Irrigation System

Yep. The point is that, if the area you are collecting from will meet your water needs over the year, then you need to store at least a quarter of that to be sure of having water at all times. If it won't meet your needs, then you have to increase the collection area, but the proportion of the annual rainfall that you need to store remains the same.

Colin Bignell

Well, the main supply pipes will be buired, and also, I will drain it all down for the winter The pump and control part is going to be installed at the lowest point anyway, so draining it down will not be a problem!

I will also make sure the pump has a quick connection on the water and electrics, so it can be removed and stored in the winter.

Sparks...

Er, but since you need to decide "if the area you are collecting from will meet your water needs over the year" before deciding you need to be able to store a quarter of it at any one time this hasn't got one very far in the exercise has it?

That is why, in practice, the area that applies is probably the area of roof you have available and why the OP might need to top up with mains water. In any case, it is only a rule of thumb.

Colin Bignell