The rains that bring May flowers sort of fizzled after what looked momentarily promising…

Across California, federal CVP reservoir storage currently stands at 4.986 million acre-feet (MAF), which is about 58% of the 15-year average, a slight increase in a comparison of averages from this same time a month ago.  In fact, when compared to this same time last month, federal reservoir storage actually increased by 278,000 acre-feet (AF) over the past 30-days despite receiving only modest levels of new precipitation around April 20-22.  Oroville Reservoir, the largest State reservoir also increased its storage by 241,000 AF over this same period.

One of the key features of California’s water system is the ability of water managers to control water balancing across the State.  Regardless of whether it’s the flood season, irrigation season, or under balanced or imbalanced delta water quality conditions, the system can be adjusted in so many different operational ways.   And that reaction can be quick; the mid-month rains notably increased East Side stream flows such that total delta inflows exceeded 13,500 cfs on April 24th (while less than 8,000 cfs on April 15th).  The other key reality is that water conditions are highly spatial in nature, meaning that specific drought or water surplus conditions in one location do not necessarily translate into identical conditions elsewhere.

For example, while many parts of the western U.S. worry about drought, the American River basin (which includes the State Capitol of California, Sacramento) is enjoying “normal” water conditions.  Folsom Reservoir storage stands at 768,000 AF, this is 7% above the 15-year average for the reservoir for this time of year.  Having Folsom Reservoir carryover at 7% above average on May 1st, is always considered a welcome positive sign.  Accumulated precipitation received to date at Blue Canyon in the upper American River basin stands at 100% of normal.  In short, it’s been a good precipitation year for the American River basin, even if the rest of the State has struggled.

Across the State, reservoirs have reacted differently to this year’s water availability.  While large reservoirs such as Trinity and New Melones have lost storage over the month of April, at 39,600 and 11,300 AF, respectively, other reservoirs such as Friant and San Luis have gained storage, at 49,947 and 26,905 AF, respectively. Elsewhere, the monthly accretions or depletions vary; for instance, while Lake Berryessa in the coastal system lost 28,985 AF, reservoirs in the Sierra Nevada like Tulloch and Goodwin gained storage, at 3,067 and 29,000 AF, respectively.  With increasingly warming spring/summer temperatures approaching, the annual P↓/ET annual imbalance has already started: for April, Folsom was 2.78/4.36 and at Shasta 3.89/4.36 in inches of water).

California’s snowpack continued its rapid depletion over the month even though notable accumulations were temporary over certain north-central portions of the State.  Statewide SWE stands at 7.3 inches, losing 3.7 inches over the month.  Current Statewide SWE is 32% of normal (it was 39% or normal at this time last month).

Across the northern, central, and southern regions, SWE values range between 20-38% of normal for this date.

The Northern Sierra Precipitation – 8 – Station Index stands at 39.3 inches (with current totals representing 81% of average).  For the San Joaquin Precipitation – 5 – Station Index, the current 24.3 inches represent 66% of average for this date.  For the Tulare Basin Precipitation – 6 – Station Index, the current 16.0 inches is only 62% of average.

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.


Full Natural Flow Data:

Precipitation Data:

Snow Data:

Extended Regional Forecasts:

Bulletin 120:

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For even more reservoir and water conditions, visit the Notebook’s Water and Hydrology Internet Resources page.
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