In Arizona and other western states there is pressure to count water lost through evaporation

WASHINGTON (AP) — Exposed to the blazing sun and hot, dry air, more than 10% of the Colorado River’s water evaporates, leaks, or spills as the West’s 1,450-mile (2,334-kilometer) power plant churns through the dams, reservoirs, and open-air canals.

For decades, key stewards of the river ignored the massive water loss, instead allocating their share of the river to Arizona, California, Nevada, and Mexico without subtracting evaporation. But the 10% can no longer be ignored, say hydrologists, state officials and other Western water experts.

Decades of drought in the west have pushed water levels in key reservoirs along the river to unprecedented lows. Nevada and Arizona officials say they now have to account, along with California, for how much water is actually in the river.

The challenge is to find a method that California agrees with. “It’s very difficult to reach a consensus,” said Sarah Porter, director of the Kyl Center for Water Policy at Arizona State University. She thinks it’s unlikely that states will independently reach an agreement without federal intervention. Unlike Arizona, California, Nevada and Mexico, the upstream or Upper Basin states – Colorado, New Mexico, Utah and Wyoming – already account for evaporative losses.

Now, with a federal deadline looming for Colorado River Basin states to say how they will use at least 15% less water from the river, there is renewed urgency for Arizona, California and Nevada to consider what is being lost to evaporation. One suggestion comes from Nevada: States at the end of the river would see their portion of the Colorado River shrink based on the distance it travels to reach users. The further south the river flows, the more water is lost as temperatures rise and the water is more exposed to the elements.

The Southern Nevada Water Authority estimates that Arizona, Nevada, and California lose about 1.5 million square feet of water each year to evaporation, transportation, and inefficiency. That’s 50% more than Utah uses in a year. Nevada and Arizona could be on board with this plan. Nevada stands the least to lose from this plan because Lake Mead — the artificial reservoir that supplies Arizona, Nevada, California and Mexico with water — is in its backyard.

Tom Buschatzke, general manager of Arizona’s Department of Water Resources, called Nevada’s supply fair. “Calculating the losses as suggested by Nevada is probably the fairest and most representative of the real, physical world,” Buschatzke said. “The further you are, the greater the losses.”

But crucially, California disagrees. Officials there have said Nevada’s plan would likely violate western water laws. California has rights to most of the water of the Colorado River. Equally important, in times of scarcity, water cuts come later than for other users, based on what is known as the Law of the River, a set of overlapping agreements, court decisions and treaties that determine how the river is divided. Due to its high water rights, it has so far been spared from cuts.

California water managers have said that evaporation and system losses should be considered based on this existing system. In a December letter to federal officials, Christopher Harris, executive director of the Colorado River Board of California, said that any other approach “could face significant legal and technical challenges.”

For Arizona, this could mean losses so great that some experts say Phoenix’s drinking water supply could be at risk due to reduced supplies to the Central Arizona Project, the 330-mile (541-kilometer) aqueduct system that supplies water from the Colorado River that goes underground area and Tucson.

Under Nevada’s plan, California would pay a heavy price. Aside from consuming more water from the river than any other state, its water travels some of the longest distances. California’s Imperial Irrigation District, the largest single user of Colorado River water, would lose about 19% of its interest. The region grows many of the country’s winter vegetables and alfalfa, and Imperial has said it is not at all comfortable with water cuts due to evaporation losses.

Imperial Irrigation District water manager Tina Shields said Arizona and Nevada — whose water rights are younger than California’s — supported the Southern Nevada Water Authority’s plan because it would benefit them to share the losses.

“If you have a junior, right, do that,” Shields said. “They are trying to share the issue with other users.”

According to John Fleck, a researcher at the University of New Mexico’s Water Resources Program, lower basin states have avoided recognizing these losses for so long, in part because there hasn’t been a need to in previous decades. Water was plentiful, and some states did not take all the water to which they were legally entitled.

In many cases, the infrastructure needed to supply water—huge canals, dams, and waterways—was not in place.

“The problem goes back to a time when … nobody had to deal with this problem,” said Fleck. “The difficult politics also made the issue untouchable, said Fleck. “No one was ready to take it on,” Fleck said. “It all boils down to the same thing: you have to take less water out of the system.”

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