Perth
18.3 Discharge to landscape

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 to the atmosphere from the saturated zone of each aquifer for the two model areas during the 2010–11 year are given in the following table.

 

Total discharge to the atmosphere from each groundwater store during the 2010–11 year

 

Discharge volume (ML)

Groundwater store

Perth area

Peel–Harvey area

Water table aquifer

411,182

132,190

Underlying aquifer – Leederville

25,330

0

Underlying aquifer – Yarragadee

1,318

0

Total discharge

570,020

 

Quantification Approach

Data Source

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

Provided by

Department of Water.

Method

Groundwater discharge to the atmosphere was estimated using PRAMS and PHRAMS. Both models calculate evapotranspiration, which is considered to be equal to the groundwater discharge to landscape, from the saturated zone of the aquifers. Note that this should not be confused with evapotranspiration from the unsaturated zone, which is rainfall water that has entered the ground in the unsaturated zone but is removed before it reaches the water table at the top of the saturated zone.

Both models use the MODFLOW evaporation module to calculate evapotranspiration from the saturated zone open water wetland surfaces and near surface water table areas.

Assumptions, Limitations, Caveats and Approximations

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 18.3

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

18.3 Discharge to landscape

16.3.1 Groundwater discharge to landscape

731,892

665,560

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.