Perth uses approximately 300GL of water PA.
The Govt. is constructing a ~$400Mill seawater desalination plant to produce 45GL PA.
In 2005 May to October (the runoff season) Perth dam catchments which total 3500 squ. kms had circa 958mm of rain. This equates to about 3353GL water falling free into catchments of which approx. 115GL flowed into dams, meaning that catchment efficiency (yield) was 3.4%.
My graphic at the Perth Water Users web site shows that up until the mid 1990′s catchment efficiency was 5.5 to 6%. Let’s just assume for a minute that our Governments had been sensible and maintained catchment management work, keeping yields this year at say 5.7%. An increase of 2.3% (5.7-3.4) in this years yield would have put an extra 75+ GL (2.3% of 3353 GL) into our dams, which is 160% of the projected production from the Kwinana Desalination Plant (KDP), where costs have already blown out to over $387Million.