The Prior Appropriation Doctrine has been created in order to meet the needs of western and arid states. An arid State refers to a State which has insufficient water supply or lack of rain. This Doctrine caters on a first come first serve basis, meaning whoever makes use of the water first has the superior right to the water. All appropriation laws are not similar when going from State to State, as each varies based on the demand and what the State deems as beneficial use of the water.
Expropriation is the taking away or surrendering of the permit or right an owner has to the usage of water. This usually occurs when an owner violates regulations or breaches his contract of what is stated on the permit. Overall, the Government has put its own regulations on bodies of water, but makes sure to leave room for flexibility so that the states could apply the rules according to their needs.