The Pinal County Governmental Alliance reports that California and Nevad have stored 2 million acre-feet of water in aquifers, most of it in Pinal County of Arizona. The presentation was made to the Central Arizona College governing board.
The water is from the Colorado River and is being 'banked' in groundwater aquifers for use later.
The only way this makes sense is if California and Nevada are allowing some of their Colorado River water to go down the Central Arizona Project canal to Pinal Co. where it is used to recharge the local aquifers.
When California or Nevada need that water, they can't pump it back up the canal, so they must take part of Arizona's share of river water, and let Arizona pull the California or Nevada shares out of the aquifers to replace it.