Runoff from the South Fork Stanislaus watershed stopped spilling at Lyons Reservoir on Monday — the earliest end-of-spill date on record — and while a Tuolumne Utilities District engineer said the annual water supply for more than 40,000 Tuolumne County residents is secure, he warned earlier this week “it’s going to be a tight fit to get through Labor Day.”
Each spring in the Central Sierra, some reservoirs fill all the way up from snowmelt and other runoff. Once full, those reservoirs begin to spill because they can’t hold any more water.
End-of-spill is when the snowmelt and runoff stop flowing into a full reservoir, and it’s viewed as the end of Mother Nature’s annual contribution to that reservoir. No more water is flowing in, so the reservoir stops spilling.
June 7 is the earliest end-of-spill for Lyons, including the driest year on record, 1977, when the reservoir stopped spilling in July, Glen Nunnelley, associate engineer for TUD, told the district’s elected board of directors in a public streamed meeting Tuesday.
This year’s June 7 end-of-spill at Lyons also came four days earlier than it did in 2015, another very dry year, when end-of-spill happened on June 11, Nunnelley said.
‘We’re in amongst the driest years’
Early end-of-spill can be a bad sign, and late end-of-spill can be a good sign, but it’s not that simple.
End-of-spill is just one variable water managers use to gauge, calibrate, and predict water supply in wet and dry years. Furthermore, it is not a bottom line trigger for determining drought severity. Timing of late spring storms can push end-of-spill back into July, yet that year can be one of the driest water years on record, like 1976-77.
Nunnelley qualified this year’s end-of-spill at Lyons, at length. He didn’t sugarcoat it for the TUD board.
“We’re in amongst the driest years,” he said.
“This is concerning,” Ron Ringen, the TUD board vice president, said. “I’m glad you’re keeping an eye on it.”
“This climate change thing is kind of on us,” TUD Board President Barbara Balen said.
Tuolumne Utilities District, created by voters in 1992, relies on water stored behind dams in reservoirs where infrastructure is owned and operated by Pacific Gas and Electric Co., which uses stored water to generate hydroelectricity.
The district is currently involved in multi-year negotiations with PG&E to try to acquire water rights and infrastructure from the company that are critical to TUD’s annual water supply.
‘Our water supply is secure’
Nunnelley began his presentation on TUD’s water supply by saying, “We’re providing just a brief update because of the conditions. They’re definitely dry… So the end of the line we have right now is the end-of-spill occurred yesterday. We were originally anticipating, we had some forecasts performed as early as April, we were looking at kind of mid-June, maybe no earlier than about June 10th. So this is definitely early.”
Earlier than anticipated end-of-spill at Lyons Reservoir means TUD is now reliant on stored water the rest of this year, Lyons first, then tapping into Pinecrest storage some time around Labor Day, Nunnelley said.
“We’re anticipating Pinecrest will be down to 5,607 feet in elevation,” he said. “So it’s anticipated to be below their target of 5,610 feet in elevation. And right now, Lyons Reservoir will probably drop below 1,200 acre-feet of storage.”
Cabin owners, boaters, and business owners at Pinecrest prefer that PG&E keeps the reservoir as full as possible, at 5,610 feet minimum, through Labor Day each year for recreation and aesthetics.
Lyons Reservoir has a maximum capacity of 6,200 to 6,400 acre-feet, according to state dam and reservoir records.
Nunnelley continued, “This is doable. We’re going to watch it carefully. It’s still a few months away. Things can change in our forecasts. It’s not perfect. When we’re running things tight, it’s good to watch those scenarios. The supply is there. The water is in Pinecrest, and there’s a large allocation of water to get us through the rest of the year. So our water supply is secure in that respect.”
‘Paycheck to paycheck’
To put things in context, Nunnelley said, TUD is not below any of the state’s largest reservoirs, with a large storage capacity reservoir above TUD’s service area, with several hundred thousand acre-feet and a demand nearer 20,000 acre-feet.
“We go paycheck to paycheck,” he said. “So our first concern is we have to get enough rain and snow to fill the reservoirs. Once they’re full, they can’t get more full. Then we see how early end-of-spill hits, and we may have a push against the regulatory environment. Right now, we have early end-of-spill and dry conditions, and we have stored water to get us through to the end of the year.”
Balen asked about the end-of-spill at Lyons in 2015. Nunnelley said it occurred four days later than it did this year.
Board member David Boatright asked for the earliest end-of-spill at Lyons on record. Nunnelley said, “This one.”
“Even 1977, our driest year, the end-of-spill was much later, I think, the end of July,” Nunnelley said. “This year compares to 2007. Unusually dry and low reservoirs by Labor Day.”
Nunnelley finished his presentation on a cautiously upbeat note.
“We’re no strangers to it,” he said of the current dry conditions and limited water supply. “Our demand is about what it was last year. We’ll see how the climate unfolds. There could be more or less evaporation. Could be more or less demand as we go through. It’s a tight year to watch, and we’ll get through it.”
Contact Guy McCarthy at email@example.com or 770-0405. Follow him on Twitter at @GuyMcCarthy.