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.

SEQWater has provided in recent years the releases from the Wivenhoe/Somerset dams. This graph was in a period where there were no overflowing flood water. The releases appear to cover both our requirements and the River's 66% although it did not become law until March 2007.

Photo : SEQWater releases from the Wivenhoe/Somerset system

QWC information is that 280,000ML has been allocated from the system for our consumption. This calculates out to a little above 23,000ML monthly.

The releases seem to indicate that the River was ignored. 

On the basis of their calculations of 34% for our consumption and 66% for the river and ignoring that the yield is 360,000ML and taking the allocation of 280,000ML, the following should have been the releases:-

280,000ML divided by 34% X 100% = 823,529 annually or 68,627ML monthly. 

That is the way the law is written and it makes no allowances for the sometimes large overflows that occur at flood time. 

Surprisingly, the last major "uncommon event" in May 2009 which had the Premier declaring the "drought" over, had only minor engagement with the catchments. The comparatively small rise of 16% in dam levels did not overflow the dams whereas a full engagement of an event that size would have overflowed the dams for some time. The impact on Moreton Bay, while severe, did not have the contribution of the Brisbane River's major tributaries.

The inclusion of the major floods of 1890, 1893 and 1974 in their calculations based on years 1890 to 2000, have the ability to present a major statistical aberration.

For example, major floods of 1890 (5.3 metres), 1893 (8.5 metres) and 1974 (5.4 metres) are included in this calculation. Floods of that size have the capacity to distort percentage calculations presenting a statistical aberration for annual requirements. It also has the element of a short gap of 3 years and a long gap of 81 years. The current gap is 35 years and continuing. The Brisbane River heights used are Brisbane City heights.

Photo : Combination of Bureau rainfall data and confirming Brisbane City River heights highlighting all major large scale rainfall events.