Prepared exclusively for Maven’s Notebook by Robert Shibatani

The first two months of the new 2020-2021 water-year (WY) have been dry, without substantial precipitation accumulations across the CVP.  The last notable accumulations were on November 18th.  Precipitation totals for the 48-hour period ending on November 18th however, were notable (<3 inches) for some Sierra Nevada locations in Nevada, Placer, and Plumas counties (e.g. Alta Sierra, Grass Valley, Hell Hole).  It is still early in the WY and we do have the benefit of relying on recent year’s reservoir carryover, but storage totals are being inexorably depleted.

Precipitation-to-date totals for Trinity, Shasta and Huntington Lake are 26, 26, and 24% of the average, for this date, respectively.  Blue Canyon in the American River watershed has received less than half of its average precipitation totals for this time of year.  A La Niña has established itself with below normal precipitation anticipated throughout the upcoming winter months.  Much of the State over the next 14 days has a 33-50% probability of receiving below normal precipitation which is consistent with projections for much of the western U.S.  Corresponding temperatures during this time are expected to be higher than average.

Overall north CVP storage still stands at 96% of the 15-year average with Trinity, Shasta, New Melones, Folsom and San Luis reservoirs at 95, 88, 113, 88 and 92% of their 15-year averages, respectively.  Oroville Reservoir stands at 90% of its 15-year average.  Total storage in CVP reservoirs and Oroville is about 7.05 MAF representing about 46% of the total CVP and Oroville storage capacity.  This is about 3 MAF less than the same date last year.  An important point to remember; just because the reservoirs are at or close to their 15-year averages, does not mean they are full.  In fact, they are less than half full based on total capacity.

From an operational perspective, the question becomes, will the current precipitation forecasts compel reservoir operators to request “relaxation” (variance) of their reservoir rule curve requirements for empty space?  Can operators risk holding more water in storage in excess of allowable volumes in response to anticipated dry year conditions and the likelihood of decreased inflows?  Should they?  Or would that unacceptably compromise established flood management rules?  An increasingly interesting question once one factors in a shifting hydroclimate.  As the seasonal rains begin or whenever they arrive, we will be watching closely to see which reservoirs choose to “spill”.

Prepared by Robert Shibatani

Robert Shibatani, a physical hydrologist with over 35-years combined academic, legal, consulting and water advisory expertise, is 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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