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Thu, Dec. 05

Prescott: 2019 could be a record-breaking year for water recharge
Continued withdrawals cause drop in lake levels

Water flows from the newly installed valve in the Watson Lake dam as a part of the City of Prescott’s groundwater recharge and recovery program. Public Works Director Craig Dotseth says about 6.5 million gallons of water a day are currently being released from the lake. (Cindy Barks/Courier)

Water flows from the newly installed valve in the Watson Lake dam as a part of the City of Prescott’s groundwater recharge and recovery program. Public Works Director Craig Dotseth says about 6.5 million gallons of water a day are currently being released from the lake. (Cindy Barks/Courier)

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The City of Prescott’s withdrawal of water from Watson Lake has caused a nearly five-foot drop in the lake level. A high-water mark on the granite walls shows the level of the lake earlier this year. (Cindy Barks/Courier)

A steady rush of water – totaling about 6.5 million gallons a day – is currently coursing out of a valve on the Watson Lake dam.

That continued withdrawal of surface water, in turn, has caused a drop of nearly five feet in the level of the scenic Granite Dells-area lake.

Prescott Public Works Director Craig Dotseth told the Prescott City Council this week that the withdrawal of water from Watson Lake (and the nearby Willow Lake) is a key piece of the City of Prescott’s groundwater recharge and recovery program.

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 then can be allocated to new development as “alternative water.”

So far this year, the city has withdrawn and recharged about 2,800 acre-feet of water from its two lakes, which translates to a total that is approaching the billion-gallon mark.

An acre-foot totals 325,851 gallons of water — for a total of more than 912 million gallons released from the two lakes so far this year.

POSSIBLE RECORD-BREAKING YEAR

Depending on the amount of precipitation that the community gets in the coming monsoon season, Dotseth said 2019 could duplicate or exceed the wet year of 2017 for recharge into the aquifer.

That year, the city recharged 3,861 acre-feet of water, along with another several thousand acre-feet of treated wastewater (effluent) – for a total recharge of about 7,300 acre-feet.

“The best ever has been the 7,300 in 2017, and we are on track to be close to that, or maybe we can even exceed it this year,” Dotseth told the council. “We’ll see how it goes with moisture. But we are going to be in that same realm for 2019.”

Although withdrawals from Willow Lake were halted on about June 12, the water release at Watson Lake continued. Dotseth he expects the level of withdrawal to remain at its current rate until Watson Lake gets to about seven feet below the spillway.

Then, if monsoons replenish the lakes, the withdrawals would continue until the city reaches its limit of 3,861 acre-feet a year. Currently, Dotseth said, the city is about 1,000 acre-feet from that point.

LAKE IMPACTS

After a wet winter and spring, both lakes were full to overflowing earlier in the year, but the withdrawals have had significant impacts.

On Thursday, July 11, Dotseth reported that Willow Lake was about 4.6 feet below the spillway, while Watson Lake is about 4.97 feet below its spillway.

Although both lakes are down this summer, they are nowhere near the level they were in the dry year of 2018, when Watson Lake was 10.7 feet below the dam’s spillway, and Willow Lake was 8.6 feet below spill.

RECHARGE EXCEEDS PUMPING

The wet conditions of 2019 have allowed the city to reach a point where recharging has exceeded the amount of water pumped from the ground.

“A total of 4,451 acre-feet has already been recharged,” Dotseth told the council on Tuesday. “In contrast, we have pumped 3,268 acre-feet. Right now we’re about 1,200 acre-feet ahead with recharge. We’ve recharged almost 1,200 acre-feet more than we’ve pumped for the year.”

Still, Councilman Steve Sischka said he regularly gets questions from residents who ask about the correlation of the recharge with the actual groundwater levels.

“I think we need a more definitive answer … to your best estimate, where does it go?” he asked Dotseth.

While Dotseth responded, “It’s going back into the aquifers,” he said the timeframe for how long the water takes to reach the city’s main water source at its wellfield in Chino Valley is harder to determine. “I can’t tell you if that is months or years,” he said.

When the city bought Watson and Willow Lakes from the Chino Valley Irrigation District in 1998, it began releasing water from the lakes to supplement the amount being recharged into the aquifer. The withdrawn water is directed to the city’s airport-area recharge field, where it is allowed to filter into the ground.

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