Perth

This line item refers to groundwater discharge to rivers and drains within the region.

Groundwater discharge to rivers from the Water table aquifer for the two model regions during 2009-10 are given below.

Data source

Western Australian Department of Water: PRAMS Version 3.4 and PHRAMS groundwater models.

Data provider

Western Australian Department of Water.

Method

Groundwater discharge was estimated for the Perth region (the area between Gingin and Mandurah) and the Peel-Harvey region (the area south of Mandurah) using the PRAMS and PHRAMS models, respectively.

PRAMS calculates the flow of water between drains and rivers and the aquifers. The model assumes that all groundwater flows between the aquifers and the surface water stores occurs from the aquifers (i.e. there is no recharge from connected surface water). PRAMS removes water from drain and river cells when the water table rises above the specified invert level of the drain cell. The volume of water removed is the volume of groundwater discharged to connected surface water.

PHRAMS also calculates the flow of water between drains and rivers and the aquifers. However, in this case, a simplified Drain Package has been developed within the model that calculates the discharge to these drains and rivers from the aquifer.

Two classes of drains were assumed:

• Major drains that are permanent and deep (including major rivers) - conductance is calculated at 10,000 m²/day and a depth of 2 m.
• Minor drains – conductance is calculated at 10,000 m²/day and a depth of 1.5 m.

Groundwater discharges to drains and rivers were calculated when the local groundwater level rises above the drain bed elevation.

Perennial lakes and estuaries provide major groundwater sinks in the Water table aquifer, and are subject to rainfall and evaporation. These were included in the water table aquifer balance and modelled accordingly.

Uncertainty

Ungraded.

Approximations, assumptions, caveats/limitations

• It is assumed that there is no groundwater discharge to the major reservoirs within the region. Only groundwater discharge to rivers and drains is considered in this line item.
• The two models use different techniques to calculate discharge. PHRAMS uses a single conductance figure for all rivers and drains, and only two depth categories. It is assumed that, once groundwater is above the drain bed elevation, there is a consistent flow of water out of the aquifer.
• Both models were initially developed to estimate and assess the impacts of changes in climatic conditions and varying extraction rates on the aquifers, not to prepare water accounts. These models have since been modified to also provide data for water accounting.