Drought and over-allocation are a persistent threat to the Colorado River’s water supply, the source of much of Southern Arizona’s water, and “water banking” is one way managers work to buffer the state against shortages. The method of storing water underground in natural aquifers for future use has been a crucial component of Arizona’s water management plan for more than three decades. And the practice raises questions about how and where water is stored, who owns stored water, how much can be recovered, and when and where can it be recovered.