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 : 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