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.

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

May 2009 Largely as it was in August 2007

The Bureau definition of a drought has changed from "acute shortage of water". This old definition was used by the QWC and applied to the depleted dam. It ignored the hydrological situation ( source meeting with Minister Hinchliffe and QWC officers 31st January 2009)

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

Initial look at the Bureau's hydrology maps that formed the basis of the "drought"

The Bureau of Meteorology is the chief recorder of Rainfall and other items related to weather. In August 2006, their rainfall maps were my first observation point. A click on the "drought" heading for the last three years to July 2006 showed a decile map which grades rainfall from 1 to 10 and recorded that it was in the category of "lowest on record" for the catchments of the Dams. 

I then clicked on the "percentage" map for the same period and rainfall to observe how much rain fell. It showed that the catchments had received 80 % of the long term average 1961 to 1990.

The diversity of the answers for exactly the same rainfall required explanation. You will see from the attached e-mail from a Climatologist from the Bureau of Meteorology National Climate Centre that the two results are compatible. The Dam catchments had received close to 80% of the long term average and that 80% was the "lowest on record". They later confirmed that District 40 in which the catchments reside is a stable rainfall area and permits this Statistical aberration.

The deficiency of 20% in both catchments was in the non-summer months which normally create little inflow.

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

May 2009 Additions added

Definition of a drought

In "living with drought" the Bureau states that "Meteorologists monitor the extent and severity of drought in terms of rainfall deficiency".

Our Leaders have placed before us the Meteorology definition of a drought. It was contained in the "Decile" map delivered to our homes in SEQ. "Decile maps" grade rainfall in section 1 to 10 with  1 being the lowest on record and 10 being the highest on record. 

It was used by the QCCCE when documenting the "drought" to January 2007. The base was on rainfall as no hydrology report was available at that time distance.

THE LAST TWO YEARS ( to March 2009)

A look at the available maps (in the bureau website) for the last two years reveals that

APPARENT DEPARTURE FROM METEOROLOGICAL DEFINITION IN THE LAST TWO YEARS

We are all aware that the definition of a "drought" is now tied to the dam levels with 60% ( now 74% May 2009) being the end of the "drought". This is a significant change to a "social expectations and perceptions" view of a drought.

In my view, departure from the meteorological definition obscures the underlying reason for the depletion of our dams and as a consequence the view of all solutions to it.