what a lake mead water shortage will mean in practice

Lake Mead Water Level

inkstain |  There is a clear possibility of a shortage declaration on Lake Mead in August, which would force a reduction in Lower Colorado River water deliveries, primarily to Arizona, in 2016. Nevada and Mexico would also see small shortages. Neither California, nor the states of the Upper Basin (New Mexico, Utah, Colorado and Wyoming) will see any curtailments.

This is a big deal, but it is almost entirely an Arizona big deal. Arizona currently has the slack in its system to absorb the reductions, including possibly deeper cuts if Mead continues to drop, without major disruptions. The Phoenix and Tucson metro areas are not going to dry up and blow away.

Longer details below:

Built in the 1930s in a canyon on the Arizona-Nevada border, Hoover Dam impounds Colorado River water in Lake Mead. It protects valleys along the lower river from flooding, and stores water during wet years for use during dry years in Nevada, Arizona, California, in the United States, and Baja and Sonora in Mexico. From the beginning, California has taken its share of water through an aqueduct that carries water to Los Angeles, and a second that carries farm water to the Imperial Valley. A significant share of the river’s water also supplies farms and communities in Arizona along the river itself, from Parker south to Yuma. But for the first three-plus decades of the dam’s existence, Arizona had no way to get the river’s water to its major population centers in the state’s central desert valleys.

In 1968, in return for political support from California to win federal funding for the Central Arizona Project, Arizona cut a deal: California would not block the project, which would use federal money to build a canal to carry 1.6 million acre feet of water per year up from the Colorado River and into Phoenix and Tucson. In return, Arizona would agree to subordinate its priority for the 1.6 maf water to California’s. (This Reg Manning Arizona Republic cartoon suggests some hard feelings about all this.)

That means that if the river is short, essentially all the cuts come to Arizona’s share of the Colorado River’s water. This is important point number one:

• It takes many more years of shortage before California loses a drop.

Southern California – mostly farmers in the Imperial Valley but also urban/suburban LA and San Diego, won’t lose any water. They’ve got enough to worry about right now with their own drought. Drought in the Colorado River Basin won’t add to their problems.

Important point number two:

• Arizona’s major on-river agricultural water users, primarily the Colorado River Indian Tribes and the farmers of the Yuma area, also would not lose a drop under a shortage. Their rights are older, and not crimped by the 1968 California bargain.

But what does “shortage” actually mean, and who gets to decide when the river has a shortage and deliveries are to be curtailed?

The legal authority rests with U.S. Interior Secretary Sally Jewell, but back in 2005-07 the federal government and states decided the rules for making the decision were too squishy, so they came up with a really crisp definition: when the U.S. Bureau of Reclamation’s August version of its monthly planning document known as the “24-month study” shows that Lake Mead is expected to drop below a surface elevation of 1,075 feet above sea level, a shortage shall be declared.