How tree rings help measure Arizona's 'mega drought'

After a record low winter run-off, some water experts are now calling this Arizona's worst mega-drought in recorded history, even when compared to tree-ring data that goes all the way back to the 1300s.

Twenty-three years is the blink of an eye when it comes to tree-ring chronology, but, according to Charlie Ester with the Salt River Project, that's about as long as the current drought in Arizona has been ongoing.

“As we got into this more and more, it was just so persistent,” said Ester. “We began wondering, how does this compare to what may have happened in the distant past?”

Several years ago, SRP teamed up with the University of Arizona Laboratory of Tree Ring Research and took to the northern Arizona forest, where the ponderosa pines gave them a glimpse into the last 700 years and, according to Ester, the last two decades have been the driest ever.

“The current 23-year period has gotten less run-off than any of those other periods. So in other words we are in the worst mega-drought that there is any data to support,” said Ester.

Stream gauges have been providing researchers with measured data for the last 100 years or so, by comparing that data to strategically selected tree rings. UA's Dave Meko says they're able to crunch the numbers and get an idea as to what our rivers looked like centuries ago.

"We get them from the mountains where most of the run off is occurring,” said Meko. “So it's almost like we're using the trees as substitute rain gauges.”

Meko says they can get streamflow accuracy up to 70%, not bad for 700 years of data, but while it's good for looking into the past, it's not a great indicator of what's to come.

“As much as people want to see cycles in moisture, regular, dependable cycles, we don't see them,” explained Meko.

That's why utilities like SRP are always looking ahead.

“The last day that our reservoirs are spilling water in a wet year is the first day of the next drought and if you plan that way, you're never going to be surprised by drought,” said Ester.

Even in the midst of the worst 23-year drought, SRP reservoir levels are still around 57 percent full and Ester says they’re ready even if it lasts another decade.

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