If I were to dig down a foot or so and bury a pie tin, would this be of any benefit?
Sorry Tim but I can't make heads or tails out of such limited info. Benefit to what? For what? Can you explain exactly what you are talking about and why? What is it you are trying to accomplish?
I am trying to use it to capture water as it drains. Maybe this would allow me to use less water.
Since this is a tomato forum, I'm assuming that you have sandy soil and want to increase the water retention for your tomato plants. I can see some problems with your assumption.
First, in ground tomatoes usually have huge root systems, much deeper than "a foot or so" and definitely wider than the 9 or 10 inches of a pie pan.
Second the water capacity of a pie tin that is filled with sand (or soil) would not be significant.
Amend your soil. Lots of compost and organic matter will help with water retention. You could even work in some of those water absorbing/releasing crystals. Add more compost and organic matter. It helps sandy soils retain water and heavy soils that don't drain well to drain better.
I hope that helps.
Agree with Betsy. Consider these facts. First water doesn't drain straight down. It diffuses. So you'd likely catch less than 1/4 of the water added in the pan. Plus the pan would be full of soil so even less water would go into it.
Second you are assuming the plant roots would go straight down into the pan. They don't grow that way as Betsy said.
Third, a pie pan, even if it was full of water, is nothing to an established tomato plant.
Sorry but your plan, as you have described it so far, just won't work in any way.
If you have something else in mind then you will have to provide a great many more details. Minimal info provided tends to get minimal responses.