U2.05 Gilliom 2019 ReNUWIt Annual Meeting Poster

In the State of Colorado, a strict Prior Appropriation water rights system prohibits precipitation harvesting with few exceptions. In the Colorado Revised Statutes, 100% of precipitation harvested must be replaced to the system from a different source. This precludes the possibility of stormwater contributing to renewable supply. A 2009 bill authorized up to ten pilot projects to try centralized stormwater harvesting for outdoor use in new residential developments. The pilot projects are authorized to harvest “new” stormwater runoff that was previously consumed by vegetation or evaporation in the pre-development natural catchment. In Colorado, a water rights holder is entitled only to historic conditions, so the “new” stormwater can be harvested without injuring downstream rights.

To inform temporary water rights for pilot projects, statute describes “Allowable Harvest Factors” to estimate the allowable harvest volume for a given precipitation event in a given location in Colorado. The factors estimate historic conditions for a temporary Substitute Water Supply Plan, which allows operation without replacement during the probation period. To apply for a permanent decreed water right, the pilot project must complete modeling for site-specific Allowable Harvest Factors. The State has yet to approve a set of factors for use across the state; this work is conducted in cooperation with the State of Colorado to establish said factors on a rigorous technical basis that will suffice for temporary water rights.