With many states in the Southwest facing prolonged drought, the question of who has access to water—when and how much of it—is heating up. Arizona, which ranks lowest in priority among several states for obtaining water from the Colorado River, initially agreed to cut a third of its annual use, amounting to about 320,000 acre-feet. More recently, under a new drought plan by several states, it agreed to reduce its water use by 512,000 acre-feet.
This blog post by the Arizona Municipal Water Users Association explains why, even as water use is curtailed, Arizona cities are still diverting much of their allotted water to the aquifers. The association’s members have stored enough water—more than 2.4 million acre-feet—to supply its member cities for three years, although the post notes “it would never be used up that quickly because of the diversity of our water supplies.”
As AMWUA points out, most of the water that it stores underground, primarily through the use of recharge basins or injection wells, comes from the Colorado River via the Central Arizona Project. Member cities also add treated wastewater to the aquifers.
“The law gives water providers ‘water credits’ for storing water underground,” the blog notes. “These credits allow cities, water companies, and water agencies to put water in the ground now and use an equivalent amount of stored water at a later time. The Arizona Department of Water Resources oversees underground storage in the state and tracks the water each city has stored.
“When there is a shortage of Colorado River water, water supplies for those with rights to use Colorado River water are cut incrementally over time and depending on the severity of the shortage. The last water to be cut is called ‘high priority’ water. Cities and other municipal water providers and Indian Tribes in central and southern Arizona hold rights to the highest priority water delivered though the CAP and—even during shortages—have the right to store that water in underground aquifers.
“Occasionally cities hear whispers or hints from some in the water community that cities should not be permitted to store their water underground during a water shortage and that this water should go to others, whose lower priority water supplies have been reduced. Any attempt to limit the ability of cities and tribes to store their water underground during times of shortage would be an unlawful interference with long-standing, federal contractual rights. Furthermore, limiting cities’ ability to store their water underground during times of shortage and drought would be shortsighted.”
The blog also points out that cities have a state-mandated requirement to ensure a 100-year water supply for their users—a tall order, given the current shortages. Keeping the aquifers supplied, it notes, helps avoid overpumping of groundwater, land subsidence, and other associated problems—something other regions like California have been experiencing.