Perth
10.3 Recharge from 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 recharge from the landscape into each aquifer for the two model areas during the 2010–11 year are given in the following table.

 

Volume of groundwater recharge from the landscape within the Perth region during the 2010–11 year

 

Recharge volume (ML)

Groundwater store

Perth area

Peel–Harvey area

Water table aquifer

798,634

112,595

Underlying aquifer – Leederville

52,847

0

Underlying aquifer – Yarragadee

2,317

0

Total recharge

966,393

 

Total rainfall over the entire landscape of the Perth region was 10,628,826 ML during the 2010–11 year (derived using national grid data produced as part of the Australia Water Availability Project). This means that approximately 9% of the total annual rainfall over the Perth region contributed to groundwater recharge.

Quantification Approach

Data Source

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

Provided by

Department of Water.

Method

PRAMS calculates the recharge of water into the Perth region aquifers from the unsaturated zone above (i.e. the landscape store). The model delineates a series of horizontal cells called representative recharge units, and collates data on:

  • land use, vegetation classifications, leaf area indexes and soil classifications (spatial datasets)
  • water table depths and plant root depths
  • climatic data throughout the region.

The CSIRO WAVES model was then used to calculate flows from some of the specific land use areas, such as agriculture, pine plantations and native bushland (simpler models are used for general land use areas including residential, industrial and parkland areas). The Vertical Flux Model calculates recharge on a daily basis and MODFLOW was used to do the time step calculation.

PHRAMS calculates the inflow of water into the Peel–Harvey model area and assumes that most of the inflow is recharge from rainfall. The CSIRO WAVES model is used to estimate the maximum annual recharge based on the following equation:

            Recharge = 0.8 x Rainfall – 280

The annual recharge and annual pan evaporation were distributed into monthly amounts using a monthly distribution table. The monthly accounts were then applied to the specific geological areas (which have their own recharge rates according to soil types and land cover) to calculate total recharge.

Assumptions, Limitations, Caveats and Approximations

The two groundwater models use different techniques to calculate recharge. PRAMS is a more complicated and sophisticated model than PHRAMS. PRAMS uses daily climatic data to determine total recharge during the 2010–11 year, whereas PHRAMS is based on annual data inputs.

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 10.3

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

10.3 Recharge from landscape

15.2.1 Diffuse groundwater recharge from landscape water

1,778,266

1,666,283

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.