Something I have been mulling over in my head for awhile is buried milk crates (or something milk crate like). Dig a hole the depth of a milk crate and open it up to the width and lenght of your desired resevoir. Line the bottom and sides of the hole with black plastic, or rubber or something like that. Insert the milk crates upside down into the hole and cover with a water permeable material - like a weed block fabric or something like that, cheesecloth, you get the picture. Cover the whole thing up with soil except for enough of an area to scoop or siphon the water out, kinda fuzzy on how to get the water back out - was thinking of growing a plant in the resevoir but I think it would drown.
Anyways, what happens is every time it rains, the rain water soaks down into the milk crates and gets trapped above the plastic barrier. A little bit of math here: 231 cubic inches is one gallon of water. When it rains, 1 inch of rainfall is 1 cubic inch, so in this setup, 231 square inches of area will trap 1 gallon of water in the milk crates. Given a 1 foot square milk crate, each crate will yield .62 gallons per each inch of rain. With 9 milk crates (3x3 grid) assuming our 12 inch crates will give us (36*36) 1296 square inches of area. 1296 sq in divided by 231 gives a little more than 5.6. So for each inch of rainfall, 9 milkcrates will capture 5.6 gallons of water. To fully sate this setup will require 12 inches of water (again assuming 12 inch square crates) at which point the resevoir would have amassed 67.3 gallons of water. And of course a larger array of crates will trap a proportionally larger amount of water.
Not really sure of the practicality of this setup, but it keeps creeping back into my head. If there were some way to keep the roots from drowning or some way to aerate the water I think this would be just a peachy way to grow a maintenance free plant.
kthanksbye