Expected spring #runoff into #ColoradoRiver plunges after dry April — Tucson.com

Graphic credit: Colorado River Basin Forecast Center

From Tucson.com (Tony Davis):

May’s monthly runoff prediction for the April-through-July period was 65% of average, or 4.6 million acre-feet. That’s 1 million acre-feet less than the April forecast predicted. Tucson Water customers typically use close to 100,000 acre-feet a year.

While low runoff this year is highly unlikely to trigger the river’s first major shortage as soon as 2021, it raises the possibility of one in 2022. A shortage would fall particularly hard on Central Arizona farmers. Flows of water into Powell that would be low enough to cause a shortage in 2022 are likely to occur about 10% of the time, said the U.S. Bureau of Reclamation’s most recent study of its reservoir operations, published in mid-April.

“This could be the start of a multi-year drought. Last year was a wet year, so one year like this we can handle,” said Eric Kuhn, a retired general manager of a northern Colorado water district and author of a recent book on the river. “One dry year doesn’t usually make a difference if we start in good shape. Two dry years or three dry years is a concern.”

The dry April weather wiped out the benefits of a robust winter snowpack in Upper Colorado River Basin states, federal forecasters said this week. Until now, this year’s forecast had been relatively stable, ranging from 74% to 82% of average runoff since January…

But Brenda Alcorn, a hydrologist for the federal Colorado Basin River Forecast Center, said that if the dry spell continues, the eventual runoff could well end up lower than what’s now predicted.

It could possibly fall as low as or lower than the 2018 runoff of 36% of normal, “if we have extreme dryness through the rest of the spring,” Alcorn said.

Since the mid-2010s, the Colorado’s spring-summer runoff has done “a lot of yo-yoing,” Alcorn said.

The runoff rose sharply in 2017, fell sharply in 2018 and soared to 145% of the average runoff in 2019.

While these fluctuations have staved off the shortages that most water experts now view as inevitable, the river is clearly more vulnerable to shortages than it was in 2000, when both Lakes Mead and Powell were full or nearly full.

Today, Lake Mead is 44% full, and Lake Powell is at 48%…

This latest bad spring-summer river runoff forecast, however, stemmed largely from dry, not hot weather, the Colorado Basin River Forecast Center said.

“April precipitation was generally below to much below average across the Colorado River Basin and Great Basin,” the center said in a report on its latest forecast. “It was exceptionally dry over northern Utah and southwest Colorado.”

A number of sites that measure snowpack in Utah and southwest Colorado showed their lowest April precipitation since officials started recording it up to 35 years ago…

April river flows neared record lows in parts of the Gunnison, Dolores and San Juan river basins in the two states.

April temperatures across the basin were generally near the average. Temperatures fell below average for the first three weeks of the month.

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