Department applies extra conditions to proposed Gnangara increase
Released 10 Nov 2010
In the proposed decision extraction will be targeted to get more water from the deeper confined aquifers and less from the superficial aquifers, which are more directly connected to lakes and wetlands.
The proposal is for 150.7 GLs to be taken from the combined Gnangara aquifers and 14.7GLs from the Jandakot aquifers.
"The proposal will see 12GLs less than we took from the superficial Gnangara aquifer than the last time extreme dry caused a maximum draw on Gnangara in 2006-07," Director Water Resource Use Rob Hammond said.
"We will also increase the take by 5.4GLs from the Jandakot-Leederville."
Mr Hammond said the extractive pattern has been designed to minimise impact of abstraction on key ecological values attached to the Gnangara groundwater system.
"This has been an exceptionally dry year. The wetlands and lakes will be affected by the lack of rainfall, and this will not be the only difficult decision we have to make," he said.
He said the department was doing its best to balance the need for Perth's water supplies, and the health of the Gnangara mound.
The department can authorise up to 165GLs* under the current Gnangara groundwater areas allocation plan, if circumstances are proved to be exceptional.
The Water Corporation has requested this volume after dams that supply greater Perth's scheme water recorded only 11GLs inflow in 2010.
"We have considered the case for exceptional circumstances, including the implications for Perth's scheme supply without the maximum draw, and agreed to propose a decision that considers all values and needs.
To trigger the exceptional circumstances the Water Corporation had to prove to the department that it had:
- Made all possible efforts to secure alternative supplies including maximising output from the Perth Seawater Desalination Plant
- Demand management measures for extreme dry circumstances remain in place
Mr Hammond said the shortfall in inflow to Perth's dam water supplies was one impact of the driest winter on record.
He said the department remained committed to reducing the annual draw on Gnangara to mitigate the sustained impact of a drying climate over 30 years.
"At the same time last year we had enough water to reduce the draw on Gnangara, and we did significantly.
"However this year, following record low inflow into metropolitan dams, the Water Corporation has proved to us they need the water to meet Perth's supply needs.
Mr Hammond said the government had developed a long term plan for the groundwater system's sustainability in the draft Gnangara Sustainability Strategy.
"After an extensive across government process of consultation with key water user groups, conservationists and the general public, we have a long term plan.
The final aspects of the plan were recently adjusted following public feedback and how some major aspects of the plan would be funded was the next stage of its development.
"The department and its partners remain committed to ensuring the long term sustainability of the system not only for Perth's high quality scheme water, but for all users of the system.
"There is no question that the Gnangara system in particular is under stress because of the drying climate and it has always been the intention of the Department of Water and the Water Corporation to reduce drinking water abstraction by a significant amount when possible.
"Last year, while still only about 54 per cent full overall, the dams were at their highest level for 10 years which allowed us to reduce the draw. This year, despite increased efficiency measures, we are not in that position."
The annual draw on Gnangara has historically provided 60 per cent of scheme water with the State's first desalination plant reducing that to 50 per cent in recent years.
Average abstraction in recent years has ranged from more than 158GL* to a low of 110GL* in 2009-10.
"Sustainable abstraction from the Gnangara system remains a high priority as it is essential to greater Perth's water needs," Mr Hammond said.
Note: * Abstraction figures are a combination of Gnangara and Janadakot mounds.
10 year IWSS abstraction history Gnangara and Jandakot
|Year||Jandakot (GLs)||Gnangara (GLS)||Combined (GLs)|
|2001-02 (dry year)||11.92||153.21||165.13|
|2006-07 (dry year)||8.36||160.84||169.2|
|2010-11 (dry year)||13.3||151.71||165.01|
Contact: Peter Collins
Phone: (08) 6364 6848