Brisbane water issues and catchment rainfall history 1900 to 2005

People are saying that Brisbane is experiencing its worst drought in 100 years.
Let’s take a look at what the data can tell us.
The Bureau of Meteorology (BoM) has a website with pages where you can construct maps of drought affected areas for various periods. Readers must realise that the BoM drought definition is a statistical one, based on how current rain periods relate to the worst 10% of historical data and the worst 5%. All this is explained on the BoM website. 
Making a 3 year drought map of the Brisbane River catchments for the period 1 August 2003 to 31 July 2006 (see below) we see that no areas are classified as “Lowest on Record”.   In the portion of the BoM at left, pink is a "serious" rainfall deficiency and the light red is "severe".
So we must ask,  exactly what are public figures talking about when they say Brisbane is experiencing the worst drought in 100 years ?  It could be that they are referring to another period.

Looking at rainfall historic data now and averaging annual readings from Blackbutt, Esk and Kilcoy Post Offices we find that several periods  over 30 years have delivered similar rain to recent levels.
Rainfall data is less than perfect, many stations close and an alternative has to be opened at another site, recording can start then stop, there can be gaps in data and on balance it would seem that to make claims about the “worst drought in 100 years” could be beyond what all the data could speak and looks like an attempt to divert attention from past water policy shortcomings.
Brisbane catchment rain history
Starting in the years 1977 to 1980, again in 1986-87 and also in 1993-94,  low rainfall years should have warned policymakers about the limitations of the supply system.  These clear signals of cyclic low rain years, similar to periods such as 1918-19,  are set against a background of ever increasing population and demand.  The graph below is from a Queensland Govt report.
At this point the issue of any changes to catchment yields should be understood.
Catchment yield is the perecentage of rain that makes its way into dams and is reduced by a greater density of vegetation.  It will be interesting if we can discover if yields have been altered due to changing vegetation, say over the last 20 years ?
We are trying to discover data on catchment yields. 
It is interesting to ponder that for every 100 mm of rain that falls over the ~7000 sq kms catchment above the Wivenhoe Dam, about 700 Giga Litres  (GL = a Billion litres) falls onto catchments which is about 60% of the capacity of Wivenhoe Dam.  Of course only a small percentage of this reaches the dam and this is the all  important yield.
It would seem obvious from the rain history data that policymakers should have moved in the mid-1990's to shore up supplies.
Surely Brisbane people expect their Government to get water policy right this time.
Dams have served Brisbane well for many decades, however it must be recognised that due to the location of the State border and the proliferation of National Parks, the most productive water supply catchments in the Great Dividing Range have not been utilised.
Just as past signals about limitations to rainfall were not heeded by policymakers when planned dams were cancelled,  it would be bad news for Queensland taxpayers if their Government lurched too far the other way into undue pessimism about rainfall. No doubt Government people are pressured by  CSIRO reports based on pessimistic greenhouse climate models predicting a drier  future not to mention the sales talk from desalination industry salesfolks who must be pinching themselves that the Govt. is buying their water factories yet rainfall is way over a metre per year in ranges scarcely 100 km inland.
There can be no doubt that water costs will multiply with any move away from traditional supplies and there will be a new water order with upwardly mobile prices linked to energy costs plus the risk of strikes in the desal water factories and the certainty of an increased water bureaucracy feeding off the higher charges.
It is fascinating that Toowoomba recently voted on the issue of drinking treated sewerage, yet not far from there rivers rise which flow to Cubbie Station which has had no trouble in trapping truly vast volumes of water.
One day in the future a Brisbane politician might discover that rain actually falls free from the sky and can be trapped in dams for later use.
A note on data. Of course all the rainfall data used in the graphic was bought from the BoM  and the reference to Cooyar P.O. on the map is because Blackbutt data was missing several years through WWI and into the 1920's and I substituted Cooyar readings adjusted for the typical average station difference.    Kilcoy also had 3 missing months in late 2003 which I replaced with estimates from neighbours.
I think there are still gaps here and there in the daily data and if I can obtain a more complete catchment dataset I might spend a long  day  trying to correct these.

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Posted  21 August 2006