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