Rain barrel water harvesting system at our house. You can see that the barrels are propped up on concrete blocks. The barrel to the left is on two sets of blocks, while the barrel to the right is on 1. This allows the first barrel to overflow into the second when it fills up. Each barrel holds 50 gallons and they both fill up after a hard rain of about 20 minutes or so. We rarely use our outdoor faucets due to the fact that we have a steady supply of rainwater.
One other point...propping the barrels on concrete blocks makes it each to set the watering cans underneath them.