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: With the exception of a note it remains as it was in August 2009

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A few of the principal statistics have been graphed for visual comprehension. This graph shows the Sq Klm of the catchments.

Dam capacities highlight the enormous capacities of our Dams. The Somerset was our mainstay until 1986 when the Wivenhoe was commissioned.

Again the deterioration in our Dam levels. Of minor note is that the annual report of SEQWater for the year 2005 was incorrect. The Dam levels at 30th June 2005 were deducted from the 30th June 2003 levels instead of 30th June 2004. It has been adjusted in these figures.

The SEQWater web-site provides the overall statistics of the dams. Of particular note is the completion dates of the Wivenhoe in 1985 and the Somerset in 1959.

The expected annual rainfall in the Wivenhoe at 940mm and the Somerset at 1,230mm is puzzling. You will see from the charts in the "rainfall stats" section that they both appear too high and would place them permanently in a position of "below average" rainfall. I have requested an explanation in writing but it has understandably been ignored. They are currently busy people and have been most helpful in their public pronouncements. The attached graphs further highlight this minor puzzle.

The other aspect of note is that the Annual water supply is currently under review. This attachment was their previous assessment. It suggests that the annual flow into the Wivenhoe and Somerset dams is 446,900 megalitres. That is 1,224 ML per day. Quite adequate for our unrestricted use as compared to the restrictions now in place. If you examine the "Key features" above, you will see that it currently under review. Reference is SEQWater Environment report 2001-2002 page 2.

Note : May 2009 : QWC has considerable disagreement with these calculations. They have changed their approach from HYNF to Stochastic being "best guess under the circumstances". To me it seems that they are having difficulty coming to grips with "uncommon events" which are the major supplier of our water. Their quite normal but temporary departure has caused them some concern. SEQWater had the benefit of 6 "uncommon events" in their calculations. Of course storage is the answer in the expanded Borumba Dam when they are plentiful.