To determine whether stormwater capture affects downstream water rights, judges need to first define where in a water droplet’s migration the water right begins. A water right defined at the point of entry into the waterway will be regulated differently than one where the right is defined at the point where tributary rain or snowfall first hits the ground. Unfortunately, this question has not been resolved in most states, meaning that additional court cases may be needed to fully establish the legal water rights framework for stormwater capture and reuse.

In states that follow riparian use doctrine, stormwater capture falls within riparian landowner rights. Under riparian law, there is limited need for detailed volume accounting for water rights allocation. As a result, stormwater capture is unlikely to result in a legal challenge for nonriparian landowners, even if the captured water would otherwise have flowed into a stream or river [35]. 

The consequences of stormwater capture on downstream water rights are more of a concern for states that follow prior appropriation doctrine for water rights allocation. Six prior appropriation states provide specific exemptions for stormwater capture and use from water rights permitting [35]. Only one state, Colorado, has set specific limits on the amount of stormwater that homeowners may store and use on their property: 110 gallons per household [36]. Eleven other prior appropriation states have not yet set regulations regarding stormwater capture and reuse. However, New Mexico has stipulated in state guidelines that “the collection of water harvested [through stormwater capture] should not reduce the amount of runoff that would have occurred from the site in its natural, predevelopment state.” [37]

[35] National Academies of Science, Engineering, and Medicine, “Legal and Regulatory Issues,” in Using Graywater and Stormwater to Enhance Local Water Supplies: An Assessment of Risks, Costs, and Benefits, Washington, D.C.: National Academies Press, 2016.

[36] P. Cabot, C. Olson, R. Waskom and K. Rein, “Rainwater Collection in Colorado—6.707,” Colorado State University Extension, Apr. 2016. [Online]. Available: https://extension.colostate.edu/topic-areas/natural-resources/rainwater-.... [Accessed July 5, 2022].

[37] New Mexico Office of the State Engineer, “Water Use & Conservation: Policy Development,” [Online]. Available: https://www.ose.nm.gov/WUC/wuc_policy.php. [Accessed July 5, 2022].