Perth
18.2 Groundwater outflow to outside region at coast

Supporting Information

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
Map of groundwater model areas relative to the Perth region boundary

b. Supporting Information

Groundwater discharge into the sea from each aquifer for the two model areas during the 2010–11 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).


Total regional outflow from each groundwater store during the 2010–11 year

 

Discharge volume (ML)

Groundwater store

Perth area

Peel–Harvey area

Water table aquifer

262,770

(5,112)

Underlying aquifer – Leederville

11,174

0

Underlying aquifer – Yarragadee

(4,042)

0

Net outflow

264,790

 

Quantification Approach

Data Source

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

Provided by

Department of Water.

Method

Groundwater discharge into the sea from aquifers within the Perth region 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 2010–11 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. 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.

Uncertainty Information

The uncertainty estimate was not quantified.

Comparative year

The following table presents substantive changes in the line item reported in this 2011 Account with the corresponding line item in the 2010 Account.

 

Comparative year information for line item 18.2

2011 Account ine item

2010 Account line item

2011 Account volume for the 2009–10 year (ML)

2010 Account volume for the 2009–10 year (ML)

Change from the 2010 Account to the 2011 Account

18.2 Groundwater outflow to outside region at coast

16.1.2 Groundwater discharge into sea

363,595

339,441

Methodology change: revised model output boundaries and updated groundwater model used

Change in line item name and number


The figure above shows that both groundwater model areas extend beyond the Perth region boundary. In the 2010 Account, line item volumes were derived for these entire model areas. Subsequent to the publication of the 2010 Account, the Department of Water improved the methodology in estimating the groundwater movement associated with the Perth region boundary, rather than the model boundaries.

In addition, an updated version of PRAMS was used in this 2011 Account. Major changes in this version of PRAMS compared to the previous version include (i) the incorporation of an updated conceptual model for the Leederville aquifer, (ii) the use of interpolated gridded climate data, and (iii) improved private extraction estimates.

For line items 10.1 and 18.1, this change in methodology resulted in a greater than 100% change in volume from that reported in the 2010 Account. Consequently, the comparative year values estimated by the groundwater models were all restated in the water accounting statements.