Underground Storage: How it Works and Why it Matters

Many things must be considered when it comes to long-term planning for water supplies in the desert. One critical component is storing water underground, which is a valuable investment in a sustainable water supply.

We often talk about the importance of investing in diverse water supplies, infrastructure, conservation programs, and underground storage. Highlighting and explaining the role that underground storage plays will provide a better understanding of its significance.

Over the past two decades or so, the AMWUA cities have collectively invested hundreds of millions of dollars in storing over 2.5 million acre-feet of water underground. That's enough water to meet the ten AMWUA municipalities' needs for more than two years. However, it's important to note that it would never be used up that quickly because of our water supplies' diversity. Having more than one water source means the AMWUA municipalities have greater flexibility in managing their water systems and delivering water to you. In turn, this means a more robust economy for the Valley and a more sustainable way of life for all of us.

How underground storage works

The AMWUA cities store reclaimed water  and a portion of their Colorado River water allocations delivered through the Central Arizona Project (CAP) underground to prepare for the future.

Storing water underground is known as "recharging" the aquifer. The water is stored in recharge basins, each usually a couple of acres in size, is constructed to quickly retain water to filter into the aquifer. It can consist of gravel, fractured rock, sand, silt, or a blend of such materials and vary in thickness from a few feet to 1,000 feet or more. Water is stored in the aquifer and withdrawn by well-pumping when needed.

Each acre-foot of water is stored underground in aquifers or recharge basins for use down the road. The storage of this water is called a Long-Term Storage Credit. The Arizona Department of Water Resources (ADWR) oversees underground storage, maintains an account of Long-Term Storage Credits (LTSC) for each city or person that stores water underground, oversees these accounts' additions and deductions, and monitors if a municipality contributes to the regional goal of safe-yield .

Why storing water underground is important

Recharging water underground is smart water management in the desert. Underground storage is important because it allows municipalities and water providers to plan for future needs and have backup supplies for times of shortage. Underground storage and recharge are vital components of AMWUA member cities' strategy for drought resiliency and long-term sustainability.

Many benefits come from recharging. It creates a water savings account that can be recovered when needed to help with the effects of a prolonged drought or a shortage of another water supply, such as the Colorado River. Underground storage also improves the quality of recharged surface water by its filtration through the aquifers' underlying sediments.

Underground storage is critical to long-term planning, and our collective sustainability, something the cities have recognized for decades. With prolonged drought and the threat of a drier future looming, the water stored underground becomes all that more important. Investing in recharge efforts will remain part of the cities' water management plan and protect that water for the future, so it will be there when we need it most.

Relevant underground water storage terminology:

Aquifer: An underground geological formation of sand, soil, gravel, and rock able to store and yield water.

Long-Term Storage Credits (LTSC): Credits earned by storing CAP water or recycled water underground. These credits can be recovered at a later date.

Recharge: Storing renewable water supplies underground for a later date. Arizona worked for years to have underground storage recognized as a beneficial use, preventing California from taking Arizona's allocation of Colorado River water.

Recovery: The act of pumping, or "recovering" water that was previously stored underground. Individual water users can Recover LTSC, and stakeholders are developing a plan to Recover the millions of acre-feet of water stored underground by the Arizona Water Banking Authority.

Stored Water: Water that is stored underground for recovery at a later time.

Underground Storage Facility (USF): A site where water is stored underground, typically through large basins or settling ponds where water percolates into the aquifer below.

For more key water definitions, visit the AMWUA Water Glossary .

For over 50 years, the Arizona Municipal Water Users Association has helped protect our member cities' ability to provide assured, safe, and sustainable water supplies to their communities. For more information, visit www.amwua.org .

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