The itsy bitsy spider went up the water spout...
Before you start planning how to store your 28 thousand gallons of water, remember it is not just water going through your downspout. Leaves, acorns, seeds, twigs, and other debris that makes it on to your roof and soon into your gutter. You could just let them collect in the bottom of your rain barrels, but these items will likely clog overflow outlet or water spigot.
There are some creative ways to filter these items out. I find that the Spring's blossoms and Autumn's leaves are main seasons I see items in my filter that need to be cleaned out. I am going to assume you are not going to cut down and remove all the trees that are 500 ft near your house. However every year when i am racking leaves, I consider it...
You could install gutter guards (I like specs of Master Shield ). They are meant to keep debris out of your gutters (and thus your downspouts), but they cost over $4K (last time I got an estimate in 2010). Your local home improvement stores have DIY options, but I don't recommend them. These DIY options to seem not to work when you look at real world tests (see this website). Most DIY product show that your gutters need to be lowered to get a better 'debris wash away' angle. Also the stems of leaves, pin needles, small twigs, and helicopter seeds still get stuck in the filtering mesh. See the resource section at the bottom to links that explain this better.
Some filters are nothing more then putting a mesh screen on top of the rain barrel and redirecting the downspout to this mesh. I personally don't like them because they allow mosquitoes to get into the rain barrel and breed. These seem to be the most popular options for DIY rain barrels.
An other option are filters that attach to the downspouts called 'rain barrel diverters' or 'downspout filters'. There are many different types. I have 4 trees that loom over my roof, so lots of items make it into my gutter. Most of these downspout filters options look as if they would clog easily. However I like the idea of how Rain Reserve operate. They cost about $100 (but I'm really tempted to buy one).
I because of the amount of debris that gets into my gutter, I chose to make one of my barrels into a filter. It is a basically a trashcan with a screen mesh between the lid and the barrel. This allows high amount of debris to be filtered out of the water before I need to remove the lid and scoop it out (which is great for your compost). I find that the Spring's blossoms and Autumn's leaves are main items in my filter that need to be cleaned out.
Gutter Guard Reviews: