Melbourne
18.2 Groundwater outflow to outside region at coast

Supporting Information

The volume presented in the water accounting statements (129,345 ML) represents the regional groundwater flow out of the Melbourne region through the major sedimentary and basalt aquifers at the coastline. This outflow may or may not specifically represent groundwater discharge to the sea.

In general, the dominant direction of groundwater flow in the Melbourne region is from the recharge area in the fractured rock aquifers in the north, northwest and northeast towards the Port Phillip Bay and Western Port in the south.

The regional groundwater outflow from the Melbourne region and comparison to 2010 Account values are provided in the following table.

Regional groundwater outflow from the Melbourne region and comparison to 2010 Account

Year

Flux

Volume (ML)

2009–10

Discharge at sea

24,018

2009–10

Outflow

120,076

2010–11

Outflow

129,345

Compared to last year's approach this year's approach uses only the uppermost 5 layers excluding the bedrock (layer 6) of the Port Phillip groundwater model (Department of Sustainability and Environment 2010b). Outflow from the Aeolian dunes of the eastern limit of the Nepean peninsula has also been included in the 2010–11 results. The outflow was estimated at 9,533 ML/yr. This is similar in magnitude to the outflow (12,400 ML/yr) reported in the groundwater resource appraisal for southeast Melbourne (Southern Rural Water Authority 2010).

An improvement to the representation of the groundwater levels at the coast was also carried out (0 metres Australian height datum). These changes made to the line item estimation approach resulted in an increased groundwater outflow at the coast. For comparison purposes, this year's methodology has been applied to 2009-10 data and the results are reported in the previous table, together with the original value (discharge at sea) reported in last year's National Water Account.

Quantification Approach

Data Source

Hydraulic conductivity and aquifer thickness: Port Phillip Catchment Management Authority groundwater model (Department of Sustainability and Environment 2010b). Bore locations, groundwater level data and aquifer attribution: Melbourne University Groundwater Database.

Provided by

Bureau of Meteorology.

Method

Groundwater flow was calculated using a simple geographic information system (GIS) approach based on Darcy's law. Groundwater levels were interpolated for each season using the ArcGIS top-to-raster tool from reduced groundwater levels measured at monitoring bores. Seasonal groundwater flow grids were derived from groundwater level grids, aquifer thickness and hydraulic conductivity using a modification of the ArcGIS Darcy Velocity tool. Groundwater flow across selected flow boundaries (see the following figure) was then calculated using a simple GIS analysis and seasonal values were aggregated to reporting year.


Groundwater flow boundaries
Groundwater flow boundaries

Assumptions, Limitations, Caveats and Approximations

Regional flow was estimated for layers 1 to 5 of the Port Phillip groundwater model (Department of Sustainability and Environment 2010b), which represent the sedimentary and basalt aquifers with the exclusion of the basement (layer 6). These productive aquifers are considered to be the most hydraulically conductive units and flow in other units is assumed to be insignificant.

Consistent with the groundwater model report (Department of Sustainability and Environment 2010a) groundwater flow in the Nepean Peninsula was calculated only for layer 1. It was modelled as a fresh water lens in the highly permeable unconfined aquifer using the same GIS tool.

Groundwater levels were estimated by assuming that all 5 hydrogeological layers within the Port Phillip groundwater model region are hydraulically interconnected. This assumption facilitated the interpolation of a groundwater potential surface from groundwater level measurements, as these measurements were limited in number. Groundwater levels were also assumed equal to 0 metres australian height datum (mahd) the coastline. These assumptions were used to generate seasonal groundwater level surfaces across the sedimentary area.

Bore locations and the through-flow boundaries considered in the estimations are indicated in the previous figure. Flow across the remaining boundaries was assumed negligible on an annual basis because aquifer properties like hydraulic conductivity limit flow (e.g. fractured rock basement), the northern and northeast boundaries represent a groundwater divide with no through-flow, or groundwater flow was approximately parallel to the boundary (e.g. western boundary).

 

Uncertainty Information

The uncertainty in the field measured data (e.g. groundwater levels, hydraulic conductivity) was unspecified and unknown; the impacts of such uncertainty on the groundwater flow was not estimated.

The regional flow estimations were based on the interpolated groundwater level grids produced using ArcGIS Topo to Raster tool. Use of different interpolation methods may impact the values of the groundwater level grids and hence the estimated regional flow.

Groundwater flow was estimated for a simplified boundary constructed from a series of line segments. Groundwater flow across this boundary was calculated using the method described above. The uncertainty surrounding this simplification was not analysed.

 

Comparative year

This item corresponds to line item 16.1.2 Groundwater discharge to sea reported in the 2010 Account. In the 2011 Account, improved datasets were made available after the publication of the 2010 Account. Restatement of the value published in the 2010 Account was made as the difference was material and increased the accuracy of the information provided to the users of the National Water Account. The restated comparative year volume is 120,076 and is summarised in the following table.

Restated comparative year information for line item 18.2 Groundwater outflow to outside region at coast

2010 Account line item name

Volume at 30 June 2010 reported in 2010 Account (ML)

Value difference to volume reported due to improved datasets  (ML)

Volume at 30 June 2010 reported in 2011 Account (ML)

16.1.2 Groundwater discharge to sea

24,018

96,058

120,076