The monthly reservoir report is written by hydrologist Robert Shibatani exclusively for Maven’s Notebook.

California’s March (“water”) Madness didn’t pan out as we had hoped, but the water supply status is also not as dire as many would like us to believe.  Here are the facts as of today, the first day of April 2021.

Interestingly, collective CVP storage is still “gaining”.  As of yesterday, north CVP storage stood at 6.026 MAF or 53% of total north CVP capacity.  This represented about 75% of the 15-year average.  Over March, while precipitation was well below average, key CVP reservoirs such as Shasta, Trinity, Folsom and Millerton (Friant) still gained storage, relative to the first of the month; these reservoirs gained 112,100, 17,700, 14,100 and 9,403 AF, respectively.  Currently, storage for Shasta, Trinity, New Melones, Folsom, San Luis and Millerton (Friant) stand at 69, 77, 101, 60,64 and 59% of their 15-year averages, respectively.  The SWP’s largest reservoir, Oroville, stands at 1,432 TAF or 61% of its 15-year average for this time of year.

Monitoring reservoir releases over the month, we detected no “spills”.  Releases were generally at or below their 15-year median values. Water losses or depletions from reservoirs resulted from a combination of power releases, direct evaporation, and diversions for consumptive uses (e.g., Folsom Pumping Plant for City of Folsom and San Juan Water District).

As reported by media outlets across the western States, cumulative precipitation values for California are below average.  Accumulated precipitation to date for Trinity, Shasta, the American River basin at Blue Canyon, New Melones, and Huntington Lake were 59, 44, 55, 73 and 51% of average as of yesterday.  The Northern Sierra 8-Station Index is at 22.4 inches (or 51% of average) while the 5-Station San Joaquin Index is at 17.7 inches (or 53% of average).  At this time, these numbers are not encouraging.  April and May are the final “rainy” season months and typically only generate about 1.5 inches in Sacramento and over 5 inches in Redding.

It is no secret that current reservoir storage is being maintained primarily by snowmelt.  At present, Statewide SWE is about 60% of normal.  The central Sierra Nevada has a SWE of 19.2 inches, representing 64% of normal while the northern Sierra has a SWE of 18.8 inches or 67% of normal.  Snowpack SWE across the Sierras, however, are expected to decrease dramatically over April as daily air temperatures are rising rapidly; Sacramento for example, with a projected high today of 86 degrees, will approach the record high of 90 degrees.

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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