NASA's Goddard Institute for Space Studies in New York puts on the internet
the following map of global rainfall anomalies for the last century.
The map makes it clear that in the lottery of constantly changing climate,
many regions have had rainfall declines equivalent to SW WA or greater
and conversely many regions have enjoyed increasing rain, for example much
of Eastern Australia. Much of the globe seems to have had slightly
increased rain over the last 100 years.
Bureau of Meteorology High Quality Rain Data
The Bureau of Meteorology (BoM) collects and archives Australian rain data.
In 1992 BoM staff published a survey of Australian rainfall data, Nicholls, N. And Lavery, B.M., 1992 Australian Rainfall Trends in the Twentieth Century, Int. J. Climatology, 12 153-163, and 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.
This filtered out 191 stations that were "high quality" over the history of the data thus suitable to generate a long term trend on the basis of less data gaps, statistical testing and station histories. A few years later this list was increased to 379 and these data updated through 1995 are available as monthly totals for download on the BoM ftp site.
A point of relevance to any discussion re Perth's rainfall history is that none of these high quality data are from the greater Perth region or supply dam catchment areas. This leads to the point that to be in the best position to make any scientific statements about Perth rainfall history one would ideally first undertake a new statistical analysis of the regions data.
No report on this subject is apparent looking over The Water Corporation
In recent media advertisements headed "Dam Update" The Water Corporation relates dam inflow data to Perth rainfall for this year when it is obvious that catchment area rain data is better related to stream inflow.
GHCN Global Rainfall Data
Another survey filtering out a high quality rainfall data set for Australia is that carried out globally by United States climate groups National Oceanic and Atmospheric Administration (NOAA) and National Climatic Data Center (NCDC) as part of their Global Historical Climate Network survey in the mid 1990's. The GHCN survey has more stations included than the 379 of the BoM so presumably used less demanding statistical testing. These data can be downloaded from their site.
General points on rainfall data
Rain data can be difficult to analyse statistically because it tends to be very "noisy" with large variations between years and typically few years are actually near average. Also neighbouring stations can vary significantly which just adds to the problem if you are trying to filter out non-climatic trends or drifts.
Rainfall readings can be affected by structures or vegetation changing near the rain guage and if these changes are progressive over years then drifts can be produced in data and because the data is noisy, the drifts are not easy to detect.
Sudden changes like a new building or clearing nearby trees can produce a sudden jump in data.
Composite graphic of Perth and catchment region annual
rainfall in mm
The following graphic gives a fair impression of the Perth region rainfall history with reservations that the Perth Regional Office was moved several times. Note that the rainfall scale starts at 500 mm. More updated catchment area data is being sought and will be posted when available.
Note that Dwellingup and Mundaring data are more "noisy" than Perth so the low rain in 2001 for Dwellingup is on scale with past years, not quite as low as 1940.
Interesting to note how pre-1900 Perth rain went through periods comparable with recent times so perhaps the "climate change" so much chattered about could be seen as a return to an earlier climate. It will be interesting to see if the pre-1900 Perth data stands up in other regional comparisons.
We have to face the facts that;
The Government is claiming we are in the "..worst drought since 1914..". This is another example of Government misinformation in this whole "water crisis". It is utter nonsense, 2001 was a moderately dry year in the Perth region, in fact looking at Perth rainfall records 2001 was only the 15th dryest year.
Click for graphic comparing the 1914 drought with 2001, you can see the 1914 negative rain anomaly was much deeper than in 2001.
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