If you live in Colorado, you should look around to make sure no one is watching before you stick your tongue out to catch those first drops of spring rain. Rainwater in Colorado isn't yours for the tasting, even water that falls on your property. That makes rain barrels, the large containers conservationists and gardeners use to collect precipitation for use in watering plants, illegal. And don't think about being ecofriendly by using your excess laundry water to irrigate your garden; secondary use of water is illegal too.
Like many Western states, Colorado employs a complicated system of water use known as prior allocation, which severs water rights from other property rights. The system preserves an 80-year-old compact Colorado signed with other Western states (as well as a separate federal pact with Mexico) divvying up runoff from the Colorado River. It means you can buy a parcel of land in Colorado, but the right to any precipitation that falls on that land likely belongs to someone two houses over, two counties over, or even in another state. It might also belong to a state or local government, but it probably doesn't belong to you.
Under Colorado law, then, collecting rainwater or reusing "gray water" from bathtubs or washing machines violates the rights of someone who may not see that water for months. Do you still want to use rainwater in your garden? A fact sheet published by Colorado State University advises Coloradans wishing to do so to ask one of the state's "water courts" for a ruling about whether such a use might violate someone else's rights.
Colorado Springs Gazette gardening columnist David Phillips noted last year that the law makes little sense from a conservation standpoint. In a state known for torrential summer downpours, Phillips writes, the "flood caused by water running off a few thousand acres of roofs, roads and parking lots erodes downstream ranches" and "undercuts city sewer pipes." Capturing a bit of that runoff, Phillips writes, might save some money for cities like Colorado Springs, which recently undertook a $295 million storm drain project.