Arizona is one of the few states in the nation that requires developers to prove a 100-year assured water supply before they can build new subdivisions. This rule, managed by the Arizona Department of Water Resources (ADWR), was created to balance growth with the state’s limited and fragile water resources. 
To make this determination, ADWR looks at several factors. The most important is whether enough water exists to serve a community for the next century without harming aquifers, rivers, or other water users. In Active Management Areas like Phoenix and Prescott, this often means detailed groundwater modeling. For example, the Verde River Subflow Zone was identified as a key area where underground water is so closely tied to the river that it must be treated like surface water. Wells drilled in this zone are regulated more strictly because their pumping can directly reduce river flow, which impacts both ecosystems and downstream rights holders (Arizona Department of Water Resources, 2021).
Beyond groundwater, Arizona also allows for the use of renewable supplies such as treated wastewater, surface water, or imported water. In 2024, the state adopted an Alternative Path to Designation of Assured Water Supply, which lets providers qualify for the 100-year rule even if groundwater alone is insufficient, as long as they reduce pumping and replace part of their supply with renewable sources (Arizona Department of Water Resources, 2024).
The process combines hydrologic science with policy safeguards. It requires proof of physical water availability, legal rights to use the water, financial capacity to build necessary infrastructure, and consistency with state management goals. This system helps ensure that when new neighborhoods rise in Arizona’s desert cities, they do so with a secure water future in mind.
Works Cited
Arizona Department of Water Resources. 2021 Partial Verde River Subflow Zone Delineation Report. Phoenix: ADWR, 2021.
Arizona Department of Water Resources. Alternative Path to Assured Water Supply (ADAWS) Rulemaking, Economic Impact Statement. Phoenix: ADWR, 2024.
Arizona Department of Water Resources. Assured and Adequate Water Supply Program Overview. Phoenix: ADWR, 2024.
