Perth
9.3 Groundwater discharge

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

This line item refers to groundwater discharge to rivers and drains within the region.

Groundwater discharge to rivers from the water table aquifer for the two model areas during the 2010–11 year are given in the following table.

 

Total groundwater discharge from the two model areas during the 2010–11 year

Model area

Discharge volume (ML)

Perth

75,069

Peel–Harvey

14,995

Total discharge

90,064

 

Quantification Approach

Data Source

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

Provided by

Department of Water.

Method

Both groundwater models calculate the flow of water between drains and rivers and the aquifers. The models assume that all groundwater flows between the aquifers and the surface water stores occurs from the aquifers (i.e. there is no recharge from surface water).

PRAMS removes water from drain and river cells when the water table rises above the specified invert level of the drain cell. The volume of water removed is the volume of groundwater discharged to surface water.

PHRAMS uses a simplified Drain Package developed within MODFLOW to calculate the discharge to these drains and rivers from the aquifer. Two classes of drains were assumed:

  • Major drains that are permanent and deep (including major rivers) - conductance is calculated at 10,000 m2/day and a depth of 2 m.
  • Minor drains – conductance is calculated at 10,000 m2/day and a depth of 1.5 m.

Groundwater discharges to drains and rivers were calculated when the local groundwater level rises above the drain bed elevation.

Perennial lakes provide major groundwater sinks in the water table aquifer, and are subject to rainfall and evaporation. These were included in the water table aquifer balance and modelled accordingly.

Assumptions, Limitations, Caveats and Approximations

It is assumed that there is no groundwater discharge to the major surface water storages within the region. Only groundwater discharge to rivers and drains is considered in this line item.

The two models use different techniques to calculate discharge. PHRAMS uses a single conductance figure for all rivers and drains, and only two depth categories. It is assumed that once groundwater is above the drain bed elevation, there is a consistent flow of water out of the aquifer.

Both models were initially developed to estimate and assess the impacts of changes in climatic conditions and varying extraction rates on the aquifers, not to prepare 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 9.3

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

9.3 Groundwater discharge

13.3 Groundwater discharge to connected surface water

260,725

220,149

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.