Despite a wet winter and brimming reservoirs, the drought has crept back into the Rim Country on dusty paws.
Creeks flowing off the Rim have subsided to about three-quarters of their normal flow for this time of the year, despite the deeper than normal snowpack this winter.
Still, groundwater well levels remain high, with significant increases over the winter months. Moreover, none of the water companies in the area have yet imposed water use restrictions.
But rainfall levels half of normal since January and dwindling stream flows could create localized shortages in the months to come.
The dusty dry spring has underscored the capricious nature of rainfall in the southwest and the lurking threat of drought, already demonstrated by studies of rainfall patterns over the past 500 years. Studies based on the growth rings of trees have revealed repeated droughts, including 10- to 30-year periods with average rainfall only about half of the historic average. Climate experts fear such droughts could become more severe and last longer, as a result of global temperature increases linked to the effect of pollutants on the atmosphere. The past decade has included some of the hottest, driest years on record.
Certainly, the abrupt change in rainfall starting in January made the point painfully clear.
For instance, this week the Salt River at Roosevelt Lake carried 689 cubic feet per second, about 77 percent of normal, according to Salt River Project gauges.
Tonto Creek was doing a little better, with 27 cubic feet per second — about 88 percent of normal.
However, the Verde River had dwindled to 79 percent of normal, with a flow of 79 cubic feet per second at Triangle.
SRP doesn’t separately monitor the East Verde before it merges with the main stem of the Verde near Childs, but stream levels outside of Payson have dropped in the past several days.
A bone-dry spring seems to have overwhelmed the effect of a wet winter and a deeper-than-normal snowpack.
Between January and the end of April, Payson recorded just 3.75 inches of rain — compared to a 30-year average of 7.35 inches. In April, the area got just a trace of rain — about a third of an inch. Usually, Payson gets more than an inch of rain in April.
The succession of big winter storms both this year and last did provide enough runoff to fill key reservoirs, some of which had been nearly emptied by the region’s decade-long drought.
As of Monday, Roosevelt Lake was still 99 percent full, with about 2 million acre-feet in storage.
Roosevelt provides the bulk of the water storage for the Valley, greatly exceeding the capacity of all the other Salt and Verde River reservoirs. At one point some three years ago, it was about 17 percent full. Now, many boat ramps are nearly underwater and campsites once far from the shoreline have become beachfront property.
The other reservoirs on the Salt River system all stand at 92 to 98 percent full, including Canyon Lake, Apache Lake and Saguaro Lake. Combined, those three reservoirs hold a little more than 300,000 acre-feet. One acre-foot provides the needs of a family of five for a year.
The Verde River reservoirs hold 214,000 acre-feet. However they are currently only 75 percent full — mostly because SRP keeps water levels in Horseshoe Lake low to provide habitat for the endangered Willow Flycatcher, which nests in tamarisk thickets near the lake inlet.
The quick return of dry conditions underscores the disquieting lessons learned from studies of rainfall patterns in the Southwest — and the value of Payson’s moves to lock in a long-term supply of water from the Blue Ridge Reservoir.
The major reservoirs on the Colorado River at one point were less than half full and have only partially recovered. Seven states concluded decades of lawsuits by dividing up the flow of the Colorado River.
However, they signed the settlement during a long, wet period and assumed that the river carried 16 million acre-feet on average. In fact, the river actually carries about 13 million acre-feet per year.
Even worse, tree ring studies show that during drought periods, the flow can decline to more like 11 million acre-feet.
Severe droughts have occurred several times a century throughout the past 400 to 500 years, according to a study by researchers from the University of Arizona’s Tree Ring Laboratory.
The study looked at tree ring patterns between 1579 and 1962 from 121 sites scattered throughout the Southwest, but concentrated in the Colorado River drainage.
The most severe drought recorded in the Southwest was from 1660 to 1670, but the longest sustained drought lasted from 1579 to 1598.
However, in Arizona the study found the 1950s produced the longest, most severe sustained drought, prior to the even sharper drop in the past decade.
Fortunately, even severe droughts often include a year with above-average rainfall. Thanks to the chain of Salt and Verde river reservoirs, the Salt River project can capture enough runoff in even one such year to ride out another four or five years of drought without imposing major water use restrictions in the Valley.
Ironically, Rim Country, with no reservoirs, gets far more rainfall but remains much more vulnerable to drought than the Valley.
Groundwater studies show that the groundwater wells supplying Payson, Star Valley and other Rim communities fall rapidly in the face of drought conditions. Payson well levels dropped by several hundred feet as the population boomed, but stabilized three years ago when the town adopted the toughest water conservation ordinance in the state.
Studies monitoring well levels in Payson and Star Valley reported sharp increases in well levels in the past year, as a result of two winters of normal rainfall.
The Blue Ridge pipeline would for the first time back up Payson’s water supply with a reservoir. Currently, the town uses about 1,800 acre-feet annually from its network of wells, On the average, rainfall puts about 2,500 acre-feet annually into the water table beneath Payson — although during drought conditions, those totals can fall by 50 percent for extended periods.
The Blue Ridge pipeline would provide about 3,000 acre-feet annually to Payson and another 500 acre-feet to other Rim communities. However, that water supply remains dependent on the 11,000 acre-foot Blue Ridge Reservoir, which could also be affected by extended drought.