Michael at the Groovy Green blog writes:

Colorado state law mandates that any water falling from the air is not yours. In fact, according to their site, its already been “legally allocated” — so, you don’t actually have any rights when it comes to using precipitation that falls on your property.

So, since rain is illegal to capture, that makes rain barrels obsolete. The Colorado Springs Gazette adds this gem:

The rain barrel is the bong of the Colorado garden. It’s legal to sell one. It’s legal to own one. It’s just not legal to use it for its intended purpose.