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The simplest rainwater harvest method. This requires the following items. Things you absolutely must have, and for minimal cost:
A plastic tarp
4 poles (you could use 4 strong branches)
a clean, closeable collection vessel.
Anyone can make this. It can be setup as needed, though leaving the poles up and using waterproof rope like strong cheap nylon is the easiest method. Keep the plastic tarp pristine. It's easy to use a mild bleach solution to do this. You'll be surprised how much rain you can catch based upon the length of tarp, but of course, the structural integrity due to weight will determine how far you should stretch it.
The whole point is to funnel the falling rain into the vessel. You should have multiple vessels as water is heavy. 1 gallon = 8.35 lbs. You'd hate to spill that precious water. Ideally you have several people switching out the vessel, taking turns.
This method is only for drinking water. For all other water, you're going to catch the rain from your roof.
Do a weather check on typical rain levels in you area. This will serve as a guide to how much you can reasonably catch. Realize that any method will be subject to evaporation. You must catch it, and store it, in such a way to minimize evaporation. Obviously, a lot can easily be lost, and you've got to do creative things to minimize the effect of sun on the container.
Catching rainwater for your crops is simple. You're going to modify your downspouts by simply cutting them to the proper length, insuring that it is rigidly held in place, and then placing a vessel underneath the downspout to catch that water.
1 inch of rain on a 2,000 square foot roof = 1250 gallons. You'll never catch all of it without careful planning. This is not drinking water, not without a lot of purification. It will however be great for watering your garden.
Don't be fooled into thinking you can simply use a clean metal garbage can as a means of catching the water. There will be too much water, and it will be too heavy. A standard 30 gallon galvanized metal weighs around 250 lbs. Try dragging that around without help and then putting another in place, and so forth.
You'll be rigging up some way to open and close a valve to multiple stationary barrels or vessels, most likely a pvc plastic system of pipes to them.