What are the steps to calculate how much rainwater I should collect for my garden?

Let's reason in liters/square meter of plants, to make it generic.

Let's use this for the statistics of rainfall: https://www.weather2visit.com/europe/netherlands/rotterdam.htm

Statistics of course cannot take into account exception, like the current month, when we had about 1 mm rain in 18 days.

The plant type is also important, but I don't how how to handle that.

So, how big should my barrel be per square meter of plants?

  • Enough to fill your storage system ; get as large a storage system as conditions permit. Unless you are talking about some potted plants, you can't have too much. – blacksmith37 Jul 18 '18 at 15:34

This is an impossible calculation; rainfall varies, and any statistics available are merely average figures. Equally, calculating the amount of water your garden and its plants are likely to need in any given year is just as impossible - it depends on the weather, the size and type of plants you have, and other factors, such as how recently they've been planted, whether they fruit and so on. Usually, people just set up a rain collection system where they can to collect what they can, by attaching a diverter to a downpipe from the guttering on the roof.

You are also (or should be) restricted in regard to water collection by possible health risks; your water butt or barrel is best restricted to no more than 100 gallons capacity, and out of sunlight to help stop the water becoming hot during hot weather. The risks include legionella, campylobacter, salmonella (usually from fecal contamination off the roof) and some other possible infection and contaminant risks. Storing too much water in one container, which if its very large will be difficult to clean regularly, is not a good idea. There may or may not be guidance and/or restrictions in your own country which apply to collection of rainwater, so it might be helpful to check with whatever Environmental body you have there to see what their recommendations are.

An overview and some information in this PDF document which might be helpful https://www.google.co.uk/url?sa=t&source=web&rct=j&url=http://www.ccnse.ca/sites/default/files/BCCDC-Rainwater_Harvesting_Oct_2011.pdf&ved=2ahUKEwjgoOeDtajcAhWMKMAKHSDsBTcQFjAJegQIBBAB&usg=AOvVaw3C7G3AJoJ11xSjH37RbDGe

  • Good point about contaminations, I didn't think about it. – FarO Jul 18 '18 at 12:32

In practice you're much more likely to be limited by storage space at a domestic property, unless you have an underground pumped system, as space close to the house is at a premium. The refill rate is also very variable. In the back garden I have a 100 l water butt collecting from a fairly large area of roof facing the prevailing wind. I empty and refill that several times per summer. When it falls below the level of the tap I take it away and tip out the rest by my shrubs, then allow it to dry.

At the front I have a larger one, but it collects from a smaller area on the downwind side of the house. Once below about 3/4 full it's unlikely to refill before winter.

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