It is a bit difficult to say how much rainwater we collect as once it has filled the underground tank any more simply goes down the overflow. And of course we only use it when we are here which is to say April to September. I think the tank holds 3000 litres, so not huge, and we have had to drop the pump down a bit to get it to pump since we first put it in, but it still isn't on the bottom, but we do only use it to water the polytunnel and the raised beds outside plus a very few other pots of plants.
Geologically our area is interesting. To the north of us is a region called the "Sidobre" which is massively granite, the huge local business is quarrying it and turning it into monumental masonry as it isn't much cop for anything else as it is basically just a grey colour, although one town decided to use it to pave the pavements with. But it does have a big lot of tourist attractions with huge rocks balanced on others, waterfalls, lakes with rocks in that look like the backs of whales etc. Donkeys' years ago some volcano somewhere nearby must have bust a gut and chucked thousands of tons of lava etc up in the air which landed simply all over the place.
But on a geological map of our little local area, around our hamlet, there are a bunch of circles of which our place is at the centre. Which accounts for why we and our neighbours are constantly coming across lumps of granite on our land varying in size from nearly that of a house to smaller than a golf ball. We had to be very careful where we picked to dig to put the house up. One of our neighbours' builders discovered a massive granite block just inches below the level of his basement floor, if it had been any higher they would have had to have had a massive rethink. We have two blocks like that in two different places on our land.
So for some reason the ground possesses a lot of underground springs, But not all houses' land is the same. The neighbour just mentioned has as much land as us, i.e. 6000 sq metres and he has no springs at all, although he is only half a mile from us, as he is a very keen gardener he is dead jealous.
Funny stuff water.