10.2 Groundwater inflow from outside region at coast
a. Groundwater models
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
b. Supporting Information
The volume reported in the water accounting statements (0 ML) represents the volume of seawater intrusion into the groundwater stores within the Perth region during the 2011–12 year.
The interface between the aquifer and the ocean moves throughout the year. During the summer months, when levels in the water table aquifers are relatively low, the saltwater interface moves towards the coastline. Despite this, the models predict a net annual volume of flow from the aquifers into the ocean during the 2011–12 year, i.e. seawater intrusion is zero.
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 2011–12 year. For the Peel–Harvey area, PHRAMS assumes that the estuary inlets along the coastline form part of the boundary.
Assumptions, limitations, caveats and approximations
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, i.e. 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.