AN OBSERVATION OF THE CONFLUENCE OF STATISTICAL ABERRATIONS, DAM
RATIOS, TIMING AND UNCOMMON RAINFALL EVENTS THAT COMBINED TO HAVE OUR LEADERS
DRAW INCORRECT CONCLUSIONS. THEIR INFLUENCE ON ACTIONS TAKEN.
Author : J. V. Hodgkinson F. C. A. Chartered Accountant : August 2006 to May 2010
The relationship of the Wivenhoe/Somerset dam system to its rainfall in the catchments has been quite different from the way it has been presented to us since the joint dams’ inception in 1986. The main rainfall supplying our water is the, season ignoring, rain depressions in the form of Cyclones, Monsoons and large scale rain depressions. They generally cover the whole of South East Queensland at the same time. They provide much more water than our December to March “wet season” which is well known to us.
Mr Rob Drury of SEQWater drew our attention
to this in his Courier Mail article of the 10th 
February 2007. Output from the
Government IQQM computer model, covering the 111 years 1890 to 2000, has proven Mr Drury correct. In essence 300mm of
rain in the catchments in a few days is a flood and, in the right circumstances,
can fill the dams from scratch. On the other hand 3 months of 100mm a month is a
comparative
trickle. These facts can be buried in official overall rainfall statistics the
use of which in dam performance can and did throw up statistical aberrations.
Mr Drury called these rainfall events
“uncommon events”. Bureau of Meteorology flood 
records from 1841 reveal that
they occur on average every 3.7 years with most below that average. Therefore
those above can extend well above that average such as the recent period of 2001
to 2007.
Mr Drury also provided dam level charts which were not published by the Courier Mail. They provided clear evidence of the importance of "uncommon events" and the inability of the "Summer wet season" to provide sufficient water from as far back as 1992. We will examine them.
This is my review based on official statistics and documents. These statistics and documents are presented to you so that you can arrive at your own conclusions.
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