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There’s little debate about the fact that we’re in a long-term drought. But when it comes to potential shortage in our Colorado River supplies, that’s only part of the picture. Another important component – especially to Arizona and Central Arizona Project – is something referred to as the “Colorado River Lower Basin Structural Deficit.”

Lake-Mead-210That sounds like a complex term, but the issue is relatively straight-forward. Lake Mead drops approximately 12 feet per year, even in a “normal” water supply year because the current use exceeds the available supply.

Here’s the simple math of the Lower Colorado River Basin plus Mexico:

  • California takes 4.4 million acre feet
  • Arizona takes 2.8 million acre feet
  • Mexico takes 1.5 million acre feet
  • Nevada takes .3 million acre feet

That’s 9 million acre feet total. Yet the U.S. Bureau of Reclamation only supplies 8.23 million acre feet from Lake Powell in each “normal” year.  The supply released from Lake Powell to Lake Mead is controlled by complex federal laws and an interstate compact; this attempts to address the frequently-asked question, "Why not just release more water from Powell?”  That results in a deficit, regardless of whether we’re in drought conditions. Further, Lake Mead loses around 600,000 acre-feet annually due to evaporation.

The bottom line: the net annual loss to Lake Mead is about 1.2 million acre feet.

Why does Arizona seem most concerned about this? Each year Lake Mead declines in storage due to the net annual loss.  When Lake Mead declines far enough that the Secretary of the Interior declares a Lower Basin shortage, it will be Arizona that takes the first and biggest cut and CAP that absorbs that entire cut to the state. 

Colorado River water managers have taken steps to address declining water levels in Lake Mead and Lake Powell in recent years, beginning with 2007 shortage sharing guidelines adopted by all seven states sharing the Colorado. Those guidelines call for a reduction in Lower Basin deliveries when Lake Mead falls below certain trigger elevations. More recently, in 2014 CAP began participating in a Pilot System Conservation Program, to work with water users across the Basin to conserve water in Lake Mead and Powell.  In addition, in December 2014, CAP, the Arizona Department of Water Resources and others entered into a Memorandum of Understanding to conserve up to 740,000 acre-feet in Lake Mead by the end of 2017. The MOU also recognizes the need to conserve a total of 1.5 to 3 million acre feet by the end of 2019 to reduce the near term risk of Lake Mead falling below critical elevations that could force the Secretary of the Interior to make difficult decisions about who gets water and who doesn’t.

The 2007 guidelines, the Pilot System Conservation Program and the 2014 MOU are positive steps on the road to improved Colorado River management. But a long-term solution to the structural deficit problem is imperative to ensuring the sustainability of this vital resource for all parties. 

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