Rainfall history, Melbourne, Australia.

Melbourne Rainfall History

For several months through 2000 Melbourne TV viewers have been bombarded with advertisements from our water utility, Melbourne Water, to the effect that, "Melbourne is in the middle of its worst drought ever."

O.K., we are told that water storage capacity is at about 55% and rising, as it should be in Spring and if good rains do not come in next few months than summer water restrictions could come in.

All sounds reasonable but what are the facts about Melbourne rainfall.

The graph from Yan Yean on Melbourne's northern outskirts shows many periods of low rainfall that were drier than the event we are in now. The Yan Yean data comes from the Bureau of Meteorology (BoM) High Quality Rainfall Data Set For Australia Version 2, Lavery et al 1992, see full reference below. These data can be downloaded from the BoM FTP site.

To look more closely at the recent period in question, the graph below plots monthly rainfall anomalies and it is clear that from mid 1996 to end of 1998 rainfall was moderately below average From start of 1999 to end of graph at June 2000 rain has been a little over average but deficient in the Spring of 1999 when run off could have been expected.

So where does the Melbourne Water statement come from, this "...........worst drought ever." part. It might be significant that the BoM High Quality Rainfall Data above does not contain any stations from the main area of Melbournes water catchments.
The writer has seen a rainfall record from the catchment area and the trend over 90 years is for less rainfall compared to the increasing rain trend seen in Yan Yean.
That both sets of data could be right is doubtful.
Rainfall records can deteriorate for example, by the growth of trees around the gauge or construction close by. If factors like these slowly encroach over years then a drift can set in that renders long term trends incorrect.

Further progress on this problem requires more data from Melbourne Water's catchment areas.

Clearly if the moderate dry event experienced since 1996 can drop storage levels to 50%, then at current rates of expanding consumption, Melbournes water supplies will be severely restricted if a dry spell similar to those experienced in the 1890's, 1920's and 1940's were to strike again.

In order that proper planning can take place and informed decisions be made, it must be imperative that all relevant data be publicly available and various issues addressed.

Such as;

is another dam required ?,
how could it be built against Green and NIMBY opposition ?,
without a new dam supplies could be eked out by better use of waste water, less wasting of water, water restrictions, home tanks,
clearly there are positives and negatives to the above measures and how cost effective are they ?
if Melbourne consumers are eventually going to be put on semi-permanent water restriction mode, will they enjoy knowing that "environmental flows" are released from the Thompson Dam ?.

how to balance all these factors.

References
Lavery, B.M., Kariko, A.P., and Nicholls, N., 1992
A High-quality historical rainfall data set for Australia,
Aust. Met. Mag., 40, pp33-39

Nicholls, N. And Lavery, B.M., 1992 Australian Rainfall Trends
in the Twentieth Century, Int. J. Climatology, 12 153-163

You are reading it first here.

Posted 8, September, 2000