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

************************************************************

May 2009 : Retained largely as it was in August 2007.

WHAT OUR LEADERS AND DECISION MAKERS BELIEVE THE SITUATION TO BE.

Our Premier has recently resigned and has been replaced by the Deputy Premier. However their comments at the time are relevant.

Our chief decision makers were firmly of the opinion that a major drought is the cause of our depleted dams. This was followed by the Water Commission with similar views and expressed in their advertisements.

This brochure insert appeared in the Courier Mail on the 3rd February 2007. It confirmed that our two chief decision makers at that time were attributing our depleted dams to " the worst drought on record". They also introduced the influence of a rapidly expanding population and its water requirements. The map is 6 year map and important to these observations. It is now expanded for clarity. 

The map shown is clearly a decile map. A decile map grades the rainfall in decimal categories of 1 to 10 with 1 being "the lowest on record". The last 6 years may not be the lowest but they are in the lowest 10%.

I retrieved from the Bureau of Meteorology a "percentage" map of the same rainfall for the same 6 years. It shows 80%.

The stability of district 40 in which the catchments reside permits this 80% to be classified in the lowest on record category because the catchments have never received less than 80%.

It should be recognized at this point that the Qld Govt sponsored "Climate Change Centre of Excellence" reported the catchments as receiving 76.2% and defended it as a "Drought" rivalling the "Federation drought". There was no indication that they were aware that the balance of 23.8% occurred in the low to nil inflow non-summer months which have little impact on dam levels. Refer to "Rainfall deficiency" and the " Federation drought" buttons.

Decile evidence of 6 year periods for both the Wivenhoe and Somerset dams will provide more clarity on that statement. In the Wivenhoe there are 7 periods of 6 years in the last 100 years that are within 66mm ( 693 to 759 mm) mm of the last 6 years. With one exception, there were no "Uncommon event" within those periods.

The 80% of long term average rainfall does not fit within the Bureau of Meteorology definition of a drought being "acute shortage of water" in the context of rainfall. 

Press release on recycled water 28th January 2007

Relevant comments on rainfall were made by our leaders when announcing the cancellation of the vote on recycled water. They are contained in "Rainfall 2001-06" button.

Of particular interest is the comment that it may take 5 to 10 years to fill the Wivenhoe to 40 percent. It was made in the context of "below average" rainfall which has been examined in detail in this web-site.