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.

The meaning of and the limitation on the use of reasonable use theory was deliberated in Scott v. Slaughter, 237 Ark. 394, 397 (Ark. 1963). Per Scott,

How can we track the presence of a water resource that we can’t see? Groundwater—water that is contained within fractured rocks or soil pores beneath the ground surface—is by its nature invisible. Yet it comprises over 26 percent of U.S. water withdrawals. That percentage is dramatically higher in arid regions like California, where groundwater is an important source for irrigated agriculture. In addition, groundwater from shallow private wells is a relatively small but important source of drinking water for nearly 43 million people who are not connected to a public drinking water distribution system.