Misleading Streamflow Graphic

In several  annual and other reports available on their web site The Water Corporation uses this graphic (or one very similar) of historic streamflows into catchment dams. Click on graph for full image. Note, this page written in 2002 but the graphic is updated
stream flow 1911-2003 In nearby text there are usually references implying that these reduced streamflows since 1975 are due to decreasing rainfall.  For example on page 14 of  The Water Corporation 1999 annual report downloadable from their web site text above the graphic is headed, Dependability of supply and goes on to say "Records show that over the last 25 years we have had less and less rainfall...."

Rainfall records from the GHCN for Dwellingup and Mundaring are available to 1992 and when compared to streamflow in my graphic below show that about the mid-1970's streamflows fell much more than rainfall.
This changing relationship between rainfall and streamflow could be due to many factors such as changes in stream measuring equipement, changes in sites, changes in vegetation in stream environs and catchments. The Water Corporation archives might hold the reasons for this altered stream / rain relationship in the 1970's.

If they know, they are not telling us.

Easier to blame "climate change" isn't it.

Sound science is often too hard for Government.

On the new Water Forums web site the stream flow graphic is more explicitly linked to "climate change" and decreasing rainfall.
It has to be obvious that if we can not fully understand the history of our water resources then planning for the future might be based on an unsound foundation.
It seems puzzling that the history of  Perth's water resources and rainfall can not be better expressed by Government.

I emailed The Water Corporation on 30  July telling them that it was a mistake to think that the above graphic showed streamflows declining because of rainfall.

In The West Australian on Saturday 3 August a page 13 article by Steve Pennells makes the point  that streamflow decreases can not be blamed on rainfall.   The article also goes into the issue of thinning of catchment forests to increase streamflows.
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