CAP partners with users on Colorado River overuse

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.
Most 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 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 million acre-feet from Lake Powell for delivery to the Lower Basin. That is enough water to satisfy the combined entitlements of California (4.4 million acre-feet), Arizona (2.8 million acre-feet) and Nevada (0.3 million acre-feet) as well as half of the Mexican Treaty obligation (1.5 million acre-feet).
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 million acre-feet.
As an example, from 2000 through 2013, at least 8.23 million acre-feet was released from Lake Powell every year, and in 2011 12.5 million acre-feet 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.
Colorado 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 1075.