I just put in four water tanks that take water from the garage and part house roof.
The total capacity of the tanks fill my pool and easily water the garden.
Recently, I found I was watering the garden and grass to us the water up as rain was due.
The overflow from the tanks, just goes out into the street as all rain water did prior to the tanks going in.
The tanks fill quickly and I find I'm watering grass and garden beds after it rains to use the water up. I have placed signs to say I'm using and watering with tank water..
I have not used mains water in the garden since installing the tanks and pump...
Bernie