Prepared exclusively for Maven’s Notebook by Robert Shibatani

As we enter the last month of this water year, it looks as if we may have dodged a bullet.  The long-term forecast for the remainder of the year does not identify any protracted periods of triple-digit heatwaves.  And while devastating wildfires across the State, like the Caldor Fire have captured our attention, the California water forecast for the remainder of the year looks at least manageable.

Despite very low precipitation levels across the State, this was not the driest year on record. Water Year 1977 still holds that distinction.  Shasta Dam precipitation to date, for example stands at 23.66 inches, in 1977 it was 17.69 inches.  By comparison, in 1983, a record wet-year, the Shasta gauge stood at 114.16 inches.  Rain gauges at Shasta and Trinity reservoirs in the north and Huntington Lake in the south are at 39, 52 and 43% of their annual averages, respectively.

Currently, water storage in CVP reservoirs is about 54% of the 15-year average (it was 57% on August 1).  As of yesterday, total north CVP storage was 3.245 million acre-feet (MAF), having lost some 526,000 AF over this past month.  Volumetrically, this represents about 29% of total north CVP reservoir capacity.  This number shouldn’t be too alarming, however, since, on average, system capacity is approximately only 52.5% on this date every year.  Last year at this same time, there was almost 2.72 MAF more water in storage.

California’s three largest federal reservoirs, Shasta, Trinity, and New Melones, currently are storing 1,227,000, 854,000, and 905,000 AF, respectively.  These storage volumes represent 49%, 60% and 69% of each of the reservoir’s 15-year averages for this date, respectively.  These numbers can be considered not bad, given this season’s circumstances.  Federal water managers have been diligent in controlling yield in their three largest storage facilities.  The largest State Water Project facility, Oroville Reservoir is currently storing about 800,000 AF, representing 46% of its 15-year average and over 900,000 AF lower than on this same date last year.

All reservoirs lost storage over the month of August.  For example, Shasta Reservoir lost 227,000 AF, Trinity Reservoir lost 150,100 AF, New Melones, 131,000 AF, and Folsom Reservoir lost 10,300 AF.  These losses were primarily from power releases, with lesser amounts lost to direct diversions and evaporation.  At Friant, the reservoir released over 44,514 AF to the Friant-Kern Canal with only 16,701 AF released to the river.  Compare that to Folsom Reservoir where the opposite user allocation occurred, here, 29,411 AF were released to the river through the power plant and only 5,422 AF was released to the pumping plant for deliveries both north and south of the river.

Evaporation for the month was high, but notably declined from July.  Evaporative losses at Shasta Reservoir was calculated at 7,160 AF (relative to 11,435 AF in July), for Trinity Reservoir it was 3,285 AF (5,708 AF in July), for New Melones Reservoir, 5,137 AF (7,054 AF in July) and for Folsom Reservoir, 2,836 AF (3,695 AF in July).

Prepared by Robert Shibatani

Robert Shibatani, a physical hydrologist with over 35-years combined experience as an international expert witness on reservoir-operations, climate change hydrology, commercial flood damage litigation, and water supply development.  He is Managing Partner for The SHIBATANI GROUP International, a division of The SHIBATANI GROUP Inc. and resides in Sacramento, California.


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