Melbourne

The volume recognised in the water accounting statements (21,873 ML) represents the sum of the Permissible Consumptive Volume for each water table aquifer Groundwater Management Unit in the Melbourne region. The total volume of water in aquifers is not estimated. Instead, the changes in storage for water table aquifers are recognised in the note for line items 16.1.2 and 15.2.1. Permissible Consumptive Volumes were announced in 2006, and represent the maximum annual volume that can be extracted in each groundwater management unit.

Water table aquifers in unincorporated areas outside of the Groundwater Management Unit areas have not been evaluated from a groundwater resource perspective, and do not have Permissible Consumptive Volumes. Water table aquifer volumes in the unincorporated areas outside of the Groundwater Management Unit areas cannot be quantified with representational faithfulness, and are not included.

Data source

Victorian Government Gazette, no. G 44, 02 November 2006.

Data provider

Victorian Department of Sustainability and Environment.

Method:

The Victorian Department of Sustainability and Environment estimated the volume of water table aquifers in Groundwater Management Unit areas in order to set a Permissible Consumptive Volume for each aquifer.

Permissible Consumptive Volumes for the  Groundwater Management Unit areas were determined as follows:

• Moorabbin Groundwater Management Area, Nepean Groundwater Management Area, Wandin Yallock Water Supply Protection Area and Deutgam Water Supply Protection Area Permissible Consumptive Volumes were based on the total volumes of existing licences issued for these groundwater management units.
• Frankston Groundwater Management Area was modelled using rainfall recharge, hydrograph fluctuation, through-flow and aquifer storage.
• Lancefield Groundwater Management Area was determined on the rainfall recharge method.
• Merrimu Groundwater Management Area was derived from a rounded calculation of rainfall recharge.

Uncertainty

Uncertainty is ungraded.

• Permissible Consumptive Volumes are prescribed and consequently have no uncertainty. However, Permissible Consumptive Volumes are constructs based in part on modelling and historical extraction.
• Permissible Consumptive Volumes apply only to the Groundwater Management Unit areas of Melbourne region. Estimating the size of the groundwater asset based on Permissible Consumptive Volumes means that no estimate is made of the size of the groundwater asset for land outside of Groundwater Management Unit areas.
• Estimating the size of the groundwater asset as at 30 June 2010 by summing Permissible Consumptive Volumes, therefore, is highly uncertain.

Approximations, assumptions, caveats/limitations

• Frankston Groundwater Management Area Permissible Consumptive Volume has a low confidence rating due to uncertainty in the derivation of the infiltration factor, hydrograph and through-flow calculations, and the extent of the recharge area.
• Lancefield Groundwater Management Area Permissible Consumptive Volume has a low confidence rating due to uncertainty in rainfall infiltration data, recharge processes and distribution.
• Merrimu Groundwater Management Area Permissible Consumptive Volume has a low confidence rating due to uncertainty in rainfall infiltration data, recharge processes and distribution.
• Frankston and Lancefield Groundwater Management Areas Permissible Consumptive Volume estimation assumed a rainfall infiltration factor of 5%.
• Merrimu Groundwater Management Area Permissible Consumptive Volume estimation assumed a rainfall infiltration factor of 10% in the north, 7.5% in the central area and 5% in the south.