j wrote:
I've acquired a 12' x 18' epdm pond liner. I can fit about 2/3 of
that in. It's mostly shady where it will go.

I've never had a pond of any size. Water is getting expensive here
and I wonder if I can fill this with runoff and use it to water my
modest garden. We get about 4"/month. With somewhere between 500
(easy) and 1000 (harder) SF (horizontal) of roof to help feed it.

Any ideas on depths and what to put in it? Caveats? It looks to me
that it will be marginal being able to withdraw water and it will be
hard enough to fill if it is deeper than a foot, but I have no
experience to back that up.

Jeff

To store a decent amount of water you will need a big hole. By 'big' I am
thinking at least 10 kl (12 cubic yards) or bigger. If you are thinking of
only a fraction of that you will be going to much trouble for little return.
Do some sums and work out how much water it will take to give your garden
beds 1" of rain. You seem to have picked a spot already so how deep could
the hole be there? What's the soil like? You are not going to dig that by
hand on Sunday afternnoon. Either you will be digging for a week or hiring
equipment.

For easy filling the hole needs to be close to and below the collection
roof, plastic drainpipe about 90mm (3 1/2 in) will get there. It needs to
have an overflow to a convenient drain of some kind. A pump and piping will
reticulate it to your garden. If you were thinking of watering with a
bucket you will get very fit indeed. Then there is the possibility of
children, dogs, drunks drowning in it or the pond failing local pool
ordinances if it is unfenced. The liner is only part of the cost. Plan well
before you dig.

David