.2 River outflow from the region

Outflow from the Sydney region is the streamflow reaching the sea. The estimated volume for the line item is 3,039,529 ML.

This item includes streamflows resulting from:

• 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.

Banked environmental flow releases have been separately accounted for in line item 17.16. The following table provides a summary of outflow from the Sydney region to the sea.

Outflow from the Sydney region to the sea – summary

Description	Volume during 2010–11 (ML)
Rule-based environmental flow releases from storages	293,094
Unspecified releases from storages	19,950
Spills from most downstream storages and weirs	576,421
Natural runoff from the catchment of rivers downstream of storages and weirs, and unregulated streams leading to the sea	2,142,348
Treated water releases from wastewater treatment plants	74,554
Less downstream diversions	(66,838)
Outflow to the sea	3,039,529

 

Rule-based environmental flow releases from storage reservoirs – flow reaching the sea

Authority responsible for environmental flow releases	Storage reservoir/weir from which the release was made	Environmental flow release during 2010–11 (ML)
Sydney Catchment Authority	Woronora	7,126
	Lake Burragorang (Warragamba Dam)	3,197
	Prospect	Not applicable1
	Lake Yarrunga (Tallowa Dam)	219,245
	Blue Mountains2	Not applicable1
	Pheasants Nest Weir	38,877
	Broughton Pass Weir	24,649
Total		293,094

¹Environmental flows have not been specified from these storage reservoirs.

²Blue 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.

Unspecified releases and spills from storages

Storage/weir	Other releases during 2010–11 (ML)1	Spills during 2010–11 (ML)	Total during 2010–11 (ML)
Lake Burragorang (Warragamba Dam)	2,907	0	2,907
Lake Yarrunga (Tallowa Dam)	16,4192	538,645	555,064
Blue Mountains	224	180	404
Pheasants Nest Weir	0	6,462	6,462
Broughton Pass Weir	0	7,601	7,601
Danjera2	0	23,533	23,533
Total	19,950	576,421	595,971

¹Other 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).

²When the spill rate is above the required environmental release no additional release is required. However, for operational reasons, the release from the outlet is maintained through the period of spill.

Gauged flows are not available to cover all river reaches downstream of storages and weirs, 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 2,142,348 ML for 2010–11, made up of 740,134 ML of gauged flows and 1,402,214 ML of flows below these gauges, as well as in ungauged and tidal streams. This is detailed in the following two tables.

Gauged flows

River	Gauge No.1	Flow volume for 2010–11 (ML)	Comments
Macdonald River at St Albans	212228	100,910
Colo River at Upper Colo	212290	352,476
Gross River at Burralow	212291	89,804
South Creek at Richmond Road	212297	7,876	Excludes 3,662 ML of treated wastewater released from St Marys Water Recycling Plant, this has been accounted for separately
Woronora River at Needles North Engadine	213211	24,150	Excludes 7,126 ML environmental flow release, which was accounted for separately
Shoalhaven River at Grassy Gully No. 2	215216	164,918	Excludes (from the gauged flow): 16,419 ML diverted by the Sydney Catchment Authority to Shoalhaven City Council pumping at Burrier 19,245 ML environmental flow release and  538,645 ML dam spilling from Lake Yarrunga. Excluded items were accounted for separately
Total gauged flow		740,134

 ¹The Sydney Catchment Authority manages the gauging stations.

Natural streamflow in the river reaches in ungauged and tidal streams

Description	Volume for 2010–11 (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)	1,402,214	Based on the Bureau of Meteorology calculation of line item 9.4 Runoff to surface water

 

Treated water releases from wastewater treatment plants

Description	Volume for 2010–11 (ML)
Wastewater releases to the rivers from the area managed by Sydney Water Corporation	55,648
Recycled water releases to Boundary Creek (a tributary of the Nepean River) by Sydney Water Corporation	15,989
Wastewater releases to the Shoalhaven River below Gauge No. 215216, from the area managed by Shoalhaven City Council	2,917
Total	74,554

 

Deductions from natural river flows for downstream diversions

Description	Volume for 2010–11 (ML)	Comments
Riparian right releases	22,718	Based on total estimated riparian use provided by the NSW Office of Water (9,837 ML released from Lake Burragorang was deducted from the total use)
Stock and domestic, and other lumped access licence releases in the respective river reaches	38,531	As estimated by the NSW Office of Water based on entitlement information available in Draft Water Sharing Plan for the Greater Metropolitan Region Unregulated River Water Sources (2012a)
Sydney Water Corporation diversion from the Hawkesbury River for North Richmond	5,589	Based on the value provided by Sydney Water Corporation
Total	66,838

 

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.

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 SCA's radio telemetry system so that gauging data are readily available. For the other sites, data are downloaded to SCA'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 modelled the landscape water balance to estimate inflows using the following data sources: the Bureau: climate data and geographic information system (GIS layers), Commonwealth Scientific and Industrial Research Organisation (CSIRO): raster spatial data, WaterDyn and AWRA-L model parameters, and radiation data; NSW Office of Water: GIS data on waterbody and river channel sections; SCA: GIS data on its operation area; Geoscience Australia: built-up areas and other GIS layers.

The Bureau of Meteorology

The Bureau calculated natural streamflow for the following ungauged and tidal rivers:

• the Hawkesbury River and the lower Nepean River, except catchment areas upstream of

-        Warragamba River at Lake Burragorang (Warragamba Dam) (Gauge No. 212243)

-        Pheasants Nest Weir (Gauge No. 212203)

-        Broughtons Pass Weir (Gauge No. 212233)

-        Macdonald River at St Albans (Gauge No. 212228)

-        Colo River at Upper Colo (Gauge No. 212290)

-        Gross River at Burralow (Gauge No. 212291)

-        South Creek at Richmond Road (Gauge No. 212297)

• northern Sydney rivers
• southern Sydney rivers, except the catchment area upstream of the Woronora River at Needles North Engadine (Gauge No. 213211)
• the Illawarra river
• the Shoalhaven River, except the catchment area upstream of Grassy Gully No. 2 (Gauge No. 215216).

Natural river flow in the river reaches is based on landscape water draining into the rivers modelled by the Bureau (see the method described in line item 9.4 Runoff to surface water).

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.

SCA.

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.

Bureau of Meteorology: spilling volume from Danjera Reservoir for 2010-10 was estimated using the following input data; estimated precipitation and evaporation, and modelled inflow to Danjera Reservoir using Bureau of Meteorology data, storage volumes of the reservoir provided by Shoalhaven City Council (source: level monitoring at the reservoirs with hydrometric sensors linking to council's radio telemetry system); diversions and other losses for the reservoir provided by Shoalhaven City Council (source: Shoalhaven City Council Telemetry System and daily data registers in respect of the operations of water treatment plants); SCA; SCA Hydstra system and Daily Return System (DRS) databases.

Bureau of Meteorology: spilling volume from Danjera Reservoir; SCA: Spilling volumes and other releases from the storage reservoirs and weirs managed by SCA.

Data source

Agency	Method
Bureau of Meteorology	Storage mass balance calculations using measured storage volumes, estimated precipitation and evaporation, and modelled runoff
Sydney Catchment Authority	Spill volumes derived from water levels and rating tables for spillways

Breakdown table by agency

Agency	Approximations, assumptions, caveats/limitations
Bureau of Meteorology	Based on data provided by Shoalhaven City Council, water was not supplied from Danjera Reservoir during 2010–11. Storage losses other than spills have been assumed as negligible. For other approximations and assumptions, see the following line items: 9.1 Precipitation on surface water 17.1 Evaporation from surface water
Sydney Catchment Authority	Nil

Uncertainty by agency

Agency	Uncertainty
Bureau of Meteorology	The volume was estimated from measured and modelled data. Estimated uncertainties for some of the inputs used for the calculation of spilling volume from Danjera Reservoir are not quantified. These inputs are precipitation, evaporation and modelled run-off. Therefore, uncertainty for the spilling volume from Danjera Reservoir is ungraded. Estimated uncertainty for storage volume, which was the remaining input for the calculation, is +/– 5%
Sydney Catchment Authority	The volume is based on measured data. Estimated uncertainty based on meter accuracy, and professional judgment on rating tables, site configuration issues and local issues at the time of spilling at each site is +/– 5% to 10%

The following table compares the value at 30 June 2010 reported in this account with the corresponding value reported in the 2010 Account.

 

Comparative information from the 2010 Account for line item 17.2

2011 Account line item	2010 Account line item	Value reported in the 2010 Account (ML)	Value reported in the 2011 Account (ML)	Additional information
17.2 River outflow from the region	14.2 Streamflow to downstream entity	1,196,052	1,196,052	Change in line item name and number