Written by Robert Shibatani for Maven’s Notebook

We’re now in what some call the summer’s anvil, the heart of the hot season.  And while temperatures have moderated somewhat, widespread arid conditions, declining water levels, and large as yet contained wildfires like the Dixie, McFarland, and several new fires in Sequoia and Kings Canyon national parks have made for a challenging time in State water resources management.  Throughout the U.S. western and southwestern States, seasonal flash floods have prompted declarations of local states of emergency with several missing people and a few regrettable fatalities.  Fortunately for California, we have so far been spared the consequences of widespread flash flooding but seeing it in surrounding States is a sober reminder that even during the worst droughts, monsoonal conditions can bring devastating floods quickly and without regard to human safety.

Currently, water storage in CVP reservoirs is about 57% of the 15-year average.  As of yesterday, total north CVP storage was 3.771 MAF which is approximately one-third of total north CVP capacity.  It is important to make the distinction between active storage, relative to total reservoir capacity, and storage, relative to longer-term averages (e.g., 15-year average); for they are different.  The former, a strict arithmetic portrayal of percent capacity paints a more ominous picture of a reservoir’s status.  The latter considers a wider range of reservoir storage factors reflected in past yearly operations and is perhaps a closer determinant of supply or yield risk.

The three largest federal reservoirs, Shasta, Trinity, and New Melones, currently are storing 1.454, 1.004, and 1.036 MAF, respectively.  These storage volumes represent 51%, 64% and 75% of each of the reservoir’s 15-year averages for this date, respectively.  Oroville Reservoir is currently storing only 892,000 AF, almost a million AF lower than on this same date last year.  At one-quarter of its total reservoir capacity, Oroville Reservoir storage is 44% of its 15-year average for this time of year.

All reservoirs lost considerable storage over the month of July.  For example, Shasta Reservoir lost 292 TAF, Trinity Reservoir lost 158 TAF, New Melones, 180 TAF, and Folsom Reservoir lost approximately 45 TAF.  These losses were primarily from power releases, with lesser amounts lost to direct diversions and evaporation.   Evaporation for the month was high.  Evaporative losses at Shasta Reservoir was calculated at 11,435 AF, for Trinity Reservoir it was 5,708 AF, for New Melones Reservoir, 7,054 AF and for Folsom Reservoir, 3,695 AF.

As most Californians will agree, water levels and river flows across the State were low, in fact, well below long-term averages.  Sacramento River flows at Keswick were about 9,000 cfs yesterday.  Flows in the lower American and Feather rivers were 1,018 and 2,000 cfs, respectively, considerably lower than their long-term medians for this date of 3,450 and 4,500 cfs, respectively.  Barring any substantive change (e.g., flash flooding), such reduced flows will continue through to the end of the WY.

There are two-months remaining in WY 2020-2021.  The State has arguably enough water to last until the new rainy season although there will be continuing shortages to various beneficial users including the environment.

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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