Perth’s average rainfall is around 790 mm a year. The pattern is for high rainfall from May to August, almost 70% of the year’s total rain in four months.
For your information, here is the official Bureau of Meteorology map showing average annual rainfall (in mm) of Perth and areas throughout Western Australia, as well as a chart breaking down Perth’s average rainfall by month.
How To Harvest Your Rainwater (and calculate your savings)
Throughout the country, water truly is the most precious resource we have, and it’s for that reason that so many clever Australians are setting up simple rainwater harvesting systems at their residences.
In these systems, roof guttering collects water runoff and directs it to water tanks. From that point, the stored water can be used to clean, irrigate the garden, wash clothes, or even filtered for potable household use.
Just how much water can you collect from your roof?
Here’s how to calculate it:
Number of square metres of roof X the number of millimetres of rainfall = litres harvested
If you have 150 square metres of roof, and the average Perth rainfall is 790 mm per year, then you could collect over 110,000 litres a year (150 x 790). And just a 10 mm rainfall would yield 1500 litres! In both cases, it’s safest to subtract 10% to allow for absorbtion, evaporation etc.