A watershed moment

As this blog has progressed over the last three years, its scope has grown to include the two major tributaries of the Barwon - the Leigh/Yarrowee and Moorabool Rivers - as well as the smaller creeks which also drain into each of the three rivers. Then, a while back I was asked what area was included in the watershed of the Barwon.
Good question! I knew in general terms that it would include the three rivers and any creeks and streams which flowed into them, but just how far did that extend? So I thought I'd have a bit of a look an see what maps and diagrams were available that would provide a visual image.
On the Australian National Library's website I soon found several historical maps which outlined the area including the following map published in the 1950s:


Map of the Barwon watershed. Image produced by the Victorian State
Rivers and Water Supply Commission, 1954 and held by the State Library
of Victoria (Click to enlarge)

As expected, the extremities of the watershed to the headwaters of the three rivers and the Barwon river mouth at Barwon Heads. The most southerly extent of the catchment lies to the south of the township of Forrest in the Otway Ranges, with the peaks of that mountain range providing a dividing line between the Upper Barwon and the Victorian coastline. In the north, the watershed extends from Ballarat where the Yarrowee River rises in the city, to a point north of Ballan in the Wombat Sate Forest where the east and west branches of the Moorabool River begin their journey to the sea.
For administrative purposes, the Barwon catchment area falls within the boundaries Corangamite Catchment Management Authority (CCMA). The Corangamite Catchment is responsible for some 13,340 square kilometres from Ballarat to Geelong and the Bellarine Peninsula and along the coast to the town of Peterborough which lies about 50km east of Warrnambool. It is one of ten such authorities which cover Victoria.

Map of the Victorian catchment management authorities taken from the
Department of Environment and Primary Industries website

For provision of services such as drinking water and sewerage, the Barwon and its tributaries - as far as I can tell - fall under the jurisdiction of two water authorities. The majority of catchment area is contained within the area serviced by Barwon Water which includes an area of about 8,100 square kilometres from Apollo Bay in the west to Meredith and Cressy in the north and to Little River and the Bellarine Peninsula in the north and east. This map on the Barwon Water website shows the extent of the area of supply. Those parts which do not fall under the auspices of Barwon Water are the upper reaches of the Moorabool and Yarrowee/Leigh Rivers which lie in the area controlled by Central Highlands Water servicing an area of 9,275 square kilometres.
But this still doesn't exactly answer the question: what is the combined catchment area of the three rivers - Barwon, Moorabool and Leigh/Yarrowee?

Another 1950s depiction of the Barwon watershed, titled Catchments
of Barwon River and inland Basins, created by the Victorian State Rivers
and Water Supply Commission and held by the Victorian State Library
(click to enlarge)

According to Wikipedia, the watershed of the Barwon and its tributaries makes up about 8,590 square kilometres, however I have not been able to independently verify this and suspect it may refer to the full extent of Barwon Water which includes catchment regions which are not part of the Barwon system. According to various other websites, the Moorabool River accounts for 2,300 square kilometres, the Yarrowee/Leigh River for around 890 square kilometres, however it seems very hard to find a clear answer for the Barwon itself.
The City of Great Geelong website indicates that the Barwon's catchment extends to 3,700 square kilometres. Given the area covered I believe this refers only to water flowing directly into the Barwon however the wording is ambiguous. Assuming that my interpretation is correct, then the combined catchment area of the three rivers is in the region of 7,790 square kilometres or more than three quarters of a million hectares - but less than the 8,590 square kilometres suggested by Wikipedia.
So, in short, I'm not precisely sure, however if anyone can provide a figure, I would be pleased to add it to this post.
NOTE: subsequent to publishing this post, the CCMA inform me that "Corangamite CMA's GIS (Geographic Information System) figure is 495,480 ha for the entire basin including Upper Barwon, Barwon, Murdeduke, Leigh, Moorabool" which converts to the much smaller figure of 4,955 square kilometres for the Barwon watershed plus the Murdeduke Basin.