What is the difference between assured and adequate water supply, and how does the 100-year rule work?

Arizona runs two parallel water-supply programs through the Arizona Department of Water Resources (ADWR): the Assured Water Supply program inside the state's Active Management Areas, and the Adequate Water Supply program outside them. Both ask whether there is a 100-year supply of water, but they have very different teeth, and within the assured program a "designation" and a "certificate" are not the same thing. Understanding which one applies to a parcel tells you how solid its long-term water really is.

Assured versus adequate

• Assured Water Supply applies inside an Active Management Area (AMA). The Phoenix metro sits within the Phoenix AMA. Here, demonstrating a 100-year supply is mandatory before a new subdivision can be sold, and it has real consequences if it cannot be shown.
• Adequate Water Supply applies outside the AMAs. In most of those areas a developer must determine and disclose whether a 100-year supply exists, but a finding of inadequacy does not necessarily stop lots from being sold. It is largely a disclosure program, so the burden shifts onto the buyer to read what is disclosed.

That difference matters: inside the Phoenix AMA the 100-year showing is a gate, while in adequacy areas it is closer to a label on the package.

What the 100-year demonstration requires

To meet the assured or adequate standard, a water supply generally has to be shown to be:

• Physically available for 100 years.
• Continuously available over that period.
• Legally available, meaning there is a recognized right to use it.
• Of adequate quality for the intended use.
• Financially feasible to deliver, and consistent with ADWR's groundwater management goal and any applicable management plan.

Designation versus certificate

Inside an AMA there are two ways a development can satisfy the assured-supply requirement, and the distinction is worth knowing:

• A Designation of Assured Water Supply is granted to a city, town, or private water company that proves a 100-year supply for its entire service area. If your home will be served by a designated provider, the provider carries the demonstration and individual subdivisions within its service area generally do not each have to prove supply separately.
• A Certificate of Assured Water Supply is granted to a specific subdivision that is not served by a designated provider. The developer must prove the 100-year supply for that subdivision before the lots can be sold.

For a buyer, "served by a provider with a Designation" is generally the most reassuring answer, because the supply obligation rests on an established utility rather than a single project.

Why this is moving in the Phoenix AMA

The assured-supply landscape in the Phoenix AMA has tightened. In 2023 ADWR's updated groundwater modeling found that projected demand exceeded the groundwater that could be relied on for the full 100 years in parts of the AMA. As a result, ADWR limited approval of new assured-supply determinations that depend solely on pumping groundwater in affected areas, pushing new growth toward renewable or alternative supplies. This is an active and evolving area of state water policy, so treat these specifics as current to June 2026 and confirm the latest status with ADWR.

What it means for your lot

The practical questions are the same ones that matter on any outlying parcel: what is the water source, who guarantees it, and how secure is it for the long term. A lot inside a provider's designated service area, a lot with its own assured certificate, a lot outside an AMA with an adequacy disclosure, and a lot-split parcel relying on a private well or hauled water are four very different situations, and the purchase price rarely tells you which one you are looking at.

How Jematell Homes helps

We help buyers connect a parcel's water status to what building and living there will realistically require, and we flag when an outlying lot deserves a closer look at its supply before an offer. Because this area of law is technical and changing, we point you to ADWR and qualified professionals for formal determinations rather than offering legal conclusions, and we confirm the current rules with the relevant AHJ for your specific parcel.