Updated: Jul 25
Part IV provides the background on water law in the United States (US) and more specifically in Arizona (AZ).
Many common water resource terms will be explained but you can go to the Water Lingo Tab for more definitions and information. Notes  containing references and supplemental information are provided at the end of the blog.
Now that we have the background on water resources, it is time to review who has the rights to the water. Water rights are complicated and defined by whether its surface water or groundwater and by location (state/federal).
Surface Water Law
There are two basic surface water rights systems in the US; (1) riparian and (2) prior appropriation. 
Riparian Water Rights
Riparian water rights are surface water rights inured to landowners that own property that touch a surface water, i.e. stream, river, etc.
Riparian water rights are subject to the reasonable use of the water. Reasonable use means that water user cannot interfere with the reasonable use of a downstream riparian landowner.
Riparian water rights are usually regulated via permitting systems by the states. Riparian water rights have also been termed “eastern water law” because most states in the east and midwest utilize the riparian system.
Prior appropriation water rights are determined by a priority system that gives the highest priority to the first user (senior appropriator) of the surface water. Prior appropriation has also been termed, "first in time, first in right".
Prior appropriation rights are subject to the beneficial use of the water. Beneficial use means that water must be utilized for municipal and industrial (M & I) or agricultural uses. Junior appropriators can utilize the same surface water source but cannot impinge on the senior appropriator's rights.
Prior appropriation water rights are usually regulated via permitting systems by the states. Prior appropriation water rights have also been termed “western water law” because 9 western states utilize the prior appropriation system.
Arizona is a Prior Appropriation state.
Groundwater law gets a bit more complex. There are at least four groundwater rights systems in the US, (1) absolute dominion, (2) correlative rights, (3) prior appropriation and (4) reasonable use. We will not get into all of these here because it would take several blogs to discuss. You can read more about groundwater rights systems at the references in the notes.
Arizona is a Reasonable Use state.
Under this system, a landowner can withdraw groundwater to make reasonable and beneficial use of the landowner’s property and adjacent landowners have no claim if the groundwater under their land is adversely impacted.
The Arizona courts have ruled that a landowner withdrawing groundwater that is transported to a distant property (e.g. public water agency) can still be held liable if the adjacent landowner’s land is adversely affected. This controversial decision led to the 1980 Groundwater Management Act (GMA) which further defines Arizona groundwater rights. GMA will be discussed in more detail in the next blog.
Federal Water Law
The federal government is also engaged in water rights.
The Reserved Rights Doctrine establishes that public land and tribal land set aside by the federal government have a reserved right to an adequate water supply.
The Reserved Rights Doctrine was upheld in the most famous water litigation in the US---Arizona v California (not a basketball game!). This lawsuit was about the allocation of Colorado River water.
The Colorado River actually has its own water law. The "Law of the River" includes a series of compacts, laws, court decisions, decrees, minutes (Mexico) and regulations that define Colorado River water rights.
The Law of the River and Arizona v California will be discussed in more detail in a future blog(s) on the Colorado River.