Perth
10.3 Recharge from landscape

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

The volume reported in the water accounting statements (1,650,692 ML) represents the total volume of groundwater recharge from the landscape within the Perth region during the 2011–12 year.

Groundwater recharge from the landscape into each aquifer for the two model areas during the 2011–12 year are given in the following table.

Total volume of rainfall over the entire landscape of the Perth region was 17,030,580 ML during the 2011–12 year (derived by multiplying the area-averaged rainfall of 805 mm (see Climate overview) by the area of the region (21,156 km²)). This means that approximately 10% of the total annual rainfall over the Perth region contributed to groundwater recharge.

Recharge from landscape was approximately 685,000 ML greater than that reported during the 2010–11 year. This large increase reflects the improved rainfall conditions observed throughout the region during the 2011–12 year (see Climate overview) compared to the record low rainfall recorded during the comparison year (see the 2011 Account).

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.

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 2011–12 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.