Storms bring welcome water to Sierra reservoirs, snowpack
Slow-moving atmospheric river storms soaking the Mother Lode foothills and the rest of the Central Sierra with rain and snow in recent days, combined with precipitation from previous storms since the current water year began Oct. 1, have combined to put a dent in drought conditions up and down central and northern California.
As of Monday afternoon, storms since Tuesday and Wednesday last week had brought 3.28 inches of rain to New Melones Dam; 3.6 inches of rain to Mount Elizabeth; 4.58 inches to Sonora; 3.84 inches to Columbia; 4.21 inches to Moccasin; 4.49 inches to the Stanislaus Powerhouse at Camp 9; and 4.25 inches to Buck Meadows, Katrina Hand with the National Weather Service said Monday.
As of Monday, the Central Sierra region that includes the Stanislaus River and Tuolumne River watersheds, Calaveras Big Trees, Hetch Hetchy and the Stanislaus Na
tional Forest had received 19.2 inches of precipitation since Oct. 1. That regional total was up 5 full inches in the past 96 hours, since Thursday last week.
The 19.2 inches as of Monday morning was equivalent to 170% of average for the date Dec. 27.
High in the mountains where sensors are keeping track of the snowpack, instruments Monday showed Central Sierra snow-water equivalents at 157% of normal for the date Dec. 27, and 52% of average for April 1, a key date water agency managers use to monitor each year’s water supply. Statewide, snow-water equivalents were at 153% of normal for the date Dec. 27, and 49% of average for April 1.
Updates Monday morning showed major, mancontrolled reservoirs in Calaveras, Tuolumne, and Mariposa counties holding from less than one-quarter to more than three-quarters of capacity.
New Hogan on the Calaveras River was holding 36% of capacity. On the Stanislaus River, New Melones, the state’s fourth-largest capacity reservoir, was 38% full, Donnells was 45% full, Beardsley was 57% full, and Tulloch below Melones was 85% full.
On the Tuolumne River, Don Pedro, the state’s sixth-largest water storage facility, was 78% full. Above Moccasin in the Tuolumne River watershed, Cherry was 90% full and Hetch Hetchy in Yosemite National Park was 75% full. On the Merced River, Mcclure was just 23% full.
Tuolumne Utilities District, which conveys water to more than 40,000 people in southwest Tuolumne County, said Monday that TUD will meet or exceed its annual water supply for the 2021-22 winter.
“Lyons Reservoir and Pinecrest Lake are small and are currently holding above-average storage levels, and are certain to fill and spill with the forecasted storms,” Don Perkins, the TUD general manager, said Monday. “Water supply for TUD will be met for the year with the next check-in to occur as we approach the end-of-spill.”
End-of-spill will be sometime in spring or early summer, once snowmelt runoff peaks and eases, and dams in the South Fork Stanislaus watershed cease overspilling.
As of Dec. 14, Pinecrest was holding 15,275 acrefeet, 83% of capacity, and Lyons was holding 2,730 acre-feet, 50% of capacity, Perkins said Monday.
The most recent U.S. Drought Monitor maps, updated Thursday, Dec. 23, showed a reduction in the most severe drought conditions in the Central Sierra, with the foothills of Calaveras and Tuolumne counties remaining in the most dire drought category — exceptional — while most of the Central Sierra has eased from exceptional drought to severe drought.
Drought Monitor maps are likely to change again this Thursday as more precipitation from current storms are factored into the big picture.
Whether La Niña forecasts for a drier winter are accurate remains to be seen.
As of Dec. 20, oceanographers and other climate scientists said La Niña conditions for a potentially drier-than-average winter in California remained present. Equatorial sea surface temperatures were below average across the central and east-central Pacific Ocean, and the tropical Pacific atmosphere was consistent with La Niña. In addition, La Niña was 95% favored to continue through the Northern Hemisphere winter 202122.