Perth
17.2 River outflow from the region

Supporting Information

This line item represents the volume of water that outflows to the sea.

The total annual volume of flow observed during the 2010–11 year at the most downstream station along the rivers that flow into the sea, is given in the following table.

 

Volume of flow measured at the most downstream stations during the 2010–11 year along rivers that flow into the sea

River

Station

Total annual flow (ML)

Quality codes

Swan

616084

2,193

1,2

616086

1,713

11

616011(a)

20,870

 1

Subtotal

24,776

11

Canning

616027

3,346

1,12

Murray

614030

11,254

1,2

614063

1,437

1,2,11

614065(b)

45,261

1,2,4

Subtotal

57,952

11

Harvey

613052

31,118

11

Total

117,192

12

(a)  Station 616076 was used in the 2010 Account but was closed in June 2010. Station 616011 is the nearest station to 616076 with a complete record.

(b)  The annual record at station 614065 was incomplete. The nearby station 614006 was used to infill the gaps in record (October 2010 to June 2011).

Quantification Approach

Data Source

Department of Water: Hydstra database.

Provided by

Bureau of Meteorology.

Method

The total volume of water that flows out to sea is the sum of the available volume of outflow from the four rivers that flow into the sea:

  • Swan
  • Canning
  • Murray
  • Harvey.

The river outflow was estimated using instantaneous discharge data (L/s) collected at the most downstream gauging station (nearest to the outlet to the sea) along a river. These data were converted to daily volume data (ML) to determine the total annual discharge (in ML) at each station during the 2010–11 year.

The most downstream gauging stations along the main channels of the Swan and Murray rivers are 616011 and 614065, respectively. However, the following figure shows that two tributaries flow into the Swan and Murray main channels at a confluence downstream of these two gauging stations. Therefore, the total outflow from the Swan and Murray rivers was the sum of the total annual discharge at the most downstream station plus the total annual discharge measured along the two tributaries.


Location map of gauging stations used to calculate total outflow to sea
Location map of gauging stations used to calculate total outflow to sea

Assumptions, Limitations, Caveats and Approximations

It is assumed that the river outflow to the sea is equal to the volume of discharge measured at the most downstream station along a river. That is, there is no adjustment made for the contributing area below the gauging station used to calculate the outflow.

This contributing area below the gauging stations is 9,173 km2, approximately 43% of the total area of the Perth region. Based on a drainage-area ratio equation, estimated outflow is approximately 207,000 ML, which is 1.75 times that reported in the Statement of physical water flows (117,192 ML).

However, given that the ungauged component of the Perth region mainly lies on the Swan Coastal Plain, which is an area of relatively high rainfall-recharge, it is unlikely that this area will generate such a large amount of runoff. Instead, it is considered that the reported outflow to sea may be underestimated by up to 10–20%.

Uncertainty Information

The Department of Water assigns quality codes to flow data in accordance with the following table:

 

Quality codes for flow data collected by the Department of Water

Quality code

Description

1

Very good record

2

Very good record – Corrections applied

3

Good record – Corrections or estimations applied

4

Estimated record – Good

5

Estimated record – Fair

6

Estimated record – Poor

10

Estimated record – Not reviewed / quality not known

11

Theoretical rating

12

Estimated rating

 

The total volume of water that outflows into the sea has a quality code of 12. This quality code is the lowest quality of data recorded for all the station data. This was only recorded for a relatively short period of time (<30 days) at station 616027. Most of the flow data recorded at the gauging stations was of a quality code 1 or 2.

Comparative year

The following table presents substantive changes in the line item reported in this 2011 Account with the corresponding line item in the 2010 Account.


Comparative year information for line item 17.2

2011 Account line item

2010 Account line item

2011 Account volume for the 2009–10 year (ML)

2010 Account volume for the 2009–10 year (ML)

Change from the 2010 Account to the 2011 Account

17.2 River outflow from the region

14.2 Streamflow into downstream entity

769,234

769,234

Change in line item name and number