Perth
18.2 Groundwater outflow to outside region at coast

The volume reported in the water accounting statements (306,260 ML) represents the total volume of groundwater discharged to the sea from aquifers within the Perth region during the 2012–13 year.

Groundwater discharge into the sea from each aquifer for the two model areas during the 2012–13 year are given in the following table. This is the net volume of groundwater discharge into the sea that occurred from the aquifers (i.e., volumes in parentheses indicate net inflow from the sea).

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

Groundwater movement between aquifers within the Perth region and the ocean was estimated using PRAMS and PHRAMS. Both models apply a constant head at the coastline to estimate the volumes flowing through each boundary over the year. For the Peel–Harvey area, PHRAMS assumes that the estuary inlets along the coastline form part of the boundary.

PRAMS and PHRAMS calculate lateral flow between the ocean and the modelled areas. Both use a constant head at the coastline of zero (given the level of 0.0 m Australian Height Datum is mean sea level).

Flows from the estuaries within the Peel–Harvey modelling area were estimated and included in the reported amount; that is, the estuary systems in the Peel–Harvey area were considered part of the 'coastline' boundary in PHRAMS. These estuary systems are relatively minor in the Perth modelling area and therefore were not considered in PRAMS.

Both models were initially developed for the purpose of estimating and assessing the impacts of changes in climatic conditions and varying extraction rates on the aquifers, not for the purposes of preparing water accounts. These models have since been modified to also provide data for water accounting.