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The water system that has quenched the East Valley’s thirst through more than a century of droughts and floods is well positioned to deal with the coming effects of climate change, according to a new federal study.

The study by the Bureau of Reclamation  found that the Salt River Project watershed is far more resilient than the much larger Colorado River system that also supplies some of the East Valley’s water.

In fact, according to SRP water managers, the system appears to be better situated now than it was 40 years ago – despite explosive growth and development that changes the face of the region almost daily.

The data offer assurance that despite a “mega-drought” that has afflicted the Southwest for the past quarter-century, homes and businesses within the SRP service area won’t go dry anytime soon.

That service area is huge. 

It covers 387 square miles stretching from the northwest Valley through Phoenix and into the Southeast Valley. SRP is a major supplier for Chandler, Gilbert, Mesa, Tempe and Scottsdale, although those cities draw from other sources as well.

SRP water comes our way from northern and eastern Arizona via the Salt and Verde rivers. Four dams on the Salt – including the granddaddy, Roosevelt Dam – and two on the Verde create reservoirs that can be tapped any time you need to make an ice cube. The dams also serve as a flood-protection system.

The rivers drain a watershed of some 12,500 square miles stretching from northwest of Flagstaff to near Springerville, then southward almost to Mesa.

By comparison, the Colorado River watershed is huge – some 246,000 square miles.

And while it might seem the bigger watershed would be more robust, the Bureau of Reclamation study found the opposite is true. 

That bureau, by the way, oversees water-conservation projects in the American West and actually owns the dams that SRP operates.

“The big takeaway from that long report … is that the Salt and Verde river stream flow is less sensitive to warmer temperatures than the Upper Colorado River stream flow,” said Bo Svoma, an SRP meteorologist.

“On the Colorado, stream flow reductions due to warming are commonly cited to be about 6.5 percent per degree Celsius, and on the Salt and Verde rivers that number is between 1 and 2 percent per degree Celsius,” he said.

The reason, he said, is that much of the runoff in the SRP system occurs between December and April, when days are cool and the sun is low on the horizon. 

That results in less evaporation than in the Colorado system, where the runoff occurs between April and June.

Charlie Ester, SRP’s manager of watershed management, said it’s true that a smaller watershed is not immune from the effects of drought and is, in fact, “much more volatile” than the vaster Colorado system.

“When a drought hits, we get slammed hard,” he said. “Look at 1996-2002. Those were the driest years we’ve ever seen. But then, 2005. It was a dramatically wetter year and in one wet season it filled up our entire system. 

“So, where a small watershed is more profoundly affected on the dry side, it’s also more profoundly affected on the wet side. We can fill our system in one season. You can’t do that on the Colorado.”

As of last month, water levels in the SRP reservoir system stood at 94 percent of capacity. The nearly 1.9 million acre-feet of water in storage could supply SRP’s service area for more than two years even without another drop of rain.

And that is not to mention a wealth of groundwater that lies below our feet.

If you drive Loop 202 westbound across north Mesa, you may notice riparian areas in the Salt River bed that weren’t there as recently as 25 years ago. 

SRP and other agencies have aggressively recharged aquifers to the point that the water table in some places has bubbled to the surface. SRP relies on that groundwater for emergency deliveries in case its surface supplies do run low.

This, Ester said, has occurred even amid what climatologists believe is Arizona’s worst “mega-drought” in some 600 years.

“We’ve recharged, I believe, close to three million acre-feet of water,” Ester said.

One reason SRP can do that is that despite the East Valley’s booming population, water use has declined dramatically over the past four decades.

“When I first started at SRP in the mid-80s, (annual water demand) was well over a million acre-feet,” Ester said. “Some years we would deliver 1.2 million to 1.3 million acre-feet. We struggle now to deliver 800,000 acre-feet of water.”

Deliberate conservation by water users, as well as more efficient appliances, accounts for a lot of that, he said. Plus, an acre of homes uses less water than an acre of irrigated farmland.

The effect is so dramatic that whereas an acre-foot used to be defined as serving a family of four for a year, now it’s defined as what two such families require.

Despite the new research showing the resiliency of the SRP water-delivery system, both Svoma and Ester have concerns about the effect of climate change on Arizona.

“When the Southwestern U.S. transitions out of its drought and Arizona starts getting more frequent wet winters, that will be happening in a warmer world,” Svoma said, adding:

 “And if you have a warmer world there’s more water vapor in the air because the oceans are warmer and they’re evaporating more. … More water vapor in the air with storms means heavier precipitation.”

Thus, he said, the Valley could be at a greater risk of flooding on the order of the 1993 gusher that washed away a bridge under construction on Mill Avenue in Tempe.

Ester has his own worry – namely, the health of the forests that make up much of the SRP watershed.

 Poor forest management has fed several massive fires over the past 20 years, creating sediment that runs into the reservoirs and limits their capacity.

Arizona needs better forest management, he said, to eliminate catastrophic fires and “get it back to the point where fire can achieve a more natural role that it played over eons of time.”

The federal study of the Salt River Project is available at: usbr.gov/watersmart/pilots/docs/reports/Final_Reservoir_Operations_Pilot_Report-Salt_and_Verde_Az.pdf

(1) comment

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