Perth
18.4 Discharge to surface water

The Department of Water use the Perth Regional Aquifer Modelling System (PRAMS) Version 3.4 and Peel–Harvey Regional Aquifer Modelling System (PHRAMS) to estimate the natural water movement to and from the groundwater store within the Perth region.

As shown in the following figure, PRAMS estimates groundwater movement for the area north of Mandurah. PHRAMS estimates groundwater movement for the Peel–Harvey area south of Mandurah. The volumes reported in this account refer to the natural water movement that occurs within each model area within the Perth region boundary.

Map of groundwater model areas relative to the Perth region boundary

This line item volume (197,044 ML) represents the total volume of groundwater discharge to rivers and drains within the region during the 2011–12 year.

Groundwater discharge from the water table aquifer for the two model areas during the 2011–12 year is given in the following table.

Both groundwater models calculate the flow of water between drains and rivers and the aquifers. The models assume that all groundwater flows between the aquifers and the surface water stores occurs from the aquifers (i.e. there is no recharge from 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 surface water.

PHRAMS uses a simplified drain package developed within MODFLOW to calculate 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 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.

It is assumed that there is no groundwater discharge to the major surface water storages 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.