The doctrine of prior appropriation means that:

The doctrine of prior appropriation is a principle used primarily in the western United States for allocating water rights. It asserts that water rights are granted based on the principle of "first in time, first in right." This means that the state has the authority to regulate and allocate water usage rights to individuals and entities. Under this doctrine, the rights to use water are typically granted to those who are first to put the water to beneficial use, regardless of the proximity of their land to the water source.

This system contrasts with the doctrine of riparian rights, where landowners along the water source have rights to the water based on their proximity to it. By emphasizing that the state controls and can distribute these rights, the doctrine of prior appropriation ensures that water resources are managed in a way that supports overall resource availability and sustainability. This makes the control and distribution of water usage rights a governmental function, adhering to the needs of the population and the environment.