Perth

Groundwater recharge from the landscape into each aquifer for the two model regions during 2009-10 are given below.

Total rainfall over the entire landscape of the Perth region was 13,188,531 ML (derived using national grid data produced as part of the Australia Water Availability Project). This means that approximately 13% of the total annual rainfall over the Perth region during the reporting period contributed to groundwater recharge.

Data source

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

Data provider

Western Australian Department of Water.

Method

Groundwater discharge was estimated for the Perth region (the area between Gingin and Mandurah) and the Peel-Harvey region (the area south of Mandurah) using the PRAMS and PHRAMS models respectively.

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 region aquifers 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, which were then applied to the specific geological areas (that have their own recharge rates according to soil types and land cover) to calculate total recharge.

Uncertainty

Ungraded.

Approximations, assumptions, caveats/limitations

• The two groundwater models use different techniques to calculate recharge. PRAMS is a more complicated and sophisticated model compared to PHRAMS, in that it uses daily climatic data to determine total recharge over the reporting period, whereas PHRAMS is based on annual data inputs.
• 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.