Calculating Your Average Rainfall & Ideal Tank Size

There are many factors you need to consider when choosing your tank size. These factors include how much water you typically use around your property, the size of your roof and whether you will be relying solely on rainwater for your water needs.

But before you worry about any of that, you need to know your average rainfall, as based on historic data.

National or local weather services are an excellent source of historical rainfall data for your area. In Australia, the Bureau of Meteorology offers this information online and free of charge. When considering historical rainfall data, try and look at data for the last 100 years or the longest period available. When looking at this information, consider monthly and yearly data as well as overall averages. This includes considering the longest periods without rainfall.

Looking at the data in this way is important for two reasons. First, some areas get a great deal of their yearly rainfall at a particular time of year – such as locations with a “wet season”. In these places, a tank or tanks with a larger storage capacity would be necessary to harvest more rainwater during these times and store it for use during times without rainfall.

Secondly, averages can conceal periods of drought or even just year-to-year variations. For example, one town had an average annual rainfall of 1,562mm over 20 years, but this ranged from 1,089mm to 2,892mm in the individual years during this time. In such areas, you may want to consider using a tank or tanks that can handle the larger volume, not the average.

As a guide to collection capacity, consider that each 1mm of rain = 1 Litre (L) of water per square metre (m2) of roof area. Balancing the volume of rainwater you need with the volume you can harvest will inform your decisions regarding tank numbers and size. You should also design your Rain Harvesting system with appropriate products to minimise water loss.

Click here to learning more about the 12 Steps of Rain Harvesting.