This has me thinking about water for my garden in an emergency. We live in a desert and don't get a lot of rain through the summer. In order to be productive, my trees, berries and seeds would definitely need water even if it wasn't available. I probably wouldn't want to use my clean drinking water supplies. They would be depleted too rapidly.
Collecting rain water would be a great way to ensure that you have some water for your garden. I've seen some great ideas and different set-ups as I've browsed the web. I've compiled a few here to give you some ideas.
1) A large container such as a trash can or water barrel to collect the rain.
2) A lid or top. You need to be able to keep kids out. A lid (or at least a screen) will also keep mosquitoes from multiplying.
3) An overflow function. You don't want the water backing up into your downspout, pouring into your window wells or gathering around your foundation. You might think this isn't necessary, but Sunset Magazine states that one inch of rain water on a 1000 square foot roof will yield 600 gallons. In a good rain storm, you might be needing that overflow after only a few minutes. A good overflow system will handle a large volume of water and move it far from your foundation.
4) Water accessibility method. This can be a fancy hose set-up at the base or it can be just as simple as a lid that you lift to scoop out the water.
You probably don't want to locate your barrel in the front of your home where it might be an eye-sore. A location in your back or side yard that is also close to your garden would be ideal. Two or more barrels can be connected for additional water storage.
Here are some pictures and ideas:
This is a pretty simple setup from Workbench Magazine. The hose on the side is for overflow. You could make it longer if needed. Instructions can be found here: https://docs.google.com/file/d/0B2gdNar_rMtAMjQxNzUxYmItYjU1ZC00YzYzLTgwNjMtZjg1ODA2MjYxMzdk/edit?pli=1
This is a fairly elaborate setup from Family Handy Man. I love the use of two barrels and the wooden base. I also really like the overflow mechanism used. Instructions Link: http://www.familyhandyman.com/DIY-Projects/Green-Home/Saving-Water/how-to-build-a-rain-barrel
You can buy a ready-to-go commercially prepared rain barrel. Just search for "rain barrel" and you will see many options. Some are even beautiful like this 65 gallon barrel above which is about $150 on Overstock. Amazon is also a good source. They have kits and other supplies as well.
Here are some basic instructions from HGTV. Actual barrel building starts at 1:30 in the segment.
This is also a fantastic how-to video from Kansas State.
I'd love to hear how you've made this work at your home!
Update: Here is how Troy made his own 10 barrel rain collection system: http://iprepared.blogspot.com/2013/11/a-working-rain-collection-system.html