1 cubic foot of water weighs 62.4 lbs.
1 cubic foot equals approximately 7.5 gallons. (Glover says 7.48052)
A gallon of water weighs 8.3 lbs.
An inch of rain on a square foot is 1/12 of a cubic foot which equals 0.62 gallons.
An acre-inch is 0.62 X 43560 sq ft/acre = 27,007 gallons.
I knew a guy who was panicking about Y2k back in 1999. He bought a whole bunch of MRE food and 8 or 10 barrels, 50 gallons each, to store water. He spent thousands of dollars.
He stopped by one day and asked me what I was doing to prepare for water. I pointed to a 20 X 25 tarp. He looked confused. I showed him how I could lay out the tarp on our sloped lawn so that a rain would wash into a natural trough that could easily drain into buckets. We live in the Pacific Northwest, where you can count on a 1/2 inch rain easily in a month, and more in the winter.
1/2 inch on 500 square feet yields 155 gallons. I knew from sailing provisioning that a gallon per day per person is plenty if you are not exerting hard. Even if you only collected half of this in a month, you could keep two people going with minimal effort. If need be, I could dump the water in our extra bathtub for storage.
And the tarp was cheap.
I wasn't really worried even about the tarp. There were fresh water streams nearby. A bit of filtration and maybe boiling would be sufficient for water in an emergency.
Y2k was a bust, of course. I understand that there are bargains on those water barrels.
The tarp principle is useful for arid climates. 19 mil pond liner goes for around $0.50 a square foot. I saw a 1000 square feet of visqueen for around $100. I think of people in desert climates without decent supplies of water. But a region that gets a downpour once or twice a year could collect a lot of water inexpensively. I once did it experimentally on a small scale (300 square feet) in an area that received 9 inches of precipitation per year. After one good rainstorm I collected 120 gallons. Multiply that by 10 or 20 and you can maintain a family.