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 January 2012    

FLOOD PROOFING BRISBANE from damaging floods to the point of extinction. MITIGATING flooding in Ipswich and Gympie. Putting REAL MEANING into "Drought proofing SEQ"

Preliminary view of our main water supply. To emphasize its importance it appears at the heading in all pages in this web site 

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. 

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.
 
WEB address: wivenhoesomersetrainfall.com
All comments welcome : hodgkin@powerup.com.au

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May 2009 : There is no change to this section since August 2007.

An examination of this email from a Climatologist with the Bureau of Meteorology in Melbourne was the first indication that the Catchments had received close to 80 per cent of the 30 year Long Term average 1961 to 1990. Of further particular interest is that the "mean" measurement of rainfall was used.

This was followed up by comparison of the six years 2001 to 2006 and contained in the "Decision makers" button.

So that there is no confusion between the terms "mean" and "median", I have attached the definitions supplied by the Bureau of Meteorology web-site.

The "mean", being the term used by the Bureau in the email, is adopted particularly in the light of the second paragraph of "median".

It also advises that over a long period of time they will be close and the use of either is acceptable.

The Bureau's 1975 handbook edition of Queensland Climatic averages also advises that "the median for the year usually differs from the sum of the monthly medians because it is obtained from all of the available yearly values and not by a summation of the monthly median values." Cross check is therefore difficult.

The Queensland Climate Change Centre of Excellence's report of March 2007 uses the term "average or median". Of particular interest in this section is their conclusion that the catchments, for the 70 months March 2001 to January 2007, received 76.2 per cent of average rainfall. In view of the BOM definitions, comparison with the "mean" should be "Close". This QCCCE report also compares the deficiency of rainfall with the "Federation drought". This is examined in the "Federation drought" button.

The Bureau of Meteorology E Mail confirms that the rainfall for the three years to July 2006 was close to 80 per cent. My initial and updated review of Rainfall Stations in the dams confirms both the QCCCE report and the BOM E Mail.

The purpose of this section is to show that the rainfall not received, 23.8 per cent (100 - 76.2), occurred mainly in the eight non-summer months. The eight non-summer months rarely provide inflow into the Dams.

Bearing in mind that the long term average against which the Bureau compares rainfall is 1961 to 1990, I have selected this period as the starting point of comparisons. A further reminder is that the Rainfall Stations selected have been in continuous operation since 1961 and most for many years prior to that year.

The information starts at 1965 as the charts we are about to examine are also used as part of the "decile" charts used to observe six year periods. See "Decision makers" button. 1961 does not easily fit into division by 6 being the six years 2001 to 2006 against which the comparisons are made. 

There is clearly a statistical aberration in the six years 1971 to 1976 being the 1974 flood period. I have shown the results with that period excluded for both the Wivenhoe and Somerset Dam charts. 

Somerset Dam

The summary at the bottom right hand corner shows that the rainfall received by this dam for the period 2001 to 2006 to be 76.2 per cent. It is in agreement with the above. I now prefer to eliminate the 1971-76 aberration and review the outcome of Summer V Non-Summer. The four Summer months received 91.3 percent of average and the eight non-summer 68.8 percent of average. 

The outcome is almost average Summer rainfall in the Somerset Dam for the six years 2001 to 2006.

Wivenhoe Dam

The summary at the bottom right hand corner shows that the rainfall received by this dam for the period 2001 to 2006 to be 84.6 per cent. It is 8.4 percent above the 76.2 % of the QCCCE. I now prefer to eliminate the 1971-76 aberration and review the outcome of Summer V Non-Summer. The four Summer months received 99.7 percent of average and the eight non-summer 77.2 percent of average.

The outcome is average Summer rainfall in the Wivenhoe Dam for the six years 2001 to 2006.

The CONCLUSION