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SOUTH COUNTY, January 29, 2018 – In spring 2017 at Combie Lake, icy water gushed over the spillway and into the Bear River, 24 miles south of Grass Valley. It was the same at other reservoirs that store water for western Nevada County.
The Sierra Nevada snowpack of 2017 was the third-deepest on record, according to state meteorologists. Snow in the local watershed stood at 196 percent of average on May 1 that year, according to local officials. But little of that year’s abundant snowmelt benefitted western Nevada County. Reservoirs, filled earlier by wintertime rain, had little room left for the spring surge, and most of it flowed on downstream.
It’s an ironic example of the effect of rising winter temperatures on Sierra Nevada snowpack, the main source of water for 95,000 people and $98 million in agriculture served by the local Nevada Irrigation District.
“If we get rain during the winter instead of snow… if our reservoirs are full, we spill that water.”
– Keane Sommers, Nevada Irrigation District
Like other water districts along the Sierra Nevada, NID’s water storage system is built on a historic pattern in which water comes earlier in the year than people need it: Wintertime precipitation stays on the mountain as snow until late spring. Snow slowly melts to fill high-mountain reservoirs. Consumer demand picks up in mid-April, when agricultural water deliveries start; the snowmelt refills the reservoirs as water flows out to consumers. Consumer demand peaks in early summer and continues through mid-October, slaked by stored snowmelt.
But the winter of 2017 had a different pattern: Snow-dumping storms were followed by warm, tropical rains – several times. At middle elevations, rain ran off quickly. At higher elevations, rain melted already-fallen snow, increasing storm run-off, said regional climatologist Nina Oakley of the Western Regional Climate Center, in Reno.
That weather came on top of a 122-year warming trend in the Sierra Nevada: Wintertime average low temperatures have risen 2 degrees since 1895, shifting snowfall to rainfall across the range.
Keane Sommers, who manages NID’s hydroelectric system, noted what happened in the winter of 2017 in the local watersheds that feed NID’s reservoirs. “If we get rain instead of snow during the winter, it comes off nearly immediately,” Sommers said.
As a result, rain and snow run-off filled NID reservoirs much earlier in 2017 than usual, Sommers added. NID reservoirs – like others across California – had little room for the snowmelt that poured downhill.
“There’s little agricultural demand and little treated-water demand” during winter and early spring, Sommers added. “If our reservoirs are full and there’s no demand, we spill that water.”
In a typical year, the frozen reservoir of NID snowpack holds about 125,000 acre-feet of water – enough to supply NID customers for a typical year. But much of 2017’s bumper crop of snow flowed right out of NID’s system, Sommers said.
And, amid a warming climate projected to shrink Sierra snowpack by nearly half in 30 years, it’s unclear when the next big-snow year will come again.
“Our customers need that water,” said NID General Manager Remleh Scherzinger.
Rising temps advance spring snowmelt
Rising Sierra Nevada temperatures are both reducing snowpack and advancing the time when it melts.
In Nevada Irrigation District’s watershed, snowmelt historically crests in late May. But in recent years, it has come sooner – in mid-December during the 2016-17 water year, and in late March during the 2015-16 water year, according to NID records.
That’s exactly what researchers predict, using models that project the effects of rising temperatures on Sierra snowpack
“More snow will melt prior to April 1,” wrote Daniel Cayan of UC San Diego. “Expected effects… include earlier and more volatile runoff, which could produce more frequent and sometimes more intense flooding.”
Earlier spring run-off already has been observed in the watershed supplying the local Nevada Irrigation District.
“The melt is happening earlier by two to three months” said NID Hydroelectric Manager Keane Sommers. “It’s coming off earlier and faster.”
It’s a shift Sommers had noticed for the previous four years – and a departure from the pattern the previous hydroelectric manager had taught him to expect, he added.
“A growing challenge will be to retain and store this mountain runoff,” Cayan concluded in a 2016 article in Source magazine, written with Penelope Grenoble.
The Great Tree Migration
Trina Kleist is a Grass Valley freelance writer whose clients include Nevada Irrigation District. She may be contacted at firstname.lastname@example.org or (530) 575-6132.