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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May 2009 : No change since August 2007. 

The basis of this web site is Bureau of Meteorology rainfall Stations in the catchments and the Web-site of SEQWater, the Dam managers. Their Annual Reports reveal that they are a truly professional body. Their public announcements have been helpful and they are to be commended.

This is an example of the data provided by the Bureau. It is obtained through identification of the Rainfall Station numbers and then by request to the data section. A small fee is required. You will observe that the information is in EXCEL spread sheet format which gives easy access to assessing the required information.

The Bureau of Meteorology measures rainfall against the 30 year average (or mean)  1961 to 1990. To maintain statistical purity, I have used all the rainfall stations that have been in continuous operation from 1961 to 2006. The remaining stations were mostly closed prior to 2006 but have been inspected for possible aberrations.

These charts were initially used to confirm the rainfall maps. The Qld. Govt. Climate Change Centre for  excellence has since confirmed the overall percentage as 76.2%. The relationship of the Somerset and Wivenhoe dams is one of the central ingredients of this examination. The Somerset is on the Stanley river and the Wivenhoe on the Brisbane River at the junction of the Brisbane and Stanley Rivers. It backs up to the foot of the Somerset.

They have separate catchments but as we will see the Somerset is the dominant partner.

The North Pine Dam is completely separate to the other dams. (SEQWater verbal advice). It experiences similar rainfall patterns. A reading of the "water grid" indicates that the proposed Traveston dam will feed into the North Pine dam.