The wild swings in rainfall in central Arizona this year have once more underscored the importance of reservoirs in a freakishly uncertain world — including Roosevelt Lake and the Blue Ridge Reservoir.
So far this year, the unusually dry 224,000 acre-feet of runoff in the Salt and Verde rivers has reached just 16 percent of last year’s happily wet 1.4 million acre-feet. Normally, the two rivers carry about 683,000 acre-feet.
Last year, the Salt and Verde River watersheds had their 20th wettest year in recorded history. So far this year, the same watersheds have recorded the 22nd driest year in history.
Fortunately, Roosevelt Lake remains 77-percent full, thanks to above-normal rainfall in 2010. The lake holds two-thirds of the storage for the whole SRP system.
In an aside, SRP officials this week also noted that they have had to intermittently shut down the pumps delivering Blue Ridge Reservoir water to the East Verde River. Officials said they shut down the pumps to fix communications problems between the pumps lifting water out of the Blue Ridge Reservoir and the sensors in the power-generating turbines at Washington Park where the water gushes into the East Verde.
Blue Ridge is now just 65-percent full and will drop a daunting 80 feet by the time SRP turns off the pumps once snow shuts down easy access. The East Verde continues to run chocolate brown, largely as a result of mud and silt still washing into the creek where the Water Wheel Fire charred slopes between Beaver Valley and Whispering Pines two years ago.
Officials said on Tuesday that they had restarted the pumps, but only at about 25 cubic-feet per second. SRP runs about 11,000 acre-feet of water down the East Verde during the nine months the pumps operate, letting the water run down into the Verde River and on into reservoirs supplying Phoenix. At the moment, that includes the 3,500 acre-feet reserved for northern Gila County, which Payson can’t take until it finishes a 15-mile-long pipeline along Houston Mesa Road.
In another lucky break for the drought-plagued region, the Colorado River watershed had one of its wettest years on record. The melting snowpack in the Rocky Mountains has boosted the flow of the river 45 percent above the average for the past century. That makes this the best runoff year for the Colorado River since 1995, with the 15th greatest flow since record keeping began in 1909.
As a result, officials say the surface of Lake Mead will likely rise 30 feet, easing fears of a severe shortage and water rationing that could affect 30 million people from Los Angeles to Phoenix.
The freakish shifts in rainfall have made a mess of most attempts to forecast water supplies, but have also underscored the value of backup reservoirs during periods of unstable weather. For instance, once Payson builds the Blue Ridge pipeline, the town will more than double its sustainable water supply. Equally important, the Blue Ridge pipeline will connect Payson to a completely different watershed — which will provide a buffer against the dramatic variations in rainfall even within a small region.
For instance, this year the Verde River system actually carried more water than the Salt River, which usually carries a far greater volume of water.
SRP officials said the source of the wildly unpredictable shifts in rainfall lies in changes in the temperature of surface waters in the Pacific Ocean. A cyclical but still poorly understood ocean surface warming trend called La Niña typically affects rainfall all the way from Australia to Arizona.
Charlie Ester, SRP’s manager of water resources, said the 2011 winter rainfall figures actually followed the normal pattern for a La Niña year.
“Since 1950, there have been 18 La Niña winters and the majority of those have been dry, with six being normal and four being above-normal,” said Ester. “We’ve also had a pretty dry monsoon season on the watershed so far, which is a bit unusual following a dry winters, so we’ll see if the early trend that we may return to another mild La Niña season holds up for next winter.”
The Salt River had its 13th driest season ever, while the Verde River had it’s 34th driest year on record.
Fortunately, SRP’s reservoirs worked as designed — capturing the excess runoff in the wet periods and hoarding that excess through the dry spells.
Roosevelt Lake holds two-thirds of the storage capacity that sustains the Valley and remains 77-percent full, despite dry times on the Salt River, said Ester.