Perth
18.1 Groundwater outflow to outside region

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

Lateral outflows from each groundwater store within the two model areas during the 2010–11 year are given in the following table.

 

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

 

Outflow volume (ML)

Groundwater store

Perth area

Peel–Harvey area

Water table aquifer

17,254

2,053

Underlying aquifer – Leederville

5,834

97

Underlying aquifer – Yarragadee

6,364

0

Total outflow

31,602

 

Quantification Approach

Data Source

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

Provided by

Department of Water.

Method

Both groundwater models derive the lateral flux within the Perth region by defining the appropriate water balance zones and calculating the volume of water flowing across each boundary on an annual basis.

Assumptions, Limitations, Caveats and Approximations

It is assumed that all groundwater flow to and from outside the region is via the north and south boundaries. There is no groundwater flow between the model areas and the Darling Range in the eastern part of the region due to the geology of that area.

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 18.1

2011 Account line 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

18.1 Groundwater outflow to outside region

16.1.1 Regional groundwater outflow

30,311

94,688

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.

This change in methodology resulted in a greater than 100% change in volume from that reported in the 2010 Account. Consequently, the comparative year value estimated by the groundwater models was restated in the water accounting statements.