Open Letter to Sue Murphy - CEO Perth Water Corporation Feb 2018 
Dear Ms Murphy,
Several statements you make on page 7 of your 2017 Annual Report are downright misleading, cherry picking or convey a biased impression of rainfall.

[1] You say - "Our climate gets drier and drier." - Well Perth Catchments rain looks fairly consistent and useful over 40 odd years.The 5 year average is higher now than it was in the late 1970's.

And Perth Catchments annual rainfall 1,110mm  is wetter than Townsville 1,072mm (1981-2010 30 year averages)
Perth is way wetter than  Melbourne - annual rain 725mm compared to 603mm (1981-2010 30 year averages)
Adding  2 long term rain charts for later discussion.

and

Interesting that Mundaring shows less high rain years from the end of WWI to 1970's compared to Perth.
[2]   You say in your para 2  Ms Murphy  "After reasonable winter rains in 2016 – still well below average..."  You are obviously including the high rain years from WWI to  ~1970 in constructing your average.  I wonder  what value there is in wistfully pining after wet years that are now well and truly historic - not happening now.   That is why in my top chart I refer to the post 1975 data which one could argue is the rain regime we are in.  There is justification for treating the mid-1970's as a step point in rain trends. Much international climate research refers to the 1970's Pacific climate shift see this 1992 paper by Richard Kerr which draws attention to the PDO (Pacific Decadal Oscillation).   I think it is likely Ms Murphy that the cause for Perth and SW WA rain suddenly weakening around the mid-1970's  lies with poorly understood  oceanic and circulation factors related to the 1970's Pacific climate shift.
At this point it is interesting to take a look at  the rain history for Australia and various Australian regions sourced  from the BoM. The link takes you to my page where you can quickly check the disparate rain trends for the various Australian States and regions - note that most show an increasing rain trend.  A change around the 1970's is often seen - usually positive - and NSW is interesting with a big increase in rain ~1950. Another point must be made here Ms Murphy - that these BoM charts demonstrating a wide variety of rain trends here and there around Australia means that it is  invalid for you or the WA Govt Water Departments  and WaterCrats to view the post WWI to 1970 high rain years around Perth as "normal". They could just as likely be more correctly viewed as "high rain years" and "normal rain" might be more like we are seeing post 1975 and pre-WWI.

[3]  You say "June 2017 was one of the hottest and driest on record for the South West of Western Australia".  I am sorry but that is blatant cherry picking Ms Murphy. Other months in your July 2016 to June 2017 Annual Report period were cooler than average which you could have easily mentioned to be more balanced. In fact the whole 12 months has a mean temperature anomaly of minus 0.15°C. A point your readers would never have realised.  Very misleading Ms Murphy.!! You should withdraw and pulp your 2017 Annual Report and write a more balanced CEO's Year in Review.

The 2018 annual report from WaterCorp will have to fudge over the current cool summmer in Perth because that would then make two cool summers in a row - shock horror. 
Perth weather: Our mild summer breaks 25-year record 8Feb18
will carry on later -  Oops - I see an update,  they must be busy at the Perth BoM - Perth weather hits record number of cool summer days but a scorcher is coming 14Feb18 -  "Perth has experienced its longest run of summer days below 35 degrees in more than three decades."
But praise be to the prophet - at least a scorcher is coming.