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

SEQWater has by inference indicated that Summer Rainfalls were the main areas for high inflow into the Wivenhoe and Somerset Dams, uncommon events excluded.

They nominated three Rainfall Stations, two in the Wivenhoe and one in the Somerset.

They are correct. Attached is a summary of all the recording Stations in the Dams, some dating back to the 1880's. The Summer rains have without fail registered 50% or better in the 4 Summer months December to March. The balance, of course, was received in the non-summer months.

Almost all high impact rainfall that creates inflow was received in the Summer months. There are a few notable exceptions and attention is drawn to them where required.

The information is split into two sections. Rainfall up to 1964 and rainfall after that date. There is a slight variation between the two that do not alter the point of this exercise. I have referred to it in the "Climate Change" button.

Mary Valley ( Proposed Traveston Dam ) adjacent catchment area.

The Bureau of Meteorology rainfall statistics for the Mary Valley to the proposed Traveston Crossing are enclosed. They show that the four Summer Months exceed the eight non-summer months.

This is not surprising as the Mary Valley experiences the same rainfall pattern as the Somerset/Wivenhoe system. This has important consequences when dealing with "uncommon events".

CONCLUSION

The consistency of the data as between rainfall Stations and periods has facilitated the comparisons in both Dams dating back several years to the early 1900's.

It had a major effect on determination of the cause of our depleted dams in the years 2001 to 2006.

The Summer Rains, with the exception of uncommon events, have produced almost all the inflows into the Dams. There are minor, but some important exceptions usually on the edge of the non-summer months namely April /October /November.

Summer Rainfall has been used throughout the web-site and the exceptions that occurred in the non-summer months noted where applicable.