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 : There is no change to this section since August 2007

SEQWater has by inference indicated that Summer Rainfalls were the main areas for high inflow into the Wivenhoe and Somerset Dams, uncommon events excluded.

They nominated three Rainfall Stations, two in the Wivenhoe and one in the Somerset.

They are correct. Attached is a summary of all the recording Stations in the Dams, some dating back to the 1880's. The Summer rains have without fail registered 50% or better in the 4 Summer months December to March. The balance, of course, was received in the non-summer months.

Almost all high impact rainfall that creates inflow was received in the Summer months. There are a few notable exceptions and attention is drawn to them where required.

The information is split into two sections. Rainfall up to 1964 and rainfall after that date. There is a slight variation between the two that do not alter the point of this exercise. I have referred to it in the "Climate Change" button.

Mary Valley ( Proposed Traveston Dam ) adjacent catchment area.

The Bureau of Meteorology rainfall statistics for the Mary Valley to the proposed Traveston Crossing are enclosed. They show that the four Summer Months exceed the eight non-summer months.

This is not surprising as the Mary Valley experiences the same rainfall pattern as the Somerset/Wivenhoe system. This has important consequences when dealing with "uncommon events".

CONCLUSION

The consistency of the data as between rainfall Stations and periods has facilitated the comparisons in both Dams dating back several years to the early 1900's.

It had a major effect on determination of the cause of our depleted dams in the years 2001 to 2006.

The Summer Rains, with the exception of uncommon events, have produced almost all the inflows into the Dams. There are minor, but some important exceptions usually on the edge of the non-summer months namely April /October /November.

Summer Rainfall has been used throughout the web-site and the exceptions that occurred in the non-summer months noted where applicable.