Perth
17.2 River outflow from the region

The volume reported in the water accounting statements (509,541 ML) represents the total volume of water that outflows to the sea from rivers within the Perth region during the 2011–12 year.

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. The total annual volume of flow observed during the 2011–12 year at the most downstream station along the rivers that flow into the sea, is given in the following table.

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 2011–12 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

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, i.e. 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 900,000 ML, which is 1.75 times that reported in the Statement of Water Flows (509,541 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%.

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

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 recorded for approximately three months at Station 616086. Most of the flow data recorded at the other gauging stations were of a quality code 1 or 2 (see table in 'Supporting information').