Is Rainwater a Public Good?

Maybe not in Colorado.

While laws about rainwater collection are often murky, Colorado's are quite clear: Homeowners do not own the rain that falls on their property. The Rocky Mountain state uses a convoluted mix of first-come, first-serve water rights, some of which date back to the 1850s, and riparian rights that belong to the owners of land lying adjacent to water. A single person catching rain wouldn't make a difference to water rights holders, according to Brian Werner of the Northern Colorado Water Conservancy District. But if everyone in Denver captured rain, he says, that would upset the state's 150-year-old water-allocation system. The Colorado Department of Natural Resources estimates that 86 percent of water deliveries go to agriculture, which is already stressed by dwindling supplies. And because 19 states and Mexico draw water from rivers that originate in the Colorado Rockies, backyard water harvesting can have widespread implications (of course, the same goes for water that comes from the tap in these regions).

Although it is explicitly stated that one person collecting rainwater probably would not make much of a difference, the implication here is that it is not technically a fully non-excludable, non-rival good.