Willow Lake withdrawals recharging aquifer
Ongoing rainfall has kept lake’s level fairly high, officials say
In a switch-up this year, Willow Lake has been bearing much of the responsibility for the City of Prescott’s surface-water recharge into the area’s aquifer.
Already in 2019, more than a quarter-billion gallons of water has been released from Willow Lake and directed to the city’s recharge facility near the airport, where the water is allowed to filter into the ground for aquifer recharge.
The city’s water policy calls for the annual withdrawal of water from its northeast Prescott lakes to replenish the aquifer. The recharged water results in water credits for the city, which can then be allocated to new development as “alternative water.”
Normally, the bulk of the city’s withdrawals and recharge happen at Watson Lake, which is deeper and tends to fill up quicker because of a larger watershed area.
But an unusually wet winter and spring — combined with needed repairs at the Watson Lake dam — have served to shift much of the recharge to neighboring Willow Lake this year.
Public Works Director Craig Dotseth reported that 710 acre-feet of water had been withdrawn already this year from Willow Lake. (An acre-foot totals 325,851 gallons of water – for a total of more than 231 million gallons released from Willow Lake so far this year.)
The water is currently flowing from a valve at the base of the Willow Lake dam, and is being allowed to flow down a channel toward the airport recharge facility.
The 710 acre-feet of water is in addition to about 828 acre-feet that flowed over the spillways of the two dams earlier in the year — water that also was directed to the city’s recharge field.
In all, Dotseth said, the city has already recharged 1,538 acre-feet of surface water from the lakes in 2019.
Despite the Willow Lake withdrawals, Dotseth said the lake level has remained fairly high because of ongoing rainfall.
The city’s withdrawals from Willow Lake started on April 9, after water stopped overflowing the lake’s spillway.
Both of the lakes had been overflowing for weeks prior to that from runoff from a series of winter snowstorms – Watson since Feb. 28, and Willow since March 3.
Willow Lake is currently 1.3 feet below its dam spillway, after rising about a half-foot after last week’s rainfall. Dotseth said the city likely would continue withdrawing water from Willow Lake — possibly until the lake is about four feet below its spillway.
Although the lake levels are expected to drop because of the release-and-recharge program, Dotseth said, “They definitely are going to be higher than normal (because of the wet year).”
At Watson Lake, the city is currently releasing water to meet its annual requirements for 375 acre-feet of prior surface-water rights for Granite Dells Ranch. Therefore, water from Watson is currently not being recharged into the aquifer.
The start of that obligation came after weeks of repair work on the Watson Lake dam, which temporarily precluded the city from releasing water from Watson Lake.
Dotseth said the dam work is now substantially complete. A final walk-through took place on Monday, May 13.
While the city’s use of surface-water recharge helps to augment its water portfolio, water consultant Herb Dishlip noted this past week that the capabilities fluctuate greatly from year to year.
“Some years, it’s dry; some years, there’s an abundance,” Dishlip said, pointing to a graph that showed annual withdrawals from the lakes fluctuating from a low of zero in 2007 to a high of more than 4,000 acre-feet in 2017.
City Councilman Steve Blair questioned city Water Resource Manager Leslie Graser about the limitations the city faces during wet years, such as 2017 and 2019. He maintained the city should be able to withdraw and recharge more in wet years.
But Graser pointed out that the city is limited by the state in the amount of surface water it can release and recharge. The annual recharge total is limited to 3,861 acre-feet per year. (The release limit is slightly higher because of expected losses through evaporation.)
“We only have a surface water right to a certain volume,” Graser said.
Blair responded, “It makes no sense when we have a wet year.”
Other council members maintained that Prescott has planned well for its water use, and noted that water conservation programs have helped to reduce the amount of water used by city residents each year, even as the population rises.
Dishlip added that Prescott has had to plan more extensively because of its lack of Central Arizona Project (CAP) water from the Colorado River. Because Prescott does not have that source of water, as do Phoenix-area communities, Dishlip told the council, “You have had to work harder.”