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By Ted Cooke, Acting General Manager, Central Arizona Project


In our interactions with customers, stakeholders and the general public, Central Arizona Project representatives have noted that most people think the long-term drought is the reason for the declining water levels in Lake Mead.

They’re only partly right.

Lake-Mead-151Most of the decline over the past 15 years has occurred because the Lower Colorado River Basin states of Arizona, California, Nevada, as well as Mexico, collectively use more water than Lake Powell normally releases each year—about 1.2 million acre-feet (MAF) more. The result is a “structural deficit” that causes Lake Mead’s elevation to drop about 12 feet every year, drought or no drought.

In a “normal” year, the U.S. Bureau of Reclamation releases 8.23 MAF from Lake Powell for delivery to the Lower Basin. That is enough water to satisfy the combined entitlements of California (4.4 MAF), Arizona (2.8 MAF) and Nevada (0.3 MAF) as well as half of the Mexican Treaty obligation (1.5 MAF).

But Lake Mead loses around 600,000 acre-feet annually due to evaporation. There are side inflows and system losses below Lake Powell that also factor into the equation. On top of that, the Lower Basin must meet the other half of the Mexican Treaty obligation. The bottom line: the net annual loss to Lake Mead is about 1.2 MAF.

As an example, from 2000 through 2013, at least 8.23 MAF was released from Lake Powell every year, and in 2011, 12.5 MAF was released after an above-average winter snow pack. But the water level in Lake Mead still fell more than 100 feet. That decline is due to the structural deficit.

Why does CAP care about the structural deficit? Both the Mexican Treaty delivery and Lake Mead evaporative losses are joint obligations of the Lower Basin states, but both are currently met by simply taking water out of storage in Lake Mead. When Lake Mead declines far enough that the Secretary of the Interior declares a Lower Basin shortage, it will be Arizona—and CAP in particular—that takes the first and biggest cut. That means that Arizona is being forced to cover California’s share of evaporative losses and Mexican Treaty deliveries. That is not what was intended when CAP was assigned the junior priority on the Colorado River in the 1968 Basin Project Act.

Lake-Mead-191Colorado River water managers have taken steps to address declining water levels in Lake Mead and Lake Powell in recent years, beginning with shortage sharing guidelines adopted by all seven states sharing the Colorado and the Secretary of the Interior in 2007. Those guidelines call for a reduction in Lower Basin deliveries when Lake Mead falls below certain trigger elevations, beginning at elevation 1,075. 

More recently, 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 MAF 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 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. One thing is clear: continuing “business as usual” in the Lower Basin poses an unacceptable risk to all Colorado River water and power users. It is time for western water managers to think about “what’s next” in managing this vital water supply to more than 30 million users across the West.

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