Written exclusively for Maven’s Notebook by hydrologist Robert Shibatani
Its currently raining here in Sacramento along with much of the north State and north-central Sierra Nevada with snowfall beginning last night from the Oregon border and continuing throughout the morning as far south as Huntington Lake as of early afternoon today.
This first December storm is a harbinger for wetter conditions expected through the weekend and into next week. And this rainfall comes not a moment too soon as many reservoirs and lakes across the State continue to recede during what has become a particularly dry autumn.
As of November 27th, total CVP reservoir storage was 3.25 million acre-feet (MAF), representing about 27% of total federal reservoir storage capacity or, 56% relative to the 15-average. This represents a decline of about 110,000 AF from the beginning of last month.
Current CVP reservoir storage for Shasta, Trinity, New Melones, San Luis, Folsom and Millerton are 1.404, 0.529, 0.586, 0.190, 0.250 and 0.298 MAF respectively. Oroville Reservoir has dropped below 1 MAF this past month for the first time in a while and is currently storing about 0.974 MAF, although still representing some 71% of its 15-year average for this date.
Not all CVP reservoirs, however, lost storage during November. New Melones and San Luis reservoirs gained 2,600 AF and 13,131 AF, respectively, while the others, such as Shasta, Trinity, and Millerton reservoirs lost 15,400 AF, 6,700 AF and 30,600 AF, respectively.
Folsom Reservoir lost 42,300 AF of storage and, over the month, water surface elevation in the reservoir dropped by 40.1 ft. Net storage loss is a cumulative condition and represents the balance between releases and inflow. Below is a schematic of Folsom Reservoir operations on November 15th with particular attention to water temperatures and proportional allocations through the various release/spill routes (e.g., releases on this day were primarily through each of the three power penstocks and one of the lower river outlets). On this date, the pumping plant was pumping 54 cfs, reservoir elevation was 380.62 ft, and the reservoir had a net loss of about 2,100 AF.
The reservoir released some 85,245 AF over the month and this was made up primarily by releases through the power penstocks (e.g., 60,429 AF), the river outlets (e.g., 21,392 AF), the pumping plant (e.g., 3,382 AF) and “spills” (e.g., 42 AF).
While situations will undoubtedly change in December, accumulated precipitation across the CVP is still depressed and far below average; 2.8, 3.4, 6.4, 1.9, and 5.1 inches, for Trinity, Shasta, Blue Canyon, New Melones, and Huntington Lake, respectively. Interestingly, current totals, as of this date, are largely below those of the critically-dry 1977.