Colorado rain water laws....
"The use of water in this state and other western states is governed by what is known as the prior appropriation doctrine. This system of water allocation controls who uses how much water, the types of uses allowed, and when those waters can be used. A simplified way to explain this system is often referred to as the priority system or first in time, first in right.
An appropriation is made when an individual physically takes water from a stream or well (when legally available) and puts that water to beneficial use. The first person to appropriate water and apply that water to use has the first right to that water within a particular stream system. This person, after receiving a court decree verifying their priority status, then becomes the senior water right holder and that water right must be satisfied before any other water rights are filled. In Colorado, the state engineer and director of the Colorado Division of Water Resources, has the statutory obligation to protect all vested water rights. The process of allocating water to various water users is traditionally referred to as water rights administration, and is the responsibility of the Division of Water Resources.
Of course, the appropriation system is much more complicated than described above. Some priorities on major stream systems in Colorado date back to the 1850s and most of the stream systems have been over-appropriated, meaning that at some or all times of the year, there will not be enough water in that stream system to satisfy all vested water rights. Practically speaking, this means that in most river drainages, a person cannot divert rainwater and put it to a beneficial use without a plan for augmentation that replaces the stream depletions associated with that diversion.
Senate Bill 09-080, which was passed and signed during the 2009 legislative session, allows limited collection and use of precipitation for Colorado landowners. The changes apply only to residential properties that are supplied by a well (or could qualify for a well permit). Landowners will complete a permit application, supplied by the Division of Water Resources, that will provide notice of their intent to collect precipitation and a description of how they intend to do it.
To qualify for a permit, you must meet a minimum of the following criteria:
* The property on which the collection takes place is residential property.
* The landowner uses a well, or is legally entitled to a well for the water supply.
* The well is permitted for domestic uses according to Section 37-92-602 or Section 37-90-105, C.R.S..
* There is no water supply available in the area from a municipality or water district.
* The rainwater is collected only from the roof.
* The water is used only for those uses that are allowed by, and identified on, the well permit.
In addition, HB 09-1129, is also a new law that allows developers to apply for approval to be one of ten statewide pilot projects that harvest rainwater and put it to beneficial, but non-essential, use in the subdivision. These projects may only operate according to an engineered plan, submitted to the state engineer for approval and eventually, to the water court. This new law does not apply at all to individual homeowners. "
http://www.ext.colostate.edu/pubs/natres/06702.html