A good year to talk about this (except if you live in Texas). Does anyone use a cistern or water tank to collect surface water? We are considering catching runoff from the roofs for garden watering.
Observed cisterns:
In Southeast Alaska, A very high rainfall area, many people use cisterns for their water supply. Most were large tanks of 20 or so feet in diameter and of a height to allow roof drainage into them.
Most of this web page is devoted to the Bush's septic systems at their Crawford (Texas) ranch, but there is a mention of their rainwater collection system with a 40,000 gallon underground cistern.
I understand that the roof runoff is not collected using gutters, but via french drains surrounding the house.
We divert rainwater from our barn's roof into two 1100 gallon tanks sitting on either side of the barn, and fill another 550 gallon tank by pumping water into it from one of the larger tanks. A general rule of thumb is that for every inch of rain you can collect up to 600 gallons of water per 1000 square feet of catchment area, depending, of course, upon how efficient your system is. Ours is quite simple: gutters on two sides of the barn, draining directly into the two large tanks. Someday, time and money permitting, I'd like to direct the overflow into additonal tanks.