AN OBSERVATION OF THE CONFLUENCE OF STATISTICAL ABERRATIONS, DAM RATIOS, TIMING AND UNCOMMON RAINFALL EVENTS THAT COMBINED TO PRESENT OUR DAMS AS VICTIMS OF A SEVERE DROUGHT. THIS IS NOT THE CASE. THEIR INFLUENCE ON CONCLUSIONS REACHED AND ACTIONS TAKEN.

Author : J. V. Hodgkinson F. C. A. Chartered Accountant : August 2006 to December 2009

The Underlying cause of our depleted dams had the appearance of "drought". However most people would not class 80 percent of rainfall compared with the Bureau of Meteorology long term average 1961 to 1990 as a "drought". This is the case in the catchments. Even less people would class it as a "drought" when closer examination of the rainfall shows that the inflow producing four Summer months had been quite normal with 99.7% in the Wivenhoe and 91.3% in the Somerset catchments. The 20% deficiency was in the low inflow producing eight non-summer months.

This incorrect conclusion of "drought" has clouded the real underlying cause of our depleted dams. It is the random nature of large scale rain depressions and they are our main water supply. They occur on average every 4 years and have the capacity to fill our dams in a few days. The last gap was six years 2001 to 2007 and caused a major and expensive panic. Further evidence is available in the tab DEPLETED DAMS or Click here

Dealing with those large scale events that extend beyond the 4 year average requires a very different approach than dealing with a "drought". The current water resource strategy has ignored the short history of the Wivenhoe dam.

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.