What
happened in our catchments to deplete our dams?
A consolidation
of information contained in this website. It can be used as a companion to
“Borumba dam expansion alternative”.
******************************************
Information
confirming that no drought existed in the catchments of the Wivenhoe/Somerset
Dams for the period 
February
2001 to 2007. The depletion was caused by the random nature of our main water
supply “large scale rainfall events” such as Monsoons, Cyclones and large
scale rain depressions. They have no relation to seasonal conditions and can
happen at any time generally within a 4 year average. The Courier Mail article
of the 10th February 2007 sets out the rainfall requirements.
Photo :
Courier mail article explaining our water sources
The result of this
determination is that the solution to cover the period of depletion in the dams
is diametrically opposed to the solution required for a “drought”.
The "drought" proposition is still retained in the "South East
Queensland Water Supply Strategy" and is an integral part of that strategy.
While the words are well chosen and well intentioned, the public assessment was
that it "does nothing" at least for the present. The Strategy ignores the short history of our water supply
incorporating the addition of the Wivenhoe Dam (1986) and its potentially severe
damaging consequences dealing with the recent depletion as a "drought".
The
evidence
1. Initial
enquiry resulting in this email from
the Bureau of 
Meteorology
confirming that rainfall in the catchments for the three year period 01/08/03 to
31/07/06 was close to 80% of the long term average. It was compatible with being
in “decile 1” being the lowest on record.
This means that Rainfall in three year periods had never been less than close to
80% of the long term average 1961 to 1990.
Photo : Email from the bureau of meteorology
Decile
maps are susceptible to statistical aberrations. For example, 10 rainfall
measurements from 991mm to 999mm would place 991mm in the lowest on record
decile 1 and 999mm in the highest on record decile 10. The Bureau offers no
warning on the maps that this could be the case.
2.
A copy of the six years 2000 to 2006 “decile
map” promoted to every household in South East Queensland (SEQ) is
attached. It is now accompanied by the corresponding “percentage”
map that I had prepared by the Bureau of Meteorology for the same period and the
same rainfall. It reads 80% of the long term average.
Photos : Decile graph showing lowest on record ; Percentage
graph for the same water and the same period
It
is now clear that the “decile” map is a statistical
aberration and was used to convince the residents of SEQ that our depleted
dams were the victims of a severe drought in the catchments.
Most people of SEQ would have held a different view if the “percentage”
map was the one published.
I
was not satisfied and I required further verification. I retrieved from the
Bureau of Meteorology all the data from all relevant rainfall stations in the
catchments, some dating back to the 19th Century with the following
result:-
3.
Schedule showing the distribution of rainfall for all operating 
rainfall
stations in the catchments since records were kept. It clearly shows that the 4
summer months December to March receive slightly in excess of 50% of the annual
rainfall. The other 8 months receive the balance but, being spread out over the
8 months, does not have the same ability to create inflow into the dams.
Rainfall
stations in the catchments showing the difference between summer and non-summer
rainfall
4.
Summaries of the Bureau rainfall data for the Wivenhoe and Somerset catchments
for the period incorporating most of the long term average 1961 to 1990 and the
period to 2006. Years 2001 to 2006 are the period of the “worst drought in 100
years”
They
show that the summer rainfall was quite normal in the catchments with 99.7% in
the Wivenhoe and 91.3% in the
Somerset
. The 20% deficiency was in the non-summer months that normally create little
inflow.
Photo
: Wivenhoe and Somerset catchments rainfall showing the summer rainfall for the
period of "drought" quite normal.
5.
Population increase is worth noting. The attached schedule 
drawn from Government
websites shows that the population was predictable and was as predicted. There was no sudden uplift.
6.
Attached is the Queensland Climate Change Centre of Excellence (QCCCE) report
“The Drought to 2007”. It confirms that 76.2% of the long term average was
the lowest on record.
Unfortunately
the QCCCE did not have hydrological records of the Federation drought, 1898 to
1903, with which the comparison of the “worst drought on record” was made.
The comparison was made solely on Rainfall.
Photo :
QCCCE observations of the "drought". Confirming the rainfall at 76.2%
of long term average.
Those
rainfall records are attached. They clearly show that the “Federation
drought” did not conform to the usual 50% of rainfall in the summer months.
The rainfall in the summer months was down to 30% thus spreading the rainfall
over the whole twelve month periods. That they created little inflow is clearly
evident from the attached graphs.
Photos :
Comparison of Federation drought rainfall with the "drought" period
2001 to 2006. Evidence that the current period quite normal and the Federation
drought abnormal.
Not all of
Queensland was involved in the Federation drought. 
North Queensland was battered
by two cyclones in that period. The report of the QCCCE adds weight to the
Bureau of Meteorology's opinion that district 40 in which the catchments reside
is a stable rainfall area.
Photos :
1899 Cyclone and the 1900 cyclone in North Queensland. 112 degree Fahrenheit in
Adelaide and 104 degrees in Melbourne
7.
The depletion of our dams was caused by the random nature of our main water
supply “Large scale rainfall events” that are Monsoons, Cyclones and large
scale rain depressions. They are known by SEQWater as “uncommon events”. It
is a misnomer as they occur on average every 3.7 years.
They are our main water supply as the attached schedule of 
“years
to fill” indicates. They can be hidden in overall rainfall statistics as 300mm
over a few days is a flood whereas 3 months of 100mm each is a trickle but the
same measurement of rainfall.
Photo :
Years to fill the dams without withdrawals for all our water sources.
The next two graphs are conclusive proof that it was the
random nature of our main water supply that caused the depletion of our dams. It
is worth printing them out and comparing the Wivenhoe experience in both graphs.
Their
random nature is best illustrated by this graph drawn up 
from
Bureau of Meteorology information and labelled “Frequency of large scale
events”. It lists the 43 events since 1841. The pink
shows the average of 4 years and the blue
illustrates those in excess of the average.
It
is quite clear that the last 6 year period was predictable and was not unusual.
There were 7 periods in excess of the 6 year period and it is the blue periods
to which our water supply security should be directed.
When
read in conjunction with the attached dam level graph of 
the
Wivenhoe the cause of our depleted dams comes into stark perspective. It was not
a drought but the random nature of our main water supply “large scale rainfall
events”.
8.
The underlying problem is that our dams are too small to control these events
for the benefit of our ecology and the residents of South East Queensland. The
answer is laid out in the accompanying “Borumba dam expansion alternative”
on the HOME page.
J.
V. Hodgkinson
F.C.A.
December 2009
