Phoenix and Tucson are entering into a water sharing arrangement that officials say stands to benefit both desert cities.
Phoenix lacks the storage capacity for a portion of the city's unused Colorado River water, so the arrangement calls for Phoenix to send the excess water to Tucson via the Central Arizona Project canal system.
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Once transported about 100 miles to the Tucson area, the water from Phoenix then would be stored in the aquifer.
That deposit of water would raise the water level in the Tucson aquifer, and Tucson could save money by reduced costs for pumping ground water to supply its residents.
During future water shortages, Phoenix could be repaid for the water it sends to Tucson by drawing from Tucson's share of Colorado River water in the CAP system. And Tucson could draw from the Phoenix water stored underground in the Tucson aquifer.
"Securing our water future is one of our highest priorities for Phoenix residents and our economy," said Phoenix Mayor Greg Stanton.
The Phoenix City Council on Wednesday approved two intergovernmental agreements previously approved by Tucson and the Metropolitan Domestic Water Improvement District in Tucson.
The CAP is a 336-mile-long canal system that carries Colorado River water from near Lake Havasu City in western Arizona to cities, farmers and Indian communities in central and southern Arizona.
The Phoenix-Tucson arrangement begins in 2015 with a small pilot project that will run two or three years. If successful, it could lead to a larger-scale, long-term program, officials said.
Phoenix now uses only about two-thirds of its allocation of Colorado River water.
Tucson relies on a combination of ground water and Colorado River water. The Phoenix area uses both of those sources plus water from reservoirs fed by rivers and watersheds in central and eastern Arizona.
The arrangement between Phoenix and Tucson could be a model for other Arizona cities, said Tom Buschatzke, assistant director for water planning with the Arizona Department of Water Resources.