Humboldt University has a short *.pdf introductory on rain catchment systems. These aren’t that hard to design and build, they cover a few applications.
Another source of info is here.
The use of metal roofing is best, and the storage tanks could be buried, especially in cold country to prevent freezing. A series of simple filtration systems to keep out debris is recommended and probably necessary for almost all locations.
It’s possible in most regions to collect enough rainwater to simply forgo a well or utility source. 1″ of rain on 1000 sq. ft. of surface collection area equals 550 gallons of water. Even in low rainfall areas of 18″ – 24″ of rain, you could conceivably collect 10,000 – 13,000 gallons of water.
An increase in collection area and storage would yield as much water as you could use. Smaller collections systems could be built off of sheds, barns, or any roof surface. Even sloped ground surfaces with underground storage tanks could work.
A thousand square feet is a 100 x 100 ft area, most roof systems on residential houses can provide this. Metal roofs are best, but even shingled roofs (wood or tar) will also work quite well. Screens and basic filtration systems are added to prevent debris from clogging the rain gutters or storage tanks.
Water should be free — nobody should have to pay for water. What isn’t free is making sure the water you use is clean, safe and available. You can assemble a very basic system from components found at garage sales, or you can build something elaborate with more expense.
I’ve been collecting cisterns and rain barrels (used), having spent only $100 or so for these things so far. I still need rain gutters to direct the water off the roof (at least to the corners), and I plan to bury my cistern. But I should be able to build the entire thing for a $1000 or so, if I do all the work myself. I’m not sure yet if I’ll use a polyethelene liner or something else for the cistern “tank”, but I’ll come up with something.
I already have a 3500 gallon cistern in use for the well, so my rain catchment will probably be used for the soon-to-be-built greenhouse and added garden space. My roof surface area is fairly substantial, about 1820 sq. ft., which means with our average rainfall here, I could easily capture enough rainwater to meet daily needs for the household and the garden / greenhouse (over 50,000 gallons).
Climate change is making the need for water conservation and catchment systems more and more essential. Learning how (easy) this can be can make a huge difference in where people can manage to live.