New Flexibility on the Central Arizona Project Canal: The Tucson/Phoenix Exchange and the System Use Agreement

Abstract

A Colorado River shortage is coming. The exchange program between Phoenix and Tucson-area subcontractors is an elegantly simple solution to protect against a municipal shortage because it takes advantage of preexisting legal frameworks and fills capacity in already-built infrastructure. Surface-water dependent Phoenix delivers Central Arizona Project (CAP) water it cannot presently use to recharge facilities that the groundwater-based Tucson area does not presently need to fill. As noted below, the current Tucson/Phoenix exchange is both an intergovernmental agreement (IGA) between Tucson and Phoenix, and a pilot project between Tucson, Phoenix, and the Metropolitan Domestic Water Improvement District (Metro). It could readily morph into a series of exchange agreements between additional Tucson area parties and MetroPhoenix area parties, as well. At the largest scale, the “Tucson/Phoenix exchange” means using Southern Arizona’s productive, and clean aquifers—and more precisely, the large amount of clean storage space in the vadose zone above those aquifers—to store water for the Phoenix area. During an “exchange,” the Tucson area would pump stored water and place an order to have Tucson-area CAP water directly delivered to Valley-area water treatment plants. But even this simple solution has taken at least 5 years to implement; and due to a variety of administrative challenges, Tucson has yet to deliver water to Phoenix, even on a “pilot scale.” Flexibility, collaboration, innovation, and creativity will become increasingly necessary to meet Arizona’s water needs; the Tucson/Phoenix exchange shows a pathway that—now that it has been almost completely invented, revised, collaborated, dissected, permitted, and approved—is ready to be a substantial component of Arizona’s shortage implementation strategy.