The comment by CP on my 2007 article -There never was a rain shortage to justify seawater desalination for Perth’s water supply – has prompted me to post the May 2012 reply from WA Minister The Hon Bill Marmion.
First – here are some relevant links – 2002 article in The West Australian Water bosses snubbed plan
This page has a graphic showing the effect of thinning – third graphic down.
“Effects of catchment thinning at Higgins near Dwellingup from WC online report”
Reply from the WA Govt Water Corporation to an article in the Rockingham local paper June 2005 The Sound Telegraph – thats Cockburn Sound.
On 14 March 2012 I emailed the WA Minister -
Dear The Honourable Bill Marmion,
You should know that Perth dam catchments are blessed with rainfall averaging over 900mm per year – and that is just in the May to October runnoff season.
Yet by decades of not managing catchment vegetation efficiently your water supply authorities are progressively decommissioning your network of dams – great facilities mostly put in place by the sweat, foresight and prudence of earlier generations.
This graphic demonstrates the relentless loss in catchment efficiency.
I have more online with my article “2011 update 37 year Perth dams catchment rainfall trend – nearly a Thousand GL of water wasted over 15 years by not managing catchments”
Please make sure you checkout my 3 page word doc linked at my online article;
“There never was a rain shortage to justify seawater desalination for Perth’s water supply”
December 4th, 2007
To improve the resilience of the Perth water supply using the most environmentally friendly and low cost methods – here is what you could do Minister.
 Order that dam catchment vegetation be managed so as to aim for a catchment efficiency figure of near 6%.
 Order that the Gnangara pines be cut forthwith in favour of native bush – the pines value as saw logs is probably less than the value of the Gnangara Mound water that they are wasting, taking water values as per the average of your desalination plants.
I would suggest that after those measures take full effect – dam levels should recover to where you might be able to close down some of your very expensive seawater desalination capacity. That would save many millions of taxpayer dollars.
You should also be able to postpone the expensive venture to tap deeper groundwater. More savings.
Please let me know if you have any questions Minister.
Here is the reply from Minister Marmion.
Dear Mr Hughes
Thank you for your email dated 14 March 2012 regarding the management of
I recognise that vegetation has been contributing to a reduction of inflow into surface water reservoirs of Perth’s forested surface water catchments.
A trend in reducing rainfall since the 1970s is also a considerable factor in this reduced catchment recharge . The assertion that rainfall to our dam catchments is over 900mm per year is generally true . However, this rainfall is declining markedly through the south west and is below 900mm in the significant Mundaring Weir catchment.
It should also be noted that rainfall reduction and streamflow to dams is not a linear relationship.
Once groundwater disconnects from the streams there is a significantly greater reduction in streamflow.
A recent study by the CSIRO (currently in review , pending publication) has found that the denser vegetation in Perth ‘s forested catchments is responsible for approximately one-third of the run off decline to our reservoirs. The remainder is mainly attributable to rainfall decline.
The Department of Water (DoW) has been researching this issue for some time and has also been working with other agencies and researchers on options to address
vegetation management issues . A prime example is the DoW and Water Corporation’s Wungong Catchment Thinning Trial, which is investigating the impacts of increased forest thinning on improving surface water catchment recharge in Perth’s forested water catchments.
The DoW has also contributed to the development of the Department of Environment
and Conservation ‘s (DEC) next Forest Management Plan regarding the issue of forest vegetation management and its impact on forested surface water catchments. I understand the draft plan is expected to be available for public comment by late 2012.
The DoW is also coordinating a scientific review of the rehabilitation option for mine bauxite rehabilitation in the northern jarrah forest to inform future practice.
You may also be interested in the following CSIRO report which provides information on current and future water yields of both aquifers and surface’water catchments,
I appreciate your interest and demonstrated concern for the future of Perth’s water supply.
HON BILL MARMION MLA
MINISTER FOR ENVIRONMENT; WATER
11 May 2012