Unpacking the City’s 100-Year Assured Water Supply
As shimmering heat waves rise from the asphalt and the Sonoran Desert landscape basks under an intense sun, a question often bubbles to the surface for residents and visitors alike: How does a sprawling metropolis like Phoenix thrive in the middle of a desert? The answer lies not in a single well or river, but in a complex, multi-layered, and meticulously managed system built on decades of foresight—a system legally recognized as a 100-Year Assured Water Supply.
This designation isn’t just a hopeful projection; it’s a legal requirement and a cornerstone of Arizona’s water management policy. Let’s dive deep into how Phoenix secures and manages its most precious resource.
The Legal Bedrock: What is a 100-Year Assured Water Supply?
The concept originates from Arizona’s landmark 1980 Groundwater Management Act. This legislation was created to curb the unsustainable over-pumping of groundwater, which was causing land to sink and threatening the state’s long-term viability. The Act established Active Management Areas (AMAs), including one for Phoenix, where strict water conservation rules apply.
Within these AMAs, any new subdivision development must demonstrate an Assured Water Supply. To earn this designation from the Arizona Department of Water Resources (ADWR), a provider like the City of Phoenix must prove it has:
- Sufficient Supply: A legally and physically continuous supply of water to meet the needs of the proposed development for 100 years.
- Financial Capability: The financial means to construct and maintain the necessary infrastructure to deliver that water.
- Water Quality Standards: The water must meet state and federal health and safety standards.
- Consistency with Management Goals: The water use must be consistent with the AMA’s goal of reaching “safe-yield” (not pumping out more groundwater than is put back in).
For the City of Phoenix, this means proving it has a secure water portfolio to meet the demands of not just its current residents but also any new growth for a century into the future.
The Diverse Portfolio: Where Phoenix Gets Its Water
Phoenix’s water strategy is built on the principle of diversification. Relying on a single source would be a gamble; instead, the city draws from a robust and varied portfolio. In a typical year, the vast majority—between 97% and 99%—of Phoenix’s water comes from surface water sources.
1. The Salt and Verde Rivers (via SRP) – The Historical Lifeline
This is Phoenix’s oldest and largest water source. The Salt River Project (SRP) manages a system of reservoirs on the Salt and Verde Rivers, capturing snowmelt from the mountains of northern and eastern Arizona. This resilient, renewable supply accounts for nearly 60% of the city’s total water. Phoenix holds significant and secure rights to this water, making it the bedrock of the city’s portfolio.
2. The Colorado River (via CAP) – The Critical Connection
The Central Arizona Project (CAP) is a 336-mile-long aqueduct that brings Colorado River water to Central Arizona. This source makes up about 40% of Phoenix’s water supply. While a vital component, this source is also the most vulnerable. Due to a historic, two-decade-long drought and chronic overuse, the Colorado River system is under immense stress. Arizona has already seen its CAP allocation cut significantly, and further reductions are a key focus of long-range planning.
3. Groundwater – The Strategic Reserve (Approximately 2%)
Constituting a small but critical fraction of the city’s annual drinking water, groundwater acts as a strategic backup supply. While the 1980 Act was designed to reduce reliance on it, Phoenix operates dozens of wells for use during periods of surface water shortage or emergencies. The city’s primary goal is to store far more water underground than it pumps out, ensuring the long-term health of the aquifer.
Proactive Management: More Than Just Sources
Securing water rights is only half the battle. The City of Phoenix Water Services Department employs a sophisticated, multi-pronged approach to manage, conserve, and extend its supplies.
1. Underground Storage and Recovery (Recharge)
This is one of Phoenix’s most powerful drought-proofing tools. During wet years or when demand is low, the city takes its unused SRP and CAP water and “banks” it underground in natural aquifers. This process, known as recharge, is done at multiple facilities across the Valley. This stored water can then be recovered by wells during times of shortage. Phoenix has stored hundreds of thousands of acre-feet of water for the future—a massive buffer against drought.
2. Water Reuse (Reclaimed Water)
Every day, Phoenix treats nearly 90 million gallons of wastewater to a very high standard, creating what is known as reclaimed water. This water is not put back into the drinking water system directly. Instead, it is used for agricultural irrigation, cooling at the Palo Verde Generating Station (which in turn frees up other supplies), and creating vibrant wetland habitats like the Tres Rios Wetlands. This recycling program essentially allows the city to use its water twice.
3. Comprehensive Conservation Programs
Phoenix has been a leader in water conservation for decades. The city understands that the cheapest and most reliable source of water is the water we don’t use. Initiatives include:
- Tiered Water Rates: The more you use, the more you pay per gallon, incentivizing conservation.
- Public Education: Campaigns like “Water—Use It Wisely” provide residents with practical tips for saving water.
- Rebates and Audits: The city offers rebates for installing low-water-use landscaping (xeriscape) and provides free water efficiency audits for homes and businesses.
These efforts have paid off. Despite a population that has grown by over 400,000 people since the year 2000, the City of Phoenix uses the same amount of water today as it did over 20 years ago. This remarkable achievement is a testament to the success of its conservation-focused culture.
Facing the Future: Challenges and Confidence
The challenges are real. The ongoing drought on the Colorado River is the most significant threat to the region’s water supply, directly impacting Phoenix’s CAP allotment. Climate change projects warmer temperatures, which will increase evaporation from reservoirs and reduce snowpack, the primary source for the Salt and Verde Rivers.
However, the City of Phoenix’s planning is not static. Its Drought Management Plan has multiple stages of response ready to be implemented, focusing first on cutting its own non-essential uses and ramping up public conservation calls before ever impacting customer taps.
Because of its diverse portfolio, its massive underground water savings, its robust conservation ethic, and its legal 100-year Assured Water Supply designation, Phoenix is in a stronger, more secure position than many other communities in the West.
